Shorting neutral to Earth confusion

  • #26
Averagesupernova
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Naspook said, "If the connection is made at the distant device a GFCI device will trip due the unsafe condition if you are lucky."
As others explained, that could happen if part of the return current went through ground rather than neutral.

But the opposite is scarier: If an unlucky person touched a life wire at the remote site, current would flow through them to ground (or earth if you like). But as ground and neutral are connected there, the harmful current may return through the extra earth-neutral connection and through the neutral wire. After all, a solid copper neutral wire likely offers a path of less resistance than traveling through the soil. And the remote grounding rod might have a better connection. That means that the GFCI will detect matching opposite currents on life vs. neutral, and fail to trip.

Does this scenario make sense? If yes, then I'd be safer disconnecting the remote neutral-ground short.
With a GFCI connect between the source and the point where the ground and neutral are tied together, this will just about always result in the GFCI tripping. This is because the return current which is normally confined to ONLY the neutral is shared between the ground and neutral which the GFCI sees as an imbalance between hot and neutral. A very common thing to happen is that a service panel is updated and AFCI (arc fault), GFCI, or a combination of both breakers are installed. An AFCI breaker has some GFCI protection but it does not detect as low of an imbalance as a true GFCI breaker does. Anyway, in the above scenario we will have those breakers tripping for no apparent reason. None of the old breakers tripped. The problem is often in an outlet box where the bare ground wire has shorted to the neutral terminal. This was not a problem before AFCI and GFCI breakers were installed. It was likely this way since the original install and was not a problem.
 
  • #27
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Yes, that was the case.


Thanks!

Yes, and they call cookies "biscuits" and french fries are "chips".

Thanks all of you, I was wondering if terminology had changed or if it was just that 40 years ago I was only exposed to American books and with the internet, terminology became mixed, it seems the latter is responsible for a lot of the mixed terminology I see now. This is what happens when you stop learning and then pick up after 40 years.
Many countries descended from the British Empire (long live the Queen :-) use the term Earth for Ground, oh and you have it back to front, you call biscuits cookies.
 
  • #28
OldEngr63
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Do you mean Us Navy ships have delta connected gen? As i understand ships normally have Y gen with isolated neutral. High voltage 6.6 kv ones are earthed with a high Resistance.
The US Navy is not 100% consistent from ship to ship, but when I worked for NSWCCD, delta connection was common for many ships.
 
  • #29
The US Navy is not 100% consistent from ship to ship, but when I worked for NSWCCD, delta connection was common for many ships.
Thank you sir I honestly did not know that. I work on merchant ships and have never heard of delta genset. But then again I'm young. Would you be kind enough to tell me the benefits/disadvantages?
 
  • #30
nsaspook
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Thank you sir I honestly did not know that. I work on merchant ships and have never heard of delta genset. But then again I'm young. Would you be kind enough to tell me the benefits/disadvantages?
Benefits: Ungrounded delta can take one leg shorted to hull and still work at full power, with a two leg short down the line the switchboard can disconnect one shorted leg at the generator an still have two phases on-line. Disadvantages? Not many on a warship where having some power could be the difference between life and death. Most of our critical equipment had a 'battle-short' switch because 'it's better to risk having a technician or two killed trying to fix something than to have the whole ship destroyed"
Battle%20Short%20Button.jpg

Battleshort
 
  • #31
Benefits: Ungrounded delta can take one leg shorted to hull and still work at full power, with a two leg short down the line the switchboard can disconnect one shorted leg at the generator an still have two phases on-line. Disadvantages? Not many on a warship where having some power could be the difference between life and death. Most of our critical equipment had a 'battle-short' switch because 'it's better to risk having a technician or two killed trying to fix something than to have the whole ship destroyed"
Battle%20Short%20Button.jpg

Battleshort
Thank you for the info. As i understand it wouldn't a Y 3 phase isolated neutral work the same way? One phase short to hull is no problem. If 2 phases short then usually the genset trips though it can reconfigured to isolate one phase.
Or am I just being dense? Lol
 
  • #32
dlgoff
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BTW, there's no ground wire at the remote barn -- it's all old 2 conductor wiring & 2 prong outlets.
Not just old barns but old homes where neutral was ground; as in water pipe. My first encounter with electricity was at about age 5 when getting a drink from the outdoor hose faucet barefooted on a hot summer day.
Funny how pipes can have a high resistance to ground. :oldsurprised: Maybe that was what "sparked" my interest in electrical things.
 
  • #33
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Thank you for the info. As i understand it wouldn't a Y 3 phase isolated neutral work the same way? One phase short to hull is no problem. If 2 phases short then usually the genset trips though it can reconfigured to isolate one phase.
Or am I just being dense? Lol
In an ungrounded delta, each phase floats. So if you ground one leg, all three still work normally. Ground is just referenced to one of the legs when it becomes grounded. The circuits still act normally. At least theoretically. (I'm not sure how wise that would be though.)

In a Y, grounding one leg will place the neutral at a high voltage. Then the currents through the neutral are different (higher?). In addition someone touching the neutral thinking it's safe will be in for a shock. :oldsurprised:
 
  • #34
In an ungrounded delta, each phase floats. So if you ground one leg, all three still work normally. Ground is just referenced to one of the legs when it becomes grounded. The circuits still act normally. At least theoretically. (I'm not sure how wise that would be though.)

In a Y, grounding one leg will place the neutral at a high voltage. Then the currents through the neutral are different (higher?). In addition someone touching the neutral thinking it's safe will be in for a shock. :oldsurprised:
There is no neutral on ships. And very cool smiley!!
 

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