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Isolated Ground Systems 2005 NEC®
by Mike Holt
Please Note: There are no Illustrations included in this newsletter.
When is an isolated ground appropriate, and what are the rules?
Decisions on when and how to use an isolated ground (IG) involve design issues you can't resolve based on the NEC alone [90.1(C)]. Before you try to resolve those issues, you must understand what an IG is-and is not.
An IG is not an arrangement whereby you drive a ground rod into the earth and use that as your sole grounding connection. In fact, doing this violates 250.4(A)(5), which requires you to establish an effective ground-fault current path. The IG concept probably arose from misunderstandings of neutrals and of the differences between grounding and bonding-certainly, this "isolated ground rod" idea did.
So, what is an IG? To answer that, we turn to IEEE-142, 5.5.54. The basic design involves setting up a bonding system (for a given circuit or set of circuits) and keeping that electrically isolated all the way to the service equipment or source of a separately derived system.
Initially, designers also kept the grounding (earthing) system separate from the source grounding (earthing) system. They would drive separate ground rods for these systems and not bond them back to the source. They mistakenly thought this would result in "separate" grounding systems-the "isolated" one being "clean" compared to the "main grounding system."
But we know that such a practice tries to defy Ohm's Law, Kirchoff's Law, several NEC references, and basic physics. Rather than solve a "dirty ground" problem, this arrangement creates serious safety and operational problems. Thus, you have no choice but to bond that "separate" ground rod system back to the source after all. So, an "IG" is actually a "separately insulated bonding conductor to the power source"-not a separately isolated grounded one.
An IG system sometimes is designed with an independent counter-poise ground (ground rods) that is bonded to the equipment grounding (bonding) conductor). The NEC recognizes this independent grounding connection to the earth as a "supplementary electrodes." According to EPRI studies, a supplementary electrode is useless and its presence could actually create a condition where sensitive electronic equipment could be damaged by lightning.
Because a supplementary electrode does not fulfill any of the NEC-required functions, you don't have to bond it to the building grounding electrode system, nor do you have to size it per 250.66, and you don't have to make it comply with the 25 ohm resistance requirement of 250.56 [250.54] (Figures 250-54 01 CC250-18.cdr and 250-54 02 CC250-19.cdr).
You cannot use the supplementary electrode as the effective ground-fault current path required by 250.4(A)(5) and 250.4(B)(4). By definition, this supplements-but does not replace-NEC-required electrodes. Do not confuse the requirements for supplementary electrode [250.54] with the requirements for the underground metal water pipe supplemental electrode [250.53(D)(2)].
An example of a supplementary electrode is a ground rod installed next to a machine tool. Such an electrode serves no electrical purpose. Yet, some equipment manufacturers require independent electrodes. They insist that their equipment be electrically isolated from the structure's electrical system [no equipment grounding (bonding) conductor].
This dangerous practice violates 250.4(A)(5), which prohibits the use of the earth as an effective ground-fault current path. If the metal enclosures of sensitive electronic equipment were isolated or floated as required by some equipment manufacturers, dangerous voltage on metal parts would remain from a ground fault (Figure 250-149).
You can isolate a metal raceway (containing circuit conductors for sensitive electronic equipment) from the electrical equipment it supplies by using a nonmetallic raceway fitting located at the equipment. However, the metal raceway must contain an insulated equipment grounding (bonding) conductor to provide the effective ground-fault current path to the power source [250.96] (Figure 250-147).
To IG or not to IG?
The idea behind an IG system, is that by bonding equipment with an insulated equipment grounding (bonding) conductor to the power source, you prevent contaminating equipment on one circuit with electrical noise from another circuit. If you have a noise problem and want to fix it by installing an IG, keep in mind that the theory and evidence supporting such an approach are thin at best. When all else fails, you could try an IG and see what happens.
Computer circuits are prime candidates for the "install IG receptacles first, ask questions later" approach. But, the results are more cosmetic than substantial and tend to mask underlying problems.
