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Sound Reinforcement - Forums for Live Sound Professionals - Your Displayed Name Must Be Your Real Full Name To Post In The Live Sound Forums => AC Power and Grounding => Topic started by: Mark McFarlane on December 11, 2013, 10:17:53 am

Title: URGENT: 24V between ground and neutral at festival event
Post by: Mark McFarlane 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.
Title: Re: URGENT: 24V between ground and neutral at festival event
Post by: John Roberts {JR} 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
Title: Re: URGENT: 24V between ground and neutral at festival event
Post by: Tim McCulloch 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."
Title: Re: URGENT: 24V between ground and neutral at festival event
Post by: Mike Sokol 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
Title: Re: URGENT: 24V between ground and neutral at festival event
Post by: Mark McFarlane 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."
Title: Re: URGENT: 24V between ground and neutral at festival event
Post by: Chris Hindle 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.
Title: Re: URGENT: 24V between ground and neutral at festival event
Post by: Stephen Swaffer 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.
Title: Re: URGENT: 24V between ground and neutral at festival event
Post by: Tim McCulloch 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.
Title: Re: URGENT: 24V between ground and neutral at festival event
Post by: John Roberts {JR} 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.
Title: Re: URGENT: 24V between ground and neutral at festival event
Post by: Mark McFarlane 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.
Title: Re: URGENT: 24V between ground and neutral at festival event
Post by: John Roberts {JR} 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
Title: Re: URGENT: 24V between ground and neutral at festival event
Post by: Mark McFarlane 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
Title: Re: URGENT: 24V between ground and neutral at festival event
Post by: Keith Broughton 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....
Title: Re: URGENT: 24V between ground and neutral at festival event
Post by: Steve M Smith 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.
Title: Re: URGENT: 24V between ground and neutral at festival event
Post by: Geoff Doane 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
Title: Re: URGENT: 24V between ground and neutral at festival event
Post by: Lyle Williams on December 12, 2013, 04:16:46 am
Loose neutral.  Mid point of wye being dragged away from the nominal 0v point.
Title: Re: URGENT: 24V between ground and neutral at festival event
Post by: Mark McFarlane 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?
Title: Re: URGENT: 24V between ground and neutral at festival event
Post by: Geoff Doane 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
Title: Re: URGENT: 24V between ground and neutral at festival event
Post by: Mark McFarlane 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)
Title: Re: URGENT: 24V between ground and neutral at festival event
Post by: John Roberts {JR} 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

Title: Re: URGENT: 24V between ground and neutral at festival event
Post by: Lyle Williams on December 12, 2013, 10:35:31 pm
With a loose neutral, the voltage difference between where neutral is and where it is supposed to be is dependent on the loads and reactivity on each phase.  With completely equal pure resistive loads on each phase, a loose neutral would stay around zero volts relative to ground.  The position of neutral may be highly dynamic if unbonded.

A loose neutral is absolutely not a high impedance low current situation. 
Title: Re: URGENT: 24V between ground and neutral at festival event
Post by: Lyle Williams on December 12, 2013, 10:42:36 pm
It is also possible that this isn't a loose neutral.  It could be that neutral is bonded to ground at the transformer/generator, but your local ground isn't linked back to that ground.

If live to neutral looks clean (sinwave-ish) and at around the correct nominal voltage, then it is probably independent grounds.  If live to neutral is a mess (maybe wrong by 24v?) then that would lead to the loose neutral diagnosis.

In any case, this is something for the local electrician.  I can't see much of the detail from the other side of the world.
Title: Re: URGENT: 24V between ground and neutral at festival event
Post by: Mark McFarlane on December 12, 2013, 11:41:19 pm
The cable to the gene is 4 wires. The gene is 3 phase. If we use one of the cables to carry ground, and only connect 2 phases, will there be a problem, e.g. uneven load causes some other problem?

Is there a solution with only 4 wires going from the stage to a 3 phase generator, e.g. could we run a ground rod at the stage (60m from the gene)?  I know that's not correct, but is it better than no ground at all?
Title: Re: URGENT: 24V between ground and neutral at festival event
Post by: Mark McFarlane on December 12, 2013, 11:51:06 pm
It is also possible that this isn't a loose neutral.  It could be that neutral is bonded to ground at the transformer/generator, but your local ground isn't linked back to that ground.

If live to neutral looks clean (sinwave-ish) and at around the correct nominal voltage, then it is probably independent grounds.  If live to neutral is a mess (maybe wrong by 24v?) then that would lead to the loose neutral diagnosis.

In any case, this is something for the local electrician.  I can't see much of the detail from the other side of the world.

