219575445 Drafting Electrical Drawings

Engineering Encyclopedia Saudi Aramco DeskTop Standards

Drafting Electrical Drawings

Note: The source of the technical material in this volume is the Professional

Engineering Development Program (PEDP) of Engineering Services. Warning: The material contained in this document was developed for Saudi

Aramco and is intended for the exclusive use of Saudi Aramco’s employees. Any material contained i n this document which is not already in the public domain may not be copied, reproduced, sold, given, or disclosed to third parties, or otherwise used in whole, or in part, without the written permission of the Vice President, Engineering Services, Saudi Aramco.

Chapter:D rafting FileReference:AGE10804

Foradditionalinformationonthissubject,contact N.H.Alahaimeron873-0876

Engineering Encyclopedia

Drafting Drafting Electrical Drawings

C ON TEN TS

PAG ES

INFORMATION Electrical Drawings

1

Electrical Symbols

1

DraftingPracticesUsingGraphicalSymbols TypesofElectricalDiagrams

9

PanalarmAnnunciatorSystem

26

EmergencyShutdown(ESD)System

28

Truth Tables

43

Ladder Diagrams

45

ElectricalBuildingConstructionDiagrams

59

Circuit Loads

68

PowerGenerationandTransmission WORK AID GLOSSARY

Saudi Aramco DeskTop Standards

8

96 158 162



ELECTRICAL DRAWINGS

Saudi Aramco uses many different kinds of electrical drawings and each has its particular use. The drawings use graphic symbols to indicate the components being used in the electrical circuits. There are many symbols in use. It is not possible to cover all of them in this module. Equally, it is not practical for you to try to remember them all. However, you should be aware that the symbols exist and you should know where a description of their meaning can be found. Electrical Symbols

Some of the symbols used by Saudi Aramco can be found on Mandatory Drawing 990-PAB036766 . Some of these symbols will be shown and referred to throughout this module. They can be easily called up on Computer Aided Drafting and Design (CADD) systems. Some of the basic symbols, together with descriptions, are shown in Figure 1.


1




2




3




4




5




6




7



Drafting Practices Using Graphical Symbols

(a)

The orientation of a symbol on a drawing does not alter the meaning of the symbol. This is true even if the symbol is drawn backwards . A symbol is made up of all its various parts.

(b)

The weight (or width) of a line does not affect the meaning of the symbol. In some cases a heavier line may be used for emphasis.

(c)

Symbols are not drawn to scale. They can be drawn to any size compatible with the scale of the drawing.

(d)

Arrowheads can be drawn closed or open, except when showing a "protective gap" (a gap placed between line parts and the ground which limits the maximum over-voltage that may occur.)

(e)

The standard symbol for a t erminal (o) can be a dded to a ny one of the graphic symbols where connecting lines are attached. This added terminal symbol is not a part of the graphic symbol itself.

(f)

In order to make a drawing simpler, graphic symbols for devices such as relays or contactors may be drawn in parts. However, if this is done the drawing must show how the parts are related.

(g)

Most often, it does not matter at which angle a connecting line is drawn to meet a graphic symbol.

(h)

Broken lines with short dashes: - - - - - - , may be used to show paths or equipment that will be added to the circuit later, or those which are connected to the circuit but are not part of it.

(i)

If details such as type, impedance, and rating are to be given, they should be drawn next to a symbol. If abbreviations are used, they should be in accordance with the American Standard Abbreviations for Use on Drawings. Letters that are joined together and use parts of graphic symbols are n ot abbreviations.


8



Types of Electrical Diagrams Single-Line Diagram

A single-line or one-line diagram uses single lines and simplified graphic symbols to show electric circuits, or systems of circuits, and the components used. A single-line diagram can be used to show essential components and their function in simplified form. Figure 2 shows an example of a single-line diagram. They may show single lines even though three wires (for 3-phase supply) are used.


9



Connection or Wiring Diagram

A connection or wiring diagram shows how the circuit's componen ts are connected. It may include connections inside or outside the components. It gives as much detail as is needed to make or trace connections. A wiring diagram usually shows how a component looks and where it is placed. Figure 3 shows a connection or wiring diagram for a refrigerator.


10



Interconnection Diagram

An interconnection diagram is similar to a wiring diagram, but only shows connections on the outside of a component. It shows connections between components; the connections inside the components are usually omitted. Figure 4 shows an interconnection diagram for a rotational system.


11



Block Diagram

Block diagrams are made up of squares or rectangles ("blocks"). They are joined by single lines. The blocks show how the components or stages of the circuit are related. Arrowheads are drawn at t he ends of the terminal lines. They show the direction t he signal travels from input to output. The identification of a st age is lettered wit hin its block or just outside it. The blocks may be used together with symbols and a schematic diagram. Block diagrams are often drawn as a first step in designing new equipment. Many different layouts may be sketched before deciding which to use. Figure 5 shows a block diagram.


12



Schematic or Elementary Diagrams

These diagrams use graphic symbols to show how a circuit is connected and what it does. They do not have to show the size or shape of the components in the circuit, nor where the parts of the circuit actually are. It follows, therefore, that there is more than one way to lay out any schematic drawing. Figure 6 shows a simple schematic diagram. In this diagram, the battery supplies direct current (dc) to the circuit. The positive (+) and negative (-) terminals show the polari ty of the battery terminals. The long line in the battery symbol is always POSITIVE (+). The solid lines represent conductors or wires that carry the direct current through the circuit. The resistor acts like friction in a pipeline. It reduces the amount of current t hat can flow through a circuit. The ammeter measures the current in the circuit (Note that ammeters must always be connected in series., i.e. the current being measured must be able to flow directly through the ammeter.) The voltmeter measures the voltage across two points in a circuit. In Figure 6 it is measuring the voltage across t he battery. (Note the voltmeters must always be connected in parallel , i.e. it must be connected "across" the two points between which the voltage is being measured.) The fuse protects the circuit from being damaged by too high a current flow. The fuse will burn out if the current is higher than that for which the fuse is rated.


13



Schematic or Elementary Diagrams(Cont'd)

Figure 7 shows a schematic in which power is supplied by an alternating current (ac) supply. Alternating current is shown by the symbol in Figure 8. The sine wave represents the alternating current. The symbols on a schematic are usually numbered. On Figure 7, for example, the resistors are R1 and R2; lamps are L1, L2 and L3; switches are S1 and S2; the fuse is F1; the ammeter is M1 and the voltmeter is M2 (the M stands for "meter" in each case).


14



Series and Parallel Connections

Figure 9 illustrates the difference between series and parallel connections. In (A) two resistors (R 1 and R 2) have only one common connection. They are therefore in series and the same current, I, passes through both of them.

In (B), resistors R 1 and R 2 share two common connections. The current, I, from the battery is divided between R1 and R 2. The resistors are connected in parallel.


15



Relays

A relay is an electrically-operated switch. Figure 10 shows a relay. The coil induces a magnetic field when a current is passed through its wir e. The soft iron core increases the strength of this magnetic field. The iron core also acts like a magnet and attracts the armature. Moveable contacts are attached to the armature. When the coil is energized, the armature is pulled down to the iron core. This causes the normally closed (NC) contacts to open, and the normally open (NO) contacts to close. A spring is used to hold the armature away fr om the iron core. It keeps the NC contacts closed when the coil is deenergized. Insulating material isolates the contacts from each other and from the rest of the relay body. The contacts conduct electricity when they are closed. They act as switches to open and close circuits.


16



Relays (Cont'd)

Figure 11 shows the symbols used by Saudi Aramco to show relays. drawn in their deenergized state - ALWAYS!


Contacts are always

17



Relays (Cont'd)


18




19



Relays (Cont'd)

The symbols shown above are found on schematics of Saudi Aramco alarm and emergency shutdown (ESD) systems. The main purposes of the alarm and ESD systems are to warn operators that a process or a machine is not operating properly and to shut down a machine, a process or an entire plant if dangerous conditions develop. Figure 12 shows how a NO relay can be used in a circuit.


20



Relays (Cont'd)

There are really two circuits in Figure 12. One is a direct current circuit with a 12 v supply, and the other is alternating current with 110 v supply. When the switch is closed, 12 volts will flow through the relay, R1. The magnetic field that is induced will cause the normally open contacts to close. This will cause the ac circuit to be completed and an alternating current will flow to light the lamps. Relays have to be plugged into bases. The base is not a part of the relay, but the relay cannot be used without the base. The base is like a termination block for the relay. The relay has pins which plug into the base. Relays often have diagrams printed on them, l ike the one shown in Figure 13-A. It shows how the wires in the relay base should be connected. These connections are the same as those of the relay.


21



Relays (Cont'd)

Therefore, the diagram also shows how t he pins are connected inside the relay. The relay coil is seen to be connected to terminal 2 and 10. This is where the power to the relay would be connected. Terminals 1, 3 and 4 are a group of contacts. Terminals 1 and 3 are NO. Terminal 1 and 4 are NC. Terminals 5, 6 and 7 form another group of contacts and so do terminals 8, 9 and 11. Figure 14 shows a relay circuit for a high pressure alarm system. The circuit shown is for when the pressure is normal. Pressure Switch No1 (PS1) senses that the pressure is normal. The switch stays closed. This allows current to flow through pins 2 and 10 and keeps the coil of relay R 1 energized. Current also flows to keep the coil of relay R 3 energized.


22




23



Relays (Cont'd)

When R 1 is energized, the following contacts are made: Pin 1 and Pin 3 Pin 6 and Pin 7 Pin 9 and Pin11 Tracing the current flow shows that: (a) Pins 1 and 3 are not used because there is not a complete circuit and no current can flow; (b) Pins 6 and 7 are used because there is a complete circuit. The green light, L 2 will illuminate to show that pressure is normal; (c) Pins 9 and 11 are used because there is a complete circuit. Current flows through relay R 2 and energizes it. Note that the coil is energized even though the pressure is normal. This is done so that any break in the circuit (that is, any failure in the circuit) would deenergize the coil, R1. This would cause the relay's spring to close contacts 6 and 5. Current would then flow t o light up the high pressure warning lamp. This is called a FAILSAFE system. Upon subsequently finding that pressure was normal, the operators would know that the fault lies in the relay. Similarly if pressure really does go high PS1 will trip open, as shown in Figure 15 and deenergize relays R 1 and R 3. The only complete circuit will then be through pins 5 and 6 and the high-pressure warning light, as shown in Figure 15.


24




25



Panalarm Annunciator System

All Saudi Aramco processing plants have special lights and alarms called ANNUNCIATORS. Annunciators cause alarms to be given (usually as flashing lights and horns) when any equipment or process begins to malfunction. The alarms are part of the "Panalarm" system. Figure 16 shows some of the symbols used on alarm systems and elsewhere. Note how the switches for different process variables have different symbols.

THE PARTICIPANT MAY NOW ATTEMPT TO COMPLETE EXERCISE No.2, PAGE 173.


26




27



Emergency Shutdown (ESD) System

ESD systems are designed to shutdown equipment, processes or entire plants automatically if dangerous conditions occur. Figure 17 shows an ESD system.


28



Emergency Shutdown (ESD) System (Cont'd)

A)

A production trap separates gas from crude oil. The normal operating pressure is between 85 and 100 psi. The separated gas is sent to a gas compression plant, or to a flare.

B)

A dual pressure switch is used to operate the ESD system. The switch has two settings: 100 psi and 120 psi.

C)

If the pressure in the trap goes to 100 psi, the pressure switch will trip. Relay R open. This breaks the circuit to the Panalarm.

D)

The high-pressure alarm light illuminates to warn the operators. A horn also sounds.

E)

If the pressure continues to rise relay R

F)

x 2. W hen R 2 opens, the circuits to the Solenoid Operated Valves (SOV) are broken. This causes the solenoid cores to drop. The air supply to the pressure control valves is then stopped.

G)

The PCV at G is AO/AFC. Therefore, the stopping of the air supply to the valve causes the valve to close. This stops gas from being sent at too high a pressure to the gas compression plant.