In most cases, an IG is a waste of money. IEEE 1100, Powering and Grounding Sensitive Electronic Equipment (Emerald Book) states, "The results from the use of the IG method range from no observable effects, the desired effects, or worse noise conditions than when standard equipment bonding configurations are used to serve electronic load equipment [126.96.36.199]."
Usually, an engineer can prevent or solve noise problems simply by following best practices and industry standards for electrical installations. Unfortunately, some engineers discover this after trying the IG approach and finding out they have the same problem they started with.Before you decide an IG will solve your problems, ensure your electrical infrastructure follows Chapter 3 wiring methods. A thorough review of your system against Chapter 2 would probably resolve any problems that still remain. If not, various IEEE standards provide more steps you should take before going the IG route.
One reason for considering an IG is excess noise on a sensitive circuit. Such noise is more likely to be of a higher amplitude on the current-carrying conductors than on the ground circuit. So, careful attention to wire separation and routing will do far more for you than "isolating" bonding connections will ever do.
Another reason for considering an IG is the idea you are going to "design out" any chance of picking up noise through the ground (bonding) connection. Because your IG must eventually tie into the grounding system, it's not really isolated after all. It's just "separately routed." Is the idea that you are accomplishing something by doing this just an illusion? Draw the circuits out, and see what you think.
Except for some anecdotal accounts, we have scant evidence that an IG cures any problems. But, as IEEE-142 points out in Chapter 5, the IG can make existing problems worse while creating new ones. So very few systems may benefit from an IG, and few-if any-require one.
But what about the various accounts of existing installations where problems disappeared once an IG was installed? Can't we draw some conclusions from those real-world results? In many cases, these IG installations are part of a larger bonding system repair project-so, we don't know exactly what fixed the problems. It's possible that IGs did cause symptoms to subside at a given facility-but as IEEE-1442 points out, IG tends to mask problems rather than fix them.
If you decide to try IG, beware of the slippery slope it may put you on. Of the IG systems that are properly designed, few are installed correctly and even fewer are properly maintained. On top of everything else, engineering opinions differ as to what is a proper design.
Some IG guidance
So, what does this mean for you, if you think an IG may solve your power quality problems? For starters, there is no standard design you can adopt. However, there are standards you can refer to for guidance on the basic principles and requirements involved. Begin with the NEC and IEEE-142.
What about IG receptacles? By design, these have the grounding terminal insulated from the metal mounting yoke. Therefore, you must connect the grounding terminal of an IG receptacle to an insulated equipment grounding (bonding) conductor that provides the effective ground-fault current path to the power source winding [250.146] (Figure 250-199).
IG receptacles must be identified by an orange triangle located on the face of the receptacle [406.2(D)]. Sometimes the entire receptacle is orange, with the triangle molded into the plastic face in a color other than orange. IG receptacles installed in nonmetallic boxes must be covered with a nonmetallic faceplate, because a metal faceplate cannot be bonded to an effective ground-fault current path [250.4(A)(3)] (Figure 406-2).
IG receptacles require additional attention to wiring methods. For example, the outer metal sheath of interlocked Type MC cable isn't listed as an equipment grounding (bonding) conductor [250.118(10)]. Therefore, you can't use this wiring method to supply an IG receptacle unless the cable contains two equipment grounding (bonding) conductors (Figure 1). However, you can use interlocked Type AC cable containing a single insulated equipment grounding (bonding) conductor, because the metal armor of the cable is listed as an equipment grounding (bonding) conductor [250.118(8)] (Figure 2).
Now that you are familiar with what an IG is and what some of the requirements are, you will probably think twice before installing one. Your first thought should be about whether your installation conforms to the NEC and related standards. Before you attempt to fix any problems by installing an IG, look very carefully at industry standards, best practices, and conformance issues. If your installation is in conformance, your second thought should be about how to correctly design, install, and maintain that IG.
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