An isolated ground receptacle (IGR) can reduce electrical noise, but if installed incorrectly, it can create a dangerous installation. This receptacle differs in construction from its self-grounding counterpart. The grounding terminal for an IGR is insulated from its metal mounting yoke. This means you must connect the grounding terminal directly to an effective fault current path by an insulated equipment grounding conductor. To ensure the device you're installing is an IGR, look for the Code-required orange triangle located on the face of the receptacle [Sec. 410-56(c)].
The NEC says the insulated equipment grounding conductor for an IGR may originate at the neutral point of the power source, and it may pass through boxes and panelboards without termination, but neither configuration is required [Secs. 250-96(b), 250-146(d), 250-148 Exception, and 384-20 Exception].
The NEC does not require you to run the insulated equipment grounding conductor for an IGR back to the neutral point of the power source. In some cases, running it to this point would be too difficult, impractical, or expensive. Thus, the typical grounding termination point for an IGR in an existing facility is the equipment grounding terminal at the panelboard that supplied the circuit.
Per the Code, the grounding terminal for an IGR could terminate to the metal outlet box that contains it (Figure, point C). The NEC doesn't dictate where you terminate the grounding terminal for an IGR — just that you terminate to an effective fault current path. Nor does the NEC require each IGR to be on its own dedicated branch circuit.
However, the Code does require you to ground the metal enclosure. This is automatic with a regular receptacle; you must provide an additional grounding means when using an IGR (unless you bond the enclosure to the IGR). The Code does not permit you to use interlock, or standard, type MC cable when wiring an IGR because the outer interlock sheath is not recognized as an effective fault current path to ground metal enclosures. However, you can use AC cable with an insulated equipment grounding conductor, because type AC cable armor is listed for use as an equipment grounding conductor.
Some manufacturers require you to “ground” their equipment to an “isolated” ground point in the earth, independent from the building's electrical system. Sec. 250-2(d) prohibits — as application of Ohm's Law — using the earth as the sole equipment grounding conductor or fault current path, because doing so is dangerous. If the metal enclosures were truly isolated, or floated, a line-to-case fault could not clear, and those enclosures would remain energized with dangerous touch voltage.
