Why would a short circuit ruin GFCI breakers on different circuits

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KyleFowler

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Location
Maryville, TN
Occupation
Electrician
The other day on a jobsite someone ran a hanger self tapping screw into one of the two paralled lines for a 800 amp 480v feeder. Thank God it was off when they did it however it made for a nice bang when it was turned on, simultaneously tripping the 2000 amp main and blowing one of the utility transformer fuses, fun times. Anyway, after fixing that we discovered that all 40 of the Sq D QOB 120v GFCI breakers would no longer trip when using any GFCI plug testers. Installed new GFCI breakers and voila everything is good now. We tried busting a few of the breakers apart to see what happened with no smoked parts or anything weird to be seen. Still cool to see inside a GFCI breaker though lol.

So what happened? Obviously the big short caused it but it didn't blow out any lights or other stuff. Just curious. Thanks in advance
 

Joe.B

Senior Member
Location
Myrtletown Ca
Occupation
Building Inspector
I've heard that a big short like that can cause an EM pulse, maybe that would affect the little circuits that provide the ground fault protection. Would a large enough EM pulse cause inductance or coupling similar to a nearby lightning strike? Obviously much smaller but the effect could still be enough to fry the little circuitry I would think.
 

synchro

Senior Member
Location
Chicago, IL
Occupation
EE
Depending on the setup. ferroresonance may have occurred in the distribution transformer when the utility fuse blew. That could have then caused an overvoltage which took out the GFCI breakers.
 

hillbilly1

Senior Member
Location
North Georgia mountains
Occupation
Owner/electrical contractor
The other day on a jobsite someone ran a hanger self tapping screw into one of the two paralled lines for a 800 amp 480v feeder. Thank God it was off when they did it however it made for a nice bang when it was turned on, simultaneously tripping the 2000 amp main and blowing one of the utility transformer fuses, fun times. Anyway, after fixing that we discovered that all 40 of the Sq D QOB 120v GFCI breakers would no longer trip when using any GFCI plug testers. Installed new GFCI breakers and voila everything is good now. We tried busting a few of the breakers apart to see what happened with no smoked parts or anything weird to be seen. Still cool to see inside a GFCI breaker though lol.

So what happened? Obviously the big short caused it but it didn't blow out any lights or other stuff. Just curious. Thanks in advance
Test button wouldn’t work? Or one of those plug in testers wouldn’t trip the breaker? If the test button worked, but the plug in tester wouldn’t, XO is not bonded in the transformer. Strange it started working though after changing the breakers, unless someone noticed the bond wasn’t there and bonded it.
 

retirede

Senior Member
Location
Illinois
Test button wouldn’t work? Or one of those plug in testers wouldn’t trip the breaker? If the test button worked, but the plug in tester wouldn’t, XO is not bonded in the transformer. Strange it started working though after changing the breakers, unless someone noticed the bond wasn’t there and bonded it.

Interesting point. Makes me wonder if the X0 bond was compromised by the fault incident and didn’t get discovered and repaired until the decision was made to change out the breakers?
 

hillbilly1

Senior Member
Location
North Georgia mountains
Occupation
Owner/electrical contractor
Interesting point. Makes me wonder if the X0 bond was compromised by the fault incident and didn’t get discovered and repaired until the decision was made to change out the breakers?
Or it was an issue that didn’t get discovered until after the incident? Seen many a transformer that didn’t get bonded work for years until another issue popped up. Also I’m making the assumption this is just one panel, and it is full of gfci breakers, which the op said no other loads were damaged if it was a surge. And if it was a surge, I would believe it would have been only on one phase, not all three, unless it did come through the neutral.
 

retirede

Senior Member
Location
Illinois
Or it was an issue that didn’t get discovered until after the incident? Seen many a transformer that didn’t get bonded work for years until another issue popped up. Also I’m making the assumption this is just one panel, and it is full of gfci breakers, which the op said no other loads were damaged if it was a surge. And if it was a surge, I would believe it would have been only on one phase, not all three, unless it did come through the neutral.

Which is why I asked if the plug-in tester was used prior to the event.
 

KyleFowler

Member
Location
Maryville, TN
Occupation
Electrician
It blew out GFCI breakers in multiple panels. I haven't worked on this jobsite alot however I have worked there enough to have personally seen one of the transformer XO terminals grounded previously. I'm sure they all got grounded.

I've also been on a service call recently where an XO wasn't ever grounded. Neutral to ground was 55 volts! Been like that for 5 years I was told.

Back to the subject, I really think that it was ferro resonance. The transformer was barely loaded with just a few lights compared to the 1000kva transformer outside and those GFCI's were previously tested I was told. I trust the lead electricians, they have 60+ years of combined experience. Both are close to retirement.

I researched ferro resonance and it really fits the bill on what happened. I assume that the lights were saved because they had surge protection built in.
 
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