Circuit Protective Devices

Circuit Protective Devices Learner Work Book

Name: Group: Tutor:

LEARNER WORK BOOK

Circuit Protective Devices REV4.1

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LEARNER WORK BOOK

Table of Contents Foreword ........................................................................................................4 Circuit Protective Devices Unit Overview ....................................................5 Practical Skills .................................................................................................... 5 Knowledge Requirements .................................................................................. 5

Protective Devices .........................................................................................6 Cartridge fuses to BS 1361, BS 1362................................................................. 7 HBC Fuses (BS 88)............................................................................................ 8 Miniature circuit breakers (MCB’S) (BS EN 60898) Formerly BS3871 ................ 9 Residual Current Circuit Breakers BSEN61009 and Residual Current Devices BSEN61008 ..................................................................................................... 10 Semi-enclosed / re-wireable fuses BS3036 ...................................................... 11 Moulded case circuit breakers - MCCBs BS EN60439 ..................................... 12 Air circuit breakers - ACBs BS EN 60947 ......................................................... 12 Type ‘D’ and Neozed fuses .............................................................................. 12

Current Ratings of Circuit Protective Devices ..........................................14 Selecting the correct rating of device................................................................ 15

The Operating Principles of Circuit Protection .........................................18 Fusing factor .................................................................................................... 19 HBC Fuses (BS 88).......................................................................................... 20 Cartridge fuses to BS 1361, BS 1362............................................................... 20 Semi-enclosed / re-wireable fuses BS3036 ...................................................... 20 Residual Current Circuit Breakers BSEN61009 and Residual Current Devices BSEN61008 ..................................................................................................... 21 Miniature circuit breakers (MCBs) (BS EN 60898) Formerly BS3871 ............... 24 Thermal and magnetic operation...................................................................... 25

How to Identify Ratings of Protection Devices..........................................29 Prospective fault current rating (Ipf).................................................................. 29 Overload rating................................................................................................. 31 Identifying these ratings ................................................................................... 32 Semi-enclosed / re-wireable fuses BS3036 ...................................................... 32 Cartridge fuses to BS 1361, BS 1362............................................................... 33 HBC Fuses (BS 88).......................................................................................... 34 Miniature circuit breakers (MCB’S) (BS EN 60898) Formerly BS3871 .............. 35 Residual Current Circuit Breakers BSEN61009 and Residual Current Devices BSEN61008 ..................................................................................................... 36

Discrimination and Positioning ..................................................................37 Discrimination................................................................................................... 37 Positioning of protective devices ...................................................................... 40


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LEARNER WORK BOOK

Foreword In this unit you will learn about circuit protection. Circuit protection is an immensely important subject to grasp and is relevant to every electrical installation. In a healthy electrical system, insulation between line conductors and neutral conductors or between live conductors and earth is sound, with current flowing from the supply, through the load and returning via the neutral to its source. Overcurrent protection is provided by means of a circuit breaker or fuse. These devices are designed to operate within specified limits, disconnecting the supply automatically in the event of an overload or fault current (short circuits or earth faults). ‘Overcurrent protection device’ applies to fuses and miniature circuit breakers and in this section we will be looking at the various means by which overcurrent protection devices provide protection to the cables and circuits from damage by too much current fl owing. The appropriate device must operate within 0.4 or 5.0 seconds. This workbook is to be accompanied by PowerPoint “Circuit Protective Devices”


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LEARNER WORK BOOK

Circuit Protective Devices Unit Overview

Practical Skills To achieve the learning outcome the candidate must be able to: Calculate the size of circuit protection in relation to design current Select and correctly install circuit protection on a test board using a schedule of circuits

Knowledge Requirements To achieve the learning outcome the candidate must know: The types, operating principles and application of different types of fuses, MCBs, RCDs, RCBOs The current ratings for various circuit protection devices How to identify current ratings of various circuit protective devices in relation to overload and short circuit How to ensure correct measures are taken to ensure effective operation of devices in relation to discrimination and positioning


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LEARNER WORK BOOK

Protective Devices In existence are many, many different shapes and sizes of protective devices. These devices are used to detect overloads, and to break a circuit’s supply for protection from short circuits. The priority of the protective device is to protect the circuit conductors, not the appliance or the user. Each type of fuse (and all other over current devices) has a time-current characteristic which shows the time required to melt the fuse and the time required to clear the circuit for any given level of overload current. Where the fuses in a system are of similar types, simple ratios between ratings of the fuse closest to the load and the next fuse towards the source can be used, so that only the affected circuit is interrupted after a fault. Of all the types available they all fall into four main categories: 1. Semi-enclosed (re-wireable) fuses to BS 3036 and cartridge fuses for use in plugs to BS 1362. 2. High breaking capacity (HBC) fuses to BS 88 and BS 1361. These fuses are still often known as high rupturing capacity (HRC) types. 3. Circuit breakers, miniature and moulded case types to BS EN 60898 4. Circuit breakers containing residual current protection to BS EN61008 and 61009

What type of circuit protection have you seen? Where did you see them? Write down your answers.

We will now take a look at the various types available and in the next section we will look at their operating characteristics.


