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Author Topic: URGENT: 24V between ground and neutral at festival event  (Read 18568 times)

John Roberts {JR}

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Re: URGENT: 24V between ground and neutral at festival event
« Reply #10 on: December 11, 2013, 01:17:02 pm »

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.

I suspect two 14ga wires will be adequate to take out a branch fuse/breaker. Melting two 14 ga wires open should take more than 15-20A transiently.

I still like GFCI/RCD for all back line gear, so the meat puppets are protected even with no safety ground. 

JR
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Mark McFarlane

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Re: URGENT: 24V between ground and neutral at festival event
« Reply #11 on: December 11, 2013, 01:46:12 pm »

I suspect two 14ga wires will be adequate to take out a branch fuse/breaker. Melting two 14 ga wires open should take more than 15-20A transiently.

I still like GFCI/RCD for all back line gear, so the meat puppets are protected even with no safety ground. 

JR

I usually spec GCFI for outdoor events. Slipped my mind this time.  I'll ask tomorrow, but they already think I am being a PITA
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Mark McFarlane
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Keith Broughton

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Re: URGENT: 24V between ground and neutral at festival event
« Reply #12 on: December 11, 2013, 02:41:01 pm »

'We haven't had a problem before" (as the space shuttle disintegrated in the air.)
Such a B/S answer!!
Maybe a dead person will light a fire under them....
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Steve M Smith

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Re: URGENT: 24V between ground and neutral at festival event
« Reply #13 on: December 11, 2013, 03:00:50 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

So would I... and neither would I!!!


Steve.
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Geoff Doane

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Re: URGENT: 24V between ground and neutral at festival event
« Reply #14 on: December 11, 2013, 10:05:15 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


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

I'd try it with a lamp load though.  Any handy 120V lamp should work, although you'll have to jury rig some kind of plug to go between neutral and ground.

My guess is that once it's plugged in (the test lamp), the voltage will drop to zero, because there is no neutral-ground bond, and neutral is just floating.  Not catastrophic by itself, but neutral could be pulled in any direction by some other leaky loads on other phases.  You don't have the same protection against fault currents in a chassis that the ground gives you in a properly wired system.

And the third world certainly doesn't have a lock on the phrase "We've never had a problem before".  :o

GTD
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Lyle Williams

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Re: URGENT: 24V between ground and neutral at festival event
« Reply #15 on: December 12, 2013, 04:16:46 am »

Loose neutral.  Mid point of wye being dragged away from the nominal 0v point.
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Mark McFarlane

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Re: URGENT: 24V between ground and neutral at festival event
« Reply #16 on: December 12, 2013, 05:18:31 am »

Well, as a backup they brought in a generator (for me), the food vendors get the bad power.  The generator is self contained, maybe 3' wide and 12 feet long, on skids, in a metal container.  Sorry I don't have more info, I only have afew miuntes, Jumping betwee 2 cities for 2 gigs on a normal workday when I should be in the office....

Took a quick glimpse at the wiring underway, oops, no ground wire. 3 hot phases and a neutral, only 4 wires in the cable.


I'm guessing a dedicated ground should be run back to the gene?

We can disconnect one of the hot phases and use that cable as the ground wire,

FWIW, I visited an event at the same location a few weeks ago and they just pounded a ground rod in at the distribution panel, about 70m from the gene.  Not a good idea, right?
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Mark McFarlane
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Geoff Doane

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Re: URGENT: 24V between ground and neutral at festival event
« Reply #17 on: December 12, 2013, 07:07:54 am »

Well, as a backup they brought in a generator (for me), the food vendors get the bad power.  The generator is self contained, maybe 3' wide and 12 feet long, on skids, in a metal container.  Sorry I don't have more info, I only have afew miuntes, Jumping betwee 2 cities for 2 gigs on a normal workday when I should be in the office....

Took a quick glimpse at the wiring underway, oops, no ground wire. 3 hot phases and a neutral, only 4 wires in the cable.


I'm guessing a dedicated ground should be run back to the gene?

We can disconnect one of the hot phases and use that cable as the ground wire,

FWIW, I visited an event at the same location a few weeks ago and they just pounded a ground rod in at the distribution panel, about 70m from the gene.  Not a good idea, right?

The important thing (and what was likely missing from the original scheme) is that neutral must be bonded to ground at one, and only one, place. Preferably at the generator (or transformer) itself. If they didn't "get it" with a transformer, I'm not sure they will with the generator either.

Godd luck!

GTD
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Mark McFarlane

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Re: URGENT: 24V between ground and neutral at festival event
« Reply #18 on: December 12, 2013, 04:51:55 pm »

The important thing (and what was likely missing from the original scheme) is that neutral must be bonded to ground at one, and only one, place. Preferably at the generator (or transformer) itself. If they didn't "get it" with a transformer, I'm not sure they will with the generator either.

Godd luck!

GTD

Unfortunately I got a call while i was doing soundcheck for a jazz gig tonight from another electrician who said the generator has the same 24V between neutral ground. Very strange.  I'm taking a nap and then meeting them at 6AM to look for myself.

I do not have any liability in this event, I have filed a formal letter with the site manager on my safety concerns, to which the response was 'its not problem, we do this all the time, just did it a few weeks ago,...)  I know I should walk away, but the event is being put on by a close friend, we have been working together for 4 months on planning which has been quite complex, and its a fairly serious chunk of cash in my pocket. Should be 3,000-5,000 people there through the day coming from 4 or 5 neighboring countries. 

Most of the bands have played this venue before, they just never had anyone check the power, they showed up and plugged in.

Will there be any serious risk to my equipment with the floating neutral? (ya, I can't believe I am asking either)
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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 #19 on: December 12, 2013, 10:14:11 pm »

I doubt 25v will hurt anybody. It is probably high impedance/low current...  Do not use a wet tongue but do bond ground to neutral preferably at the primary panel. Otherwise bond it anywhere you can, even with an extension cord when no one is looking.

JR

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Re: URGENT: 24V between ground and neutral at festival event
« Reply #19 on: December 12, 2013, 10:14:11 pm »


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