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What's Wrong Here?

May 20, 2010 5:12 PM, By Brian J. McPartland, NEC Consultant

This photo shows a violation of the rules given in 406.2(D), which in part states, "Receptacles incorporating an isolated grounding conductor connection intended for the reduction of electrical noise (electromagnetic interference) as permitted in 250.146(D) shall be identified by an orange triangle located on the face of the receptacle." Immediately following this basic requirement, part (1) of this section of the Code says, "Receptacles so identified shall be used only with equipment grounding conductors that are isolated in accordance with 250.146(D)." The pertinent portion of 250.146(D) to note here is the second sentence, which says, "The receptacle grounding terminal shall be connected to an insulated equipment grounding conductor run with the circuit conductors."

Correlation of these rules requires any receptacle that is identified as an isolated ground receptacle to be used only where it is actually attached to an equipment grounding conductor that is effectively, electrically isolated from the raceway and metal box in which the receptacle is installed. In this case, the insulated equipment grounding conductor exiting the metallic outlet box is attached to a field-installed grounding detail that provides continuity between the "isolated" grounding conductor and the ground path provided for the metal outlet box.

The basic idea behind the requirements of 406.2(D) is to prevent someone from being "fooled" into thinking that the IG receptacle actually has an isolated ground, as may be required for certain cord- and plug-connected electronic equipment, such as a cash register. Often, where a manufacturer calls for an isolated ground, failure to actually provide an isolated ground will essentially void the manufacturer's warranty.

Why would anyone use an isolated ground receptacle when it was not required? Maybe the installer had nothing else on the truck — and rather than go to the supply house to pick up a non-isolated ground receptacle and then return to the job to install this single device, he decided to simply use the isolated ground receptacle instead. Based on the requirements outlined in the 2008 NEC, the use of an IG receptacle in this situation is now a Code violation.

My last comment is related to the larger equipment grounding conductor connected to the grounding detail. To tell the truth, I'm not really sure what's going on here. Typically, equipment grounding conductors are to be routed with the circuit conductors [250-134(B)] to ensure a low-impedance fault-return path. In alternating-current systems, separation of the grounding conductor from the circuit conductors will result in a higher impedance because mutual cancellation of the magnetic fields is only possible where the grounding and circuit conductors are in close proximity to each other.

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