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LEARNER WORK BOOK

Cartridge fuses to BS 1361, BS 1362 The cartridge fuse consists of a porcelain tube with metal end caps to which the element is attached. The tube is then filled with granulated silica. The BS 1362 fuse is generally found in domestic plug tops used with 13 A BS 1363 domestic socket outlets. There are two common fuse ratings available, the 3 A, which is for use with appliances up to 720 watts (radios, table lamps, electric blankets) and the 13 A fuse which is used for appliances rated over 720 watts (irons, kettles, fan heaters, electric fires, lawn mowers, toasters, refrigerators, washing machines and

Name where BS1362 fuses are most commonly used


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LEARNER WORK BOOK

HBC Fuses (BS 88) The HBC fuse is a sophisticated variation of the cartridge fuse and is normally found protecting motor circuits and industrial installations. It consists of a porcelain body filled with silica, a silver element and lug-type end caps. Another feature is the indicating bead, which shows when the fuse element has blown. It is a very fast acting fuse and can discriminate between a starting surge and an overload. These types of fuses would be used when an abnormally high prospective short circuit current exists.


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LEARNER WORK BOOK

Miniature circuit breakers (MCB’S) (BS EN 60898) Formerly BS3871 A circuit breaker is a device for isolating the supply from a circuit on load in the event of an overload or short circuit fault occurring on that load. Under normal circumstances it acts as a switch capable of breaking the full load current feeding the circuit. Due to improved design and performance, the modern MCB now forms an essential part of the majority of installations at the final distribution level. From about 1970 the benefits of current limiting technology have been incorporated into MCBs, thus providing the designer/user with predictable high performance over current devices. Types of MCB: Thermal and Magnetic; Magnetic hydraulic; Assisted bimetal


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LEARNER WORK BOOK

Residual Current Circuit Breakers BSEN61009 and Residual Current Devices BSEN61008 RCD is the generic term for a device that operates when the residual current in the circuit reaches a predetermined value. **Usually this value is 30mA but other ratings are available. Residual current devices (RCDs) are a group of devices providing a modern approach to the enhancement of safety in electrical systems. They provide extra protection to people and livestock by reducing the risk of electric shock. Although RCDs operate on small currents, there are circumstances where the combination of operating current and high earth-fault loop impedance could result in the earthed metalwork rising to a dangerously high potential. An r.c.d. is a device which can provide protection against: Fire resulting from earth faults Earth faults where circuit resistance is too high for protection by conventional devices, i.e. circuit breakers or fuses Where greater protection against electric shock, for example socket outlet circuits likely to supply portable equipment outdoors Fuses and circuit breakers are totally unable to provide this level of shock protection. The descriptions below indicate two of the different types of RCD available and a description of each device. Residual Current Circuit Breakers BS EN 61009

Residual Current Devices BS EN 61008

Advantages and disadvantages of RCDs / RCCBs As mcbs but with the added advantage of detecting the smallest amounts of leakage current. This can also be regarded as nuisance tripping and a disadvantage.


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LEARNER WORK BOOK

Semi-enclosed / re-wireable fuses BS3036 A re-wireable is exactly that. It is a fuse that you can re-wire in the event of it operating. Early re-wireable fuses had a very low short circuit capacity and were very dangerous when operating under fault conditions because the fuse element melts and splashes the melted copper around and can cause fires. Later re-wireable fuses incorporated asbestos to protect the fuse holder during the fusing period, thus reducing the risk of fire from scattering hot metal when rupturing.

Do you know what type of protective devices you have in your home? If so state which ones and where they exist. You may be required to share your answer with the class.


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LEARNER WORK BOOK

Moulded case circuit breakers - MCCBs BS EN60439 Mould case circuit breakers perform much the same function as the miniature circuit breakers we looked at above but are used in installations where higher levels of fault current are present such as in smaller substations. They are usually adjustable to allow for an amount of over-current. They are rated up to 1000 amps.

Air circuit breakers - ACBs BS EN 60947 Air circuit breakers are used to interrupt circuits while current flows through them. Air circuit breakers may use compressed air to blow out the arc, or alternatively, the contacts are rapidly swung into a small sealed chamber, the escaping of the displaced air thus blowing out the arc. As with MCCBs they too are usually adjustable to allow for ranges of over-current. They are rated up to 10,000 amps.

Type ‘D’ and Neozed fuses Both these fuses are manufactured in Germany and have been developed to European testing regulations where all European testing authorities have approved them. The Neozed is the successor to the ‘D’ type fuse. You may in the course of your work come into contact with either type of fuse.


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LEARNER WORK BOOK Now complete the questions below.

1. What is the purpose of a protective device?

2. Explain what is meant by time-current characteristic

3. Name the five main types of circuit protection and state their BS number.

4. Describe what a BS1361 / BS1362 device is and state an advantage and disadvantage of using one.

5. Describe what a BS88 device is and state an advantage and disadvantage of using one.

6. Describe what a BSEN60898 device is and state an advantage and disadvantage of using one.

7. Describe what a BSEN61009 device is and state an advantage and disadvantage of using one.

8. Describe what a BS3036 device is and state an advantage and disadvantage of using one.


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LEARNER WORK BOOK

Current Ratings of Circuit Protective Devices Individual ratings vary with each device. Current ratings range between less than 6A to greater than 1000A. They generally increase in rating by 25%. The most common values can be found in BS7671 Appendix 3 Using the tables in your “Tables from the Regulations and On Site Guide” notes write down the current ratings of the main types of over current protective devices.