H)

At the same time, the stopping of the air supply causes the PCV at H to open. (The

2

1

will

will trip open, as shown in Figure 18.

valve is AC/AFO. Therefore, theflare. air supply the valve.) to be vented from the trap t o the This failure reducesopens the pressure in theThis trapallows until itgas is back to normal. Note how the diagrams show air going to the top of the valve diaphragm in the AC/AFO case and to the underneath of the diaphragm in the AO/AFC case


29



Logic Circuits

Logic circuits are used t o show on a diagram how all the parts of an ESD system work. A logic circuit can be thought of as one of the following: •

AYES/NOcircuit

•

A TRUE/FALSE circuit

• •

AnON/OFFcircuit AnAND/ORcircuit

In ESD systems it is usually convenient to think of the circuits as AND/OR. As an example, look at the circuit in Figure 19. In order for the lamp to light, switch A closed. This is called an 'and' circuit.

an d

switch B

an d

switch C

an d

switch D must be

This circuit can be represented usi ng logic symbols. Remember that logic diagrams are not wiring diagrams. They are used to show what happens in electrical control circuits.


30



Logic Circuits (Cont'd)

Figure 20 shows the same circuit using logic symbols.

The symbol shown in Figure 21 means AND. 'AND' means that all the inputs must be switched on before there will be an output. There may be many input li nes, but there can be only one output.


31



These symbols are called LOGIC GATES. This is because they are designed to let signals pass or to stop them. Gates may be switches, relays, transis tors or other kinds of electronic devices. Logic Gates.

Figure 22 shows more AND gate signals. It can be seen that a signal is needed at all inputs before there can be an output signal.

Figure 23 shows an OR circuit.

In this circuit the lamp will light if either switch A OR switch B is closed.


32



Logic Gates (Cont'd)

Figure 24 shows the standard Saudi Aramco symbol for an OR logic gate.

Figure 25 shows how OR gates work. If a signal is applied either to A or B inputs, an output signal will be produced by the gate circuit.


33



In summary, we can say that AND and OR gates are used to make decisions. The AND gate will not s end an output signal unless i t receives signals from all its inputs. Logic Gates (Cont'd)

The OR gate sends no output signal until it receives a signal from at least one of its inputs. Figure 26 shows how an AND logic gate is used to control a pump start-up. Two process variables are being monitored: pressure and level. Pressure is monitored to protect the vessel from damage by high pressure. Level is monitored to protect the pump. If there is not a high enough level the pressure head on the pump, will be too low. The pump may then be damaged by cavitation caused by a low flow rate. The input signal to the AND logic gate comes from two transmitters, a pressure transmitter and a level transmitter. If the pressure in the vessel is less than 100 psi and the level is more than five feet, the pump can be started.


34




35



Figure 27 shows the same vessel using an OR logic gate in its alarm system. If the pressure goes more than 100 psi or the level goes less than five f eet, an alarm will be given. Alternatively an OR gate logic can be used to shut down the pump if the pressure goes high high or if the level goes low low. Logic Gates (Cont'd).


36



Figure 28 shows how the output signals from one or more OR gates can be the input signals for another OR gate. Logic Gates (Cont'd).

If any of the process conditions shown on the figure causes a switch to trip (that is high pressure OR low level OR high high pressure OR low low level) the horn will sound in the control room. Figure 29 shows the process at a Gas and Oil Separation Plant (GOSP) Three process variables are being monitored. These are: •

Pressure of the gas that is separating from the crude oil

•

Level of the crude oil in the High Pressure Production Trap (HPPT) and in the Intermediate Pressure Production Trap (IPPT).

•

Temperature of the bearings of the electric motors that drive the booster and shipper pumps.


37




38



Logic Gates (Cont'd).

Figure 30 shows a simplified version of the logic circuit for this GOSP's

ESD system. The controlling devices are shown on the left-hand side of the diagram. Note that: •

The ESD pressure switches are set at 90 and 120 psi

•

The level switches are set at 12 feet

•

The temperature switches are set at 180° F.

•

The controlled devices are seen on the right-hand side of the diagram


39




40



The logic circuit shown in Figure 30 can be translated into a wiring arrangement. The AND symbol tells us that for everything to run normally correct input signals are needed from EVERY input shown. If even one signal is missing, the plant will shut down. Figure 31 shows the actual wiring of the logic circuit. Logic Gates (Cont'd).


41



Note from Figure 31 that the relays are wired together and all have contacts that are NO. This means, as already stated, that if even one input is missing, a pair of relay contacts will be open and there will be an open circuit. No current can then flow and t he plant will shut down. Logic Gates (Cont'd).

Also note that the individual coils of the six relays are connected in parallel. Each coil is controlled by a single switch which energizes the coil when the process variable is normal. This keeps the relay contacts closed. The set of six relay contacts are wired in seri es with a seventh relay, R7. R7 will be energized only when all the other six relays, R1 through R6 are energized. When R7 is energized, it s contacts are closed and current can flow to all the control valves and pumps and to the MOV ZV. Sequence of Operation

Relay 1 through 6 must be energized before R7 can be energized. If any of the switches S1 through S6 open, R7 will be deenergized and the system will shut down. As an example of the operation of the system, imagine that the bearing on the shipper pump overheats. When the temperature r eaches 180°F, switch number 6 in the ESD circuit will open. This means that the contacts on R6 will open. This breaks the 110-vac circuit. R7 then becomes deenergized and no current can flow to equipment. This causes the process to shutdown within a few seconds.


42



Truth Tables

Computer and control systems use other logic circuits in addition to

AND and OR.

The numerical input to a computer is made up of decimal numbers, but the logic operations inside the computer use the binary number system. This system uses only t wo digits, 0 and 1. As we have seen, all electrical circuits are basically binary in nature, since circuits are either switched On or Off. Logic symbols have one or more input lines but only one output line. These inputs-output can be described in "Truth Tables", as shown in Figure 32.


43



Truth Tables (Cont'd)

For example, the AND logic gate does not have a 1 output until both A and B input lines are at condition 1. Similarly, the OR logic gate has a 1 output when a 1 condition (i.e. a signal or pulse) is applied to either A or B inputs. These situations can be indicated by Truth Tables as shown on the right-hand side of Figure 32. The letter shown at the gate outputs are statements made in Boolean algebra. Boolean algebra is different from ordinary algebra. The variables are denoted by letters, but take only one of two logic levels or values, either 0 or 1. An AND circuit is indicated by placing the two letters next to each other, as shown in Figure 32 (a); This does not indicate t hat the letters are multiplied together, as it would in ordinary algebra. An OR circuit is indicated by separating the two letters by a plus sign, as shown in Figure 32(b). The plus does not mean addition as i n ordinary algebra. An inverter or NOT circuit is indicated by placing a bar over the l etters. The bar indicates that the value has been inverted. An exclusive OR circuit (Figure 32 (d)) must have either inverted A with B or A with inverted B for operation. A NAND circuit is shown in Figure 32 (E). NAND means NOT AND. The Figure shows that the AND symbol has a small circle at t he output. This means that the output is inverted. NOR mean NOT OR. Figure 32 (F shows that the output of an OR circuit has been inverted.

The inverter symbol is a triangle to indicate that it is an amplifier (i.e. it can take an input signal and give an output signal of a higher value. The vertical side of the triangle indicates the input. The small circle i ndicates that the amplifi ed output is inverted (or reversed) in polarity. For this reason, an inverter logic gate is called a NOT gate. The bar placed over the output indicates that its value has been inverted. If the circle is placed on the input side of the logic gate, it mean s that the polarity is inverted before its enters the gate.


44



Ladder Diagram

Figure 33 shows typical ladder diagrams.

It is usual to show the cir cuit in a deenergized condition on ladder diagrams. This means that the system shown will have no level, no flow and will be at ambient temperature.


45



Ladder Diagrams (Cont'd)

When direct current is being used, the positive power line is drawn on the left. The controlling devices (swit ches, contacts) are also drawn on the left. Controlled devices are drawn on the right. Note on Figure 33 that each line has a line number on the left. The numbers outside on the right give the line numbers where the contacts of relays may be found. Also note that there are two different power supplies in the circuit. One power supply provides 24v dc and the other provides 110v ac. These two power supplies are not connected together. The controlling devices on t he left of the diagram operate on the low dc voltage. The controlled devices on the right operate on the higher, ac voltage. Vertical and horizontal Ladder Diagrams

Vertical ladder diagrams are most often seen on vendor-supplied drawings. Horizontal ladder diagrams are most often used by Saudi Aramco. The ability to read ladder drawings is essential in order to trace wiring, signals and operations of relay logic circuits. Figure 34 shows a simple, vertical l adder diagram. As already stated, it is standard practice to draw the circuits in the deenergized stat e. Voltage is applied across L 1 and L 2. Current flows through temperature switch TS-1. This switch is closed when temperature is normal. The current that passes through TS-1 energizes relay R 1. R1 has two sets of contacts in use, R 1 A and R 1 B. The numbers 2 and 3 on the outside and to the right of R 1 tell us to look at lines 2 and 3.


46




The bar under the number 3 (3) means that the contacts on line 3 are normally closed (NC). Contacts R 1 A on line 2 are normally open (NO). When R1 is energized, contacts R 1 A close and the light L 1 comes on. Contacts R 1 B open, and light L 2 goes out. If the temperature goes over the set point, TS-1 opens. This deenergizes R 1. Contacts R 1 A open and L 1 goes off. Contacts R 1 B close and L 2 comes on. A ladder diagram can be used to follow an actual circuit f rom point to point. Figure 35 shows an example of a ladder diagram for a relay circuit. It is actually the ladder diagram for the ESD system circuit shown in Figure 36. See how much simpler the ladder diagram is.


47




Note on Figure 35 the numbers around the relay coil contacts. These are the pin numbers for the relays in the circuit To trace the sequence of operations, we can begin at line 1. When pressure is normal, PS-1 is closed and R 1 is energized. The ladder tells us t o go to line 3 where the relay contacts of R 1 are normally closed (NC.) When R 1 is energized, the NC contacts will open. The high pressure alarm light L 1 will be off.

When PS-1 opens, R 1 will be deenergized and the NC contacts in line 3 will be closed. This will allow current to flow and alarm light L 1 illuminates. If pressure continues to rise until PS-2 opens, the open switch PS-2 causes R The contacts for R 2 are on lines 4 and 5. In line 4, the relay contacts are NC.


2

to deenergize.

48




49




When R 2 is deenergized R 2 contacts in line 4 will close. This allows current t o flow and the high high pressure light L 2 will illuminate. In line 5, the NO contacts will be open and current cannot flow to the solenoid operated valve (SOV). The deenergized solenoid shuts off the air flow to the AO/AFC control valve and the valve closes. Figure 37 shows a more complicated ladder diagram.


50




51




Remember that these diagrams are usually shown in a deenergized state. All seven relay contacts are seen in the diagram to be normally open (NO). They will close when the coils in their relays are energized. To follow the sequence of operations, start at line 1. S 1 is a pressure control switch. Look at the symbol for the switch (shown again below).

The symbol indicates t hat as pressure rises it will push open the swit ch. So, when the pressure is normal and the circuit is energized current will flow through relay R1. (This will be through pins 2 and 10.) The figure 7 on the outside and to the right of the line tells us to refer to line 7. Line 7 shows six sets of N O relay contacts wired in series with relay coil R 7. R7 can be energized only if all other relay contacts (that is, R 1 through R 6) are closed. The outside right of li ne 7 refers us to line 8. Line 8 shows that when relay contacts R 7 are closed, alternating current can flow t o the controlled devices. Note that all these devices are wired in parallel. This means that if some of the devices do not work, the others will still receive power.


52




The ladder diagram actually shows an AND circuit since S 1 and S 2 and S 3 and S 4 and S 5 and S6 must all be closed (as in normal operation) before current can flow to the controlled devices. If even one of the switches open, everything will shut down. Figure 39 (handout No. 2, Drwg. No. D-R84-NA-853443) shows a relay panel diagram. It is drawn horizontally. The panel is part of the ESD system for plant R84 at the Juaymah Gas Plant. A very important s ection on these di agrams is the NOTES section. Note 1 describes the conditions under which the diagram is drawn, i.e ambient temperature, no level, no pressure, no flow, no voltage and valves fully closed. These are the conditions under which the switches and contacts are positioned as shown on the diagram. Note 2 explains the relay numbering system, shown again below, Figure 38.

Note 3 is the line number index. It shows that the lines start at Number 1 on sheet 1 and go through line 686 on sheet 11.


53



FIGURE 39 (HANDOUT)


54




Note 4 shows the connections of contacts in the relay panel. The REFERENCE DRAWING section refers us to all other drawings t hat are related to thi s diagram. The LEGEND is also important. See Figure 40. It explains the symbols used on the diagram. The legend also gives the location and numbers of connection points.