Unfortuantely the local electricians said '24V between neutral and ground. on the shore power was perfectly fine, without having any reason. I asked them what the voltage would be between hot and ground if I flipped the breaker, they said 0, I flipped the breaker and measured 24 volts.  Deer in the headlights,...

But back to the genie power, which is what I have now,  I just went to meet the electrician at 6AM, He didn't show up. Here's how it is currently wired

4 wires from stage to genie.
1 neutral
2 hot
1 ground

Ground wire is tied to the generator frame and to a steel rod of unknown length pounded into the ground next to the gene.
Gene is on wood skids.

Gene is 3 phase.

Someone measured this for me last night and said 24V between ground and neutral.
Title: Re: URGENT: 24V between ground and neutral at festival event
Post by: Mark McFarlane on December 12, 2013, 11:53:01 pm
more pics.

I'm late for church, ya'll we be sleeping by the time I return.

Cheers, thanks for what you have done
Title: Re: URGENT: 24V between ground and neutral at festival event
Post by: Chris Clark on December 13, 2013, 01:05:51 am
I'm not the most up to par with electrical, but isn't phase balancing rather important especially to generators? (ie, pulling only two of the three phases would be bad unless the generator has a setting for that, which I think I've seen on some newer ones)
Title: Re: URGENT: 24V between ground and neutral at festival event
Post by: Stephen Swaffer on December 13, 2013, 01:28:43 am
I wouldn't think not using one phase of the generator would cause any issues - the imbalance would tend to cause the generator to run hotter than normal-but that looks like a 500 Amp breaker with wiring sized much smaller,so if the generator is relatively lightly loaded, I would monitor voltage as I power things up-if they start act goofy, then I would rethink my plan.  Offered with the caveat that I am not an expert on 3 phase generators!

My concern with the floating neutral would be that if you have a piece of gear with a polarized cord that has its neutral tied to its chassis (I would hope not with modern gear-but who knows?) then the internal wiring pc board trace etc becomes your neutral bond which with a 500 Amp main should be #2 or larger copper-if something happens that that bond is needed your gear won't handle it!  I agree with JR-if you can use a lamp test between the neutral and ground first great but personally I would be able to concentrate on running the show a lot better knowing the neutral was bonded to the ground.

Frankly, I would be more concerned about the neutral being bonded to ground than in having a separate ground wire from gen to stage-IF the stage has grounding electrodes.  Though it is technically not right and apparently caused problems to influence code changes a very high percentage of farms have outbuildings running on overhead triplex (2 hots, one neutral) with driven grounds at the buildings-and inspectors in my area allow existing overheads to remain as long as ground rods are driven so it must be fairly safe.  But that might just be me.
Title: Re: URGENT: 24V between ground and neutral at festival event
Post by: Mark McFarlane on December 13, 2013, 07:24:57 am
FYI, all is well and the show will go on.  The gene in question is a 170kva.  All I need is about 40-50 amps max.

I got a Phd Electrical Engineer to come to the site.  We ended up using only one phase, reran a few underground cables from the stage panel to FOH and stacks that had polarity switches mid-cable (due to underground splices), and bonded the neutral to ground at the temporary stage panel. He said, basically,  "consider the generator as your municipal electrical service and the stage panel as your home's breaker box, with the neutral and ground being bonded at your home."

Another interesting tidbit. We are in the desert.  There is apparently no way to actually get a decent earth ground without digging a bunch of holes with copper plates in a series of arrays. Sand (quartz) is essentially non conductive.

So ground and neutral are bonded at the stage, then a ground wire goes 60m to the body of the generator (the gene technician says the neutral is not bonded to the frame), then the ground goes from the frame to a rod pounded into the ground (which the electrical engineer said was complete waste because the sand is an ineffective sync).

