Best practices for process instrumentation instrumentation cabling Connectviy glues he nework ogeher; Cabling, grounding, cable routng, and he mitgaton of noise and inerference Fast Forward
Proper grounding hierarchy mitigates signal noise and interference.
Raceways such as conduits and trays have to ground at both ends. The two (AC, DC isolated master ground bus bars should connect to the plant grounding grid.
By Saeed M. AL-Abeediah
The health and effectiveness of any plant!s Process Automation "ystem (PA" (PA" relies on many factors.
Among these factors is the proper selection of PA" components, seamless integration, control schemes, control system installation, and last but not least, proper electrical installation and connectivity of field instrumentation devices.
This last factor, which glues the entire PA" system together, involves cabling, grounding, cable routing, and mitigation of e#ternal influences such as noise and interference. The best practices for dealing with process instrumentation cabling and the health and integrity of instrumentation loops mirrors the re$uirements stipulated in various applicable industry standards such as %&PA ', )***+-, AP) RP /, P)P PCC*0-, and "audi Aramco *ngineering standards.
1e will loo2 at the classes of instrumentation circuits and wiring suitable for each class, signal noises, techni$ues that minimi3e the impact of noise and interference on instrument signals, and conclude with a proposed process automation grounding scheme that PA" vendors helped develop. The load-side wiring system The remote control, signaling, and power limited circuit is defined in %&PA ' as the portion of the wiring system between the load side of the over+current device or the power+limited supply and all connected e$uipment. These circuits are in three classes. )t is important to note most of instrumentation signals fall under the Class+/ circuit, e#cept for the -/ 4ac and -- 4dc loops, which are Class+- circuits. "ome may argue the -/ 4ac and -- 4dc signals fit better under Class+5. 6owever, unless the power supply is Class+5, the industry practice is to categori3e them as Class+-. )n some facilities, the choice is to use Class+- wiring across the board. This avoids signal categori3ation issues. This may not be cost effective, but it is definitely safer.
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6ow does the wiring for v arious circuit classes differ7 The wiring re$uirements vary. &or e#ample, for Class+- circuits, a cable with 8 4 insulation rating is the choice, whereas a cable with an insulation of 5 4 is re$uired for Class+/. 1hen these circuits are in classified areas such as oil, gas, and petrochemical facilities, %&PA ' mandates additional cabling re$uirements beyond the insulation ratings. 9ne should use special cable types with specific mar2ing for these loops. 6ere are the cable types suitable for each circuit, assuming the installation is in a classified area.
Signal noise and interference :odern digital instruments prove to be more sensitive to noise and interferences when compared with the old analog instrumentation devices. )n addition, modern control systems are also more sensitive to any signal distortion when compared with old single loop controllers. This dictates avoiding old wiring practices and techni$ues that may allow the transfer of noise into the control loops. ;efore we address factors necessary to minimi3e signal interference, it is worthwhile to list some of the common types of signal noise and interference. Magnetic coupling< This type of coupling is also 2nown as inductive coupling. The interference magnitude is proportional to the mutual inductance between the control loop and the source of interference current. "uch noise is out there when several wires of different circuits are together in parallel runs in the same cable or in raceways. Electrostatic coupling< This coupling is also capacitive coupling because the m agnitude of the interference is proportional to the capacitance between a control lead and a source of interference or noise voltage. )t is similar to the magnetic coupling in the sense that it manifests primarily in parallel wiring. The length of the parallel wiring e#acerbates the effect of the noise. 1e see this more often with parallel AC discrete (switching circuits, especially when the loop lengths e#ceed -, feet. )t is worthwhile to note that in some literature, they call this phenomenon =distributed capacitance.!
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Electromagnetic coupling< This problem occurs when control circuits rout within the electromagnetic radiation profile of interference sources that radiate electromagnetic energy during their normal operation. *#amples of such sources are radio transmitters, television stations, communication e$uipment, AC motors, and e#posed power transmission lines. ;ased on )***+-, the voltages induced by electromagnetic coupling we call =near+field effects! because the inter ference is close to the interference source. The effect of such noise is dependant on the susceptibility of the control system and the strength of the produced electromagnetic field. ommon impedance coupling < This type of noise commonly occurs when more than one circuit shares common wiring, such as when a common return lead wire is used for multiple field devices such solenoids or relays. This type of noise is also common when trying to consolidate the commons for DC" or P0C loops in one wire. The length of the shared wiring aggravates such noise. ommon mode< This type of noise manifests primarily because of different grounding potentials at various locations of the plant. )t sometimes occurs even if the receiving instruments or input module has a high common mode re>ection rating. )t is more common when shields are not properly connected or when they connect at more than one place. )t is more prevalent in thermocouple loops, especially when the thermocouple is a grounded type.
!educing signal interference Although complete elimination of noise may not be practical in all cases, there are wiring techni$ues that will help reduce noise and its impact on the overall health of the loops. These techni$ues include proper cable construction, classification of signals into specific susceptibility levels, signal segregation, signal separation, and proper grounding. Cable construction< As a rule of thumb, it is highly recommended twisted and individually shielded pairs or triads be utili3ed for all analog signals such as ?+/ mA, thermocouple (T@C, millivolt signals, RTD, strain gauges, and pulses. )n addition, the same cable construction should wor2 and serve for all true digital signals. &or proper protection, the shield coverage should be -.
