Special section: Networking
Best practices for process instrumentation cabling
Connectivity glues the network together; Cabling, grounding, cable routing, and the mitigation of noise and interference
By Saeed M. AL-Abeediah
The health and effectiveness of any plant’s Process Automation System (PAS) relies on many factors.
Among these factors is the proper selection of PAS 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 PAS system together, involves cabling, grounding, cable routing, and mitigation of external 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 requirements stipulated in various applicable industry standards such as NFPA 70, IEEE-518, API RP 552, PIP PCCEL001, and Saudi Aramco Engineering standards.
We will look at the classes of instrumentation circuits and wiring suitable for each class, signal noises, techniques that minimize the impact of noise and interference on instrument signals, and conclude with a proposed process automation grounding scheme that PAS vendors helped develop.
The load-side wiring system
The remote control, signaling, and power limited circuit is defined in NFPA 70 as the portion of the wiring system between the load side of the over-current device or the power-limited supply and all connected equipment.
These circuits are in three classes.
It is important to note most of instrumentation signals fall under the Class-2 circuit, except for the 120 Vac and 110 Vdc loops, which are Class-1 circuits. Some may argue the 120 Vac and 110 Vdc signals fit better under Class-3. However, unless the power supply is Class-3, the industry practice is to categorize them as Class-1.
In some facilities, the choice is to use Class-1 wiring across the board. This avoids signal categorization issues. This may not be cost effective, but it is definitely safer.
How does the wiring for various circuit classes differ?
The wiring requirements vary. For example, for Class-1 circuits, a cable with 600 V insulation rating is the choice, whereas a cable with an insulation of 300 V is required for Class-2.
When these circuits are in classified areas such as oil, gas, and petrochemical facilities, NFPA 70 mandates additional cabling requirements beyond the insulation ratings.
One should use special cable types with specific marking for these loops. Here are the cable types suitable for each circuit, assuming the installation is in a classified area.
Signal noise and interference
Modern digital instruments prove to be more sensitive to noise and interferences when compared with the old analog instrumentation devices. In 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 techniques that may allow the transfer of noise into the control loops. Before we address factors necessary to minimize 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 known as inductive coupling. The interference magnitude is proportional to the mutual inductance between the control loop and the source of interference current. Such 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 magnitude of the interference is proportional to the capacitance between a control lead and a source of interference or noise voltage. It is similar to the magnetic coupling in the sense that it manifests primarily in parallel wiring. The length of the parallel wiring exacerbates the effect of the noise. We see this more often with parallel AC discrete (switching) circuits, especially when the loop lengths exceed 1,000 feet. It is worthwhile to note that in some literature, they call this phenomenon ‘distributed capacitance.’
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. Examples of such sources are radio transmitters, television stations, communication equipment, AC motors, and exposed power transmission lines. Based on IEEE-518, the voltages induced by electromagnetic coupling we call ‘near-field effects’ because the interference 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.
Common 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 DCS or PLC loops in one wire. The length of the shared wiring aggravates such noise.
Common mode: This type of noise manifests primarily because of different grounding potentials at various locations of the plant. It sometimes occurs even if the receiving instruments or input module has a high common mode rejection rating. It is more common when shields are not properly connected or when they connect at more than one place. It is more prevalent in thermocouple loops, especially when the thermocouple is a grounded type.
Reducing signal interference
Although complete elimination of noise may not be practical in all cases, there are wiring techniques that will help reduce noise and its impact on the overall health of the loops. These techniques 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 utilized for all analog signals such as 4-20 mA, thermocouple (T/C), millivolt signals, RTD, strain gauges, and pulses. In addition, the same cable construction should work and serve for all true digital signals. For proper protection, the shield coverage should be 100%.
For 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 junction box and grounds out at the marshalling cabinet.
In all cases (except for grounded T/C), the shield drain wires shall be cut and taped in the field, and grounded at the marshalling cabinets. It is vital to ensure the shield drain wires terminate properly and drain wires for different loops do not touch each other within the junction boxes or marshalling cabinets.