Fuses to BS3036 Rating (amps)

Type B, C and D circuit breakers to BS EN 60898 and RCBOs to BS EN 61009 Rating (amps)

General purpose fuses to BS88 Rating (amps


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LEARNER WORK BOOK

Selecting the correct rating of device Determining how much current will flow in the circuit is the first step required to correct device rating selection. This current is known as the design current and is the full load current of the circuit. It is calculated using one of the formulae below depending on the type of load. You need to ensure all units have to be calculated at the same value (i.e. kW have to be divided by kV; W have to be divided by V) Where:

I = the design current in amps (A) P = the circuit power in watts (W) V = the circuit voltage in volts (V) Cos Φ = the power factor

Resistive loads The following formulae apply to single and three line supplies:

P I= V

I=

Single-line 230v Resistive

P 3 ×V

Three line 400v Resistive

Inductive and / or capacitive loads The following formulae apply to single and three line supplies:

P I= V × cos Φ Single-line 230v Inductive and or Capacitive

I=

P 3 × V × cos Φ

Three line 400v Inductive and or Capacitive

In a.c. circuits, the effects of either highly inductive or highly capacitive loads can produce a poor power factor (cos Ф) (inductive and capacitive loads will be explained later). For now it is satisfactory to know that in circuits where there are inductive and electronic components such as coils and capacitors there are losses. These losses slightly increase the amount of current the equipment uses. You will have to allow for this in motor and discharge lighting circuits. Note √3 = 1.732 Once the design current has been determined we select the setting that is equal to or greater than the current value. This usually requires us to choose the next highest in value. Therefore for BS88 fuses with 14, 18 and 31 amps of current we would select 16, 20 and 32 amps devices respectively.


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LEARNER WORK BOOK Example 1. A single-line lighting circuit has a total power consumption of 2000 watts using 100watt filament lamps. Calculate the design current and BS88 fuse rating. i) Select the correct formula. (Single line; 230v, resistive)

I=

P V

ii) Input the data into the formula and work it out to two decimal places and be sure to add the unit (A).

I=

2000 = 8.70 A 230

iii) Select the single pole (single line) device rating equal to or greater than the calculated value of current. In this case a 10 amp fuse would be selected. Example 2. A three-line inductive load has a total power consumption of 30,000 watts (30kW) with a power factor of 0.95. Calculate the design current and the BSEN60898 Type D device rating. i) Select the correct formula. (Three line; 400v, inductive)

I=

P 3 × V × cos Φ

ii) Input the data into the formula and work it out to two decimal places and be sure to add the unit (A).

I=

30000 = 45.58 A 3 × 400 × 0.95

iii) Select the triple pole (three line) device rating equal to or greater than the calculated value of current. In this case a 50 amp motor rated (Type D) MCB would be selected.


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LEARNER WORK BOOK

Complete the exercise below. You are asked to calculate the design current and select the correct rating of protective device.

1. A BS88 device is required to protect a single line under floor heating circuit (resistive) that has a total power rating of 6kW

2. A resistive 230v lighting circuit with total power of 1000 watts is to be protected by a semi enclosed fuse

3. A 230v discharge lighting circuit of four flood lights with a total load per fitting of 270 watts (manufacturers lamp information) is protected by Type C BSEN60898 device

4. A 400v 32A IP44 socket supplying a portable machine whose rating is 15kW is to be connected to a BS88 fuse board 5. What will be the rating of the BSEN60898 device that protects a 10.5kW power shower in a domestic property?

6. Twenty five inductive loads rated at 400v 0.75kW each (with cos Φ of 0.95) are to be controlled by one panel with individual MCBs. What size BS88 fuse will protect the main panel?


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LEARNER WORK BOOK

The Operating Principles of Circuit Protection Overload currents occur in circuits which have no faults but are carrying a higher current than the design value due to overloaded machines, an error in the assessment of diversity, and so on. When a conductor system carries more current than its design value, there is a danger of the conductors, and hence the insulation, reaching temperatures which will reduce the useful life of the system. Examination of the characteristics of these devices (see your “Tables from BS7671 and the on-site guide” appendices) indicates that they are not the 'instant protectors' they are widely assumed to be. For example, an overloaded 30A semi-enclosed fuse takes about 100 seconds to 'blow' when carrying twice its rated current. If it carries 450 A in the event of a fault (fifteen times rated current), it takes about 0.1 s to operate, or five complete cycles of a 50 Hz supply. HBC fuses are faster in operation, but BS 88 Part 2 specifies that a fuse rated at 63 A or less must NOT operate within one hour when carrying a current 20% greater than its rating. Circuit breakers are slower in operation than is generally believed. For example, BS EN 60898 only requires a 30 A miniature circuit breaker to operate within one hour when carrying a current of 40 A. At very high currents operation is described by the British Standard as ‘instantaneous’, which is actually within 0.01 seconds. All protective devices, then, will carry overload currents for significant times without opening. The designer must take this fact into account in his calculations. The circuit must be designed to prevent, as far as possible, the presence of comparatively small overloads of long duration. The overload provisions of the Regulations are met if the setting of the device: 1. Exceeds the circuit design current. 2. Does NOT exceed the rating of the smallest cable protected.