55




The top left-hand corner of Handout No. 2 shows CIRCUIT DESCRIPTION and LINE NUMBER boxes. The circuit description describes the function of each line or group of lines on the diagram. The line number gives a reference for each line in the diagram. This makes it easier to follow the sequence from one circuit to another. The top and bottom connection wires have + and - signs, on the left of the diagram. These indicate the power supply. The positive line r uns across the top and the negative li ne across the bottom. Underneath the negative power line there are small boxes, like the one shown underneath line 160. This is shown again below in Figure 41.

There is one box for each relay in the circuit. The boxes identify the contacts used in the relay and where the contacts are found. For example, the box in Figure 41 is for R105 in line 160. Contacts number 1 are NO. They can be found on line 163. Contacts number 2 are NC and are found on line 239. Contacts 3 are NO and are on line 478.


56




Note the circuit description COMP SHUTDOWN (critical failure) - "Comp" stands for compressor. This is another example of a FAIL-SAFE system. All the contacts in line 163 must be closed before any current can flow to operate plant devices. The circuit is designed so that, with the exception of R110-1 the contacts are closed ONLY when operation is normal. The sequence of operation for several circuits can be followed on the diagram. Studying line 162 shows two sets of relay contacts at the top, R35-3 MOD1 and R56-3 MOD2. The note shows that these two relays are found on Drawing number NA-B514129. The symbol 4 refers to Note 4, which shows the connections for these contacts in the relay panel. The next two contacts seen on line 163 are R104-1 (with its coil located on line 159, as shown by the symbol), and R105-1 (with its coil on line 160.) Looking at line 159 shows that R104-1 is controlled by a handswitch (11-HS-155). handswitch can be used by operators to shut down the compressor from a field location.

The

Another handswitch, 11-HS-196 on line 161, can be used to deenergize relay R105-1. This would also cause the compressor to shut down.


57




The next four contacts on l ine 163 are controlled by different field-mounted switches. These switches will trip when such critical variables as vibration or lube oil pressure go to dangerous levels. The relay circuit s for these switches are shown on other sheets of the diagram. When any of the switches is open, a relay will be deenergized. The relay's contacts will then open and current flow to equipment will be stopped. The compressor will then s hut down Note that there are manual bypass switches for each of these four relays. The switches are shown as BS2.1 262

and

BS4.1 264

etc.

The switches allow the relays to be bypassed so that the compressor can continue to operate if a relay needs to be taken out of the circuit for repair or any other reason. Look now at contacts R110-1 on line 163. These are controlled by the relay coil on line 172. The circuit description says that line 172 is part of the PERMISSIVE TO START circuit . Four relays are shown on this circuit. If any contacts for these relays are open, then contacts R110-1 will also be open (since no current will be flowing). The relay contacts in the permissive to start circuit can close ONLY when all the conditions for the safe start-up of the compressor have been met. A close study of the diagram shows that when all the contacts on line 163 have closed (including the permissive o start contacts 110-1) relay 107contacts will beare energized. This causes set of contacts, R107-1 ont line 164, to close. When these closed, the circuit cana bypass R110-1. This design prevents on open permissive-to-start switch from shutting down the compressor AFTER the compressor has started. In other words, once the compressor has started, the permissive-to-start circuit no longer has influence on it. The reason for this design is that once a large piece of equipment has been started, all the conditions for startup have been met (such conditions as pressure, level, temperature and so on). It is not necessary to shut down operating equipment because startup conditions have changed.


58



Electrical Building Construction Diagrams

Most of the lighting symbols used in archit ectural drafting are based on the circle. The circle represents the round, hexagonal or rectangular metal box to which the electrical connections are attached. Figure 42 a shows that a simple cir cle is the symbol for a ceiling lamp fixture. Any departure from this simple fixture requires additions to be made to the symbol. For example, Figure 42b has the letter "S" added to it to show that the ceiling lamp fixture has a pull-chain switch. Other additions to the symbol can be seen.


59



Electrical Building Construction Diagrams (Cont'd)

Figure 43 shows that all symbols for outlet receptacles are also based on the circle.

A single line drawn across the circle indicates that only one plug can be connected to the receptacle. The letter "G" indicates that the receptacle contains a grounded pin jack. Note that the three lines drawn across the symbol for the range outlet (Figure 43h) indicates that three CONDUCTORS are to be connected to the receptacle. It does not mean that there are three receptacles.


60



Electrical Building Construction Diagrams (Cont'd)

Figure 44 shows the letters used to indicate different kinds of switches.

Figure 45 shows the circuit diagram for a lamp controlled by a single-pole switch.


61



Figure 46 shows a single pole, double-throw switch (SPDT). Two such switches are used to control a lamp from two different locations. Figure 46 shows the circuit diagram for this arrangement. The 3-way Switch S 3.

A study of the circuit shows that the power supply to the lamp can be broken or restored from either switch at any time. This is a double-pole, s ingle-throw (DPST) switch. It is always closed in either position; it is never open. See Figure 47. The 4-way Switch S 4.


62



Single-Line Diagrams

Because most power conductors are cabled or grouped in conduits, architectural wiring diagrams are drawn single line. The single line represents a multiconductor cable. It does not represent a single conductor. Figure 48 shows the difference between electronic and architectural wiring diagrams. The single line in the architectural drawing means that two conductors are in a conduit or cable. If more conductors are carried in the conduit, their number is indicated by short diagonal lines drawn across the single line. Unbroken lines means that the conduit is concealed in the ceiling or wall. Broken, or dashed, lines mean that the conduit is concealed in the f loor. Electronic diagrams do not indicate where the wires are to be placed. The broken line that goes from the building to an outside lamp shows that the cable is exposed (or strung overhead). The symbol indicates that the lamp is on a pole.


63



Single-Line Diagrams (Cont'd)

Figure 49 shows a wiring diagram for a kitchen.

Note the following • Three short diagonal lines have been added to cables that carry three conductors


64




•

The lamp denoted by R (for "recessed") is controlled by two 3-way switches, one close to each door.

•

The lines with arrows and numbers represent numbered branch circuits to a service switch or a panel board. The number indicates which terminal the circuit is connected to. The lines are called "home runs" because they make up a complete circuit.

Figure 50 lists Saudi Aramco symbols for street lighting.


65



Service Entrance

Power to a building comes through underground or overhead lines. power to the building is called the service entrance .

The point of entry of the

The service entrance swit ch, seen in Figure 51, is usually found inside the building. It provides both overcurrent and disconnecting devices. Two insulated entrance cable wires are connected to the disconnect block (at A).


66



Service Entrance (Cont'd)

The block contains the main fuses. It also disconnects the 230-volt service when pulled out from the box. The third conductor i s the ground wire. It is not insulated and must always be grounded through the switch box to an external copper or galvanized iron ground rod. A voltage of 115 volts exists between the ground wire and each of the other two, insulated wires. The 115-volt circuits are protected by four plug fuses.


67



Circuit Loads

The capacity of copper wire to carry a current depends on the size of the wire. Examples of the current that can be carried by different sized copper wires are given in Figure 52. The wire gauge size (AWG means American Wire Gauge) is determined from the number of outlets and the electric power in watts (W) that is used by the appliances. Average power and current demand of appliances are shown in Figure 53.


68



Circuit Loads (Cont'd) Average Current and Power Drawn by Domestic Loads

___________________________________________________________ Power Current (watts) (amperes) ___________________________________________________________ Airconditioner,room,3/4ton Blanket, electric Dryer,clothes(230-volt) Freezer Furnace,hotair,oil-fired Heater,WaterStandard(230-volt) Iron Lamp table Mixer, food Pump, water Radio/recordplayer Range(230volt) Refrigerator Sewing machine

1,200 170 5,000 350 800 2,500 1,000 150 150 700 110 10,000 320 75

10.4 1.5 21.7 3.0 7.0 10.9 8.7 1.3 1.3 6.1 1.0 43.5 2.8 0.6

Television, color 330 2.9 Toaster,automatic 1,100 9.6 Vacuum cleaner 630 5.5 Washer, dish 1,200 10.4 Washingmachine,automatic 510 4.4 Waste disposer 440 3.8 ___________________________________________________________ FIGURE 53


69



Circuit Loads (Cont'd)

Figure 54 details the symbols that may be used on architectural electric drawings.


70




71



Electrical Industry and Power Supply Components

Saudi Aramco uses many different symbols on its plant control and power supply drawings. Figure 55 shows the metering and instrumentation symbols. Also given are switchgear, control and miscellaneous symbols. ("Switchgear" refers to all switching interrupting, regulating, monitoring and other devices.) Many of the symbols are self-explanatory; those that are not are described below: Contact-Making Clock. This is an instrument that momentarily closes a circuit to a demand meter at periodic intervals. A contact breaking clock does the opposite, it opens a circuit to a

demand meter. Note: A demand meter is a metering device that indicates or records the demand, maximum demand, or both. This is an oscilloscope that uses a cathode ray tube (like a television tube) as the indicating device. Cathode Ray Oscilloscope.

Note: An oscilloscope is an electronic instrument that projects the forms of electromagnetic waves on a cathode-ray tube. Definite Time Relay. This is a relay with a purposely introduced delay action which remains

substantially constant regardless of the magnitude of the quantity that causes the action.


72



Electrical Industry and Power Supply Components (Cont'd)

Frequency Meter. An instrument for measuring the frequency of an alternating current. Recording Demand Meter. A demand meter that records on a chart the demand for each

demand interval. Recording. The process of converting the electrical signal to an image on the record medium. Synchroscope. An instrument for indicating if two periodic quantities are synchronous. Telemeter. An instrument for allowing a measurement to be interpreted at a distance from the

primary detector. A transformer that advances or retards the phase-angle relationship of one circuit to another. Phase Shifting Transformer.

Integrator Relay. A relay that operates on the energy stored in a long pulse or series of pulses

of the same or varying magnitude, eg. a thermal relay. Varmeter. A meter for measuring reactive power. Interlock. A device actuated by the action of some other device with which it is directly

associated to govern succeeding operations of the same allied devices. A step-down transformer which is usually used in circuits that are characterized by low power levels and which contribute to a control function, such as in heating, air conditioning and industrial controls. Control Power Transformer.

Consists of one or more thermojunctions in thermal contact with an electric heater so that the voltage developed at its input by thermoelectric action gives a measure of the input current to the heater. Thermal Converter.

The combination of two or more thermal converters, when connected with auxiliary equipment so that its combined dc output gives a measure of the active power in the circuit, is called a thermal watt converter .


73





74





75





76





77





78



The Inductor

When there is a change in current flow through a coil, the change alters the magnetic field around the coil. This change in magnetic field induces another current to f low in the coil. This second current opposes the first current. Therefore, coils can be used to resist rapid changes in the current flowing through them while allowing steady, dc current to pass freely. In other words, coils present high resistance to ac and low resistance to dc. The ability of a coil to produce an induced current is called its 'INDUCTANCE'. The unit of measurement for inductance is the Henry, denoted by L Figure 56 shows the symbol for a coil.

Inductors are often used in filter circuits because they oppose, or filter out, ac at high frequencies. Tuning Coils are used to obtain a required signal. The symbols for tuning coils are shown in Figure 57.


79



Chokes

Chokes are coils that are used to limit or suppress fluctuating signals while allowing a steady signal to pass. Figure 58 shows the symbol for a choke

Transformers

Transformers are essential to Saudi Aramco power dist ribution systems. They use the magnetic field produced by one coil (the primary) to induce a voltage, in a second coil (the secondary). Transformers may be ' step-up' or 'step-down', depending on the r atio of primary to secondary coils. Step-up means that the output voltage is higher than the input. Step down means the output voltage is lower. The symbols used to depict transformers are shown in Figure 59 below.


80



Transformers (Cont'd)

Saudi Aramco symbols for transformers are shown in Figure 60 below:

Power or Distribution Transformers

.

These are used to step-up or step-down ac voltages.

These make use of a common part of a winding for both the primary and secondary. They have a tap brought out from a point on the winding. The tap is necessary for the operation of the transformer. Autotransformers.


81



Autotransformers (Cont'd).

Figures 61 and 62 show that the operation of an autotransformer is exactly like that of a conventional transformer except that one lead of the primary and one of the secondary are tied together in the correct way.