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&or discrete signals (on@off such as process switches, limit switches, relay contacts, solenoid circuits, and indication lights, one should use twisted pairs. An overall shield is fine for multi+pair@triad cables, provided the overall shield drain wire cuts off at the >unction bo# and grounds out at the marshalling cabinet. )n all cases (e#cept for grounded T@C, the shield drain wires shall be cut and taped in the field, and grounded at the marshalling cabinets. )t is vital to ensure the shield drain wires terminate properly and drain wires for different loops do not touch each other within the >unction bo#es or marshalling cabinets. Classification of wiring based on noise susceptibility level (NSL) < )***+- classified wiring levels into four ma>or classes or noise susceptibility levels. 1e at "audi Aramco developed a slightly modified categori3ation to simplify segregation and separation. The )*** %"0 levels settled into three levels based on our practical e#periences. 0evel -< 6igh to medium susceptibility with analog signals of less than 4 and discrete instrument signals of less than 5 4. *#amples of these signals are<
&oundation fieldbus
?+/ mA and ?+/ m A with 6ART
RTD
Thermocouple
:illivolt@pulse
Discrete input and output signals, e.g., pressure switches, valve position limit switches, indicating lights, relays, solenoids, and the li2e
All wiring connected to components associated with sensitive analog hardware li2e a strain gauge
0evel /< 0ow susceptibility with switching signals greater than 5 4, analog signals greater than 4, and -/+/? AC feeders less than / amps. *#amples of this level are<
Discrete input and output DC signals li2e pressure switches, valve position limit switches, indicating lights, relays, solenoids, and others
Discrete input and output AC signals including pressure switches, valve position limit switches, indicating lights, relays, solenoids, and the li2e
-/+/? AC feeders of less than / amps
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0evel 5< Power AC and DC buses of +- 4 with currents of /+ amps "ignal segregation )n instrumentation cabling, it is a good practice to segregate various signals from each other. &or optimum segregation, each ty pe of signal (within each %"0 shall tr ansmit on dedicated cables and rout to dedicated >unction bo#es. &or e#ample, ?+/ mA signals shall rout on s eparate cables from all other signals under %"0+-. The same applies on all other signal types. &rom the >unction bo#es to the control room, the cables for each %"0 level can share the same cable tray or trench.
They can also share the same marshalling cabinet provided the cables get enough air and ade$uate terminal strip identifications are in place. )n addition, all emergency shutdown s ignals should have their own cables, >unction bo#es, and marshalling cabinets. They also have to be segregated based on signal type as discussed above. Separation between different NSL < The recommended separation distances are from )***+- and P)P standard PCC*0-. )t is important to note the 3ero separation distances between signals of the same %"0 do not mean different signals within the same %"0 can use the same cable. "eparate cables must carry and serve different signals even if they are of the same %"0. Common return wire for multiple signals < Btili3ing a common wire for multiple signals is a common bad wiring practice that results in many covert noise problems. This wiring practice is common in wiring multiple solenoids associated with e$uipment, :94 wiring, relays, and in some cases in DC" or P0C loops.
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The temptation to use such wiring finds supporters especially when designing or e#ecuting pro>ects or when there are in+house pro>ects that would utili3e spare wiring. )t would be >ustified based on cost savings but always has a negative impact on the integrity of the associated loops. To protect against common impedance coupling, each signal should have its own r eturn wire e#tending from the source to the destination. Avoid using one or two return wires for multiple signals. "rocess automation system )n automation systems, proper grounding plays a significant role in the overall health and integrity of process signals. )t protects the automation systems from potential damages due to surges, voltage fluctuations, lightning, and short circuits. )n addition, proper grounding hierarchy helps mitigate signal noises and interferences by providing a low resistance path for these unwanted voltages and currents that could result in safety ha3ards or degradation to process control signals. 1hen attending to field problems associated with signal noise, erratic spi2es, or interference problems, we found the ma>ority of these problems stemmed from poor grounding. To ensure proper grounding of instrumentation systems, one must follow a clear grounding scheme. 9ne should carefully evaluate the overall grounding system when diagnosing a problem or when designing for new plants. These areas ar e grounding in the field, interconnection wiring, and grounding within central control or process interface buildings. Grounding in the field < )n the field, the enclosures of all instrument devices have to connect to ground, typically the plant overall grounding grid, or bond to an electrically conductive structure that is connected to the grid. Raceways such as conduits and trays have to ground at both ends. Handling of shield drain wires< 9ne should properly cut and tape the shield and its drain wire in the field, near the instrument. &rom the field instruments all the way to the marshalling cabinets, the shield drain wires should be treated and terminated similar to the signal wires. *#posed parts of the drain wires within >unction bo#es or m arshalling cabinets should be inside insertion >ac2ets to protect against the possibility of multiple drain wires touching each other. 9nce the loop reaches the marshalling cabinet, the shield drain wires have to consolidate and terminate at the DC and shield grounding bus bar. )n addition, all spare pairs or triads e#tending between the field >unction bo#es and marshalling cabinets should terminate at both ends. )n some cases, it would be useful to ground the spare wiring in the marshalling cabinets to minimi3e pot ential noise pic2 up. Grounding in control or process interface buildings < )n the control room or process interface buildings, the process automation system cabinets and marshalling cabinets must be e$uipped with two
grounding bus barsone for AC and one for DC common and shield drain wires. The DC and shield grounding bus bar shall be electrically isolated from the cabinet structure. All shield drain wires and DC common wires must merge and connect to the isolated bus bars. )t is vital to ensure shield drain wires ground at one end, typically in the marshalling cabinets. rounding them at both ends may result in ground loops, which happen to be one of the main causes of signal noise. The isolated DC and shield grounding bus bars within all cabinets should then be consolidate into a master instrument grounding bus bar within that building. "imilarly, all AC bus bars within these cabinets should come together at a master safety+ground bus bar within the building.
The two master ground bus bars should then be connected to the plant overall grid.
AB#$T T%E A$T%#!
Saeed M. AL-Abeediah (saeed.abeediahEaramco.com is a senior engineering consultant in the process and control systems department and P)D@instrumentation unit at "audi Aramco.
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