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LEARNER WORK BOOK

Fusing factor It is evident that each of the protective devices discussed in the previous section provide different levels of protection, e.g. re-wireable fuses are slower to operate and less accurate than MCBs. In order to classify these devices it is important to have some means of knowing their circuit breaking and ‘fusing’ performance. This is achieved for fuses by the use of a fusing factor.

Fusing factor = Fusing current Current rating This is the ratio of the fusing current, which is the minimum current that will cause the fuse to blow and the stated current rating of the fuse or MCB (which is the maximum current that the fuse can sustain without blowing). Fusing currents can be found in Appendix 3 of BS 7671. These tables are logarithmic and the scales increase by factors often, not uniformly as may be expected, and therefore the interpretation of these scales will require some practice. The rating of the fuse is the current it will carry continuously without deterioration.

Fusing factors for the above devices can generally be grouped as follows: BS3036 1.8–2.0

BS1361 1.6–1.9

BS88 1.25–1.7

MCBs up to 1.5.

The higher the fusing factor, the less accurate – and therefore less reliable – the device selected will be. You may, while looking at fuses, have noticed a number followed by the letters kA stamped onto the end cap of an HBC fuse or printed onto the body of a BS 1361 fuse. This is known as the breaking capacity of fuses and circuit breakers. When a short circuit occurs, the current may, for a fraction of a second, reach hundreds or even thousands of amperes. The protective device must be able to break or make such a current without damage to its surroundings by arcing, overheating or the scattering of hot particles. The breaking capacities of MCBs are indicated by an ‘M’ number e.g. M6. This means that the breaking capacity is 6 kA or 6,000 A. The breaking capacity will be related to the prospective fault current. Consider a protective device (fuse or MCB) rated at 20 A. This value of current can be carried indefinitely by the device and is known as its nominal current setting, In. The value of current that will cause the device to operate, I2, will be larger than In and will be dependent on the device’s fusing factor. This fusing factor figure, when multiplied by the nominal setting In, will give the value of operating current I2. For fuses to BS 1361, BS 88 and circuit breakers to BS 3871 and BS/EN 60898, the fusing factor has been approximated to 1.45. Therefore our 20 A device would operate when the current reached 29 A (1.45 × 20).


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LEARNER WORK BOOK

HBC Fuses (BS 88) The fuse element consists of several parallel strips of pure silver with notches cut at pre-determined positions. A short length of pure tinfoil is wrapped round the centre notch of each strip. The silver strips are spot-welded to silver-plated copper end rings. This type of construction gives the required fusing factor as well as a time/current characteristic suitable for the apparatus to be protected. It also provides freedom from deterioration and limits the temperature rise. This is because the addition of pure tin to each silver strip of the fuse-element lowers the melting point of the combination and enables the fuse to operate at a fusing factor in the order of 1.4 without excessive temperature rise at the road at the rated current. The use of a composite silver/tin fuse-element permits the cross-sectional area to be much larger than that of a pure silver fuse-element, with the result that its thermal capacity is greater, giving a longer time lag with comparatively small over currents.

The notching of the fuse elements ensures that the circuit is cleared quickly with short circuit currents The tin is wrapped round the silver cord so that the fuse elements work with all the freedom from deterioration of a pure silver fuse element in ordinary service conditions. When an over current occurs, the tin and the silver alloy melt, breaking the circuit. Freedom from deterioration is further ensured by the spot welding of the ends of the fuse element. The cartridge has a substantial ceramic core. With a silver-plated copper end ring secured to each end to form the end connections of the fuse element. Electro-tinned brass end caps are pressed over the copper end rings and the cartridge is finally sealed by means of outer end rings of electro-tinned steel spun into grooves in the fuse core. A fibre washer is interposed between the spun on outer end ring and the brass end cap. All cartridges are filled with silica sand to ensure quick and effective arc extinction in all conditions of operations.

Cartridge fuses to BS 1361, BS 1362 Cartridge fuses are sometimes regarded as HBC fuses and as such as constructed in much the same way so that they achieve the same type of operation.

Semi-enclosed / re-wireable fuses BS3036 When current flows in a circuit, heat is given off. If the current is lower than the fuse rating then the fuse can dissipate (get rid of) the heat into the surrounding air. If however, current greater than the rating of the fuse flows in the circuit, then the fuse will be unable to dissipate (get rid of) the heat as easily and the fuse wire overheats. When the fuse wire reaches a set temperature, it will melt.


