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Good grounding practices - Part 1

Nov. 13, 2018
Why we ground, and resistance specifications.

Read Part 2:Ā Why and how to make power and shielding connections.Ā 

Grounding can be the ultimate commonĀ cause due to its ubiquitous nature ofĀ connecting everything electrical together,Ā and a great unknown because much of itĀ and its operation are invisible to us. ThisĀ article discusses what makes up a goodĀ instrument grounding system and why. ItĀ doesnā€™t cover all the details about instrumentĀ grounding systems (which wouldĀ take a book or two), but rather some ofĀ the basic principles that lead to goodĀ practices, as well as their technical basis.

First, we must divide the instrumentĀ systems into the incoming power side,Ā which is nominally the AC side, and theĀ instrument side, which is nominally theĀ DC side (Figure 1). The DC side can beĀ further divided into the DC power sideĀ (nominally 24 VDC), control and signal.Ā The AC and DC power sides are normallyĀ isolated through transformers, while onĀ the DC side, the power and signal areĀ shared. Our discussion will be primarilyĀ about the DC side, but for reasonsĀ described later, the grounding system isĀ typically shared by both sides.

Power and ground

Figure 1: Instrument systems can be divided into the AC side and the instrument side, which is nominally the DC side. The DC side can be further divided into the DC power side (nominally 24 VDC), control and signal. The AC and DC power sides are normally isolated through transformers, while on the DC side, the power and signal are shared. The grounding system is typically shared.Ā 

Why we ground

The main reasons we ground our systemsĀ are:

  1. Personnel safety,
  2. Electrical system protection,
  3. Lightning protection,
  4. Electrostatic discharge protection,
  5. Electrical noise control,
  6. Intrinsically safe circuits,
  7. Power quality, and
  8. To provide a reference plane for ourĀ electrical and electronic circuits andĀ systems.

Personnel safety: This is primarily aĀ concern on the AC (high-voltage) sideĀ of the instrument system. MaintenanceĀ personnel test and repair instrumentĀ systems, and the operator may also interact with the instrument systemā€™s frontĀ end and field instruments. Because ofĀ the low voltage (nominally 24 VDC), theĀ instrument side is often worked withoutĀ concern about electrical shock. AnĀ instrument tech may be in for a rudeĀ surprise, even a fatal one, when workingĀ on an instrument circuit where anĀ ungrounded DC instrument circuit hasĀ come into contact with a higher voltageĀ source (e.g. 120 or 277 VAC) that didnā€™tĀ trip the high-voltage circuit overcurrentĀ protection.

The National Electrical Code (NEC)Ā Article 250 and other application-specificĀ NEC articles provide requirementsĀ for grounding for personnel safety. ThisĀ is a U.S. code, also known as NationalĀ Fire Protection Association (NFPA) 70,Ā which is used worldwide. However,Ā each country or legal identity may haveĀ their own electrical code, or provide additionalĀ requirements to the NEC. YouĀ canā€™t take exception to requirements ofĀ the NEC or similar codes just becauseĀ the system is an instrument system,Ā not an ā€œelectricalā€ system. EquipmentĀ grounding and bonding are used toĀ help ensure that there's a low impedanceĀ path back to the source duringĀ the fault conditions. This allows theĀ system overcurrent protection to openĀ up, protect the electrical system andĀ remove dangerous voltage from the circuitĀ in a timely manner.

Common codes and standards forĀ grounding for personnel safety are:

  • NFPA 70, National Electric CodeĀ (NEC), Article 250 and specific applicationĀ articles.
  • IEC 60364, Electrical Installations forĀ Buildings, Part 5, Section 54
  • IEEE 142 Std. ā€“ IEEE RecommendedĀ Practice for Grounding of IndustrialĀ and Commercial Power SystemsĀ (commonly called the Green Book)
  • IEEE Std. 80 - IEEE, ā€œGuide forĀ Safety in AC Substation Groundingā€

Electrical Instruments in HazardousĀ Locations by Ernest C. Magison has anĀ excellent chapter human electrical safety.Ā Soares' "Book on Grounding and Bonding"Ā is an excellent general reference onĀ grounding and has a chapter on groundingĀ of electronic systems.

Electrical system protection: TheĀ NEC also provides requirements forĀ electrical protection to limit the damageĀ to equipment and wiring. This isĀ also safety-related as it minimizes theĀ potential of a fire caused by an electricalĀ source. A properly designed groundingĀ system for the AC side of our instrumentĀ system can minimize the potentialĀ damage to equipment from an electricalĀ fault, surge, lightning strike, etc., andĀ contribute to the reliable operation ofĀ the equipment.

Common codes and grounding standardsĀ for this are essentially the same asĀ those for electrical safety given above.Ā NEC Article 250-50 also requires that allĀ grounding electrodes that are present atĀ each building or structure served shallĀ be bonded together to form the groundingĀ electrode system (commonly called aĀ ground grid in petrochemical facilities).

