ProSoundWeb Community

Please login or register.

Login with username, password and session length
Advanced search  

Pages: [1] 2 3   Go Down

Author Topic: 15.4 volts between Ground and Neutral  (Read 13779 times)

Mike Sokol

  • Moderator
  • Hero Member
  • *****
  • Offline Offline
  • Posts: 3361
  • Lead instructor for the No~Shock~Zone
    • No~Shock~Zone Electrical Safety
15.4 volts between Ground and Neutral
« on: August 01, 2015, 10:44:26 am »

Just got this email from a NSZ reader yesterday. After talking to him we found that the campground wiring measures around 15 volts between the Neutral and Ground in every power pedestal outlet. The EGC to an  ground rod is a fraction of a volt, but the Neutral measures 15 volts above earth potential. While it's normal to see a few volts difference between Ground and Neutral on a loaded circuit, I've never measured 15 volts unless something was disconnected. And this appears to be on EVERY pedestal in the park, so it must be happening at the Service Panel. Do you think someone forgot to bond the Neutral and Ground together in the service panel? That's the only thing that makes sense. This is a National Park so the Army Corp of Engineers is apparently in charge of the campground wiring. I guess they make wiring mistakes too... :o

BTW: The RV owner complained she was feeling a shock, so that's why a technician is looking at this. But the chassis/shell of the RV measures a fraction of a volt to earth ground. So something is amiss.

Hey Mike, 
I am reading 15.4 volts between the ground an neutral on an rv I am servicing for a client. I have combed through your no shock zone details and have tested everything u recommend for hot skin and reverse polarity checks.  Any thoughts on what might be causing the reading?
« Last Edit: August 01, 2015, 10:46:40 am by Mike Sokol »
Logged
Mike Sokol
mike@noshockzone.org
www.NoShockZone.org

John Roberts {JR}

  • Hero Member
  • *****
  • Offline Offline
  • Posts: 16785
  • Hickory, Mississippi, USA
    • Resotune
Re: 15.4 volts between Ground and Neutral
« Reply #1 on: August 01, 2015, 11:24:19 am »

Absolutely no idea why any voltage on neutral could be felt by the customer? Is it possible the ground is open/floating? I'd confirm with a low impedance measurement ( a few hundred ohms between neutral and ground).

!5V (low Z) between neutral and ground at an outlet sounds high but is explainable.

!5V (low Z) between neutral and ground at the panel where is should be bonded (0V)  is clearly unacceptable.

JR
Logged
Cancel the "cancel culture". Do not participate in mob hatred.

Mike Sokol

  • Moderator
  • Hero Member
  • *****
  • Offline Offline
  • Posts: 3361
  • Lead instructor for the No~Shock~Zone
    • No~Shock~Zone Electrical Safety
Re: 15.4 volts between Ground and Neutral
« Reply #2 on: August 01, 2015, 11:35:39 am »

Absolutely no idea why any voltage on neutral could be felt by the customer? Is it possible the ground is open/floating? I'd confirm with a low impedance measurement ( a few hundred ohms between neutral and ground).

!5V (low Z) between neutral and ground at an outlet sounds high but is explainable.

!5V (low Z) between neutral and ground at the panel where is should be bonded (0V)  is clearly unacceptable.

JR

The scary thing is this voltage between Ground and Neutral appears to be at dozens or more campsites. So the problem has to be pretty far upstream to affect so many pedestals. What I normally see in a campground is an open EGC wire somewhere in the daisy chain wiring common to this type of distro. In that case, the Ground wire is floating and its potential becomes elevated above earth ground due to normal hot-to-chassis leakage currents. But this fail appears to be backwards with the EGC at earth potential and the Neutral elevated. Do you think someone swapped the Ground and Neutral wires upstream in a sub-panel, and now the Neutral wire is floating somehow? Something really doesn't made sense with these measurements, but the RV technician who contacted me seems to know his way around a meter pretty well and was totally perplexed by what he was seeing.

