WHY are neutral and grouding spearate in subs?

hrajotteMay 3, 2012

I know that in subpanels, the neutral and grounding bars must be separate, with the panel bonded only to the grounding conductor.

But, what is the electrical reason for this? What dangers are there if a subpanel is not wired properly?

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Any equipment hooked to the sub-panel ground will have the voltage drop of the path back to the main panel on its safety ground.

The grounding conductor is intended to be as close to earth ground as possible, and this means NEVER having it carry normal operating currents.

    Bookmark   May 3, 2012 at 3:55PM
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I do not have time to type an exhaustive answer. So here is some of the answer. The neutral is a current-carrying conductor, part of the circuit. The neutral is insulated to full circuit voltage as a minimum. The Equipment Grounding Conductor (ground) does not normally carry current and is "Bonded" (connected) to the conductive housings of equipment- such as the outer surfaces of washers, dryers and refrigerators. If an energized conductor contacts the housings of such equipment, the ground provides a better path for the current back to the source than does the human body. If the neutral is broken, and the ungrounded conductor ("hot wire") remains intact on a circuit that is powering some utilization equipment, the broken end of the neutral nearer to such equipment is then energized at 120 volts, even if the equipment stops working/running. If the ground is also broken, the housings of equipment are then energized at 120 volts if the neutral and ground are interconnected at ANY point after the main service panel. My guess is that there are about 100 million electrical services in the USA. Any scenario that we can imagine is virtually assured of happening in at least some of those services.

    Bookmark   May 3, 2012 at 4:04PM
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Thank you both. I believe bus driver's sentence gave me the picture:
"If the ground is also broken, the housings of equipment are then energized at 120 volts if the neutral and ground are interconnected at ANY point after the main service panel."
I asked out of curiosity because I had this situation in my house. Soon after we bought it years ago, we had an electrician doing some other work and he noticed our sub was wired wrong and corrected it.
Would it be safe to say that a wrongly wired sub is mainly a hazard to people, or could connected equipment be damaged, too?

    Bookmark   May 4, 2012 at 9:53AM
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Ron Natalie

It's mostly a safety problem, though devices could be damaged in a fault situation (but usually there's something seriously wrong anyhow if the breaker is going to protect the device).

Actually, the quoted sentence is NOT the key part. The key part is as brick_eye stated. Since the EGC does not carry a current, it has the very low impedence path back to ground in case of a faulting current. The regular grounded (neutral) conductor can be carrying a substantial amount of current and hence may not provide this.

    Bookmark   May 4, 2012 at 10:26AM
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