ProSoundWeb Community

Please login or register.

Login with username, password and session length
Advanced search  

Pages: 1 2 [3] 4   Go Down

Author Topic: URGENT: 24V between ground and neutral at festival event  (Read 18720 times)

Lyle Williams

  • Hero Member
  • *****
  • Offline Offline
  • Posts: 1556
Re: URGENT: 24V between ground and neutral at festival event
« Reply #20 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. 
Logged

Lyle Williams

  • Hero Member
  • *****
  • Offline Offline
  • Posts: 1556
Re: URGENT: 24V between ground and neutral at festival event
« Reply #21 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.
« Last Edit: December 12, 2013, 10:47:29 pm by Lyle Williams »
Logged

Mark McFarlane

  • Hero Member
  • *****
  • Offline Offline
  • Posts: 1865
  • Middle East
    • Arkose Records
Re: URGENT: 24V between ground and neutral at festival event
« Reply #22 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?
« Last Edit: December 13, 2013, 01:18:27 am by Mark McFarlane »
Logged
Mark McFarlane
ARKOSERECORDS
Turn down what's too loud.

Mark McFarlane

  • Hero Member
  • *****
  • Offline Offline
  • Posts: 1865
  • Middle East
    • Arkose Records
Re: URGENT: 24V between ground and neutral at festival event
« Reply #23 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.
« Last Edit: December 13, 2013, 01:19:07 am by Mark McFarlane »
Logged
Mark McFarlane
ARKOSERECORDS
Turn down what's too loud.

Mark McFarlane

  • Hero Member
  • *****
  • Offline Offline
  • Posts: 1865
  • Middle East
    • Arkose Records
Re: URGENT: 24V between ground and neutral at festival event
« Reply #24 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
Logged
Mark McFarlane
ARKOSERECORDS
Turn down what's too loud.

Chris Clark

  • Full Member
  • ***
  • Offline Offline
  • Posts: 145
  • Jack of many trades, Master of few.
Re: URGENT: 24V between ground and neutral at festival event
« Reply #25 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)
Logged
Technical Services Coordinator
Entertainment System Designer
Darien Lake Theme Park, Darien Center NY

Stephen Swaffer

  • Hero Member
  • *****
  • Offline Offline
  • Posts: 2258
Re: URGENT: 24V between ground and neutral at festival event
« Reply #26 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.
Logged
Steve Swaffer

Mark McFarlane

  • Hero Member
  • *****
  • Offline Offline
  • Posts: 1865
  • Middle East
    • Arkose Records
Re: URGENT: 24V between ground and neutral at festival event
« Reply #27 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 :).
« Last Edit: December 13, 2013, 07:28:22 am by Mark McFarlane »
Logged
Mark McFarlane
ARKOSERECORDS
Turn down what's too loud.

Mike Sokol

  • Moderator
  • Hero Member
  • *****
  • Offline Offline
  • Posts: 3356
  • Lead instructor for the No~Shock~Zone
    • No~Shock~Zone Electrical Safety
Re: URGENT: 24V between ground and neutral at festival event
« Reply #28 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. 
Logged
Mike Sokol
mike@noshockzone.org
www.NoShockZone.org

John Roberts {JR}

  • Hero Member
  • *****
  • Offline Offline
  • Posts: 16416
  • Hickory, Mississippi, USA
    • Resotune
Re: URGENT: 24V between ground and neutral at festival event
« Reply #29 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.
Logged
Don't tune your drums half-ass. Listen to what a properly "cleared" drum sounds like.   http://circularscience.com/

ProSoundWeb Community

Re: URGENT: 24V between ground and neutral at festival event
« Reply #29 on: December 13, 2013, 09:57:02 am »


Pages: 1 2 [3] 4   Go Up
 



Page created in 0.064 seconds with 23 queries.