82



These have two secondary windings. Figure 63 shows that the windings can be connected in two ways. In a "series aiding" connection, t he voltage is the sum of the voltages for both windings. In a "series opposing" connection, the voltage is the difference of the voltage for both windings. Dots are sometimes used to indicate the terminals that have the same phase. 3 - Winding Transformers.

Step - Voltage Regulators.

These are transformers in which the voltage of a regulated circuit is

controlled in steps by means of taps and without interrupting the load. Load Tap Changes (LTC) An LTC is a selector switch that is used to change transformer taps even when the transformer is operating.


83



Transmission lines carry high voltages and heavy currents. This makes it both dangerous and expensive to t ake measurements directly on the lines. For this reason two types of instrument transformers are used for monitoring the lines. These are the current transformer and the potential (or voltage) transformer. The symbols used to depict these transformers are shown in Figures 64, 65 and 66. Current and Potential Transformers.


84



Current and Potential Transformers (Cont'd)

A current transformer is an instrument transformer. It is used to connect ammeters and current coils of instruments into hi gh-current lines. The cable in which current is to be measured passes through a hole in the transformer. This allows the cable to act as the primary and is why the secondary is drawn on the line in the current transformer symbol. The secondary winding is insulated f rom the primary. Its turns ratio is adjusted to give a supply of five amperes to the instrument circuit when rated, full-load current flows through the primary. Figure 65 shows a current transformer connected to a 4,800 volt supply.

The current transformer i s shown to be rated at 100 to 5 amperes (100/5). This means that when 100 amps are flowing through the line, 5 amps are flowing through the transformer's secondary circuit. This is a stepdown of 20 to 1. The figure shows that the ammeter is registering a reading of 4 amps. Therefore, line current is 20 x 4 or 80 amps.


85



These are usually used in equipment where the primary conductor is an integral part of the equipment. Bushing Current Transformers.

These connect a voltmeter to the low-voltage si de. The voltmeter can then be used to obtain a measurement for the high-voltage line. Figure 66 shows the symbols used to depict potential transformers. Potential Transformers.


86



Figure 67 shows how the current induced in a bushing CT is used to operate instruments. In the information given about transformer T2, OA/FA means Forced Air Cooled/Oil Immersed Self Cooled-OLTC means On Load Tap Changing. Z is impedance. MVA is mega volt-amps and is a measure of power.

The current in the supply line induces a current in the CT. All the instruments are in series, so the same current passes through each of them. •

AS is an ammeter switch. The value of the supply-line current can be read directly on

the ammeter, A. It has a scale of 0-3000 A. •

A wattmeter, W, shows the power of the circuit.

•

A watthour demand ( WHD ) meter shows the power demands at 15-minute demand intervals.

•

A varmeter, VAR, shows the value of reactive power in the circuit. The varmeter is connected to a phase-shifting transformer.

•

A transducer converts the varmeter reading to an appropriate signal and transmits it to a telemetry analog indication system.

•

A voltmeter switch, VS, can be used to show the voltage readings on a voltmeter V.

•

A transducer sends the voltage measurements to a telemetry analog indication system.


87



The Capacitor

The capacitor is an electronic component that can be used to store electricity and to control the flow of current. The ability of a capacitor to store electricity is called "CAPACITANCE". measured in "Farads", (F.)

Capacitance is

A simple capacitor is made of two plates that are separated by an insulating material called a "dielectric". Electrons can flow onto, and be stored on, one of the plates. The plate can store electricity until the potential difference between it and the unstored plate is the same as the voltage that was causing the electron flow. Then, the electron flow st ops. The ability of a capacitor to store electricity initially and then stop current flow means that a capacitor prevents a direct current from flowing in a circuit. However alternating current will flow since the capacitor plates continually change polarity. Figure 68 shows the symbol for a polarized fixed capacitor and for a nonpolarized fixed capacitor.


88



The Capacitor (Cont'd)

Figure 69 shows the symbol for a "trimmer capacitor" which is used in the final adjustment of a circuit. It compensates for sl ight differences in the values of components.


89



The Semiconductor

A semiconductor is halfway between being a conductor and an insulator. It allows current to pass in one direction only. Semiconductors are also known as "diodes". Figure 70 shows symbols for different kinds of diodes.


90



The Semiconductor (Cont'd)

The rectifier diode is a half-wave recti fier. That is, it allows only voltages of the same polarity to pass. This is shown in Figure 71.

Full-wave rectification can be obtained if a circuit is added to rectify the half- wave voltages. A full-wave rectifying circuit is shown in Figure 72.

The Zener diode ( D3 in Figure 72) is a voltage regulator. It will not conduct until the applied

voltage reaches a "breakdown" voltage. The voltage then remains essentially constant and independent of the load current. Any additional voltage that may be produced is absorbed by resistor R 1.


91



The Semiconductor (Cont'd)

The tunnel diode is used as an amplifier, oscil lator or fast swit ching device. (An amplifier is a

device that enables an input signal to control power and which can give an output signal greater than the input signal. An oscillator is a circuit that can change dc to ac at a frequency and wave shape determined by the circuit's components.) The Varactor diode symbol includes the capacitor symbol. This is because the diode contains

a voltage - sensitive capacitor. The varactor diode is used in high-frequency oscillators. The DIAC is an ac switching semi-conductor. As the arrows on the symbol indicate, it can be

used to conduct in either direction. Diacs are used in motor speed control systems. The Power Diode symbol is the same as for the r ectifier diode. It allows power to function in

only one direction. The Photo diode converts light energy into electrical energy.

The arrows on the symbol

represent light energy. The light - emitting diode emits light when current flows through the diode.

It is used as the

digital readout in calculators and electrical measuring instruments.


92



The Transistor

A Transistor is a semiconductor that has at least three terminals. amplifiers, detectors or switches.

They may be used as

Figure 73 shows the symbols for transistors. The symbols are different, depending on how the transistor has been constructed. The constructions may be with negative - positive negative ( npn) regions, or with positive - negative - positive ( pnp) regions. B stands for base, C for collector and E for emitter.

The base is usually the input signal terminal. The collector is usually the output ter minal. The emitter terminal is shown as a diagonal li ne with an arrow. The emitter is the region fr om which electric charge is injected into the base. The arrow always points in the conventional direction of current flow i.e. from positive to negative.


93



The Use of Inductors, Capacitors and Semiconductors.

Many electronic circuits contain currents of differ ent frequencies at the same time. Capacitors and inductors can be used to remove, or filter out, intermediate frequencies. Circuits that do this are known as filter circuits. Capacitors filter out unwanted low frequencies. Inductors filter out the high frequencies. Figure 74 shows a filter circuit. It is an LC filter circuit (L for inductance, C for capacitance).

An alternating supply is applied to a transformer primary. The alternating current induced in the secondary is rectifi ed by two rectifier diodes. As the figure shows, the rectifiers change the alternating current to direct current with a fluctuating ripple.


94



The Use of Inductors, Capacitors and Semiconductors (Cont'd)

Capacitor C 1 filters out the ripple (remember, capacitors will not all ow dc to pass.) The ripple is sent to ground. The choke L 1 offers a high resistance to the ripple frequency. At the same time, capacitor C 2 passes more of the ripple to ground. The output of the circuit contains only a slight trace of the ripple. Use of Transistors

One of the most common uses of transistors is in amplifier circuits. These circuits allow an input signal to control power and they can give a current output that is greater than the input (this is the amplification part of the circuit.) Figure 75 shows an amplifier circuit. The transistor symbol is shown in the figure. The circuit shown is designed to amplify an input signal.


95



Power Generation and Transmission Electric Generators

The construction of electric generators and motors is very similar. If a machine converts mechanical energy to electrical energy, it is call ed a generator. If it converts electrical energy to mechanical energy, it is called a motor. All electric generators and motors need a magnetic field for their operation. The magnetic field in some dc and ac motors is provided by a coil. The coil is called a winding, or, more correctly, a field winding. The symbols used t o show generators and electric motors often include the symbol for a coil. Figure 76 shows the symbol for a field winding.

Generators produce electricity through the relative movement of an "armature" coil and a magnetic The the magnetic field is obtained between the north and south poles of a magnet. The polesfield. are called "pole pieces". The pole pieces are made magnetic by passing current through field windings that are coiled around them.


96



Electric Generators (Cont'd)

Figure 77 shows the basis of a generator for producing alternating current. only one coil and one pair of pole pieces are shown.

For simplicity

Devices called brushes and slip rings are used to tap off the current. If direct current is needed, a device called a "commutator" is used to change the ac to dc. All generators, whether ac or dc, have a rotating part and a stationary part. In most dc generators, the coil from which the output is taken is mounted on the rotating part, which is called the "armature". The coils that produce the magnetic field are mounted on the stationary part, which is called the "field". On most ac generators however, the opposite is true. The field windings are mounted on the rotating part, now called the "rotor", and the armature coil is wound on the stationary part now called the "stator".


97




Windings may be connected in different ways. These are: series, shunt and compound. Figure 78 shows a series-wound dc generator. The armature is turned by a prime mover. The field windings are in s eries with the armature coil. As the armature turns, t he slight residual magnetism that is left in the field from previous operation induces a current in the armature coil. The current in the armature also flows through the field windings. This increases the strength of the magnetic field, which in turn increases t he current in the armature coil. In this way the generator can be brought up to its rated output. Generators that use part of the induced current to supply current to the field windings are called "self-excited" generators. In some generators the windings are separately excited.


98




Figure 79 shows a shunt-wound generator. parallel with the generated current.


"Shunt" means that the field windings are in

99




Figure 80 shows a compound-wound generator. This generator is a combination of a series and shunt generator. The field may be connected either in short shunt or long shunt. In short shunt, the shunt field is in parallel only with the armature. In long shunt it is in parallel with both the armature and the series field.


100



3- Phase Power Supply

The generators used to supply power usually have three, single-phase windings, as indicated in Figure 81 below.


101



3- Phase Power Supply (Cont'd)

Each winding produces it own voltage. The values of the voltages are 120° out-of- phase with each other. Instead of six leads coming out of the generator, three of the leads can be connected together. There are two ways to make the connections; one way gives a wye (or star) connection, the other gives a delta connection. The connections are shown in Figure 82 below.

In a wye connection, the point of connection is called the neutral. Voltage from this point to any of the line leads is the phase voltage (V P).


102




In a wye connection, the line voltage (V L) is 1.73 times the phase voltage. The line current i s the same as the phase current. In a delta connection it is the other way round, the line voltage is the same as the phase voltage. The line current is 1.73 times the phase current. It is possible to go from wye to delta and vice versa by using transformers. On a 3-phase supply, the transformers may be banked together. Figure 83 shows the delta and wye transformation. shown. Delta-delta transformation is also shown.

The symbols for the connections are

Figure 84 shows the Saudi Aramco symbols for wye and delta transformer winding connections.


103





104




The mathematical relationships are as follows (where N =ratio of primary to secondary winding turns): For ÆÆ or YY, Line voltage out = N x Line voltage in For Y Æ, Line voltage out = N x Line voltage in Ã3 For ÆY, Line voltage out = N Ã3 x Line voltage in Explanation of the symbols are given below:

The Æ connection is used by Saudi Aramco to transform a 69 kV supply from a wye connection to 13.8 kV on a delta connection. The ÆÆ connection is used to transform the 13.8 kV to 2.4 kV.

The Æ connection is used to transform 480 v to 208 or 120 v. The 208 volts is the line-to-line voltage and is used to operate motors. The 120 v is the phase voltage and i s used for lighting. It is possible to achieve 3-phase transformation of energy by using two transformers only. An open delta connection will do this. Such connections are used if one of the three transformers in a delta-delta bank becomes defective, as shown in Figure 85. The defective transformer is taken out of the system and the open-delta connection is used to keep the 3-phase supply in operation until repairs can be made. Open Delta.

The capacity of the open-delta connection is only 58% of the capacity of the closed delta connection.


105



Open Delta (Cont'd).

These are connections in the wye of polyphase windings, each branch of which is made up of windings that generate phase- displaced voltage. 3-Phase Zigzag Connections.


106



Electric Motors

A dc motor converts electrical energy to mechanical energy. Figure 86 shows the principle of

operation.