This has given rise to the one of theĀ most controversial aspects of groundingā€”whether it is wise or necessaryĀ to connect the DC side of the instrumentĀ system to that noisy, nasty electricalĀ safety ground. In the early days ofĀ DCS systems, manufacturers commonlyĀ called for an isolated, clean ground. ThisĀ requirement has for the most part beenĀ superseded, but still raises its ugly headĀ occasionally, both as a manufacturerā€™sĀ requirement and in questions raised onĀ various Internet forums. The answer toĀ the question by NEC is a solid ā€œyes.ā€Ā Later, we'll talk about why this is actuallyĀ a good idea, as well as the fact that thereĀ is no such thing as a ā€œcleanā€ ground.

Lightning protection: Lightning is alwaysĀ a concern for instrumentation systems,Ā and increasingly so with new technologyĀ that has ever-smaller-dimensionĀ digital circuitry, smaller signal-to-noiseĀ ratios, and tighter common mode specifications.Ā These make our digital instrumentationĀ more sensitive to lightning, RFĀ generation, induced currents and powerĀ quality disturbances.

The common standards applied toĀ lightning protection are:

  • NFPA 780 ā€“ ā€œStandard for the InstallationĀ of Lightning ProtectionĀ Systemsā€
  • API 2003 ā€“ ā€œProtection Against IgnitionsĀ Arising out of Static, Lightning,Ā and Stray Currentsā€

Electrostatic discharge protection:Ā This is primarily a concern in handling,Ā touching or being in close proximity toĀ digital electronic chips and cards. AnalogĀ electronics are not as sensitive.

Standard manufacturerā€™s recommendedĀ grounding practices for handlingĀ digital equipment should be followed. It'sĀ also common practice for raised-floorĀ installations to specify a resistivity for theĀ floor tile or a resistance to ground. ForĀ example, IBM specifies no greater thanĀ 2 x 1010 ohms to the ground reference.Ā ANSI/ESD S20.20 has a specification ofĀ ā‰¤ 1 x 109 ohms. As you can see, a littleĀ ground goes a long way when dealingĀ with static electricity, but it's a necessaryĀ consideration. This is normally satisfiedĀ by specifying the proper floor tile,Ā designing a good floor stringer groundingĀ system, and using grounding wrist strapsĀ when needed.

Static electricity can also generateĀ radio frequency interference (RFI) thatĀ can interfere with the operation of instrumentation,Ā with lightning being theĀ extreme case. I'm aware of a controlĀ room installation where the operatorā€™sĀ chair seat backing crinkled as theĀ operators adjusted themselves in theirĀ chairs, which generated small static electricityĀ discharges and generatedĀ RFI that interfered with their controlĀ displays.

Table 1: Noise coupling types
Electrical noise control: Noise isĀ any electrical signal present in a circuitĀ other than the desired one. All electricalĀ and electronic circuits have noise,Ā which becomes interference when itĀ has an undesirable or detrimental effectĀ on the operation of a circuit or system.Ā Always a concern in instrumentationĀ and electronic systems, noise isĀ not as significant in electrical systemsĀ (though modern electrical equipmentĀ often has digital monitoring and communicationĀ systems).

A common refrain is that ground is aĀ place to drain your noise. The ground isĀ not a sump for noise, and can actuallyĀ be a source of noise. A basic principle ofĀ circuit electricity is that electricity alwaysĀ seeks to return to its source. This principleĀ applies to noise: once coupled intoĀ a circuit, noise always works in completeĀ circuits, and ground can serve as a returnĀ path for noise.

Most noise of interest is coupled intoĀ the instrumentation circuits by four methods:Ā capacitive, inductive, radiated orĀ conducted (Table 1). We will talk moreĀ about this when we talk about groundingĀ of shielding.

Intrinsically safe circuits: For facilitiesĀ that use intrinsically safe (IS)Ā circuits to satisfy the requirements forĀ instruments in classified hazardousĀ areas, grounding can be an issue forĀ certain types of intrinsic safe barriers.Ā Zener barriers (Figure 2) requireĀ a high-integrity ground to shunt anyĀ dangerous electrical energy. The nominalĀ grounding specification is a maximumĀ resistance of 1 ohm. The intrinsicĀ safety ground is connected to the plantĀ safety ground grid. For the intrinsicĀ safety ground, the concept of an equipotentialĀ ground plane is important toĀ ensure that the ground potentials in theĀ intrinsically safe circuit are as equal asĀ we can make them to prevent a sparkĀ due to a voltage differential betweenĀ circuit parts and ground. TransformerĀ or galvanically isolated barriers typicallyĀ do not require a ground connection.

