The benefits and risks of the isolated ground wiring
method are a function of many site variables
Confusion continues to surround the use, performance, and NEC requirements of isolated ground (IG) wiring. Can it be used anywhere, or is it restricted to electronic load equipment? Does it really reduce electrical noise? What does the Code say about IG wiring? These are the questions we'll address in this article, hopefully clearing up any misconceptions and providing guidance on how to comply with the NEC.
Editor's note: This article first appeared in the February 1996 issue of EC&M. The late Warren Lewis — a true pioneer in the power quality industry — originally authored it. The article has been updated to the 2005 edition of the NEC.
This means of IG connection is somewhat controversial, because making an NEC-acceptable electro-mechanical interface to electronic load equipment is somewhat elusive. Covered in Sec. 250.96(B), the intent of this section is to give the same opportunity of using IG-style wiring on direct-connected electronic load equipment as cord-, plug-, and receptacle-interfaced versions have had for many Code cycles. There was no “push” based on any demonstrated need for the Code-making process to permit IG-style wiring.
A typical direct-connected, IG-style wiring method is based on a metallic conduit or raceway that is suitable for use as an equipment grounding conductor path. Examples would include electrical metallic tubing (EMT), intermediate metal conduit (IMC), and galvanized rigid steel conduit (GRS).
The insulating means used between the branch circuit's metal conduit/raceway and the electronic load equipment metal enclosure is undefined in the NEC, except for the requirement that it be listed by a nationally recognized testing laboratory (NRTL). This has been discussed and clarified to mean that the insulating fitting need not be specifically listed for use as a direct connection IG insulating fitting; it can be any listed nonmetallic electrical fitting that would do the job in a manner acceptable to the authority having jurisdiction (AHJ).
It would seem, then, that listed electrical fittings used with rigid PVC electrical conduit would be acceptable, while plastic plumbing fittings would not.
There has been a lot of confusion over where the IG-style connection can and cannot be used in an electrical system installed under the requirements of the NEC. This is clarified in the Figure (click here to see Figure), where the acceptable location is shown in contrast to all of the unacceptable ones. Note that there are two locations that are acceptable: at the outlet end of the branch circuit used to connect to electronic load equipment and at an IG receptacle at which cord-connected electronic load equipment is connected.
This means that any attempt to place an IG-style connection (receptacle or direct connection) onto any other point on the wiring system — or to serve non-electronic equipment with it — is an NEC violation of either 250.146(D), for receptacles, or 250.96(B), for direct connections (see What the 2005 NEC Has to Say About IG Wiring below).
The above information is important, since inspectors have reported seeing plastic bushings installed in metal enclosure knockouts and punchouts, where 4-inch GRS conduits of multiple paralleled feeders that serve high-ampacity switchboards terminate. One inspector reported seeing such bushings installed at both termination points of a 3-inch EMT 480/277V feeder. These are hazardous practices not permitted by the NEC. Unfortunately, until the IG methodology is fully understood by all, these practices will continue to arise.
Loads affected by common-mode currents and voltages propagated over the AC power branch circuit fall into high- and low-impedance categories. High-impedance loads are susceptible to electrical noise in the form of voltage. Low-impedance loads are susceptible to electrical noise current.
The IG wiring method changes the lower impedance of the typical solid grounding (SG) branch circuit into a much higher impedance. Therefore, it attenuates noise current but permits an increase in noise voltage on the path.
Looking into the load-end of an SG branch circuit, you will “see” a relatively low impedance, which permits a fairly heavy noise current to be propagated along the path but with minimum voltage drop. Conversely, looking into the load-end of an IG circuit, you will “see” the reverse condition: maximum voltage drop and minimized current flow.
From the above, you can infer there is a compatibility consideration between the victim load's impedance and its susceptibility or immunity to electrical noise in the common mode. This is where many debate whether an SG or IG wiring method should be used. The general rule to improve electronic load immunity or to reduce susceptibility to common-mode electrical noise is use inverse connections (e.g., connect a low-impedance load to a high common-mode impedance branch circuit and a high-impedance load to a low common-mode impedance branch circuit).
The length of the IG circuit from the solidly grounded AC source to its outlet end affects the impedance of its path from one end to the other. This, in turn, affects the amount of electrical noise current (as opposed to electrical noise voltage) the IG path can propagate along its route. Therefore, it stands to reason, that whatever effect the IG wiring method is going to have with a given load, it will be proportional to the length of the IG circuit. A similar effect exists on the SG circuit, but in inverse fashion.
Based on this information, it should be apparent that placing an IG receptacle 6 inches away from the branch-circuit panelboard, and having the panelboard a few feet from its serving solidly grounded transformer (which forms the separately derived AC system), is not going to have much effect. There simply isn't enough IG length in the circuit to develop any useful common-mode impedance on the equipment grounding conductor path for the arrangement to do any good — or bad. However, if the IG branch circuit is 100 feet or 200 feet long, then the effects (good or bad) would be appropriately produced.
This should tell you something about paying extra to install IG receptacles on circuits that are only a few feet (or inches) long. In particular, there is no useful gain in installing IG receptacles onto the same case/enclosure that contains a solidly grounded power-conditioning device, such as a transformer used with a UPS, a voltage regulator, or similar item of self-contained power conditioning equipment.