Current is sent through the armature coil. This causes the armature to act as a magnet. The armature poles are attracted to field poles of unlike polarity. This causes the armature to rotate. At the moment when unlike poles of the armature and fiel d are facing each other, a commutator reverses the armature current. This in turn, reverses the polarit y of the armature magnetic field. Like poles of the armature and the field now repel each other. This sequence causes continuous rotation of the armature. The armature can thus be used to give a mechanical energy output.


107



Electric Motors (Cont'd)

Dc motors can be series-, shunt, or compound-wound, exactly the same as dc generators. The symbol for dc generators and motors are shown in Figure 87.

Note: An exciter is the source of all or part of the field current for the excitation of an electric machine. Ac motors are usually considered to be of two types:

(1) The synchronou s motor and (2) the induction motor. Both types use a rotating magnetic field for their operation. Figure 88 shows how a rotating magnetic field is generated.

Alternating current is applied to the windings of a 3-phase stator. The windings are 120° apart. The magnetic fields that are generated are 120° out of phase. These three fields combine to produce one field. This field is continually rotating as the alternating current goes first in one direction and then in the other.


108




The Synchronous Motor uses a 3-phase stator to produce a rotating magnetic field. A rotor is

placed in the field and dc is applied to the rotor. A force of attraction is produced between the polarity of the rotor and that of the rotating field. This causes the rotor to rotate. However, before a synchronous motor can run by itself the rotor speed must be made synchronous with the rotating field. A squirrel cage induction motor i s used to bring the rotor up to synchronous speed. Figure 89 shows the symbols for synchronous and squirrel cage motors.

The Induction Motor uses a rotating magnetic field to induce a current in the winding on a

rotor. The rotor current induces a magnetic fi eld that interacts with the rotating field to cause rotation of the rotor. The symbol for a wound induction motor is shown in Figure 90. Again, the horsepower of the motor is put inside the circle.


109




Figure 91 shows a list of the symbols used by Saudi Aramco to depict ac electrical machines, including 3-phase wye and delta connected machines.


110



AC Motor Control Circuits

A motor control circuit controls the starting, protection, running, speed regulation and stopping of a motor. The circuits are complicated, but diagrams can be simplified by drawing them in ladder form. Figure 92 shows an ac motor starter circuit.


111



AC Motor Control Circuits (Cont'd)

If the full line voltage is connected directly to a motor, the starting current will be much higher than the normal running current. This can cause supply line disturbances. Fuses are installed in the circuit for protection against high starting and overload currents. Figure 92 shows the power circuit and the control circuit. The control circuit is isolated from the power circuit through a step-down transformer. The step-down is from the high voltage (HV) power supply of 480 v to 120 Volts. Contactor relay coils (S and R) and a ti ming relay (A) are used in the circuit. It is usual to start motors by connecting them across an autotransformer. This ensures that a lower starting voltage is used. As the motor approaches operating speed, the autotransformer is disconnected from the circuit . This arrangement can be seen in the power circuit of Figure 92. Operation of the ac motor starter is as follows: 1.

First the disconnect switches (DS) must be closed. The motor cannot start because contactors S and R are all open. However, current from one phase of the HV line can flow through the primary coil of the control transformer.

2.

The start switch in the control circuit is momentarily closed. Therefore, the current induced in the control transformer secondary winding flows through the control circuit.

3.

Current in the control circuit energizes the motor-driven timing relay. The current flows through the normally closed (NC) time opening (TO) contactor A and through coil S. At the same time, the energized motor-driven relay causes the normally open (NO) contactor A (which is i n parallel with the start switch) to close. This means that the start switch can now be released and current will still flow through contactor A to energize the control circuit. This kind of circuit, where it remains energized even after the start switch is released, is called an "INTERLOCK CIRCUIT."


112



AC Motor Control Circuits (Cont'd)

4.

When coil S is energized, all normally open (NO) S contactors in the power circuit close. This causes the motor to be connected to the power supply, through the autotransformer. (The autotransformer coils ensure that the voltage applied to the motor is less than the supply voltage.) Hence the motor begins to rotate. (Note that the NC S contactor which is in series with coil R will be open.)

5.

However, because the motor-driven relay coil is energized, the TO contactor A, which is in series with coil S, will open after a short time. This deenergizes coil S and causes the NO S contactors to open. Thus the autotransformer connection to the motor is broken. (Note that NC contactor S will now close.)

6.

At the same time, the TC contactor A, which is in series with coil R, will close. This causes coil R to be energized. All NO R contactors in the power circuit now close. This connects the motor directly to the power supply for normal running. Any of the following conditions will stop the motor: 1.

The disconnect switches (DS) are opened.

2.

The stop switch is opened. This stops current flow through the control circuit.

3.

A power line short circuit blows line fuses.

4.

Overload (OL) causes normally closed contactors to open in the control circuit.


113



DC Motor Control Circuits

DC motors are used when more accurate control of motor speed is needed. Speed control is usually obtained by adjusting the magnetic field or the armature voltage, or both. Figure 93 shows the starter circuit for a dc shunt-wound motor.


114



DC Motor Control Circuits (Cont'd)

Operation of the dc shunt motor is as follows: 1.

First, the DS switches are closed. This allows current to flow through the field circuit only.

2.

Current through the Field Loss (FL) relay coil causes contactor FL to close in the control circuit.

3

When the START switch is momentarily closed, current flows through contactor coil M.

4.

The current flow through conductor coil M causes the two M contacts in the armature circuit to close. It also causes the two M contacts i n the control circuit to close.

5.

When the M contact which is across the start button closes, the button can be released.

6.

Current through the dashpot time-delay relay IA causes the IA contacts in the control and armature circuits to close after a short time delay. This causes one-third of the starting resistance (IR) to be shorted out. This decrease in resistance causes more voltage to be applied across the armature, and the motor speed is increased.

7.

With contacts IA closed, current can flow through relay 2A

8.

After a similar time delay, contacts 2A close, causing two-thirds of IR to be shorted out. This, again, results in a voltage increase and higher motor speed.

9.

Similarly, with 2A contacts closed relay 3A is energized and, after a time delay, contacts 3 A are closed. This causes all of resistance IR to be shorted out, causing the motor to operate at full speed.


115



Plant Control Wiring Diagrams

Plant control wiring diagrams use many of the symbols you have already seen. The figures that follow show symbols that are new to you. Plant switches have to break circuits that carry high currents. Also, because high voltages may be used, arcing can occur when the circuit is broken. For this reason, plant power switches are heavier and larger than normal. Switches.

Figure 94 shows some of the symbols for plant switches. Note that the 'horn gap' switch (Figure 94 c) is designed to help reduce arcing.


116



Resistors may be represented by rectangles, as you have already seen. Designations are usually added to make the resistor identification cl ear. The symbols for non-linear resistors are shown in Figure 95. Resistors.


117



Fuses are usually shown as rectangles on Plant Wiring Diagrams. The circuit line is drawn lengthwise through the rectangle. This helps to dist inguish the symbol from that of a resistor or relay coil. Fuses.

Figure 96 shows the symbols used for fuses.


118



These are heavy duty relays. As we have already seen, the symbol f or a contactor coil is usually a circle containing a code let ter or number for i dentification. The identification is important because the coil and its contacts may be in different circuits. Contactors.

Figure 97 shows Contactor Relay symbols.

Standard relay symbol abbreviations are s hown in Figure 98 below.


119



Contactors (Cont'd)

Necessary explanations are given below. Interposing Relay.

An auxiliary relay at the master or remote station served by the contacts:

(1)

To energize a circuit of an element of remote station equipment when the selection of a desired point has been completed and when suitable operating signals are received through the supervisory equipment from the master station.

(2)

To connect the telemeter transmitting and receiving equipments respectively at the remote relay stations. Note that the interposing relays are considered part of a supervisory system.

Responds to the relative phase position of a current with respect to another current or voltage reference. Directional Relay.


120



Circuit Breakers

Circuit breakers are used to open an electric circuit automatically if dangerous conditions occur. These conditions are usually due to overload, underload, high voltage, low voltage and reverse current. The automatic operation of a circuit breaker is obtained through the use of electromagnets or by heat expansion. Two types of breaker are used by Saudi Aramco: (1) Air circuit breaker, in which the breaker is opened in a surrounding of air or gas. This is done to reduce arcing when heavy currents are interrupted. (2) Oil circuit-breaker in which the breaker is immersed in oil, also to reduce arcing. Figure 99 shows other symbols for circuit breakers.


121



Circuit Breakers (Cont'd)

The blowout coil shown in Figure 95e is usually part of an air circuit-breaker. The current through the coil creates a magnetic field that helps to put out the arc that forms when the contactors open. Separable Connectors (Draw-out Connectors)

When electrical parts have sometimes to be removed, they are connected with separable connectors. Separable connectors are shown by the symbol in Figure 100.

Resistance Grounding.

Figure 101 below shows resistance grounding.

If continuity of operation is important so that a fault may be tolerated for a short time, resistance grounding may be used. The 3.5-ohm resistor provides a 10- second delay before the circuit breaker is operated.


122



Electrical Power System Device Numbers and Functions

The devices used for switchgear control and protection are identified by standard device numbers. The number may refer to the actual function of the device or to the electrical quantity it is dealing with. The numbers are shown inside cir cles. Figure 102 shows a section of the circuiting for a control circuit. The numbers inside the circles identify the devices. Sometimes, suffixes are added to make the i dentification more accurate. The figures outside the circles refer to how many of the devices are being used in the circuit. The broken lines indicate devices that are connected to the circuit, but are not part of it.


123



Electrical Power System Device Numbers and Functions (Cont'd)


124



The list that follows is a copy of the Saudi Aramco Engineering Standards which deals with the device number s. The numbers of the mos t commonly used device s have been marked by an asterisk. 7.


7.1

General

7.1.1

Relays and other devices are allocated identifying numbers, sometimes with an appropriate suffix, according to the functions that they perform.

7.1.2

The device function numbers are used on electrical diagrams, in instruction books and specifications.

7.1.3

These device functions m ay refer to the actual function the device performs in an equipment or they may refer to the electrical or other quantity to which the device is responsive. Hence, there may be in some instances a choice of the function number used for a given device. The preferable choice, in all cases, is the one which is recognized to have the narrowest interpretation so that it most specifically identifies the device in the minds of all individuals concerned with the design and operation of the equipment.

7.1.4

When alternate names and descriptions are included under the function, only the name and description which applies to each specific case should be used. In general, only one name for each device such as, relay, contactor, circuit breaker, switch or device, is included in each function designation. However, when the function is inherently not restricted to any specific type of device and where the type of device itself is thus merely incidental, any one of these alternative names, as applicable, may be substituted. For example, if for device function 6 a contactor is used for the purpose in place of a circuit breaker, the function name should be specified as Starting Contactor.

7.1.5


Device function num bers and suffixes as listed in the following sections are also listed in ANSI standard C37.2 - 1970 - Manual and Automatic Station Control Supervisory and Associated Telemetering Equipment.

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7.


7.2

Device Function Numbers

* 1.

Master Element is the initiating device, such as a control switch, voltage relay, float switch, etc., which serves either directly, or through such permissive devices as protective and time-delay relays to place an equipment in or out of operation.

* 2.

Time-Delay Starting or Closing Relay is a device which functions to give a desired amount of time delay before or after any point or operation in a switching sequence or protective relay system, except as specifically provided by device functions 62 and 79.

3.

Checking or Interlocking Relay is a relay which operates in response to the position of a number of other devices, or to a number of predetermined conditions, in an equipment to allow an operating sequence to proceed or to stop or to provide a check of the position of these devices or of these conditions for any purpose.

* 4.

Master Contactor is a device, generally controlled by device No.1 or equivalent and the necessary permissive and protective devices, which serves to make and break the necessary control circuits to put an equipment into operation under the desired conditions and to take it out of operation under other or abnormal conditions.

* 5.

Stopping Device is a device whose primary function is to place and hold an equipment out of operation.

6.

Starting Circuit Breaker is a device whose principal function is to connect a machine to its source of starting voltage.

7.

Anode Circuit Breaker is one used in the anode circuits of a power rectifier for the primary purpose of interrupting the rectifier circuit if an arc-back should occur.


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7.


7.2

Device Function Numbers (Cont'd)

8.

Control Power Disconnecting Device is a disconnecting device, such as a knife switch, circuit breaker or pull-out fuse block used for the purpose of connecting and disconnecting, respectively, the source of control power to and from the control bus or equipment. NOTE: Control power is considered to include auxiliary power which supplies such apparatus as small motors and heaters.