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LEARNER WORK BOOK

Residual Current Circuit Breakers BSEN61009 and Residual Current Devices BSEN61008 How these devices operate An RCD will detect an imbalance in either the three line and neutral currents in a TP&N circuit or P - N current in a single-line circuit. An imbalance in this context means that the sum of the circuit current does not equal zero. This situation will be interpreted by the RCD as an earth fault, between either a neutral or a line conductor and earth. Conversely if there is a current of 6 amps flowing in both line and neutral coils the circuit is said to be balanced and is considered equal. These devices can achieve great sensitivity and can give a measure of protection against direct contact, although it should be emphasised that the protection referred to is against electrocution not electric shock.

The main feature of the RCD is the toroid. As you can see below, there is an iron (easily magnetised) core. This core is wound with a search coil, which is connected to a trip mechanism. The toroid also has two other coils (line and neutral) wrapped around it. These are the line and neutral conductors inside the device. Flux produced by the line coil (clock-wise)

Line coil

Search coil

Flux produced by the neutral coil (Anti-clockwise)

Neutral coil


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LEARNER WORK BOOK The R.C.D under healthy circuit conditions

Balanced current creates a balanced core. The magnetic flux produced by each core is equal and opposite in value. Therefore the circuit is considered healthy. Follow the principle below. The same current flows in the line and neutral conductor. The current in each conductor produces a changing magnetic field around itself. The change in the magnetic field around the conductors produces a magnetic circuit in the line and neutral coils. The interaction of the two magnetic fields produced by the conductors cancel each other out and the search coil cannot ‘sense’ (have a current induced in it) any difference between the two.


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LEARNER WORK BOOK The R.C.D under earth fault conditions

Imbalance created due to fault to earth. The magnetic field produced by each coil is now unequal in value. As you can see, there is now a difference between the line and the neutral current. The reason for this is that current will follow any path provided for it. Under earth fault conditions there is at least one other path, the earth. Therefore the circuit is considered unhealthy. Follow the principle below. A different current flows in the line and neutral conductors. The currents in the line and neutral produce a changing magnetic flux around themselves. These magnetic fluxes produce a magnetic circuit in the core. As the two currents are different, so the magnetic fluxes are different, and the magnetic circuit in the core is not cancelled out and the search coil ‘picks up’ the difference. If the two currents are out by a set amount then the search coil will operate a small relay that will trip a switch, opening the circuit.

Everything operates on balance. If the currents in the coil are not the same then the RCD will operate. If the currents are the same then the RCD will not trip. This means that we have to be aware that an RCD will not trip under an overload condition. If there is a fault between line and neutral, and the current flowing is 500A, then because the magnetic circuit in the core of the RCD is balanced then it will not trip. It may well blow up but it won’t trip. The RCD is therefore very different from other protective devices in that it will only operate under earth fault conditions and no other.


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LEARNER WORK BOOK

Miniature circuit breakers (MCBs) (BS EN 60898) Formerly BS3871 A circuit breaker is a device for isolating the supply from a circuit on load in the event of an overload or short circuit fault occurring on that load. Under normal circumstances it acts as a switch capable of breaking the full load current feeding the circuit. As opposed to a fuse, which melts to isolate the supply, a circuit breaker is a mechanical device. It can either be magnetic, thermal or thermal magnetic. The majority of circuit breakers have thermal device for overload protection and magnetic device for short circuit protection. The major advantages of circuit breakers are that if anything goes wrong it will switch off, but once the fault has been corrected power maybe readily restored by switching the circuit breaker on again. There is no question of having to hunt for fuse wire, or having to keep a stock of replacement cartridge fuses, or having to find suitable tools to perform the operation.

A single visit to the distribution board enables the faulty circuit to be identified and if the fault has been removed for the power to be restored. Furthermore, when the fault has been cleaned it is always possible to restore power if circuit breakers are used, but with fuses if the correct size of fuse wire or the correct replacement cartridge is not available there is always the risk of makeshift replacements being used. BS EN 60898 defines the rated current (In) of a circuit breaker for household applications as the current that the breaker is designed to carry continuously (at an ambient temperature of 30 °C). The circuit breaker is labelled with the rated current in amperes (A), is preceded of followed by a letter "B", "C" or "D" that indicates the instantaneous tripping current, that is the minimum value of current that causes the circuit-breaker to trip without intentional time delay. • • •

Type B – operate at 3 to 5 times In Type C – operate at 5 to 10 times In Type D – operate at 10 to 20 times In


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LEARNER WORK BOOK

Thermal and magnetic operation Before we discuss how an MCB operates we first need to understand the operation of a couple of the key components. Bi-metallic strip A bi-metallic strip is used to convert a temperature change into mechanical displacement. The strip consists of two strips of different metals that expand at different rates as they are heated, usually steel and copper. The strips are joined together throughout their length by rivets, by brazing or by welding. The different expansions force the flat strip to bend one way if heated, and in the opposite direction if cooled below its normal temperature. The metal with the higher expansion is on the outer side of the curve when the strip is heated and on the inner side when cooled.

Thermal Operation The thermal aspect of the breaker uses a bimetallic strip, which heats and bends with increased current, and is similarly arranged to release the latch. This type is commonly used with motor control circuits. Thermal breakers often have a compensation element to reduce the effect of ambient temperature on the device rating.


