Subpanel Puzzler

[LOW1]

i have a 100 amp subpanel in a boathouse. It is fed by a 100 amp GFCI breaker in our cabins main panel.

I was relocating some switches and outlets in the boathouse and tripped the GFCI breaker. so I shut off all breakers in the subpanel including the subpanels main shutoff.

But with everything shutoff in the subpanel I could not get the main panels GFCI breaker to reset.

I found my short, fixed it and now all is well. But why would the GFCI breaker not reset when everything it feeds has been shutoff?

 

[PCustoms]

Did the GFCI trip or did the breaker trip?

If it's not resetting, either the Breaker is bad or your fault is in the line feeding the boathouse. Disconnect the load side of the breaker, does it reset?

 

[KenC]

Likely your short was the cause. Leak to ground downstream from the breaker.

 

[rlitman]

...I found my short, fixed it and now all is well. But why would the GFCI breaker not reset when everything it feeds has been shutoff?

A neutral-ground connection can trip a GFCI, even when the hot has no power. All you need is a tiny difference in ground voltage between the main and sub-panels (this is normal) for enough N-G current to trip a GFCI.

 

[LOW1]

Did the GFCI trip or did the breaker trip?

If it's not resetting, either the Breaker is bad or your fault is in the line feeding the boathouse. Disconnect the load side of the breaker, does it reset?

The breaker in the main panel tripped.

I found the short. It was downstream from the subpanel.

 

[wyliesdiesels]

remove the wires from the feeder breaker and see if you can reset it. if you can reset it, then shut it off, connect the feeder wires, then try to turn it on. if it trips then you have an issue on your feeder.

 

[PCustoms]

The breaker in the main panel tripped.

I found the short. It was downstream from the subpanel.

So it works now?

If not, do as @wyliesdiesels and I suggested.

If it resets you have a short downstream. You can then connect the wires in the main, and disconnect them at the sub (cap the bare wires) to see if it's in that feed or downstream of the sub.

 

[justsam]

A neutral-ground connection can trip a GFCI, even when the hot has no power. All you need is a tiny difference in ground voltage between the main and sub-panels (this is normal) for enough N-G current to trip a GFCI.

Not sure I understand the mechanism for this to occur on a non energized circuit. GFCI is not looking for a delta in N-G current but rather a delta in hot to neutral current. Certainly a N-G connection would indeed cause a GFCI to trip on an energized circuit as a difference is created.

 

[rlitman]

Not sure I understand the mechanism for this to occur on a non energized circuit. GFCI is not looking for a delta in N-G current but rather a delta in hot to neutral current. Certainly a N-G connection would indeed cause a GFCI to trip on an energized circuit as a difference is created.

If the hot is turned off, it may have zero current flowing through it, but all it takes is 5mA of current flowing through the neutral now to trip a GFCI. And all you need for this to happen is some potential between the ground at the connection downstream of the GFCI, and the neutral-ground bond in the main panel. A volt or two of potential is completely normal (start looking at N-G readings with a DMM), and even galvanic corrosion of the wires acting like a battery can have enough energy to cause the trip.