IS Zener barrier

Figure 2: Zener barriers in intrinsically safe (IS) systems require a high-integrity ground with a maximum resistance of one ohm. The concept of an equipotential ground plane is important to prevent a spark due to a voltage differential between circuit parts and ground.Ā 

The standards for intrinsic safety are:

  • ISA RP 12.06.01, ā€œRecommendedĀ Practice for Wiring Methods for HazardousĀ (Classified) Locations InstrumentationĀ Part 1: Intrinsic Safetyā€
  • ANSI/ISA 60079-11 (12.02.01) ā€“Ā ā€œExplosive Atmosphere ā€“ Part 11:Ā Equipment protected by intrinsic ā€œiā€ā€
  • NEC Article 504 ā€“ ā€œIntrinsic SafeĀ Systemsā€

Power quality: A stable ground referenceĀ is important for power quality,Ā particularly with distributed control systems.Ā Good grounding practices are alsoĀ necessary for surge protection devicesĀ to work properly. Following the groundingĀ practices of the NEC and the IEEEĀ std.1100, ā€œRecommended Practice forĀ Powering and Grounding ElectronicĀ Equipment,ā€ commonly called the IEEEĀ Emerald Book, will help achieve goodĀ power quality and good grounding engineeringĀ practice.

Circuit reference plane: GroundingĀ is also used to establish a common,Ā stable voltage reference, so complexĀ and sometimes widely distributed instrumentĀ systems can understand eachĀ otherā€™s signals. Circuits work much betterĀ if they have a common referenceĀ between them.

Common ground

Grounding can be a technically difficultĀ subject primarily because of itsĀ complexity (breadth of scope, infiniteĀ number of potential connections, internalĀ and external actors and bad actors,Ā etc.,) and its uncertainty due to invisibility (canā€™t see what is going on below the surface, the groundĀ is different anywhere you look, unknown influences, limitedĀ available models that can help the practicing engineer, etc.). ItĀ also can raise the specter of many electrical engineersā€™ leastĀ favorite subjectā€”electromagnetic fields and Maxwellā€™s equationsā€”when circuit theory isnā€™t enough. Fortunately, much ofĀ the basics can be understood by analogy and a bit of circuitĀ theory. This can lead us to some good engineering practices inĀ regards to grounding instrumentation systems.

We discuss three topics that commonly arise when engineering instrument grounding. The first is here and the others are in Part 2:

  1. What should the ground resistance be?
  2. Do I have to connect my clean instrument ground to thatĀ dirty power ground?
  3. Do I connect my shields to ground at one end or both?

What should the ground resistance be?

A common grounding question is what the ground resistanceĀ should be for a DCS/PLC system (this question applies to theĀ DCS/PLC ground prior to any connection to other groundingĀ systems). The National Electric Code Article 250.53 specifiesĀ that a second ground rod is required if the resistance of aĀ single ground rod is greater than 25 ohms. The various DCSĀ manufacturers have a recommend resistance range from oneĀ to five ohms. Communication sites specifications are typicallyĀ on the order of five ohms or less.

The question also arises as to whether we should be concernedĀ about impedance rather than resistance. For instrumentĀ systems that have high-frequency components in the groundĀ circuit, impedance is generally a concern for the above-groundĀ part of the ground system as conductors tend to change fromĀ resistors to inductors as the frequency goes up. It is not asĀ much a concern for the below-ground part of the instrumentĀ ground system.

Table 2: Recommended grounding resistance
Lightning has high-frequency components, and the responseĀ of the overall ground grid is impedance-driven, which shouldĀ be taken into account in the design of the power ground grid.Ā A common engineering specification for a DCS is one ohm orĀ less to ground. There is, however, no technical reason why aĀ DCS system will not operate at a higher ground resistance. ForĀ example, a DCS will operate correctly if it's been constructed onĀ top of rock.

In general, you should make every attempt to meet theĀ manufacturerā€™s recommended ground resistance specification.Ā If this is not possible, the DCS ground should be equal toĀ or better than the associated power system ground resistanceĀ specification. Various grounding resistance specifications areĀ given in Table 2.

Testing of grounds to determine resistance is beyond the scope of this article. Testing the instrumentation ground prior to connection to the main power grounding grid, using traditional means such as the ā€œfall of potentialā€ or three-pointĀ method, is generally adequate. Some designs have a switchĀ for testing purposes on the connection line between the instrumentĀ ground and the system power ground grid with aĀ spark gap around the switch for safety purposes.

There are some good primers on earth resistance testingĀ by Megger, Fluke and Aemc. It's recommended that the testerĀ use a AC test current source. You should not try to use aĀ clamp-on ground tester for the initial test, but it can be usedĀ for subsequent checks after connecting to the main powerĀ grid and doing a benchmark test. The clamp-on ground testerĀ is an excellent tool for determining if grounding has degradedĀ by comparing readings to an initial benchmark.

It can't be overstressed that a grounding system must maintained, which means that it has to be periodically inspected and tested. This also applies to the power system ground grid that itā€™s connected to. If you let your grounding system degrade, you're asking for trouble, and not just from an instrument system perspective, but from an overall electrical perspective, too.

About the author:

Frequent contributorĀ William (Bill) L. Mostia, Jr. PE, Principle Engineer, WLM Engineering Co., can be reached at [email protected].

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