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LEARNER WORK BOOK

Solenoid A solenoid is a loop of wire, often wrapped around a metallic core, which produces a magnetic field when an electrical current is passed through it. Solenoids are important because they can create controlled magnetic fields and can be used as electromagnets. In engineering, the term solenoid may also refer to a variety of devices that convert energy into linear motion.

Magnetic operation The magnetic aspect of the circuit breakers are implemented using a solenoid (electromagnet) whose pulling force increases exponentially as the current increases. The circuit breaker's contacts are held closed by a latch and, as the current in the solenoid increases beyond the rating of the circuit breaker, the solenoid's pull releases the latch which then allows the contacts to open by spring action. Some types of magnetic breakers incorporate a hydraulic time delay feature where in the solenoid core is located in a tube containing a viscous fluid. The core is restrained by a spring until the current exceeds the breaker rating. During an overload, the solenoid pulls the core through the fluid to close the magnetic circuit, which then provides sufficient force to release the latch. The delay permits brief current surges beyond normal running current for motor starting, energizing equipment, etc. Short circuit currents provide sufficient solenoid force to release the latch regardless of core position thus bypassing the delay feature. Ambient temperature affects the time delay but does not affect the current rating of a magnetic breaker.


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LEARNER WORK BOOK Thermo-magnetic components Below is a photograph of the internal details of an energised 32-ampere European DIN rail mounted thermal-magnetic miniature circuit breaker. Circuit breakers such as this are the most common style in modern domestic consumer units and commercial electrical distribution boards throughout Europe. Unfortunately, while the size and shape of the opening in the front and its elevation from the rail are standardised, the arrangements for busbar connections are not, so installers need to take care that the chosen breaker fits the bus bar in a particular board. We will now take a look at the internal components of a thermo-magnetic MCB and discover how it operates via the power point presentation.

4 3 2 1

7

6

8

4 5

1. Actuator lever - used to manually trip and reset the circuit breaker. Also indicates the status of the circuit breaker (On or Off/tripped). Most breakers are designed so they can still trip even if the lever is held or locked in the on position. 2. Actuator mechanism - forces the contacts together or apart and is spring loaded. 3. Contacts - Allow current to flow when touching and break the flow of current when moved apart. Controlled by three different activities. 4. Terminals – to connect to the busbar and for the final circuit to connect into. 5. Bimetallic strip – carries current and heats in accordance with the amount of current. It will de-activate the spring-loaded actuator mechanism in the event of over-current. 6. Calibration screw - allows the manufacturer to precisely adjust the trip current of the device after assembly. 7. Solenoid – under high level fault currents will magnetically draw a plunger in that is mechanically connected to the contact bar and actuator mechanism. 8. Arc divider / extinguisher – discharges and cools high levels of fault current across copper plates that are insulated from each other.


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LEARNER WORK BOOK Now complete the questions below.

1. Explain what is meant by “fusing factor”?

2. Name the two factors that need to be met so that the overload provisions of the regulations are satisfied.

3. With regard to BS88 fuses explain why the fuse element is notched. What is used to fill the fuse and why is it used?

4. Explain the operating principles of a BSEN61008 device and draw the circuit diagram

5. Describe the operating principles of the thermo and magnetic components of a BSEN60898 device and explain how the device operates under overload and short circuit conditions.


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LEARNER WORK BOOK

How to Identify Ratings of Protection Devices Protective device is the name used for a wide variety of electrical components, from the ‘common’ fuse found in many homes, to other more complex devices like circuit breakers. In this section we have looked at the types of protective devices that exist and why it is that these are chosen. It is important to know what protective devices are not installed for. They are not installed to stop people getting an electric shock. Neither are they an excuse for bad workmanship, hoping that if anything goes wrong then the ‘fuse will blow’. Protective devices are installed to protect the installation from fault currents and overloads. Q. What is an overload and what is a fault current and why is it important to know what the difference is between them? A. They are both electrical terms and have precise definitions. (See below)

Prospective fault current rating (Ipf) BS7671 states that a prospective fault current is: The value of overcurrent at a given point in a circuit resulting from a fault of negligible impedance between live conductors having a difference of potential under normal operating conditions or between a live conductor and an exposed-conductive part. You can see that there is a direct connection between the two conductors (either live/live; live/neutral or live/earth). There is no load connected and there is very little resistance between the two conductors. If you recall your work with Ohm’s Law, try to follow this, assuming the resistance between the supply DB and the faulty final circuit is 0.1Ω and the supply voltage is 230V. Work out the level of earth fault current

Uo If = Zs You can see that the fault current flowing due to an earth fault can be enormous. An over-current device must be able to withstand this massive amount of current. Therefore selecting the correct device will depend upon the expected amount of fault current. This is considered through various methods depending upon the whether the installation is single or three line.


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LEARNER WORK BOOK A circuit will always have a certain amount of impedance or resistance. The lower this impedance is the more fault current will flow. For now though it is suitable to understand using Ohms law that the lower the impedance the higher the fault current. Each and every device is manufactured with a breaking capacity. This means that it will be made to be able to withstand a certain level of fault current. If an over-current device is installed where its breaking capacity is less than the expected fault current it can have devastating effects: The over-current device can explode. It can melt and weld in the closed position. The device can catch fire. Explain why would the device melting and welding in the closed position be dangerous?

