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Author Topic: Is this a floating neutral, or another problem?  (Read 1033 times)

Pete Jones

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Is this a floating neutral, or another problem?
« on: March 08, 2017, 07:42:21 am »

I had a good Google of this but I think I need help from the experts to fully understand what's going on here:
So yesterday one of my colleagues was doing a show in a large old warehouse space. He plugged in the distro to the 32a 3 phase socket provided (we are in Australia, so 240v). The metres on the distro read 240-250v phase to neutral which is normal. I think they then plugged in some equipment which didn't power up, so he goes to the distro to investigate.

He found that with everything unplugged from the distro (ie no load) and the main 3 phase breaker on it still reads 240v, but each time he turns on a breaker for an individual circuit that phase drops to 50-75v and the other 2 phases go up to 300v or more.

Is this because there is no neutral (wye 3 phase with no neutral for motors/welding) or is this what happens when you plug a wye config distro into delta 3 phase?
It matches a description of a floating neutral, but I can't think why it would happen with no load?
They obviously should have metered the mains first, but in this case would that pick up the problem, given that the distro didn't initially show any strange readings on its meters?

Also there seems to have been some damage to the distro, the breakers now trip immediately. What would have caused this?

Any help would be greatly appreciated so I can understand how to spot something like this when checking mains. Thanks!
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Mike Sokol

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Re: Is this a floating neutral, or another problem?
« Reply #1 on: March 08, 2017, 07:23:43 pm »

Is this because there is no neutral (wye 3 phase with no neutral for motors/welding) or is this what happens when you plug a wye config distro into delta 3 phase?
It matches a description of a floating neutral, but I can't think why it would happen with no load?
They obviously should have metered the mains first, but in this case would that pick up the problem, given that the distro didn't initially show any strange readings on its meters?

It does sound like a loose or corroded connection on the neutral of the WYE 3-phase transformer. I can't imagine there would be any Delta transformers in your situation, but stranger things have happened.

The way to test this is to measure the voltage from each phase to neutral, then apply a load while metering it. As you said, if the voltage across the load drops by 50 volts and the voltage on an unloaded phase goes UP by the same amount, then that's a sure sign of an open or badly corroded neutral connection. And that lost neutral connection can be all the way up at the transformers feeding the building. You need to get a POCO (power company) rep out there to confirm exactly how it's hooked up and test for continuity, etc...

Quote
Also there seems to have been some damage to the distro, the breakers now trip immediately. What would have caused this?

Something really bad is going on here. Is there any load on the breakers? If they're feeding a piece of gear that has been damaged by the over-voltage from the floating neutral, that would explain drawing too much current and tripping. Again, you need to get a PoCo rep out there before someone gets killed.

David Buckley

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Re: Is this a floating neutral, or another problem?
« Reply #2 on: March 08, 2017, 08:50:55 pm »

There is an outside possibility that the 32A three phase socket despite having five pins has no neutral connected; in industrial situations the neutral often isn't needed and some deranged electricians use four core cable, and fit a five pin socket when they should have fitted a four pin.  The wiring regs (I'm in Kiwiwland so we share AS/NZS 3000) doesn't have a specific rule prohibiting this, through whatever the Oz version of the electrical safety regulations should prohibit electrically unsafe installations, and I'd argue that this is an electrically unsafe situation.
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John Rutirasiri

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Re: Is this a floating neutral, or another problem?
« Reply #3 on: March 08, 2017, 11:03:24 pm »

Mike, this scares the hell out of me because a floating neutral can pretty much happen anytime, without warning.  If someone doesn't get killed sooner or later, then maybe an electrical fire.

I know you mentioned the use of Progressive Industry's RV surge protectors in an amp rack, which is a great idea.  Is there something that could be installed at the distro level, to detect floating neutral and kill power to the whole distro?  Some kind of over/under voltage relay?

Maybe more important than a distro is a floating neutral detection device to protect a commercial venue or residential property?

Man thanks,
John R.
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Pete Jones

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Re: Is this a floating neutral, or another problem?
« Reply #4 on: March 09, 2017, 12:07:50 am »

There is an outside possibility that the 32A three phase socket despite having five pins has no neutral connected; in industrial situations the neutral often isn't needed and some deranged electricians use four core cable, and fit a five pin socket when they should have fitted a four pin.  The wiring regs (I'm in Kiwiwland so we share AS/NZS 3000) doesn't have a specific rule prohibiting this, through whatever the Oz version of the electrical safety regulations should prohibit electrically unsafe installations, and I'd argue that this is an electrically unsafe situation.
Hi David
Yes I think this is what's happened. Apparently there were some 4 pin sockets in the same area. The sparkys probably just used what they had in the van....





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Pete Jones

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Re: Is this a floating neutral, or another problem?
« Reply #5 on: March 09, 2017, 12:13:39 am »

It does sound like a loose or corroded connection on the neutral of the WYE 3-phase transformer. I can't imagine there would be any Delta transformers in your situation, but stranger things have happened.

The way to test this is to measure the voltage from each phase to neutral, then apply a load while metering it. As you said, if the voltage across the load drops by 50 volts and the voltage on an unloaded phase goes UP by the same amount, then that's a sure sign of an open or badly corroded neutral connection. And that lost neutral connection can be all the way up at the transformers feeding the building. You need to get a POCO (power company) rep out there to confirm exactly how it's hooked up and test for continuity, etc...

