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Author Topic: Voltage difference between grounds  (Read 2007 times)

Don Krueger

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Voltage difference between grounds
« on: January 31, 2018, 03:09:26 pm »

Hi,

We play a venue a few times a year where the building has a club on the main floor (where we play) and a hall in the basement that is rented out for various events.

We have found that when there is a DJ downstairs the guitar players get shocked touching the mic and guitar strings at the same time. I'm an electronics technologist and understand that the grounds are not at the same voltage. There is nothing wrong with the safety grounds on the chassis' of the guitar amps. The last time this happened (quite a while ago) I took my meter and measured the voltage between the ground for the PA and the ground for the guitar amps and there was around 20 Vrms IIRC.

We have been able to get around the problem by plugging the amps into the same circuit as the PA and/or using some pin 1 lift adapters I made for the mic cables. Even though we are not pushing things, plugging everything into the same circuit is worrisome because I don't want the breaker to trip.

I don't have any visibility into the wiring scheme of the building or access to the basement hall during these events. I'm not sure if this is a problem with the electrical system in the building or with some equipment that is plugged in downstairs. We are playing there this weekend so if it happens again I would like try and get to the root of the problem but I'm not sure I can without more insight into the whole picture.

Any suggestions on this?

Thanks, Don
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Don Krueger

John Roberts {JR}

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Re: Voltage difference between grounds
« Reply #1 on: January 31, 2018, 03:30:18 pm »

20V sounds a little high.

If you connect a wire lead between the two grounds does it draw a spark.

JR
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Stephen Swaffer

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Re: Voltage difference between grounds
« Reply #2 on: January 31, 2018, 10:26:05 pm »

The ONLY way you get 20 volts RMS potential difference on different grounds in the same buiilding is if there is something wrong with the building ground scheme.  The NEC requires anything in a building that "could become enrgized" to be bonded.  "Bonded" being a specific term defined as intentionally connected together with a metallic path preventing a potential difference.

What SHOULD happen is the building owner(s) need to get a licensed, qualified (not necessarily a redundant modifier!) electrician innolved and sort out the grounding.  It is not terribly difficult to get right-but there are too many unknowns (how many services?  are their subpanels or separately derived services involved, etc.) to tell you how it needs to be fixed.
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Don Krueger

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Re: Voltage difference between grounds
« Reply #3 on: February 01, 2018, 11:55:05 am »

Thanks for the info. I wish I would have recorded my measurement last time so I could be sure it was that high, it was in the middle of the gig and I was rushed. If this happens again this weekend I will be more prepared and keep a record of what I measure.

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Don Krueger

John Roberts {JR}

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Re: Voltage difference between grounds
« Reply #4 on: February 01, 2018, 12:01:16 pm »

Thanks for the info. I wish I would have recorded my measurement last time so I could be sure it was that high, it was in the middle of the gig and I was rushed. If this happens again this weekend I will be more prepared and keep a record of what I measure.
The common vector for talent getting shocked by microphones is due to the microphones actually being well grounded (by the console) and the talent being energized by a leaky guitar amp (unfortunately pretty common. Check for missing  ground pins on amps.)

Safety grounds in outlets should all be bonded to neutral at the power drop (breaker panel) and not carry current routinely, unless there is a fault, so registering voltage between safety grounds suggests a possible outlet/branch wiring error.

When human lives are involved it is always a good idea to get this right.

JR
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TJ (Tom) Cornish

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Re: Voltage difference between grounds
« Reply #5 on: February 01, 2018, 12:05:12 pm »

Don, there's something wrong for sure here - probably a loose ground wire somewhere combined with a leaky device.  20 volts is too much.

You can do learn a lot by doing the receptacle testing procedure I have outlined here.

Make sure you do the load test and watch how the neutral to ground voltage changes.

Additionally, you can use a Poor Man's Distro device to bond the grounds of the two circuits you wish to use which will likely help you significantly.  You can make this yourself with a couple rack mounted power strips in the same rack with paint scraped off the rack rails and the back of the rack ears to ensure good ground contact between the two rack mount strips.
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Jonathan Johnson

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Re: Voltage difference between grounds
« Reply #6 on: February 01, 2018, 04:09:10 pm »

Don, there's something wrong for sure here - probably a loose ground wire somewhere combined with a leaky device.  20 volts is too much.

Broken ground combined with a bad MOV surge supressor?
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TJ (Tom) Cornish

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Re: Voltage difference between grounds
« Reply #7 on: February 01, 2018, 04:15:06 pm »

Broken ground combined with a bad MOV surge supressor?
It would make sense if it was a MOV on the DJ's system in the basement, maybe, however both grounds (or the upstream supply ground) would need to be mostly floating for the leakage to transfer upstairs.
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Mike Sokol

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Re: Voltage difference between grounds
« Reply #8 on: February 01, 2018, 07:24:13 pm »

As noted earlier, all the grounds should be bonded at only one point in the service panel. However, even if there were multiple G-N bonds throughout the wiring (a code violation), there would still only be maybe 2 or 3 volts difference between different grounds. So something is indeed wrong with the grounding system and should be corrected immediately. One of the most important tests you can do is a Ground Loop Impedance Test, which can be done with something like an Ideal Industries SureTest Analyzer. This will put a single-cycle 15-amp pulse on the ground wire and measure its impedance to the neutral bonding point. To be in compliance this needs to measure less than 1 ohm. I'll bet that one of the grounds isn't connected to anything, and it's voltage is floating around from normal leakage currents. However, this ground could go hot at any time and energize a lot of gear at the same time. Time to get in a good electrician and sort it out. 
« Last Edit: February 01, 2018, 07:26:27 pm by Mike Sokol »
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Mike Sokol
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Kevin Graf

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Re: Voltage difference between grounds
« Reply #9 on: February 02, 2018, 09:47:24 am »

As noted earlier, all the grounds should be bonded at only one point in the service panel.........
Lets make that:
As noted earlier, all the grounds should be bonded to the Neutral at only one point in the service panel.
As grounds may have continuity to other grounds at many places in a building. That continuity may be from bonding or just casual contact.
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Mike Sokol

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Re: Voltage difference between grounds
« Reply #10 on: February 03, 2018, 09:06:54 pm »

Lets make that:
As noted earlier, all the grounds should be bonded to the Neutral at only one point in the service panel.
As grounds may have continuity to other grounds at many places in a building. That continuity may be from bonding or just casual contact.

Yup, I sort of suggested that, but you're absolutely correct in being specific.
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Mike Sokol
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Don Krueger

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Re: Voltage difference between grounds
« Reply #11 on: February 06, 2018, 11:49:28 am »

Well, this gig came and went with no shocking problems (and no one in the basement hall).

I printed the receptacle testing doc and put it in my gig kit. If/when it happens again I'm going to measure as much as I can.
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Don Krueger
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