9. * 10.

Reversing Device is a device which is used for the purpose of reversing a machine field or for performing any other reversing functions. Unit Sequence Switch is a switch which is used to change the sequence in which units may be placed in and out of service in multiple-unit equipments.

11.

Reserved for future application.

12.

Over -Speed Device is usually a d irect-connected speed switch which functions on machine overspeed.

* 13.

Synchronous-Speed Device is a device, such as a centrifugal-speed switch, slip-frequency a voltage relay, an undercurrent relay or any typea of device whichrelay, operates at approximately synchronous speed of a machine.

14.

Under-Speed Device is a device which functions when the speed of a machine falls below a predetermined value.

15.

Speed or Frequency Matching Device is a device which functions to match and hold the speed or the frequency of a machine or of a system equal to, or approximately equal to, that of another machine, source or system.

16.



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7.


7.2


17.

Shunting or Discharge Switch is a switch which serves to open or to close a shunting circuit around any piece of apparatus (except a resistor), such as a machine field, a machine armature, a capacitor or a reactor. NOTE: This excludes devices which perform such shunting operations as may be necessary in the process of starting a machine by devices 6 or 42, or their equivalent, and also excludes device 73 function which serves for the switching of resistors.

18.

Accelerating or Decelerating Device is a dev ice whi ch is used to c lose or to cause the closing of circuits which are used to increase or to decrease the speed of a machine.

19.

Starting-to-Running Transition Contactor is a device which operates to initiate or cause the automatic transfer of a machine from the starting to the running power connection.

20.

Electrically-Operated Valve is a solenoid or motor-operated valve which is used in a vac uum, air, gas, oil, water, or similar line. NOTE: The function of the valve may be indicated by the insertion of descriptive words such as "Brake" or "Pressure Reducing" in the function name, such as "Electrically-operated Brake Valve".

21.

Distance Relay is a re lay whi ch functions when t he circuit admittance, impedance or reactance increases or decreases beyond predetermined limits.

22.

Equalizer Circuit Breaker is a breaker which serves to control or to make and break the equalizer or the current-balancing connections for a machine field, or for regulating equipment, in a multiple-unit installation.


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7.


7.2


23.

Temperature Control Device is a dev ice which functions to raise or t o lower the temperature of a machine or other apparatus, or of any medium, when its temperature falls below, or rises above, a predetermined value. NOTE: An example is a thermostat which switches on a space heater in a switchgear assembly when the temperature falls to a desired value as distinguished from a device which is used to provide automatic temperature regulation between close limits which would be designated as 90T.

24.


* 25.

Synchronizing or Synchronism-Check Device is a d evice which operates when two A.C. circuits are within the desired limits of frequency, phase angle or voltage, to permit or to cause the paralleling of these two circuits.

26.

Apparatus Thermal Device is a device which functions when the temperature of the shunt field or the amortisseur winding of a machine, or that of a load-limiting or load-shunting resistor or of a liquid or other medium exceeds a predetermined value; or if the temperature of the protected apparatus such as a power rectifier or of any medium, decreases below a predetermined value.

* 27.

Undervoltage Relay is a relay which functions on a given value of undervoltage.

28.


29.

Isolating Contactor is a contactor which is used expressly for disconnecting one circuit from another for the purpose of emergency operation, maintenance, or test.

30.

Annunciator Relay is a n on-automatically reset device which gives a number of separate visual indications upon the functioning of protective devices, and which may also be arranged to perform a lockout function.


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7.


7.2


31.

Separate Excitation Device is a de vice which connects a circuit such as the shunt field of a synchronous converter to a source of separate excitation during the starting sequence; or one which energizes the excitation and ignition circuits of a p ower rectifier.

* 32.

Directional Power Relay is one which functions on a desired value of excitation during the starting sequence; or one which energizes the excitation and ignition circuits of a p ower rectifier.

32.

Directional Power Relay is one which functions on a d esired value of power flow in a given direction or upon reverse power resulting from arc-back in the anode or cathode circuits of a power rectifier.

33.

Position Switch is a switch which makes or breaks contact when the main device or piece of apparatus, which has no device function number, reaches a given position.

34.

Motor-Operated Sequence Switch is a mu lti-contact switch which fixes the operating sequence of the major devices during starting and stopping, or during other sequential switching operations.

35.

Brush-Operting or Sl ip-Ring Sho rt-Circuiting De vice is d evice fo r raising, lowering, or shifting the brushes of a machine, or for shortcircuiting its slip-rings, or for engaging or disengaging the contacts of a mechanical rectifier.

* 36.

Polarity Device is a device which operates or permits the operation of another device on a predetermined polarity only.

37.

Undercurrent or Underpower Relay is a relay which functions when the current or power flow decreases below a predetermined value.

38.

Bearing Protective Device is one w hich functions on e xcessive bearing temperature, or on other abnormal mechanical conditions, such as undue wear, which may eventually result in excessive bearing temperature.


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7.


7.2


39.


40.

Field Relay is a relay that functions on a g iven or abnormally low value or failure of machine field current, or on an excessive value of the reactive component of armature current in an A.C. machine indicating abnormally low field excitation.

41.

Field Circuit Breaker is a dev ice which functions to apply, or to rem ove, the field excitation of a machine.

42.

Running Circuit Breaker is a device whose principal function is to connect a machine to its source of running voltage after having been brought up to the desired speed on the starting connection.

* 43.

Manual Transfer or Selector Device is a manually-operated device which transfers the control circuits so as to modify the plan of operation of the switching equipment or of some of the devices.

44.

Unit Sequence Starting Rel ay is a re lay which functions to start the next available unit in a multiple-unit equipment on the failure or on the nonavailability of the normally preceding unit.

45.


46.

Reverse-Phase or Phase-Balance Current Relay is a relay which functions when the polyphase currents are of reverse-phase sequence, or when the polyphase currents are unbalanced or contain negative phasesequence components above a given amount

47.

Phase-Sequence Voltage Relay is a relay which functions upon a predetermined value of polyphase voltage in the desired phase sequence.


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7.


7.2


48.

Incomplete Sequence Relay is a relay which returns the equipment to the normal, or off, position and locks it out if the normal starting, or operating, or stopping sequence is not properly completed within a predetermined time.

* 49.

Machine or Transformer Thermal Relay is a relay which functions when the temperature of an A.C. machine armature, or of the armature or other load-carrying winding or element of a D.C. machine, or converter or power rectifier or power transformer (including a power rectifier transformer exceeds a predetermined value.

* 50.

Instantaneous Overcurrent or Rate-Of-Rise-Relay is a relay which functions instantaneously on an excessive value of current, or on an excessive rate of current rise, thus indicating a fault in the apparatus or circuit being protected.

* 51.

A.C. Time Overcurrent Relay is a relay with either a definite or inverse time characteristic which functions when the current in an A.C. circuit exceeds a predetermined value.

* 52.

A.C. Circuit Breaker is a device which is used to close and interrupt an A.C. power circuit under normal conditions or to interrupt this circuit under fault or emergency conditions.

53.

Exciter or D.C. Generator Relay is a relay which forces the D.C. machine field excitation to build up during starting or which functions when the machine voltage has built up to a given value.

54.

High-Speed D.C. Circuit Breaker is a cir cuit breaker which starts to reduce the current in the main circuit in 0.01 seconds or less, after the occurrence of the D.C. overcurrent or the excessive rate of current rise.

55.

Power Factor Relay is a relay which operates when the power factor in an A.C. circuit becomes above or below a predetermined value.

56.

Field Application Relay is a r elay wh ich automatically controls the application of the field excitation to an A.C. motor at some predetermined point in the slip cycle.


132



7.


7.2


57.

Short-Circuiting or Grounding Device is a power, or stored energy, operated device which functions to short-circuit or to ground a circuit in response to automatic or manual means.

58.

Power Rectifier Misfire Relay is a relay which functions if one or more of the power rectifier anodes fails to fire.

* 59.

Overvoltage Relay is a relay which functions on a given value of overvoltage.

60.

Voltage Balance Relay is a r elay wh ich operates on a gi ven difference in voltage between two circuits.

61.

Current Balance Relay is a r elay wh ich operates on a gi ven difference in current input or output of two circuits.

62.

Time-Delay Stopping or Opening Relay is a time-delay relay which serves in conjunction with the device which initiates the shutdown, stopping, or opening operation in an automatic sequence.

* 63.

Liquid or Gas Pressure, Level or Flow Relay is a relay which operates on values liquid or gas pressure, flow or level, or on a given rategiven of change of of these values.

64.

Ground Protective Relay is a relay which functions on failure of the insulation of a machine, transformer, or of other apparatus to ground, or on flashover of a D.C. machine to ground. NOTE: This function is assigned only to a relay which detects the flow of current from the frame of a machine or enclosing case or structure of a piece of apparatus to ground, or detects a ground on a normally ungrounded winding or circuit. It is not applied to a device connected in secondary circuit or secondary neutral of a current transformer, or current transformers, connected in the power circuit of a normally grounded system.

65.

Governor is the equipment which controls the gate or valve opening of a prime mover.


133



7.


7.2


66.

* 67.

Notching or Jogging Device is a de vice which functions to allow only a specified number of operations of a given device, or equipment, or a specified number of successive operations within a given time of each other. It is also a device which functions to energize a circuit periodically, or which is used to permit intermittent acceleration or jogging of a machine at low speeds for mechanical positioning. A.C. Directional Overcurrent Relay is a relay which functions on a desired value of A.C. overcurrent flowing in a predetermined direction.

68.

Blocking Relay is a relay which initiates a pilot signal for blocking of tripping on external faults in a transmission line or in other apparatus under predetermined conditions, or co-operates with other devices to block tripping or to block reclosing on an out-of-step condition or on power swings.

69.

Permissive Con trol Device is g enerally a rel ay two-position manu ally operated switch which in one position permits the closing of a circuit breaker, or the placing of an equipment into operation, and in the other position prevents the circuit breaker or the equipment from being operated.

70.

Electrically-Operated Rheostat is a rheostat which is used to vary the resistance of a circuit in response to some means of electrical control.

71.


72.

D.C. Circuit Breaker is a circuit breaker which is used to close and interrupt a D.C. power circuit under normal conditions or to interrupt this circuit under fault or emergency conditions.

73.

Load-Resistor Contactor is a contactor that is used to shunt or insert a step of load limiting, shifting, or indicating resistance in a power circuit, or to switch a space heater in circuit, or to switch a light, or regenerative load resistor of a power rectifier or other machine in and out of circuit.


134



7.


7.2


* 74.

Alarm Relay is a relay other than an annunciator as covered under device No.30, which is used to operate, or to operate in connection with, a visual or audible alarm.

75.

Position Changing Mechanism is the mechanism which is used for moving a removable circuit breaker unit to and from the connected, disconnected, and test positions.

76.

D.C Overcurrent Relay is a relay which functions when the current in a D.C. circuit exceeds a given value.

77.

Pulse Transmitter is used to generate and transmit pulses over a telemetering or pilot-wire circuit to the remote indicating or receiving device.

78.

Phase-Angle Measuring or Out-of-Step Protective Relay is a relay which functions at a predetermined phase angle between two voltages or between two currents or between voltage and current.

* 79.

A.C. Reclosing Relay is a relay which controls the automatic reclosing and locking out of an A.C. circuit interrupter.

80.


81.

Frequency Relay is a relay which functions on a predetermined value of frequency, either under or over, or on normal system frequency or rate of change of frequency.

82.

D.C. Reclosing Relay is a relay which controls the automatic closing and reclosing of a D.C. circuit interrupter, generally in response to load circuit conditions.

83.

Automatic Selective Control or Transfer Relay is a re lay which operates to select automatically between certain sources or conditions in an equipment, or perform a transfer operation automatically.

84.

Operating Mechanism is the complete electrical mechanism or servomechanism, including the operating motor, solenoids, position switches etc. for a tap changer, induction regulator or any piece of apparatus which has no device function number.


135



7.


7.2


85.

Carrier or Pilot-Wire Receiver Relay is a relay which is operated or restrained by a signal used in connection with carrier-current or D.C. pilotwire fault directional relaying.

* 86.

Locking-Out Relay is an electrically operated, hand or electrically reset relay or device which functions to shut down and hold an equipment out of service on the occurrence of abnormal conditions.

* 87.

