All Code references are based on the 2005 NEC. The grounding and bonding requirements in this column apply to solidly grounded systems that operate at not more than 600V, such as 120/240V, 120/208V, and 277/480V.
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 isn't.
An IG isn't an arrangement whereby you drive a ground rod into the earth for use as your sole grounding connection. In fact, doing so 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, 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 it 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 such a practice would defy Ohm's Law, Kirchoff's Law, basic physics, and several NEC references. 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 is sometimes designed with an independent counter-poise ground (ground rods) that's bonded to the equipment grounding (bonding) conductor. The NEC recognizes this independent grounding connection to the earth as a “supplementary electrode.” According to Electric Power Research Institute (EPRI) studies, a supplementary electrode is useless and its presence could actually create a condition in which sensitive electronic equipment could be damaged by lightning.
Because a supplementary electrode doesn't 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 or make it comply with the 25-ohm resistance requirement of 250.56 (250.54).
You can't 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 doesn't replace — NEC-required electrodes. Don't confuse the requirements for supplementary electrode (250.54) with those 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 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 (Fig. 1).
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) (Fig. 2).
Some IG guidance. Many in the industry have their doubts about the IG's effectiveness (“To IG or Not to IG?” on page 64). So what does all of this mean for you if you think an IG is capable of solving 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 devices 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).
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 can't be bonded to an effective ground-fault current path [250.4(A)(3)].
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. However, you can use interlocked Type AC cable that contains 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)] (Fig. 3 on page 63).
Now that you're familiar with what an IG is and what some of the requirements are, you should be better prepared for taking steps to design and install one should you decide you need 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.
Sidebar: To IG or Not to IG?
The idea behind an IG system is that by bonding equipment with an insulated equipment grounding (bonding) conductor directly to the power source, you prevent contaminating equipment on one circuit with electrical noise from another circuit. However, some research shows that may not always be the case in practice.
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 [188.8.131.52].”
Usually, you can prevent or solve noise problems simply by following best practices and industry standards for electrical installations. However, if you decide to go the IG route, first ensure your electrical infrastructure follows all of the rules contained in Article 250 as well as Chapter 3 wiring methods. A thorough review of your system against Chapter 2 of the NEC would probably resolve any problems that remain. If not, various IEEE standards provide more steps you should take before designing and installing an IG.
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 afterall. It's just “separately routed.” Is the idea that you're accomplishing something by doing this just an illusion? Draw the circuits out, and see what you think.
Except for some anecdotal accounts, scant evidence exists to suggest that an IG cures any problems. In fact, as Chapter 5 of IEEE-142 points out, the IG can make existing problems worse and create new ones.
But what about the various accounts of existing installations where problems disappeared once an IG was installed? In many cases, these IG installations are part of a larger bonding system repair project, so it's difficult to determine exactly what fixed the problems. It's possible that IGs reduced symptoms at a given facility, but as IEEE-1442 points out, IGs tend to mask problems rather than fix them.
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 constitutes a proper design.