To establish the breaking capacity of an over-current device depends upon the device and we will look at each type on the next pages. Generally, it can be visibly seen on the device or manufacturer’s information may need to be consulted. Let us consider the breaking capacity of an MCB is 3kA (3000 amps). In order for that device to withstand a level of fault current it must not exceed 3000 amps. The maximum resistance (or earth loop impedance as it is known) should not exceed 0.07 Ω. For this reason maximum earth loop impedance values for each device is listed in the Tables from BS7671 and Onsite Guide. See table 41.2, 41.3 and 41.4.


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LEARNER WORK BOOK

Overload rating BS7671 calls an overload:An over-current in a circuit that is electrically sound. This seems a bit strange, after all what is the problem if the circuit is sound? A circuit may be sound, however the load or appliance connected to the circuit may demand more current than has been allowed for in the design. Imagine connecting a fire, which demands 20A, to a circuit that has only been designed to carry 10A. The circuit is not at fault; it is the person who has connected too large a load to the circuit who is at fault. Therefore the circuit is sound and it is the introduction of an extra element that causes it. Motors can also overload when the mechanical demand is increased and the motor over compensates to keep to the same speed. Here we see a socket that has had to supply more current that it is designed too for a long period of time. What can happen in the event of an overload?

So an over-current device is designed to carry an amount of current indefinitely. If this value is exceeded then the device should operate before a hazardous event occurs.


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LEARNER WORK BOOK

Identifying these ratings Semi-enclosed / re-wireable fuses BS3036

The over- current protection of these devices is achieved by fuse wire that can be changed by the user. The thickness of the wire determines the amount of current it can safely carry. The bodies of the devices are colour coded to display their supposed rating. Although as the fuse wire can be replaced by the user it has been known for 5 Amp devices containing larger 15 or 30 Amp fuse wire. The main BS3036 fuses used today are shown below. They are identified by their colour. 5 Amps – 15 Amps – 20 Amps – 30 Amps – 45 Amps –

White Blue Yellow Red Green

These devices are classified into three categories for short circuit rating. The ratings of these are not apparent and the manufacturers should be consulted. S1A – 1KA (1000 amps) S2A – 2KA (2000 amps) S3A – 4KA (4000 amps)


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LEARNER WORK BOOK

Cartridge fuses to BS 1361, BS 1362

The over- current protection of these devices is achieved by the fuse wire that is contained within the glass body of the device. The thickness of the wire determines the amount of current it can safely carry. The bodies of the devices are colour coded The main BS1361 and 1362 fuses used today are shown below. They are identified by their size and their colour. The ratings however are apparent by observing the body where they are also written in amps. 3 Amps 5 Amps – 13 Amps 16 Amps – 20 Amps – 30 Amps – 45 Amps –

Red White Brown Blue Yellow Red Green

These devices are classified into two categories for short circuit rating. The ratings of these are not always apparent and the manufacturers should be consulted. Type 1 – 16.5KA (16,500 amps) Type 2 – 33.0KA (33,000 amps)


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LEARNER WORK BOOK

HBC Fuses (BS 88)

The over- current protection of these devices is achieved by the fuse wire that is contained within the glass body of the device. The thickness of the wire determines the amount of current it can safely carry. The bodies of the devices are not colour coded and vary from between manufacturers. They are identified by their size the ratings are apparent by observing the body where they are written in amps. What size do you think the one shown above is?

These devices are classified into two categories for short circuit rating. The ratings of these are usually apparent by observing the body of the fuse. BS88-2.1 – BS88-6 –

50KA at 400V (50,000 amps) 80KA at 400V (50,000 amps) 16.5KA at 230V (16,500 amps)


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LEARNER WORK BOOK

Miniature circuit breakers (MCB’S) (BS EN 60898) Formerly BS3871 The over-current protection of these devices is achieved by the choice and setting of the bi-metallic strip that is contained within the body of the device. The bodies of the devices are not colour coded and vary in shape and size between manufacturers. It is apparent though from observing the body of the device as to its rating and trip characteristic i.e. its type.

This one

Its type refers to how the device will handle overload. Type B operates instantly at certain levels of overload where as a type D will handle that level of overload for a slightly longer spell of time. This is particularly useful for inductive load circuits.

What rating and type do you think the one shown above is? These devices are classified into main categories for short circuit rating. The ratings of these are always apparent by observing the body of the device. Always look for the large rating inside a rectangle.

What is the short circuit capacity of the MCB on the left?