A year ago I confirmed that a National Campground had several hundred outlets mis-wired with the Hot and Neutral swapped in the campsite pedestals. Just sayin'  :o
« Last Edit: August 01, 2015, 11:49:47 am by Mike Sokol »
Logged
Mike Sokol
mike@noshockzone.org
www.NoShockZone.org

Stephen Swaffer

  • Hero Member
  • *****
  • Offline Offline
  • Posts: 2452
Re: 15.4 volts between Ground and Neutral
« Reply #3 on: August 01, 2015, 01:28:23 pm »

I have often wondered why panels are shipped so that the ground-neutral bond has to be field installed. To me it would be safer if the electrician had to remove the bond when required.  I have seen many missing bonds so that would not surprise me.  According to Ohms law there has to be a disconnect or bad connection somewhere. 

Usually the ground wire is smaller than the neutral.  I suppose a swap is possible-but it should be a no-brainer to get that connection right-at least upstream.

I would think a more likely possibility would be a second, unintentional connection (perhaps even high impedance) somewhere and a missing ground conductor -heaven forbid they would rely on ground rods at pedestals for the ground and not run a fourth wire-but I have seen stranger-and if they ran a long 3 wire feeder and then a common ground grid at the pedestals that might make sense.
Logged
Steve Swaffer

David Buckley

  • Hero Member
  • *****
  • Offline Offline
  • Posts: 565
Re: 15.4 volts between Ground and Neutral
« Reply #4 on: August 01, 2015, 05:24:20 pm »

First thing to do is measure PSCC (prospective short circuit current) from hot to neutral and hot to earth.  If they are wildly different then yes, there is likely a poor ground/neutral bond in the service entrance.

The problem is that (last I looked anyway, which was a few years ago) there is no regulation requiring USA electricians to confirm PSCC, so most folks simply haven't got a PSCC tester.  A low resistance tester, qualified to operate on mains, that passes a significant test current is the next best thing, and measure the neutral to earth resistance, which should be low.  Third best thing, but a bit dodgy, is a car battery and a headlamp bulb as a continuity tester, and use that to check the continuity of the ground to neutral; the bulb should light.

The scary thing about TN-C-S installations (which is what the USA mandates) where there is no equipotential zone and one is in contact with "real" ground, ie that soil stuff, or worse still, in water in the presence of electrical stuff, so, for example, a marina) is where there is a potential difference between the ground pin on an outlet and the soil.  Then there is the very real possibility of being shocked by differences in ground potentials.  There is no NEC-compliant protective device available for installation purposes that can prevent ground-to-ground shocks.

This crops up in the rock and roll environment when one runs an extension cord from the house out to a band in a tent in a field.  Wired guitarist or vocalist, bare feet, damp grass...   
Logged

Mike Sokol

  • Moderator
  • Hero Member
  • *****
  • Offline Offline
  • Posts: 3361
  • Lead instructor for the No~Shock~Zone
    • No~Shock~Zone Electrical Safety
Re: 15.4 volts between Ground and Neutral
« Reply #5 on: August 01, 2015, 07:22:42 pm »

Things are getting weirder and weirder with this G-N voltage situation. The RV tech called me back to report he's now reading 30 volts between ground and neutral at other pedestals in the campground, and he's sending pictures. Plus at least of few of these pedestals are reportedly "blown", whatever that means. This park is located just a few miles from Lake of the Ozarks where two children were electrocuted while swimming between houseboats in 2012. Nobody at the time could figure out the "Unknown source of the Electricity" that killed them. I'm wondering if something odd was done by an electrician or inspector working on both sites that has created a similar shock situation in both parks.     
Logged
Mike Sokol
mike@noshockzone.org
www.NoShockZone.org

Jonathan Johnson

  • Hero Member
  • *****
  • Offline Offline
  • Posts: 3158
  • Southwest Washington (state, not DC)
Re: 15.4 volts between Ground and Neutral
« Reply #6 on: August 01, 2015, 08:08:59 pm »

A year ago I confirmed that a National Campground had several hundred outlets mis-wired with the Hot and Neutral swapped in the campsite pedestals. Just sayin'  :o

Not the topic of this thread, but since when has that stopped us from commenting on off-topic things? ;-)

I can see three probable reasons for swapped hot and neutral over a wide area:
  • Boneheaded electrician miswired every. single. outlet.
  • Bad batch of outlets from the factory, with silver- and brass- screws swapped, and the electrician only looked at the screw colors. I have seen this; that's why I always look at the face of the outlet and wire according to the width/position of the slots.
  • Hot/neutral swap upstream; outlets are wired with white to neutral contact and colored to hot contact. What's doubly bad about this is that you could have a situation where some outlets have 208V or 240V instead of 120V.