I'm running on 3 hours of sleep, so I won't be checking back in until after the gig tomorrow.  Then I can learn about how bad the solution was :).
Title: Re: URGENT: 24V between ground and neutral at festival event
Post by: Mike Sokol on December 13, 2013, 08:50:14 am
Another interesting tidbit. We are in the desert.  There is apparently no way to actually get a decent earth ground without digging a bunch of holes with copper plates in a series of arrays. Sand (quartz) is essentially non conductive.
The earth conductivity (or lack of it) for the ground connection really has nothing to do with the G-N voltage since there should be a single G-N-E bonding point at the service panel or generator. The ground rod in a power system is really there for lightning protection and keeping your local ground plane (G-N-E) close to earth voltage. The G-N voltage measured at a receptacle will only be 0 volts with no load on the branch circuit. If you add a load that drops the N-H voltage by 6 volts (say 120 down to 114 volts) then half of that drop will be on the hot leg and the other half on the neutral leg. So measuring 3 volts between the safety ground and neutral at the receptacle is normal and expected. But anything more than 5 volts implies more than a 10 volt drop on that circuit which should be tripping the circuit breaker. Of course, those numbers are for properly wired circuits and you have something wrong. But to answer your question about 24-volts between Neutral and Ground being dangerous to equipment, the answer is probably not. I do this sort of trick all the time with my NoShockZone seminar, and regularly bias the chassis ground to 120-volts above neutral. And I've never damages a mixing board or amplifier doing this. However, it's VERY dangerous to people. And while 24 volts AC is not normally dangerous, the potential for having the earth to chassis potential increase to 60 or more volts can create a deadly situation. That's what you need to watch out for. 
Title: Re: URGENT: 24V between ground and neutral at festival event
Post by: John Roberts {JR} on December 13, 2013, 09:57:02 am
Just to tie pretty bow on this, glad you got some informed help.  The fact that nothing caught fire suggests that the ground voltage was spurious low current leakage. Repeating my caveat that I am not a mains wiring expert, this was not like the home distribution panel, since the drop at the house panel has transformer isolation.

Connecting a floating ground to neutral at a sub panel is arguably better than having a floating ground only as long as the neutral line back to the power source is robust and properly attached. The reason this sub panel ground/neutral connection is not considered good practice is because of what happens if you get a bad/loose  connection in that neutral line back to the primary power drop. If neutral opens up back at the primary, the neutral and now connected ground can rise up to hot voltage. So you end up with hot voltage*** on all your product chassis.

The correct fix that your venue still needs to get right is to bond ground to neutral at the transformer/primary panel. Your band-aid ground bond at the sub panel could be more dangerous than the floating ground should the neutral path open up.   

Glad you didn't release any smoke or kill any meat puppets.

JR   

** The hot voltage from an open neutral bonded to ground is in series with all the loads so perhaps not full voltage, surely enough voltage to be life threatening.
Title: Re: URGENT: 24V between ground and neutral at festival event
Post by: Mark McFarlane on December 13, 2013, 12:28:43 pm
The earth conductivity (or lack of it) for the ground connection really has nothing to do with the G-N voltage since there should be a single G-N-E bonding point ... 

Thanks Mike as always for your thoughtful help.
Title: Re: URGENT: 24V between ground and neutral at festival event
Post by: Mark McFarlane on December 13, 2013, 12:45:11 pm
Just to tie pretty bow on this, glad you got some informed help.  The fact that nothing caught fire suggests that the ground voltage was spurious low current leakage. Repeating my caveat that I am not a mains wiring expert, this was not like the home distribution panel, since the drop at the house panel has transformer isolation.

Connecting a floating ground to neutral at a sub panel is arguably better than having a floating ground only as long as the neutral line back to the power source is robust and properly attached. The reason this sub panel ground/neutral connection is not considered good practice is because of what happens if you get a bad/loose  connection in that neutral line back to the primary power drop. If neutral opens up back at the primary, the neutral and now connected ground can rise up to hot voltage. So you end up with hot voltage*** on all your product chassis.

The correct fix that your venue still needs to get right is to bond ground to neutral at the transformer/primary panel. Your band-aid ground bond at the sub panel could be more dangerous than the floating ground should the neutral path open up.   

Glad you didn't release any smoke or kill any meat puppets.

JR   

** The hot voltage from an open neutral bonded to ground is in series with all the loads so perhaps not full voltage, surely enough voltage to be life threatening.

Gene Power
To clarify, tomorrow I'll be running off the gene, not the mains and transformer.  I'll be the only load on this 170kva gene.  Still a chance to smoke some gear tomorrow.

The neutral connection from my sub panel to the gene connection panel is good, I had them pull it, clean it and reconnect it.  How that neutral connects inside the gene is anyone's guess.

Considering the the gene neutral and ground are not connected together (according to the gene tech who probably partially read the manual and that is about all he knows, but a really nice guy), would it have been better to bond neutral to ground at the gene versus the way we did it, bonding at the one and only breaker panel at the stage?

Land Power
Regarding the site land power, which I am not using, we have a plan to get someone qualified out over the next few weeks to look at the 24V neutral-ground problem, which was described in the opening post of this discussion.  For reference, the picture below shows where I was connected in. My sub panel was run from the panel on the left, which was 120V 3 phase. The sub panel on the right I think was marked 480V 3 phase.  The box between panels was a transformer and that is where the grounding rod was connected.

