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Author Topic: URGENT: 24V between ground and neutral at festival event  (Read 18532 times)

Mark McFarlane

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URGENT: 24V between ground and neutral at festival event
« on: December 11, 2013, 10:17:53 am »

I went out tonight to advance a Festival site (football field) and check the power.

Power supply is:
(a) Local transformer the size of a VW Beetle
(b) permanent distribution panel (this runs the stadium lights),
(c) temporary 'event' distribution panel (3 phase, 120V, 60Hz ) tied to the permanent panel, The Ground connection to the permeant panel looks like two 14 gauge wires...
(d) 20-30 120V and 220V circuits running off the temp panel for various food vendors,... Home runs from the stage to the temporary panel for my circuits. 

I was there as the (third world) electricians finished running and connecting my stage circuits, they checked Hot to Neutral, measured 120Volts and said, "See, sir your power is ready".

Pulled out my Fluke and measured as follows (+/- a volt or two, I didn't write down the numbers):

What I measured:
H-N: 120V
H-G: 140V
N-G: 24 V

I had them pull the cover on the temporary distribution panel and I got the same measurements there between busses.  Neutral was not tied to ground in the panel.  Went to the permanent panel for the field, I let them measure it, they said it had the same measurements. Neutral was not tied to ground in the panel, as best I could tell.  There was a ground rod next to the permanent panel with a large cable (thumb size copper) going into the permanent panel.  The 'stadium' is about 5 years old.

I have requested someone from my companies power distribution unit to meet me at the site tomorrow.  We run refineries and somewhere there must be some talent.

Ignoring the fact that they gave me X outlets instead of X dedicated circuits, (regardless of the emails, formal specification, in-person meetings and onsite meeting where I spray painted the electrical locations, stage, tents,..., and that they spliced the supply cables with electrical tape for underground burial,...)

1) What are the safety and noise issues using this "120V to Neutral, 140 to ground" supply?
My gut feeling is that I don't plug in and walk away from the event unless the problem is rectified.

2) Is there a safe and easy solution? e.g. could we drive a ground rod at the temporary panel and tie neutral and ground together there?

A potential backup is to ask for generator power, but it may be too late.  I hate to walk from the money, it pays well, but will do so.  I have 2 days to fix the problem, although if I am walking I should tell them in the next 18 hours.
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Mark McFarlane
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John Roberts {JR}

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Re: URGENT: 24V between ground and neutral at festival event
« Reply #1 on: December 11, 2013, 10:45:47 am »

Interesting... sounds like ground is swapped for neutral on some circuits on the opposite polarity from yours, causing neutral to swing in the opposite direction from your hot.

Short answer nothing easy, but if the transformer is close and you (they) can trace all the loads connected to it, all "they" need to do is find what load is attached wrong (to ground instead of neutral) and pulling the ground.

ASSuming it is all connected properly back at the service drop, and low impedance, the ground should still serve it's purpose to sink enough fault current and trip the fuse/breaker. 25V should not kill you. But if you have time and opportunity, get them to fix this. The danger (IMO) is if the ground path opens downstream, you get full 120v on it.

You say you have your own local transformer. I am not a mains wiring expert but it seems to me that ground and neutral should be bonded (connected) at or near the primary panel/transformer drop. If it is connected they would "not" measure 25V there.

JR
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Tim McCulloch

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Re: URGENT: 24V between ground and neutral at festival event
« Reply #2 on: December 11, 2013, 10:55:37 am »

I went out tonight to advance a Festival site (football field) and check the power.


I agree with JR.  A quick check would be to unplug/disconnect all the vendor & foodservice and see if the problem goes away.... And I agree that it seems like the transformer neutral is not properly bonded/grounded.

Good luck.  You're dealing with folks who rationalize their incorrect actions with "it's never been a problem before."
« Last Edit: December 11, 2013, 12:00:59 pm by Tim McCulloch »
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Mike Sokol

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Re: URGENT: 24V between ground and neutral at festival event
« Reply #3 on: December 11, 2013, 11:14:50 am »

I went out tonight to advance a Festival site (football field) and check the power.
If you're measuring 24 volts between the neutral and ground bus, then the problem MUST be a loose or corroded N-G bond in the service panel. Driving a ground rod will do nothing to reduce this voltage. There should be a single point in the entrance panel where the ground rod, neutral bus and safety ground bus all tie together. The connection between the ground bus bar and neutral bus bar is often just a "green" screw that goes through a special hole in the neutral bar back to the metal of the service box. If that screw was forgotten or loose, then you would see these exact measurements.

The danger of a floating neutral is that your sound system may not trip a circuit breaker if there's a ground fault to chassis in a piece of gear. If that were to occur, you could created a situation where all the chassis grounds of your gear would elevate to 120-volt above earth potential, a lethal problem.

Get the electrician to find that G-N bonding point and confirm there's a bonding screw in place that's properly torqued. And shoot me a direct email to mike@noshockzone.org and we an confer a bit this afternoon. I'm doing a gig tonight but have a few hours this afternoon to think about it.

Mike Sokol
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Mark McFarlane

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Re: URGENT: 24V between ground and neutral at festival event
« Reply #4 on: December 11, 2013, 12:20:37 pm »

Thanks John, Tim and Mike.


There was no easily visible evidence of the ground and neutral being tied together either at the permanent panel (next to the transformer) our at the temporary panel (50m away), but I could have missed a lug on the neutral bus that protruded out the back to the chassis.

I ASSume the neutral and grounds should be tied together at the permanent panel, but should they also be tied at the temporary panel?

FYI, none of the temporary system was under load, nothing plugged into either the 120 or 220 outlets, so I'm not sure John's hypothesis is possible since its currently all open wires. Or probably I misunderstood.