Is there such a thing as too much of a good thing? In the case of the IG wiring method, the answer is yes. For example, if the site has a lot of potential difference between points of grounding (as over large area buildings), an IG wiring method can act to ensure that the AC power source and the electronic load get conductively connected across that potential. The result is increased common-mode current trying to flow on the IG path and a much greater common-mode voltage being developed across it. This can cause more interference with the victim load, if it has a communication or signaling circuit attached to it and is ground-referenced at some further distance away.
IG wiring methods bring with them increased susceptibility to lightning-related problems. Generally, IG circuit advocates have not appreciated this resulting susceptibility, which can be readily explained.
When lightning strikes the face of the earth, it creates a step-potential situation, known to result in thousands of volts across the soil. Similarly, lightning currents carry this kind of potential difference vertically, horizontally, and on any diagonal in a facility of given size, particularly multi-storied and large area buildings. As a result, when a lightning strike occurs, there can be a very great potential difference, for example, between a service entry and the main building's grounding point and any remote point in the building. This means that if the victim load equipment is located a long distance from the service entry, its local grounding conditions will be at a significantly different potential from those at the service equipment. If this path and its potential difference is then bridged by the connection of an IG equipment grounding conductor, there will be, by necessity, a traveling voltage and current wavefront barreling down the IG conductor from one end to the other, in a futile attempt to permit current flow and to equalize potential between the two points. This is called a lightning surge current.
Let's assume that the service entry is the originating point for the above surge current, and the victim electronic load is the terminating point for it. If there are data, signaling, or communications cables attached to the load equipment, the lightning surge current will try to flow on these circuits in a continuing attempt to find “ground.” Typically, this is quite damaging to these circuits. Because the surge current is in the common mode, no amount of line-to-line and line-to-ground surge protection equipment installed on the AC power circuit will protect the load and its attached circuits.
In addition, if there is grounded metal nearby the victim load, and if the surge current has sufficient potential driving it (due to impedance mismatch and related voltage reflections at the load-ground interface point), there can be a lightning side-flash of up to 6 feet horizontally through the air between the energized victim load and whatever grounded materials or other equipment is nearby. Side-flash phenomena are generally described in NFPA-780-2004, “The National Lightning Protection Code.”
So, if you use an SG circuit in the above case instead of the IG style, the victim load will be largely protected, since it will have become ground-referenced to its local ground conditions and will be generally unable to develop a side-flash potential problem. The same holds true for an IG circuit that is physically short, as when its AC system is not the remotely located service entry (and that is at a greatly different potential), but is instead a dry-type transformer (or similar source) installed as a solidly grounded and separately derived AC system in the same location as the load being served on the IG path.
There are no clear winners and losers between the IG and SG wiring methods since, as we've tried to explain, the risk/benefit situation is fraught with variables of a wide description. Therefore, no blanket recommendation can be made in relation to the SG or IG methods in regard to predictable benefits that either can provide. The results are related to the site's conditions, the length and routing of the circuit to be made IG or SG, and the high- and low-impedance nature of the victims' loads themselves. Keep in mind that the effects on electrical noise of the SG wiring method versus the IG wiring method apply only to common-mode noise problems, not transverse-mode ones.
Finally, what's clear is that it's very difficult and expensive to retrofit an SG design into an IG design, if you find out you need one. However, retrofitting an IG design into an SG design is relatively inexpensive, since the IG conductor easily can be either disconnected or reconnected as an additional SG conductor at any time. This translates into a recommendation that electronic loads should be provided with AC branch-circuit wiring that is IG in nature, but with the clear idea that if conditions warrant it, the circuit will be converted to the SG type (and back again at a later time) as necessary.
IG wiring with receptacles — As per 250.146(D), Connecting Receptacle Grounding Terminal to Box, “An equipment bonding jumper shall be used to connect the grounding terminal of a grounding-type receptacle to a grounded box.”
“Exception No. 4: Where required for the reduction of electrical noise (electromagnetic interference) on the grounding circuit, a receptacle in which the grounding terminal is purposely insulated from the receptacle mounting means shall be permitted. An insulated equipment-grounding conductor run with the circuit conductors shall ground the receptacle-grounding terminal. This grounding conductor shall be permitted to pass through one or more panelboards without connection to the panelboard grounding terminal as permitted in 408.40 (Exception), so as to terminate within the same building or structure directly at an equipment grounding conductor terminal of the applicable derived system or service.
“(FPN): Use of an isolated equipment grounding conductor does not relieve the requirement for grounding the raceway system and outlet box.”
IG wiring on direct connected circuits — As per 250.96(B), Isolated Grounding Circuits, “Where required for the reduction of electrical noise (electromagnetic interference) on the grounding circuit, an equipment enclosure supplied by a branch circuit shall be permitted to be isolated from a raceway containing circuits supplying only that equipment by one or more listed nonmetallic raceway fittings located at the point of attachment of the raceway to the equipment enclosure. The metal raceway shall comply with provisions of this article and shall be supplemented by an internal insulated equipment grounding conductor installed in accordance with 250.146(D) [Exception No. 4] to ground the equipment enclosure.
“(FPN): Use of an isolated equipment grounding conductor does not relieve the requirement for grounding the raceway system.”