BS3871 (Replaced by BS EN 60898) – M1 1.0KA (1000 amps) M1.5 1.5KA (1500 amps) M3 3.0KA (3000 amps) M4.5 4.5KA (4500 amps) M6 6.0KA (6000 amps) M9 9.0KA (9000 amps)

BS EN 60898 and BS EN 61009 – M1.5 1.5KA (1500 amps) M3 3.0KA (3000 amps) M6 6.0KA (6000 amps) M10 10.0KA (10,000 amps) M15 15.0KA (15,000 amps) M20 20.0KA (20,000 amps) M25 25.0KA (25,000 amps)


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LEARNER WORK BOOK

Residual Current Circuit Breakers BSEN61009 and Residual Current Devices BSEN61008 Ratings of these devices are as per BS EN 60898 devices in both over-current and short circuit protection. They can be identified in exactly the same way. So, how do we know whether a device is a residual current device? The thing that makes them unique is the fact that they bear the rating of their earth leakage protection rating too. This is shown in either milliamps (mA) or amps (A) usually shown like this: (I∆n 30 mA). The mA rating indicates the amount of earth leakage current that it takes to operate the device. See the pictures below.

Can you see the residual current rating of the device above? Write it down.

Can you see the residual current rating of the device above? Write it down.

Now answer the questions below 1. What does the overload rating on a protective device indicate?

2. What does the short circuit rating on a device indicate?


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LEARNER WORK BOOK

Discrimination and Positioning Selecting protective devices is not always a straightforward process. Selecting the incorrect rating or positioning the device in the wrong location can mean: The circuit may not operate correctly Other circuits may be affected by a fault The circuit may not be fully protected The installation may develop into an unsafe state

Discrimination In both large and small installations, there is usually a series of fuses and/or circuit breakers between the incoming supply and the electrical outlets. The relative rating of the protective devices used will decrease the nearer they are located to the current using equipment. Ideally when a fault occurs, only the device nearest the fault operates, thus ensuring minimum disruption to other circuits not associated with the fault. Discrimination is said to have taken place when the smaller rated local device operates before the larger device. Discrimination is generally only a problem when a system uses a mixture of devices. Obviously a particular type of fuse will discriminate against a similar type of fuse if it is of larger rating. Discrimination is also known as the co-ordination between fuses.

Applying discrimination is the act of designing a system in such a way that only the minimum numbers of circuits are affected by the operation of a protective device. BS7671 demands that discrimination is applied where it is necessary to prevent danger, however, even when it is not dangerous, it is good practice to apply discrimination. If a fault were to occur on circuit D we would want to have device D operate before either B or A.


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LEARNER WORK BOOK

Discrimination requires more consideration to be given to the type of protective device and it is not adequate to merely choose the next size of protective device and assume that it will be ‘big enough’. At the moment it is sufficient for you to understand that with discrimination we are trying to ensure that the protective device nearest to the fault operates first. For example: An overload current of 30A flows in circuit D. It is protected by a 6 A Type C BSEN60898 circuit breaker. Fig3.5 shows that 30 amps will cause the 6amp device to operate within 15s. The device at position B is a BS88 HRC fuse with a 10-amp rating. Fig3.3B show that 30 amps will cause the 10-amp device to operate within 6s. This would mean that in the event of 30 amps of overload on circuit D, device B would operate first which would mean circuits E and F would also lose their supply.

Consider the implications of the above example and explain what sort of things would happen as a result of device B operating before the overloaded circuit D


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LEARNER WORK BOOK As a rule we usually ensure that the device protecting a final circuit is half the size of the one that protects the consumer unit. And that device is half the size of the device that protects that consumer unit and so on. Finer discrimination can be achieved if we use a ratio of 1:1.6 Now answer the questions below Work out the minimum fuse ratings in the example distribution network on the previous page. Notice the different ways to represent devices below

1. Fuse A is a BS88 200 amp. Work out the rating of device B (BS88) 2. Fuse F is a BS3036 15 amp and device B is a Type C MCB 3. Fuse C is a 50 amp BS88 and device A is also a BS88 4. Fuse E is a Type B BSEN60898 and device B is a 20A BS88 5. Fuses D, E and F are all D10 MCBs. Size the BS88 device B. 6. All fuses are BS88. Devices D, E and F are 6 A. Devices B and C are 32A what will be Device A rated at? .


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LEARNER WORK BOOK

Positioning of protective devices It is worthwhile looking at a typical circuit and then considering where protective devices would be positioned. You can’t just stick a fuse anywhere and expect the right things to happen. Careful thought and an application of BS7671 is necessary.

From the requirements of regulation 473-01-01 it states that a protective device should be positioned where: 1. A reduction occurs in the size of the cable; 2. Installation method changes; 3. The type of cable changes; 4. The environmental conditions change.

The above requirement 2, 3 and 4 do not apply where the circuit is protected against fault current. So if the circuit disconnects within the required time it is not a requirement to install devices at those locations.


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LEARNER WORK BOOK

Now complete the questions below.

1. With regard to circuit protection explain what is meant by discrimination.

2. Using the diagram on page 56 assume fuses D, E and F are 6A BS88s. Write down the minimum sizes of BS88s for fuses B and A so that correct discrimination is achieved.

3. What is the “rule of thumb” figure used to ensure the correct rating of devices are used to ensure discrimination is achieved?

4. State 4 examples where it might be necessary to introduce a protective device into a circuit.

5. Assuming a circuit is sufficiently protected from fault current when is the only example when you are required to introduce a protective device into a circuit.


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Circuit Protective Devices

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