Something really bad is going on here. Is there any load on the breakers? If they're feeding a piece of gear that has been damaged by the over-voltage from the floating neutral, that would explain drawing too much current and tripping. Again, you need to get a PoCo rep out there before someone gets killed.
That's what happened, except it dropped by nearly 200v, the other phases increased by only 50ish. Is this maybe because it's a 240v l-n 415v p-p? Or could it be - even worse - they have tied the neutral pin to the safety ground in the 4 wire motor power?

I definitely agree it's a very unsafe situation.

I told my colleague I'd think twice about touching the distro if I saw this again!
 
Edited to add voltages.
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Stephen Swaffer

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Re: Is this a floating neutral, or another problem?
« Reply #6 on: March 09, 2017, 12:57:36 pm »

Mike, this scares the hell out of me because a floating neutral can pretty much happen anytime, without warning.  If someone doesn't get killed sooner or later, then maybe an electrical fire.

I know you mentioned the use of Progressive Industry's RV surge protectors in an amp rack, which is a great idea.  Is there something that could be installed at the distro level, to detect floating neutral and kill power to the whole distro?  Some kind of over/under voltage relay?

Maybe more important than a distro is a floating neutral detection device to protect a commercial venue or residential property?

Man thanks,
John R.

John,

There are monitoring relays-for the most part geared towards motors-because motors typically already have a magnetic contactor that can be controlled by the relay.  Once you detect a problem, you have to do something about it.  You could install a warning light-but who hasn't driven a car with a warning light and really paid no attention?  Otherwise, you need a contactor to de-energize the distro or a remote trip breaker-both fairly expensive add ons to a distro.  Add to that nuisance trips that would kill the whole show instead of perhaps just dropping one phase.

The very best way is to simply learn how (as Mike has detailed several times) to test a distro or outlet you plan to use.  As an electrical contractor, I always verify power being correct before walking away from a job-as do POCO emplyees (at least the ones I am around).  It only takes a few seconds and is much more thorough and will give you more information than relying on an engineering control.

Floating neutrals can indeed be very hazardous-even worse is a neutral that is lost before the neutral-ground bond on a building as that has the potential to energize anything that should be grounded.  There is a reason I often grabbed gloves as soon as something seemed odd when troubleshooting!
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Steve Swaffer

Mike Sokol

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Re: Is this a floating neutral, or another problem?
« Reply #7 on: March 09, 2017, 11:51:48 pm »

Floating neutrals can indeed be very hazardous-even worse is a neutral that is lost before the neutral-ground bond on a building as that has the potential to energize anything that should be grounded.  There is a reason I often grabbed gloves as soon as something seemed odd when troubleshooting!

On a slight swerve, I've just contracted a local electrical installer to put in a 3-phase/400-amp/120-208-volt panel in a large music venue where I'm the house sound engineer, and went through several cycles of explaining to the installer about what was needed. I had spec'ed a 120/208-volt 3-phase panel (in the USA) and first they wanted to know if I REALLY needed all 3-phases . (Yes, for motors). Then their tried to get a 277/480-volt panel since that's a typical industrial unit (OK in other countries, but not useful in the USA for stage and lighting). Finally, once I got those two mistakes headed off at the pass, they tried to undersize the neutral feeding the camlocks. Again, that's standard for 3-phase motors since there's no neutral current. But this would be powering a single-phase portable dimmer rack and all kinds of PA and stage amps. I made them pull out the small neutral and put in full-capacity wiring.

The takeaway is that many (most?) industrial electricians really don't understand what we do and how our gear is powered. So check, double-check, and triple-check your power. And if anything measures the least bit funny, then DO NOT TIE IN. As noted earlier in this thread, it's pretty simple to use a good DMM to measure the power before connecting. And also make sure you get a good quality industrial-duty meter that won't turn into a bomb if it's accidentally over-voltaged. Some of the cheap junk they give away at Harbor Freight could explode if you accidentally get into a high-voltage connection. Spend the money and get a Fluke or other high-quality meter.   
« Last Edit: March 10, 2017, 06:13:49 pm by Mike Sokol »
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John Rutirasiri

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Re: Is this a floating neutral, or another problem?
« Reply #8 on: March 10, 2017, 09:32:26 am »

Stephen, Mike...thank you for the information.  I always meter the generator, spider box, distro and so far I've been lucky I haven't come across loose/disconnected neutral.
I've always wondered why for residential, there not code requiring some sort of transfer switch to cut power in case of lost neutral.  It doesn't happen often, but when it does, it can be deadly.

Best,
John R.
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TJ (Tom) Cornish

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Re: Is this a floating neutral, or another problem?
« Reply #9 on: March 10, 2017, 09:38:31 am »

The takeaway is that many (most?) industrial electricians really don't understand what we do and how our gear if powered.
Agreed - lots of certification time spent on conduit fill calculations, but not much time spent on theory and usage, other than for HVAC applications.
So check, double-check, and triple-check your power. And if anything measures the least bit funny, then DO NOT TIE IN. As noted earlier in this thread, it's pretty simple to use a good DMM to measure the power before connecting. And also make sure you get a good quality industrial-duty meter that won't turn into a bomb if it's accidentally over-voltaged. Some of the cheap junk they give away at Harbor Freight could explode if you accidentally get into a high-voltage connection. Spend the money and get a Fluke or other high-quality meter.   
Add to your list a 1500w portable heater to do a load test before you plug anything else in.
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