As for the current problem at hand, I would be looking for a missing G-N bond. In typical practice in the United States, the neutral is connected to earth ground (dirt) via (the Grounding Electrode) in two places: the service entrance panel, and the utility transformer. There is no EGC between the utility transformer and the service entrance panel; only a neutral conductor. If the bond at the panel is missing, the voltage potential measured between neutral and ground at a pedestal should be close to the voltage measured between the ground rod at the service entrance panel and the ground rod at the utility transformer.

Another possibility is a broken neutral upstream, but I would expect that to be manifest in hot-neutral voltages being significantly higher or lower than expected.
Logged
Stop confusing the issue with facts and logic!

Stephen Swaffer

  • Hero Member
  • *****
  • Offline Offline
  • Posts: 2452
Re: 15.4 volts between Ground and Neutral
« Reply #7 on: August 01, 2015, 10:45:43 pm »

Things are getting weirder and weirder with this G-N voltage situation. The RV tech called me back to report he's now reading 30 volts between ground and neutral at other pedestals in the campground, and he's sending pictures. Plus at least of few of these pedestals are reportedly "blown", whatever that means. This park is located just a few miles from Lake of the Ozarks where two children were electrocuted while swimming between houseboats in 2012. Nobody at the time could figure out the "Unknown source of the Electricity" that killed them. I'm wondering if something odd was done by an electrician or inspector working on both sites that has created a similar shock situation in both parks.   

Is the campground owner/operator involved?  This sounds like a situation that needs a skilled electrician on site ASAP-before a serious incident happens.  A complete set of voltage readings would make it easier to speculate (Phase1-N, Phase1-G, G-N, Phase2-N, Phase 2 -G) at a pedestal with no load and under a significant load (plug in an electric heater or high wattage lamp).  But honestly, IMO, this is just short of a call to 911-so it would be best for someone there to diagnose and fix rather than wait for internet collective knowledge.

If there is a bad neutral connection and/or oddball ground-neutral bonding, the voltages could vary depending on loads-and even which pedestals are loaded or not-and the wrong combination could be deadly.
Logged
Steve Swaffer

g'bye, Dick Rees

  • Hero Member
  • *****
  • Offline Offline
  • Posts: 7424
  • Duluth
Re: 15.4 volts between Ground and Neutral
« Reply #8 on: August 01, 2015, 10:59:24 pm »

Any time I've encountered such anomalies I've gone right to the inspector/permitting office and informed them of the potential for serious injury.  They usually know who did the work and get them back on the job...under close supervision.
Logged
Pay no attention to the man behind the curtain...

Mike Sokol

  • Moderator
  • Hero Member
  • *****
  • Offline Offline
  • Posts: 3361
  • Lead instructor for the No~Shock~Zone
    • No~Shock~Zone Electrical Safety
Re: 15.4 volts between Ground and Neutral
« Reply #9 on: August 02, 2015, 12:43:55 am »

Any time I've encountered such anomalies I've gone right to the inspector/permitting office and informed them of the potential for serious injury.  They usually know who did the work and get them back on the job...under close supervision.

Yes, we're doing that right now. Since this is federal land, the Army Corp of Engineers is actually the AHJ and there's a forest ranger living on the site. The ranger has been notified and asked to contact the Army Corp of Engineers to find out which local contractor actually did the electrical work. I've told them to contact me ASAP to answer any questions I can about the danger of the situation.

The RV Technician who contacted me has sent a pile of pictures showing voltages he's measuring at the pedestals, and the most he sees from the Ground wire to earth is 0.7 volts. So there's no immediate hot-skin voltage with reference to earth potential at the moment. But he's still reading Neutral to EGC voltages of 20 volts or more. I'm considering calling the ranger and asking them to have the RV owners unplug and evacuate the campground, but since there's no hot-skin voltage on the RVs right now that might be premature. But I do know the RV skin voltage could change in a second, so I've aked the technician to monitor it closely.
« Last Edit: August 02, 2015, 01:08:02 am by Mike Sokol »
Logged
Mike Sokol
mike@noshockzone.org
www.NoShockZone.org

ProSoundWeb Community

Re: 15.4 volts between Ground and Neutral
« Reply #9 on: August 02, 2015, 12:43:55 am »


Pages: [1] 2 3   Go Up
 



Page created in 0.05 seconds with 20 queries.