Differential Protective Relay is a protective relay which functions on a percentage or phase angle or other quantitative difference of two currents or of some other e lectrical quantities.

88.

Auxiliary Motor or Motor Generator is one used for operating auxiliary equipment such as pumps, blowers, exciters, rotating magnetic amplifiers etc.

89.

Line Switch is a switch used as a disconnecting or isolating switch in an A.C. or D.C. power circuit, when this device is electrically operated or has electrical accessories, such as an auxiliary switch, magnetic lock, etc.

90.

Regulating Device is a device which functions to regulate a quantity, or quantities, such as voltage, current, power, speed, frequency, temperature, and load, at a certain value or between certain limits for machines, tie lines or other apparatus.

* 91.

Voltage Directional Relay is a relay which operates when the voltage across an open circuit breaker or contactor exceeds a given value in a given direction.

92.

Voltage and Power Directional Relay is a relay which permits or causes the connection of two circuits when the voltage difference between them exceeds a given value in a predetermined direction and causes these two circuits to be disconnected from each other when the power flowing between them exceeds a given value in the opposite direction.


136



7.


7.2


93.

Field-Changing Contactor is a contactor which functions to increase or decrease in one step the value of field excitation on a machine.

94.

Tripping or Trip-Free Relay is a r elay which functions to trip a cir cuit breaker, contactor, or equipment, or to permit immediate tripping by other devices; or to prevent immediate reclosure of a circuit interrupter, in case it should open automatically even though its closing circuit is maintained closed.

95. 96. 97.

Used only for specific applications on individual installations where none of the assigned numbered functions from 1 to 94 are suitable.

98. 99. A similar series of numbers, starting with 201 instead of 1, shall be used for those device functions in system. a machine, feederexamples or other equipment whenf unctions these are are controlled from the supervisory Typical of such device 201, 205directly and 294. 7.3

Suffix Letters

Suffix letters are used with device function numbers for various purposes. In order to prevent possible conflict, any suffix letter used singly, or any combination of letters, denotes only one word or meaning in an individual equipment. All other words should use the abbreviations as contained in ANSI standard Y.1.1, or should use some other distinctive abbreviation, or be written out in full each time they are used. Furthermore, the meaning of each single suffix letter, or combination of letters, should be clearly designated in the legend on the drawings or publications applying to the equipment.


137



7.3

Suffix Letters (Cont'd)

For purposes of clarification, these suffix letters have been classified in several groupings as detailed in lists A to E. The letters in lis ts A to C, since they should generally form part of the device function number, as for example 23x, 90V or 52BT. The letters in List D which denote parts of the main device and t hose in List E which cannot or need not form part of the device function designation, are written below the device function number, as for example 20 or 43 IS A List A

These letters denote separate auxiliary devices, such as: X) Y)

Auxiliary Relay *

Z) R)

Raising Relay

L)

Lowering Relay

O)

Opening Relay

C)

Closing Relay

CS

Control Switch

CL

"a" Auxiliary-Switch Relay

OP

"b" Auxiliary-Switch Relay

U

"Up" Position-Switch Relay

D

"Down" Position-Switch Relay

PB

Push Button

*NOTE: In the control of a circuit breaker with a so-called X-Y relay control scheme, the X relay is the device whose main contacts are used to energize the closing coil and the contacts of the Y relay provide the anti-pump feature for the circuit breaker.


138



7.3

Suffix Letters (Cont'd) List B

These letters indicate the condition or electrical quantity to which the device responds, or the medium in which it is located, such as: A

Air or Amperes

C

Current

E

Electrolyte

F

Frequency or Flow

L

Level or Liquid

P

Power or Pressure

PF

Power Factor

Q

Oil

S

Speed

T

Temperature

V

Voltage, Volts or Vacuum

VAR Reactive Power W


Water or Watts

139



List C

These letters denote the location of the main device in the circuit, or the type of circuit in which the device is used or the type of circuit or apparatus with which it is associated, when this is necessary, such as: A

Alarm or Auxiliary Power

A.C.

Alternating Current

AN

Anode

B

Battery, Blower or Bus

BK

Brake

BP

Bypass

BT

BusTie

C

Capacitor, Condenser, Compensator or Carrier Current

CA

Cathode

D.C.

Direct Current

E

Exciter

F

Feeder, Field or Filament

G

Generator or Ground **

H

Heater or Housing

L

Line

M

Motor or Metering

N

Network or Neutral **

P

Pump

R

Reactor or Rectifier


140



List C (Cont'd)

**

S

Synchronizing

T

Transformer, Test or Thyratron

TH

Transformer (High-Voltage side)

TL

Transformer (Low-Voltage side)

TM

Telemeter

U

Unit

Suffix "N" is generally used in preference to "G" for devices connected in the secondary neutral of current transformers, or in the secondary of a current transformer whose primary winding is located in the neutral of a machine or power transformer, except in the case or transmission line relaying, where the suffix "G" is more commonly used for those relays which operate on ground faults.

List D

These letters denote parts of the main device, divided in the two following categories: Category 1 All parts, except auxiliary contacts and limit switches as covered later under category 2 such as: BB

Bucking Bar (For High Speed D.C. Circuit Breaker)

BK

Brake

C

Coil or Condenser or Capacitor

CC

Closing Coil

HC

Holding Coil

IS

Inductive Shunt

L

Lower Operating Coil

M

Operating Motor


141



List D (Cont'd)

ML

Load-Limit Motor

MS

Speed-Adjusting or Synchronizing Motor

S

Solenoid

TC

Trip Coil

U

Upper Operating Coil

V

Valve

Category 2 All auxiliary contacts and limit switches for such devices and equipment as circuit breakers, contactors, valves and rheostats. These are designated as follows: a

Auxiliary switch, open when the main device is in the de-energized or nonoperated position.

b

Auxiliary switch, closed when the main device is in the de-energized or non-operated position.

aa

Auxiliary switch, closed when the main device is in the de-energized or nonoperated position.

bb

Auxiliary switch, closed when the operating mechanism of the main device is in the de-energized or non-operated position

The letters e, f, etc., ab, ac, ad, etc., or ba, bc, bd, etc., are special auxiliary switches other than a, b, aa, and bb. Lower-case (small) letters are to be used for the above auxiliary switches. NOTE: If several similar auxiliary switches are present on the same device, they should be designated numerically 1, 2, 3, etc. LC

Latch-Checking Switch, closed when the circuit breaker-mechanism linkage is relatched after an opening operation of the circuit breaker.

LS

Limit Switch.


142



List E

These letters cover all other distinguishing features or characteristics or conditions, not specifically described in lists A to D which serve to describe the use of the device or its contacts in the equipment, such as: A

Accelerating or Automatic

B

Blocking or Backup

C

Close or Cold

D

Decelerating, Detonate, or Down

E

Emergency

F

Failure or Forward

H

Hot or High

HR

Hand Reset

HS

High Speed

L

Left or Local or Low or Lower or Leading

M

Manual

OFF

OFF

ON

ON

O

Open

P

Polarizing

R

Right or Raise or Reclosing or Receiving or Remote or Reverse

S

Sending or Swing

T

Test or Trip or Trailing

TDC Time-Delay Cl osing TDO Time-Delay Opening U


Up

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7.4

Suffix Numbers

If two or more devices with the same function number and suffix letter (if used) are present in the same equipment, they may be distinguished by numbered suffixes as for example, 52X-1, 52X-2 and 52X-3, when necessary. 7.5

Devices Performing More Than One Function

If one device performs two relatively important functions in an equipment so that it is desirable to identify both of these functions, this may be done by using a double function number and name such as: 27 - 59 Undervoltage and Overvoltage Relay


144



Single-Line Diagrams

Single-line diagrams use single lines and simplified graphic symbols to show electric circuits, or system of circuits, and the components. Figure 103 shows a single-line diagram of a 480-v load center, i.e. a unit where 408 v ac is used to operate equipment.


145




Connections are made to a busbar , sometimes called simply a "bus". A bus is a conductor, or an assemble of conductors, for collecting currents and distributing them to outgoing feeders. At the top right-hand side of the diagram i t says "To 4160V SWGR BUS 32 SEE DIA. ARARA-417". This means that the vertical power path line is connected to 4160V, swit chgear through bus numbe r 32. Bus 32 can be seen on diagram number AR-ARA-417. The notation 3-1/C-500 MCM means that there are three wires in the line, that the wires are single conductor (1/C) and that the size of each conductor is 500,000 circular mils (or 500 MCM). A circular mil is a unit of area equal to π/4 of a square mil, or 0.7854 mm 2. It is a unit of measure for wire sizes. Further down the power path is the symbol for a power transformer. The transformer is identified by its number (308). It is stated to be a 4160/480, three phase (3Ø), 60Hz transformer. 4160 v is the primary voltage and 480 v is the secondary voltage. The primary is shown to be delta connected. The secondary is wye connected. The transformer's s econdary coil is shown to be earthed (grounded) through a resi stor. The resistor limits the amount of current that can flow through the circuit. This resistor is the same as the one shown in the symbol for the secondary wye connection. Connected to the resistor are a Test Switch(TS) and a relay. The relay is identified by its standard function device number, 59. Saudi Aramco Engineering Standards s how this to be an overvoltage relay. This relay operates when the voltage goes over a set value. The letter N indicates that the relay is connected to earth. Below the power transformer symbol is the symbol for a current transformer (CT). The CT is used to supply current to a monitoring and control circuit. The CT is shown to have a 3000/5 ratio. This means that when 3000A is flowing in the power line, 5A is flowing in the monitoring and control circuit. This is a fixed ratio of 600 to 1 eg. if 1500A flows in the power line, 2.5 A will flow in the monitoring circuit. The monitoring circuit contains four devices. First, there is a test switch. This is connected to standard function device number 51. The standards show this to be an A.C. Time Overcurrent Relay. It operates when an alternating current exceeds a set value.


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The circuit also contains an ammeter switch (AS) and an ammeter (A) A draw-out type circuit breaker is located below the CT. It is shown to be rated at 3000 Amperes. This is the maximum current that the breaker can safely carry. Underneath the 3000 is the number 52. This is a standard device number, indicating an alternating current circuit breaker. The power path continues along Bus A. Bus A is stated to be rated at 3000 Amps. A local number, 308 has been assigned to the bus. The bus is shown to be connected to another bus, bus 307. Six branch circuits are connected to bus 308. Each branch is identified by its number, numbers 3081 through 3086. Next to each draw-out cir cuit breaker is the number 600. This refers to the standard frame size of the circuit breaker. It indicates the maximum amperage that the breaker can safely carry. The third number given on the branch circuits indicates the size of the trip element used in the breaker. The trip element determines the maximum current the breaker can carry without tripping. This ranges from 175 A to 600 A in the branch circuits. Each breaker also has the number of its standard device function. In each branch this is number 52, an a.c. circuit breaker. Four of the branches go to various parts of t he plant. The remaining two are spare. The connecting lines have notations which give the type and size of the wire being used. The (2) means that there are two sets of wires in each line. Connected to the left- hand side of the bus is another monitoring and control ci rcuit. The diagram shows a fuse in the circuit in series with a 480/120 v step-down transformer. It protects the transformer and the rest of the circuit from overloads. The symbols next to the transformer indicate that both the primary and secondary are connected open delta. The secondary is grounded. An open delta transformer is a twowinding transformer that is connected in delta. This allows a 3-phase voltage to be produced using only two windings. A standard function device (Relay number 27) is connected into the circuit. This is an undervoltage relay. It operates when voltage values go l ow. The test switch is used to test the working of the relay. A voltmeter switch and a voltmeter are also connected into the circuit.


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Protective Relaying Circuits

Protective relaying is used to detect faults and dangerous conditions in power s ystems. The relays operate circuit breakers to isolate the dangerous or abnormal part of the circuit. They do this in the shortest possible time, bu t leave all normal parts of the circuit in operation. Faults and abnormal operating conditions can occur on generators, transformers, busbars, overhead lines cables and motors. The circuits that protect these pieces of equipment may all be shown on one drawing, thus making the drawing look more complicated than it really is. Circulating Current

If a circuit is healthy, the current entering the circuit should equal the current leaving. A fault is indicated if there is a difference of more than a certain amount. A circulating current circuit can be used to compare the incoming and outgoing currents in a system or unit that is to be protected. Figure 104 shows a circulating current ci rcuit being used to protect a busbar. Two equal ratio current transformers (Cts) are connected so that their secondary currents, Ia and Ib, flow in opposite directions through the relay R that is connected across them. The relay current, therefore, is the phasor difference of Ia and Ib.