I'm gonna drive out to the field and see if anyone is still there to check for the neutral-ground bond. It seems unlikely, its 8:30PM.

I got a 'we just did  festival here 2 weeks ago and it wasn't a problem', almost identical to  "it's never been a problem before."
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Chris Hindle

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Re: URGENT: 24V between ground and neutral at festival event
« Reply #5 on: December 11, 2013, 12:30:51 pm »

I got a 'we just did  festival here 2 weeks ago and it wasn't a problem', almost identical to  "it's never been a problem before."

Mark, that's the real issue.
Just because it's "never been a problem before" doesn't mean it won't become a problem on your show.
A dead meat-puppet is ALLWAYS a problem.

Could also be that this is a new fault, and in fact, it WAS never a problem before.
I say if your refinery electrician can't sort it in an hour or so, pull a genny in.

Better safe than toasted.
Chris.
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Stephen Swaffer

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Re: URGENT: 24V between ground and neutral at festival event
« Reply #6 on: December 11, 2013, 12:43:15 pm »

I agree neutral and ground should be bonded at service and if Ohms law is in effect there, then the only thing you should see between ground and neutral at a sub panel would be the voltage drop on the neutral back to the main which really should be significantly less than 3% if the system is done right 24 volts is trouble somewhere.

I might consider running power tools/motors on this system-in a pinch and very very gingerly.  Electronics?  Probably not-unless it pays enough to replace 'em all with enough left over to pay for the headache. If it is a bad neutral connection causing the issue, your 120 VAC supplies could go to anything between 0 and 240volts.  If it as unbonded neutral/ground you have a serious safety concern.
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Tim McCulloch

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Re: URGENT: 24V between ground and neutral at festival event
« Reply #7 on: December 11, 2013, 12:44:45 pm »

Thanks John, Tim and Mike.


There was no easily visible evidence of the ground and neutral being tied together either at the permanent panel (next to the transformer) our at the temporary panel (50m away), but I could have missed a lug on the neutral bus that protruded out the back to the chassis.

I ASSume the neutral and grounds should be tied together at the permanent panel, but should they also be tied at the temporary panel?

FYI, none of the temporary system was under load, nothing plugged into either the 120 or 220 outlets, so I'm not sure John's hypothesis is possible since its currently all open wires. Or probably I misunderstood.

I'm gonna drive out to the field and see if anyone is still there to check for the neutral-ground bond. It seems unlikely, its 8:30PM.

I got a 'we just did  festival here 2 weeks ago and it wasn't a problem', almost identical to  "it's never been a problem before."

No (answer to larger text question).  One neutral/ground bond per service.  Note that the service point (meter or whatever) should be grounded... though if there were 2 transformers involved, say a high voltage (13,000v to 480v) transformer that used Delta connections, you'd have no neutral.  The transformer frame would be bonded to ground.  The step down transformer that powered your panel (480v Delta to 208/120v Wye) would have its neutral bonded to ground and 5 wire service (3 hot lines, neutral and ground) extended to all subsequent panels.  What constitutes "service" is the issue.  A transformer is considered a "separately derived service" and always gets a ground; whether or not it is bonded depends on the wiring - Delta or Wye).

I also missed that other loads had not been connected yet... so skip that part for now.  As it is, this is a classic case of the neutral floating relative to GND.
« Last Edit: December 11, 2013, 12:47:30 pm by Tim McCulloch »
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John Roberts {JR}

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Re: URGENT: 24V between ground and neutral at festival event
« Reply #8 on: December 11, 2013, 12:45:46 pm »

Thanks John, Tim and Mike.


There was no easily visible evidence of the ground and neutral being tied together either at the permanent panel (next to the transformer) our at the temporary panel (50m away), but I could have missed a lug on the neutral bus that protruded out the back to the chassis.

I ASSume the neutral and grounds should be tied together at the permanent panel, but should they also be tied at the temporary panel?

No... should be bonded only once at the primary panel. But I repeat my caveats that I am not the expert here .
Quote

FYI, none of the temporary system was under load, nothing plugged into either the 120 or 220 outlets, so I'm not sure John's hypothesis is possible since its currently all open wires. Or probably I misunderstood.
If ground is not bonded to neutral at the transformer, there may be some other current flowing in the earth ground and creating a voltage potential there. In theory the earths magnetic field will also create current flow but that should be unchanging,  not mains frequency.
Quote
I'm gonna drive out to the field and see if anyone is still there to check for the neutral-ground bond. It seems unlikely, its 8:30PM.

I got a 'we just did  festival here 2 weeks ago and it wasn't a problem', almost identical to  "it's never been a problem before."
I'd be tempted to test between ground and neutral to see how big a spark such a connection would generate   :o , but I do not suggest that you do anything stupid like that.  ;D ;D ;D

JR

PS: Big spark, big problem, no spark, no problem (probably), but I am not the mains wiring expert here.
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Mark McFarlane

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Re: URGENT: 24V between ground and neutral at festival event
« Reply #9 on: December 11, 2013, 12:55:19 pm »

.. I'd be tempted to test between ground and neutral to see how big a spark such a connection would generate   :o , but I do not suggest that you do anything stupid like that.  ;D ;D ;D
...

With my tongue and a wet finger, right?

Well, it sounds like an easy fix at the permanent panel.

My other concern is the ground from permanent to temporary panel looks like two 14 gauge wires, separate from the larger supply cable. At least they are green.  When I spec'd the power I made a big deal about having a separate safety ground in place.  Now I suspect they added these two green wires for me and probably do a bootleg ground for other events.
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Mark McFarlane
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Re: URGENT: 24V between ground and neutral at festival event
« Reply #9 on: December 11, 2013, 12:55:19 pm »


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