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Author Topic: Where to attach ground?  (Read 6022 times)

Jamin Lynch

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Where to attach ground?
« on: November 24, 2015, 12:31:01 pm »

Disclaimer: I have no plans to do this....information purposes only.

There are several old dancehalls in my area where power was always a problem. In the past some guys would tie into the nearest breaker panel. Often times there was no ground bar. They would attach the ground wire to the neutral bar.

Bad idea?
What would be an easy and safe solution?
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Stephen Kirby

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Re: Where to attach ground?
« Reply #1 on: November 24, 2015, 12:41:55 pm »

By nearest breaker panel, you mean some sub panel and not the main service? 
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Jamin Lynch

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Re: Where to attach ground?
« Reply #2 on: November 24, 2015, 12:55:13 pm »

By nearest breaker panel, you mean some sub panel and not the main service?

Usually a sub panel. They were old panels probably installed prior to ground bars being required. I guess they were never updated.

The main service was never close enough to the stage
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Jonathan Johnson

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Re: Where to attach ground?
« Reply #3 on: November 24, 2015, 01:20:18 pm »

By bonding the EGC of a distro to the neutral bar of a subpanel, you increase the risk of ground current noise if you have interconnected devices that are fed from separate panels. If all interconnected devices are powered from the same panel, it shouldn't be the source of ground current noise.

As for personnel safety, I'll defer to others.

Grounded (via the neutral of a subpanel) equipment in contact with a stage or other metallic building fixture that's bonded to another location may also result in ground current noise. If that circuit (between the grounded equipment and the grounded fixture) is completed by a human in contact with both, that could also be a source of noise. I don't know if the voltage and current potential would be great enough to present a shock hazard. (If it is a shock hazard, then it will definitely be a source of noise... perhaps a 60Hz moan from the performer's vocal chords?  :o )
« Last Edit: November 24, 2015, 01:24:30 pm by Jonathan Johnson »
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Stephen Kirby

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Re: Where to attach ground?
« Reply #4 on: November 24, 2015, 02:26:12 pm »

As I understand it, the safety issue is a failure of the neutral somewhere between the service entrance where it's tied to ground and the end use point.  A real ground provides a path for the hot voltage to return on that doesn't involve a human in the circuit.  In the posited case, if the neutral fails between the main and sub panel, the return path will look for the lowest resistance to a sink or lower potential.  If the resistance is high enough, it will just hold anything it's in contact with at that potential.  But if the resistance gets low enough, like through a waterbag, current will flow.  With the ground connected to a now open stub, it doesn't do anything.
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Jonathan Johnson

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Re: Where to attach ground?
« Reply #5 on: November 24, 2015, 03:55:05 pm »

Usually a sub panel. They were old panels probably installed prior to ground bars being required. I guess they were never updated.

The main service was never close enough to the stage

Does the subpanel have a separate grounding electrode? Is there a grounding conductor leading from the panel to a ground rod or water pipe (that is in turn properly grounded)? If so, this can reduce the shock risk to personnel, but it still can result in ground current noise.

In the United States, the National Electrical Code prohibits ground-neutral bonds EXCEPT in the "service entrance" of a building, where the bond is required. However, since a single utility transformer may supply power to multiple buildings, there may be multiple ground-neutral bonds throughout a campus or neighborhood power distribution system -- each building supplied by the transformer will have its own ground-neutral bond, in addition to the ground-neutral bond at the secondary side of the utility transformer. Some power companies also bond the ground/neutral of the transformer secondary with a utility ground/neutral on the primary side. There usually is no EGC between the service entrance and the utility transformer. (Note that for RVs, mobile homes, and other portable buildings, the "service entrance" is an external power pedestal, NOT the breaker panel inside the building. The ground/neutral bond is at the pedestal, and there is an EGC between the pedestal and the breaker panel in the building, where there is no bond.)

This means that some "neutral" current could pass on the building's GEC (grounding electrode conductor) to the ground rod, through the ground rod to another building's or transformer's ground rod, to the secondary of the transformer. Due to the difference in resistance between the neutral conductor and the earth-as-a-conductor, the voltage differential and current flow should be minimal. But it does exist.

If there should be a failure of the neutral in this scenario, the voltage differential and current flow could be quite high, perhaps sufficiently high to cause shocks to personnel. However, if this "circuit" is not protected by a breaker or GFI, the hazard is not automatically removed during a fault condition.
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David Buckley

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Re: Where to attach ground?
« Reply #6 on: November 24, 2015, 05:04:17 pm »

What would be an easy and safe solution?

An isolating transformer.  Easy, and safe.  Also moderately heavy, space occupying and expensive.
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Stephen Kirby

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Re: Where to attach ground?
« Reply #7 on: November 24, 2015, 06:53:51 pm »

Doesn't the transformer require it's own ground?  My understanding of Jamin's question is with venues where there isn't a legit ground available.
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Mike Sokol

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Re: Where to attach ground?
« Reply #8 on: November 24, 2015, 07:31:11 pm »

What I used to do in this exact circumstance 40 years ago was to use a pair of vice-grips with a big lug on the side to run a piece of #8 wire to the copper water pipes in the basement of the club. I connected the other end to my distro ground. It worked well because ALL my grounds were tied to this one grounding point.  That's not a good idea nowadays (and certainly a code violation) since water pipes are plastic. However, the idea still has merit. If you can get an approved ground lug connected to the service panel, then run a heavy piece of EGC wire to your distro ground, I think it would accomplish the same thing.

I'm seeing the same sort of related issue in a few fire halls where they've offered to let me plug my 50-amp/240-volt distro into a 3-wire "stove" receptacle. I've refused for the reasons mentioned above, but have suggested they install a proper 4-wire 50-amp/240-volt receptacle for future shows. They have the breakers, so it just needs 10 feet of the proper wire and a NEMA 14-5 receptacle. Of course, the EGC of this receptacle will be bonded to the ground bus bar in the sub-panel, which is exactly how it's supposed to be done.

I figured out a long time ago that I could make a lot of strange wiring work, but if it was in violation of code and something happened (fire, electrocution, etc...) I would be on the hook for the damages and lawsuits. So I won't wire up something that's in violation just to save a few bucks. Far better to do it right and legal.
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Stephen Kirby

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Re: Where to attach ground?
« Reply #9 on: November 24, 2015, 07:41:13 pm »

So Mike, in Jamin's situation where he's tying to a sub panel which has no ground bar connected to the service entrance ground with a discrete conductor, where do you go?  Back out of the gig unless they run a proper EGC to the sub-panel?  I think it was a hypothetical question based on past experience but looking forward to the possibility in the future.
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Re: Where to attach ground?
« Reply #9 on: November 24, 2015, 07:41:13 pm »


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