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Circulating Current (Cont'd)

If a fault occurs OUTSIDE the A-B zone that is being protect, as shown by F1 in Figure 104, Ia and Ib would continue t o flow in the same directions. However, the fault would have removed the load and so the values of Ia and I b would be many times greater. Now, the relay must not operate for a fault outside the zone it is protecting. Therefore, the relay must be designed so that it will not operate even for the maximum value of through current (Ia - Ib) that a fault external to the protected zone can cause to flow through it. Figure105 shows a fault, F2, INSIDE the protected zone.


149



Circulating Current (Cont'd)

It can be seen that any current that is being fed to a second section of busbar will try to take the easier path through the fault. Therefore, the relay now r eceives the SUM of the secondary currents. The relay must be designed to act quickly to operate cir cuit breakers upon receiving this highest value of fault current. On the other hand, if no current existed outside the protected zone, Ia would be the only current to flow through the relay. Similarly, the relay must be designed to act quickly to operate circuit breakers upon receiving this lowest value of fault current . Bus Protection

Figure 106 shows a simple bus protection circuit.


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Bus Protection (Cont'd)

A Fault is imagined to be at 'F' on Section 1. One group of cts (a), (b) and (c) form a circulating current system covering zone 1. A similar group of cts (d), (e) and (f) cover zone 2. Engineers call these "discriminating zones". A third group of cts (g), (h), (j) and (k) cover both zones 1 and zone 2 (that is, the whole of the busbars). They are used as a check system. The purpose of the check system will be explained later. When fault F occurs in zone 1, Relays R1 and R3 will operate. This allows current to flow t o operate circuit breakers in zone 1 and also the bus section circuit breaker. Suppose the circuit in Figure 106 to be healthy and carrying normal loads. Now, if one of the leads, say to ct (a), were to become open-circuited, a current would flow in relay R1 (even though the circuit i s healthy). This is because there could be no current in ct (a) to balance out the currents from cts (b) and (c). The Check System.

However, the fault in ct (a) has no effect on relay R3 and so no current can flow to operate the circuit breakers. This method of using both discriminating and check systems also protects against unnecessary shutdowns owing to vibration or manual operation.


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Figure 107 shows a section of a Saudi Aramco power distribution drawing on which busbar protection can be seen. Note that ac time overcurrent relays (relays number 51) and locking out relays (number 86) are used.

Load Transfers From Feeder To Feeder And Bus To Bus

115 KV, 13.8 KV, 4.16 KV and 480 V switchgear are designed in parts or units. Figure 108 shows a bus-tie circuit for two incoming lines.


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Load Transfers From Feeder To Feeder And Bus To Bus (Cont'd)

The bus-tie breaker is normally open (NO). Buses A and B are supplied with power by their respective feeders. Contacts breakers in the feeder lines are normally closed (NC). If a feeder line fails or maintenance is needed its NC contactor breaker can be opened, thereby closing down the line. The feeder's bus bar can be kept supplied with power by closing the bus-tie breaker. Power is then supplied by the other feeder line. Transformer Protection

Many different kinds of faults can occur on transformers. Consequently, many different kinds of transformer protective circuits can be arranged. Figure 109 shows one kind of Saudi Aramco transformer protection system. "differential" protection.


It is based on

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Transformer Protection (Cont'd)

The transformer being protected is wound delta-delta and is shown at A. Current enters the system fr om the source at B. Two cts, at C and D, provide the diff erential input to the protection system. The cts, monitor respectively, the high-voltage side current and the low-voltage side current. These have a known relationship under healthy conditions. The two currents are sent to standard device function relay number 87. This is a differential protective relay. It operates on the difference between two similar electrical quantities (in this case, current). If the current differential is abnormal, thereby indicating a fault, relay 87 operates relay 86 at E. This is a locking-out relay. It operates circuit breakers to keep equipment out of service if abnormal conditions arise. In Figure 109, relay 86 is shown to operate an oil circuit breaker and an air circuit breaker. A third ct is connected to standard device function r elay number 50, at F. The letters "GS" on the relay symbol mean "instantaneous ground". Relay 50 is an instantaneous overcurrent or rate-of-rise relay. It operates instantaneously if the ct to which it is connected shows a sudden overcurrent in the line, or if the current value begins to increase excessively. Either of these conditions could occur if, for example, there was a short in the transformer winding. The Figure shows t hat relay 50 is connected to relay 86. Therefore, relay 50 operates both circuit breakers. Three standard function relays are grouped together at G. Relay 63 is a liquid or gas pressure, level or flow relay. It operates if an internal fault in the transformer generates gases in amounts sufficient to operate the relay. Its connection to the transformer is not shown. This kind of relay is often called a Buchholz gas or Sudden Surge Relay. Relay 63 is connected to a second number 86 relay. This second relay 86 also operates the circuit breakers. Relay 63 is also connected to an alarm system. Relay 49 is also connected to the alarm system. This is a thermal relay. It operates the alarm system if the transformer overheats. Note: The symbol shown near the ct at F denotes the ducting that carr ies the electric cables.

Figure 110 shows how different protective systems may be seen on single-line Saudi Aramco drawings.


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155



Power Distribution Drawings

Power distribution is more efficient when high voltages and low currents are used. Figure 111 shows a typical power distribution diagram.


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Power Distribution Drawings (Cont'd)

Utility power is supplied at 22.9 - 13.8 kV. The supply is protected by two lightning arrestors as shown by the symbols seen in Figure 112.

Connection to the circuit is made through an oil circuit-breaker (at A). protected by two more lightning arrestors.

The circuit is

Step-down transformers lower the supply voltage to 4160 V. The secondary coil of the transformers is earthed through a fixed resistor. Inside the building t he voltage is metered (measured) and relayed. Metering is done through potential and current transformers. It is here that the voltage is regulated. The supply then passes through air circuit-breakers to a bus.The bus is separated at the center by another air circuit-breaker. This allows one side of the distribution to be disconnected from the other side. Power is fed through an auxiliary feeder, department feeders, motor feeders and a generator tie. Note the symbols indicating the use of fused disconnect swit ches. Transformers step-down the voltage from 4160 V to 480 V, where shown. Power is sent to a dc motor drive. Note the symbol for a rectif ier, showing that the ac supply is converted to dc. Power is also sent to various motor control centers (MCCs). An mcc groups several motor controllers into one enclosure, li ke a cabinet. It combines motor control equipment with electrical feeders. The control equipment includes starter s, contactors, circuit breakers, fuses, switches, relays, metering and auxiliary equipment.


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WORK AID

Hundred of symbols are used in electrical engineering. It is not possible t o cover all of them. The symbols given in this Work Aid are some of those most often seen on Saudi Aramco Electrical drawings.


158




159




160




161



GLOSSARY a m p li fie r

:

A device that enables an input signal to control power and is capable of an output that is greater than the input signal.

a r m a tu r e

:

The rotating part of a dc motor or generator.

arm ature,relay

:

The moving element of a relay

base,transistor

:

A region that lies between an emitter and a collector

b i n a r y n u m b er s y st e m

:

A system of numbers with base 2 such that any number can be expressed by 0 or 1 or a combination of these digits.

blow ouct oil

:

Extinguishes circuit breaker arc through the effect of its magnetic field.

Booleanalgebra

:

A form of algebra applied to the design of digital computers.

circuitbreakers

:

A device to open a circuit automatically on a predetermined overload of current.

c o l le c to r , t r a n si st o r

:

The region through which the primary flow of charge carriers leaves the base.

c o m m o n c o n n e c ti o n

:

Electrical connection to a conductor common to several circuits such as a ground or battery bus.

c o m p o u n d fi e l d w in d in g

:

A combination of series and shunt magnetic field windings.

c o nt a ct o r

:

Heavy duty relay whose contacts open or close a primary circuit when its coil, in a secondary circuit, is energized.

c u r r e n t t r a n s fo r m e r

:

A instrument transformer used to sample the amount of current flowing in a high-voltage conductor.

deltaconnection

:

A triangular-shaped circuit connection of three windings.

d o u b le -p o le s w i tc h

:

Capable of opening or closing both sides of a circuit or two separate circuits.

e m i t t e r ,t r a n s i s t o r

:

A region from which charge carriers are injected into the base.

filt e r

:

A circuit which conducts only predetermined frequencies


162



and rejects unwanted frequencies.


163



filterchoke

A series-connected inductance with low reactance at the filter frequency.

:

fu ll -w a v e r e c tif ic a tio n

:

Conversion of both positive and negative portions of an ac wave to dc.

f us e

:

A protective device in which excessive current melts the fuse element and clears a circuit.

ground

:

Point of lowest potential which may be a metallic chassis or common bus.

g r o u n d -f a u l t p r o te c ti o n

:

A leak in current to ground due to defective insulation of components or conductors.

H enry

:

The amount of inductance that allows 1 volt to be induced in a coil when the current changes at the rate of 1 ampere per second.

high-passfilter

A circuit that passes only predetermined high frequencies.

:

homreun

:

Cables that return directly to the entrance service box or distribution panel.

horgnap

:

Divergent high-voltage switch contacts.

i n du c t a n ce

:

The ability of a conductor to produce induced voltage when the current varies through the conductor

interlockcircuit

:

A circuit that remains activated after the srcinal parallel circuit is deactivated.

invertergate

:

An electronic circuit that reverses the logic level; 1 becomes 0, and 0 becomes 1

jump er w ire

:

Short conductors used between adjacent or almost adjacent terminals.

ladderdiagram

:

Wiring diagram in which power utilization proceeds from top to bottom.

li g h t- e m i tti n g d io d e

:

A passed two terminal semi-conductor that emits light when current is through it.


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logicblocks

lowpassfilter neutraw l ir e

The basic circuits of a computer which combine and manipulate the binary signals so as to perform most of the functions of a computer.

:

A circuit that passes only predetermined low frequencies.

:

A grounded conductor. disconnected.

:

o sc i lla to r

o v e r lo a d r e l a y (O L )

:

It should never be fused or

A circuit capable of converting dc to ac at a frequency and waveshape determined by circuit components. A control relay whose contacts will open when the load current becomes excessive.

:

p o te n t ia l t r a n s fo r m e r

:

An instrument transformer used to sample the voltage in a high-voltage circuit.

p o te n t io m e te r

:

A three-terminal variable resistance usually connected across a voltage source to vary voltage division.

prim aryw inding

Input winding of a transformer

:

r ea c t an c e

:

Opposition to an ac due to an inductor and/or capacitor.

r e s is ta n c e

:

Opposition to the flow of an electric current.

r h e o st a t

:

A variable resistance with two terminals usually connected in series in a circuit to vary cu rrent.

riserdiagram

:

Shows how electrical energy is distributed through a building from the entrance to branch load centers.

r o to r schem aticdiagram

: :

The rotating member of an ac motor or generator. An elementary diagram showing the functions and relations of electronic components in a circuit by means of graphical symbols.

s e co n d a ryw i n di n g

:

The output winding of a transformer

seriefsield

:

A winding connected in series with a motor or generator

shunftield

:

armature to produce a magnetic field. A winding connected in parallel with a motor or generator armature to produce a magnetic field.


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single-poleswitch

:

Capable of opening and closing one side of a circuit.

s in g le -t h r o w s w i tc h

:

Capable of opening and closing one side of a circuit.

s ta to r

:

The stationary member of an ac generator or motor.

t h r e e - p h a se c ir c u it

:

Having or producing three ac waves displaced in time from each other by one-third of a cycle.

t r a n s is to r

:

Semiconductor device with three or more terminals capable of the transfer and amplification of a signal.

trim mercapacitor

:

A small type of capacitor that provides fine adjustment of capacitance.

t r u th t ab l e

:

Used in symbolic logic, in which the "truth" or "falsity" of a statement is listed for all conditions.

turnrsatio

:

The ratio of the number of secondary winding turns to primary winding turns of a transformer.

voltageregulator

w ye(orstar)

:

Connection of at least three windings in a circuit

:

like a Y or three-pointed star.

connections Zenerdiode


A device that maintains the output voltage of a power supply independent of load, within limits.

:

A pn junction diode reverse biased into the breakdown region. It is used for voltage regulation.

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219575445 Drafting Electrical Drawings

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