The guy who runs events at this site (and others) was very appreciative that I identified a problem and looks forward to getting it resolved.  He stuck around on and of for 3 days, probably 10 hours total just making sure I was happy with the stage power.  He was the one who brought in the gene when I complained about the land power.

Of course, the food vendors are all still using the land power.  I vowed to myself not to test any of their circuits :)
Title: Re: URGENT: 24V between ground and neutral at festival event. Slightly OT
Post by: Mac Kerr on December 13, 2013, 04:54:41 pm
Considering the the gene neutral and ground are not connected together (according to the gene tech who probably partially read the manual and that is about all he knows, but a really nice guy), would it have been better to bond neutral to ground at the gene versus the way we did it, bonding at the one and only breaker panel at the stage?

Maybe I'm tired, and thinking clearly, but if the Neutral and  Ground are not bonded how does the safety ground blow a beaker? If the electricity can't return to its source because the safety ground circuit is not connected to the electrical circuit what is the current path that is low enough impedance to blow the breaker? The dirt that the ground rod may or may not be driven into is not an infinite sink for electrons.

Since this seems to be, from this discussion, common practice in Mark's locale, is everyone at risk?

Mac
Title: Re: URGENT: 24V between ground and neutral at festival event
Post by: John Roberts {JR} on December 13, 2013, 05:11:07 pm
Perhaps they don't have as many lawyers there as we do here....

JR
Title: Re: URGENT: 24V between ground and neutral at festival event
Post by: Rob Spence on December 13, 2013, 05:38:52 pm
One thing not specifically pointed out is that the primary reason for the ground rod is to have somewhere for a lightning strike to go.


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Title: Re: URGENT: 24V between ground and neutral at festival event
Post by: John Roberts {JR} on December 13, 2013, 06:14:45 pm
One thing not specifically pointed out is that the primary reason for the ground rod is to have somewhere for a lightning strike to go.


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Not since Mike said it this morning...

JR
Quote from: Mike posted
The ground rod in a power system is really there for lightning protection and keeping your local ground plane (G-N-E) close to earth voltage.
Title: Re: URGENT: 24V between ground and neutral at festival event
Post by: Stephen Swaffer on December 14, 2013, 01:16:37 am
 Sounds like your engineer got more than just a piece of paper for the time and money he spent at school!  Sounds like a reasonably safe setup to me though I agree that bonding at the source would be a better option-but might step on toes too.

To clarify my earlier post regarding being more concerned about bonding than running the ground wire-that wire is actually an equipment bonding conductor and serves a different purpose than the ground rods.  It also more critical when there are other metallic paths between the source and the load-metal water lines-or more likely in this scenario a metal fence.  If a person cannot touch metal from both the source and the load, a voltage difference doesn't present a huge hazard.  Still best to have the equipment bonding conductor if there is any way to get it, but in my mind lack bonding is a deal breaker.

Code allows for impedance grounded systems in certain situations.  I have never worked with them, nor do I understand the advantages of them-however that might explain why the neutral is not bonded on the gen set-to allow for use on an impedance grounded system if necessary-the manufacturer leaving properly bonding the neutral up to the end user for flexibility.
Title: Re: URGENT: 24V between ground and neutral at festival event
Post by: Mark McFarlane on December 14, 2013, 11:15:25 am
My thanks again to everyone for your thoughtful assistance. I was really stressed out this week with my normal day job and two fairly large gigs. I've had <10 hours of sleep over the past 3 nights.

Anyway, the gig is over.  It turned out to be hotter than expected (I suspect it was in the mid-high 80s) but with a clear sky (i.e. no dust) which is unusual for this desert.  A long day, as festivals always are, but the bands played well and the rig sounded better than I expected for such a large outdoor space. I suspect we'll get some complaints from the food vendors, who were about 250' in front of the mains and the sound was still fairly loud.  Probably around 4,000 people came, I should get a count tomorrow.

This is my last gig with the 01V96, the new GLD is on its way.  I affectionately rolled up the copper snake and put it to rest in my grandmothers steamer trunk.

Thanks again John, Mike, Tim, Chris, Stephen, Geoff, Lyle, Steve and Rob for your kindness and support.

I'm off to soak in the tub....
Title: Re: URGENT: 24V between ground and neutral at festival event
Post by: Jeff Bankston on December 14, 2013, 05:46:15 pm
just because there was never a problem doesnt mean a thing. famous last words > the guy said he doesnt need an instrument rating to fly. the news said he had ben in trouble with the faa several times for flying in weather without an instrument rating. he took off in over cast and crashed about 2 miles from where i live. i heard the boom. the xo on the tranformer(s) needs to be grounded. this is the single most common screw up that i and my electrician buddies have seen.