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Power Lines: Determining When “Isolated Grounding” Is Needed
Options and errors in AC wiring
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If electrical wiring, from main breaker panel to outlet, consists of Romex and plastic J-boxes, an “isolated” or “technical” ground system is already in place. This is the case In most, but not all, residential
wiring.

However, when wiring consists of metallic conduit and J-boxes, as in most commercial buildings, an isolated safety-grounding scheme can sometimes reduce audio system noise. It is most applicable in situations where conduit may come in contact with building steel, water pipes, gas pipes, or other structures because they may be grounded at distant locations (perhaps even the building next door or across the street) and will inject noise current into the safety ground system.

Special insulated ground or “IG” outlets (distinguished by a triangle marking on their face and, most popularly, orange in color) are used, which intentionally insulate the green safety ground terminal from the outlet mounting yokes or saddles.

Therefore, safety grounding is not provided by the J-box and conduit, but by a separate insulated green wire which is routed back to the main electrical panel.

Of course, the J-box and conduit must also be properly grounded by other, usually existing, means. Code requires that this or any safety ground conductor be routed in the same conduit or cable as the line and neutral circuit conductors. Although not explicitly stated in Code, this practice prevents loop inductance from limiting fault current, assuring fast breaker response should a fault occur.

Most often, wiring is not “daisy-chained” to outlets on the same branch circuit, so noisy leakage current from one device couples less to others on the same branch circuit. However, inductive coupling from phase conductors to the ground conductor (the major source of ground voltage differences between outlets) is not reduced.

Technical grounding practices are covered by NEC Article 250-74 and its exceptions. An excellent reference for system grounding, with emphasis on equipment racks, is a white paper by Middle Atlantic Products (to which I was a contributor) available at www.middle atlantic.com/pdflPowerWhitePaper4_07.pdf.


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