Classification of wiring based on noise susceptibility level (NSL): IEEE-518 classified wiring levels into four major classes or noise susceptibility levels. We at Saudi Aramco developed a slightly modified categorization to simplify segregation and separation. The IEEE NSL levels settled into three levels based on our practical experiences.
Level 1: High to medium susceptibility with analog signals of less than 50 V and discrete instrument signals of less than 30 V. Examples of these signals are:
4-20 mA and 4-20 mA with HART
Discrete input and output signals, e.g., pressure switches, valve position limit switches, indicating lights, relays, solenoids, and the like
All wiring connected to components associated with sensitive analog hardware like a strain gauge
Level 2: Low susceptibility with switching signals greater than 30 V, analog signals greater than 50 V, and 120-240 AC feeders less than 20 amps. Examples of this level are:
Discrete input and output DC signals like 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 like
120-240 AC feeders of less than 20 amps
Level 3: Power AC and DC buses of 0-1000 V with currents of 20-800 amps
Signal segregation In instrumentation cabling, it is a good practice to segregate various signals from each other. For optimum segregation, each type of signal (within each NSL) shall transmit on dedicated cables and rout to dedicated junction boxes.
For example, 4-20 mA signals shall rout on separate cables from all other signals under NSL-1. The same applies on all other signal types. From the junction boxes to the control room, the cables for each NSL level can share the same cable tray or trench.
They can also share the same marshalling cabinet provided the cables get enough air and adequate terminal strip identifications are in place.
In addition, all emergency shutdown signals should have their own cables, junction boxes, 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 IEEE-518 and PIP standard PCCEL001. It is important to note the zero separation distances between signals of the same NSL do not mean different signals within the same NSL can use the same cable. Separate cables must carry and serve different signals even if they are of the same NSL.
Common return wire for multiple signals: Utilizing 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 equipment, MOV wiring, relays, and in some cases in DCS or PLC loops.
The temptation to use such wiring finds supporters especially when designing or executing projects or when there are in-house projects that would utilize spare wiring. It would be justified 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 return wire extending from the source to the destination. Avoid using one or two return wires for multiple signals.
Process automation system
In automation systems, proper grounding plays a significant role in the overall health and integrity of process signals. It protects the automation systems from potential damages due to surges, voltage fluctuations, lightning, and short circuits.
In 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 hazards or degradation to process control signals. When attending to field problems associated with signal noise, erratic spikes, or interference problems, we found the majority of these problems stemmed from poor grounding.
To ensure proper grounding of instrumentation systems, one must follow a clear grounding scheme. One should carefully evaluate the overall grounding system when diagnosing a problem or when designing for new plants. These areas are grounding in the field, interconnection wiring, and grounding within central control or process interface buildings.
Grounding in the field: In 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: One should properly cut and tape the shield and its drain wire in the field, near the instrument. From the field instruments all the way to the marshalling cabinets, the shield drain wires should be treated and terminated similar to the signal wires.
Exposed parts of the drain wires within junction boxes or marshalling cabinets should be inside insertion jackets to protect against the possibility of multiple drain wires touching each other. Once the loop reaches the marshalling cabinet, the shield drain wires have to consolidate and terminate at the DC and shield grounding bus bar.
In addition, all spare pairs or triads extending between the field junction boxes and marshalling cabinets should terminate at both ends. In some cases, it would be useful to ground the spare wiring in the marshalling cabinets to minimize potential noise pick up.
Grounding in control or process interface buildings: In the control room or process interface buildings, the process automation system cabinets and marshalling cabinets must be equipped with two grounding bus bars—one 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. It is vital to ensure shield drain wires ground at one end, typically in the marshalling cabinets. Grounding 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.
Similarly, 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.
ABOUT THE AUTHOR
Saeed M. AL-Abeediah (email@example.com) is a senior engineering consultant in the process and control systems department and PID/instrumentation unit at Saudi Aramco.
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