GFCI theory of operation question

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Dweeber

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I did a preliminary search for GFCI and did not find a direct answer for my seemingly easy question, or perhaps I am unable to grasp what was previously discussed, either way here goes.

If a GFCI outlet trips at 5ma leakage from Hot to neutral, would that include any "leakage" into the circuit?

What I am trying to describe is placing a submersible 120v water pump into an underground electrical vault (with conductors partially submerged) that could "add" current to the 120v cord of the pump, and cause the GFCI to trip.

This assumes a problem with the pump cord, as well as the conductor insulation in the vault.

I am hoping that the GFCI would add to the safety of the person lowering the pump.

I hope this is a clear enough description.

D.
 
Any current between the hot (ungrounded) and neutral (grounded) would be considered a load, and the GFCI will not trip.

What GFCI's look for is a difference in current between the hot & neutral. Any difference more than 5ma will cause it to trip.
 
Common GFCIs do not do anything to detect current flow on the EGC, and do not attempt to open the EGC in the event of current flow on it.

In the scenario that you describe, if the external power source somehow caused current flow on the pump EGC, the GFCI would not notice, and the pump would simply run.

Using the same principals used in GFCIs, you could build a system that would sound an alarm if there were current on the EGC. I don't see any benefit to an automated system unless it shuts off power to the external cables.

-Jon
 
Dweeber said:
... would that include any "leakage" into the circuit?
Click to expand...
Kind of.

As other have said the GFCI is looking for a 5mA difference between the H and N conductors, it does not care if it is an addition or subtraction.

In your case and current induced in the H will also be induced in the N, so the net difference will be zero, meaning no trip.
 
A GFCI trips between 4-6 mA and when greater than 6 mA. It senses unbalanced current between the hot and ground, and unbalanced current between the neutral to ground.
If there is leakage from a submerged cable to ground, that will add to the total unbalance current and can cause a trip.
If you were in series with the hot and neutral, there would be no unbalanced current and you receive a shock or be electrocuted.
 
Dweeber said:
If a GFCI outlet trips at 5ma leakage from Hot to neutral, would that include any "leakage" into the circuit?

D.
Click to expand...

If I read that correctly, you are concerned about current that may come from other sources other than the GFCI protected circuit. If the pump is completely connected (ground, hot and neutral) the ground should protect the person lowering the pump carrying off any stray voltages and current within its limitations. If for some reason the current is able to get on the hot or neutral, the GFCI would trip, but the voltage and current from the other source would still be there, unless it also was GFCI protected.
 
Thanks to all replies.

Hillbilly, your response is what I was thinking. The task of pumping water from these vaults raised some gfci questions, so thanks again.

D.
 
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