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Author Topic: 15.4 volts between Ground and Neutral  (Read 13819 times)

Mike Sokol

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Re: 15.4 volts between Ground and Neutral
« Reply #20 on: August 02, 2015, 04:57:36 pm »

Some of those photos look like a floating/loose neutral.
Yeah, I'm guessing something is wrong with the neutral connection, not the EGC connection. But I do like Frank's idea of a 60-watt bulb across the voltmeter for loading. I need to build one of these with a vibration proof bulb for my test kit.
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Mike Sokol
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David Buckley

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Re: 15.4 volts between Ground and Neutral
« Reply #21 on: August 02, 2015, 05:01:18 pm »

I just went out with binoculars and db checked.  There is no ground connected to either of the wires you labeled PoCo.
Yep Frank, you are lucky, you get your power distribution the "rest of world" way.    Mike's diagram, which he notes is from "a while ago" so I certainly haven't influenced it, illustrates the American way, Multi Grounded Neutral.

There is a paper that explains all about MGN: The Hazardous Multigrounded Neutral Distribution System And Dangerous Stray Currents (PDF, 230KB), which is recommended reading if you live in MGN-land.  Its a bit political, and espouses a point-of-view, but the paper's engineering is sound from an  author who knows what he is talking about.

Edited to fix URL.
« Last Edit: August 02, 2015, 05:03:46 pm by David Buckley »
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Mike Sokol

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Re: 15.4 volts between Ground and Neutral
« Reply #22 on: August 02, 2015, 05:13:57 pm »

Mike, I would suggest updating the diagram to show that the service breaker for each hot is tied together as a single unit.
I guess this is how you draw a common-trip double circuit breaker.

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Stephen Swaffer

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Re: 15.4 volts between Ground and Neutral
« Reply #23 on: August 02, 2015, 05:14:33 pm »

Actually, the phase to neutral voltage appears to be correct.  (if any distance from the service-campground pedestal is farther away than receptacle in typical home-with any load voltage drop of 4 volts is normal). The only anomaly I see is the voltage between ground and neutral.  This would exist with a Y secondary transformer lacking a G-N bond.  Again, making assumptions about the circumstances, a large load, such as a bunch of RVs, would likely be served by a 3 phase transformer with a Y secondary.

Without a bond, but with a grounding electrode system in place you should see exactly  what he is seeing.  Hotskin could be caused by an RV with an improper internal G-N bond (bootleg ground anyone?) and a missing EGC.   
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Steve Swaffer

David Buckley

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Re: 15.4 volts between Ground and Neutral
« Reply #24 on: August 02, 2015, 05:22:54 pm »

but since digital meters came out everyone seemed to abandon them [wiggys] in favor of the high-z stuff with numerical readouts.
No so Captain; an electricians DMM will have relatively low impedance inputs.  Not as low as a wiggy, sure, but not tens of megohms like a "normal" DMM.

OK, so an electricians DMM is not as cheap as a DMM from Home Depot, but its the sort of tool an electrician should have.  Right tool for the job, buy once cry once, yada yada yada...

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Rob Spence

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Re: 15.4 volts between Ground and Neutral
« Reply #25 on: August 02, 2015, 06:30:34 pm »

I guess this is how you draw a common-trip double circuit breaker.

Yup, now I like it.


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Mike Sokol

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Re: 15.4 volts between Ground and Neutral
« Reply #26 on: August 02, 2015, 06:44:10 pm »

Yup, now I like it.

Great.... As promised, here's the diagram I drew with actual wiring resistance to show how you can measure up to 5 volts between the neutral and ground if there's a 10 volt drop in the branch circuit. Again, I drew this two years ago so I could memorize the current flow in my little brain. Note that I'm only detailing one leg of the split-phase circuit feeding a 120-volt water heater element in an RV. What do you think... reasonably clear? Any critiques?

 
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Stephen Swaffer

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Re: 15.4 volts between Ground and Neutral
« Reply #27 on: August 02, 2015, 06:55:02 pm »

That  makes sense-but the readings he is showing are showing a normal H-N voltage.  If a bad connection "pulls" a neutral voltage up 22 volts from ground, and "pulls" the hot down 22 volts there is no way that H-N voltage can read 116 unless you started just a little high!
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Mike Sokol

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Re: 15.4 volts between Ground and Neutral
« Reply #28 on: August 02, 2015, 07:20:54 pm »

That  makes sense-but the readings he is showing are showing a normal H-N voltage.  If a bad connection "pulls" a neutral voltage up 22 volts from ground, and "pulls" the hot down 22 volts there is no way that H-N voltage can read 116 unless you started just a little high!

Yes, I know this isn't what happening with the OP failure reading. I'm just showing what SHOULD be happening if you have a long circuit run with perhaps 10 volts drop H-to-N at the far end. Since 1/2 of that voltage drop can be on the Neutral run, then measuring up to 5 volts between a Neutral and Ground on a heavily loaded circuit is normal. In fact, if you read 0 volts between Neutral and Ground on a loaded branch circuit, then you probably have a bootleg ground close by. Again, I drew this up just to wrap my head around different voltage measurements I would see in power distro systems for live sound. The RV stuff is just a spin-off of my ground loop theory work.

This also suggests that since changing the load will change the voltage between the neutral and ground, that double-bonding the Neutral-Ground in a sound system will create ground loop differential voltage and resulting currents that are modulated by the music. For example, I can create a hum that's only there during a heavy bass guitar note or kick drum hit. Remember me discussing GLID (Ground Loop Inter-modulation Distortion) a while ago? This shows how and why that can happen in a mis-wired power distro for a large sound system.   
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Mike Sokol
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Re: 15.4 volts between Ground and Neutral
« Reply #28 on: August 02, 2015, 07:20:54 pm »


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