Unusual Ground Fault

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2Broke2Sleep

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Florida
*2nd attempt at writing this, other computer froze so if it posts twice I apologize.

Working at a supermarket the other day and encountered something that never happened to me before. My coworker and I had installed a new single phase 20a circuit for future lighting which we left capped off at a junction box. I landed the wires, closed up the panelboard and when I threw the single pole 20 CB I heard *BOOM*. Not your usual 'poof' but a very loud boom. I walk from the panelboard to the electric room where my coworker was and he immediately told me the main CB had tripped (200A) killing the entire panel.

Now back at the panelboard I open her up and discover the deadfront had wedged the new ungrounded conductor in between the bracket and itself creating the fault. But why the main CB?? I contemplated it for a week or so and it dawned on me: the new ungrounded conductor inside the panelboard was left long and wrapped in a loop so we could move it to a new position in the near future. Could this have had something to do with the main cb opening??? I'm still kind of stumped. The service is 208Y if that matters to anyone.

Oh, and in case you're wondering we turn on all the CB before we leave because we get callbacks sometimes for having CBs off. :happyyes:
 
Were the main / feeders circuit breakers set up properly? I cannot tell you how many times I see an electrical sub pay thousands of dollars for a coordination study only to leave all of the breakers at their factory settings and never even wonder what all those little dials and dip switches are for.

May or may not have anything to do with your situation but I see it all the time.
 
2Broke2Sleep said:
*2nd attempt at writing this, other computer froze so if it posts twice I apologize.

Working at a supermarket the other day and encountered something that never happened to me before. My coworker and I had installed a new single phase 20a circuit for future lighting which we left capped off at a junction box. I landed the wires, closed up the panelboard and when I threw the single pole 20 CB I heard *BOOM*. Not your usual 'poof' but a very loud boom. I walk from the panelboard to the electric room where my coworker was and he immediately told me the main CB had tripped (200A) killing the entire panel.

Now back at the panelboard I open her up and discover the deadfront had wedged the new ungrounded conductor in between the bracket and itself creating the fault. But why the main CB?? I contemplated it for a week or so and it dawned on me: the new ungrounded conductor inside the panelboard was left long and wrapped in a loop so we could move it to a new position in the near future. Could this have had something to do with the main cb opening??? I'm still kind of stumped. The service is 208Y if that matters to anyone.

Oh, and in case you're wondering we turn on all the CB before we leave because we get callbacks sometimes for having CBs off. :happyyes:
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The fault current was high enough that it was in the range of tripping for the main. It just happened to clear before the BC. Happens often enough although it can be somewhat frustrating.
 
The fault current reached the trip curve of the main breaker.

ptonsparky said:
The fault current was high enough that it was in the range of tripping for the main. It just happened to clear before the BC. Happens often enough although it can be somewhat frustrating.
Click to expand...

Ninja'd!

eta: OP, the length of the wire did contribute, but only insofar as to get caught between the deadfront and panel and create basically a bolted fault within a foot or so of the branch circuit circuit breaker.
 
luckylerado said:
Were the main / feeders circuit breakers set up properly? I cannot tell you how many times I see an electrical sub pay thousands of dollars for a coordination study only to leave all of the breakers at their factory settings and never even wonder what all those little dials and dip switches are for.

May or may not have anything to do with your situation but I see it all the time.
Click to expand...

That is a good point, but I don't have an answer.
 
luckylerado said:
Were the main / feeders circuit breakers set up properly? I cannot tell you how many times I see an electrical sub pay thousands of dollars for a coordination study only to leave all of the breakers at their factory settings and never even wonder what all those little dials and dip switches are for.

May or may not have anything to do with your situation but I see it all the time.
Click to expand...
200 amp 208/240 volt equipment not so likely to have any adjustments at all.

2Broke2Sleep, as Tom said, it is how much fault current flowed.

If you mostly work residential work you may not see this happen all that often if ever.

In this case you possibly had a higher available fault current then you are used to seeing, short distance to the source helps keep that level higher, then on top of that your fault was right at the panelboard. Had it been 50 feet away from the panel on 12 AWG conductor, the resistance of the branch circuit conductor alone would likely have limited the fault current enough that the main never sees enough to get into it's instantaneous trip range.
 
JFletcher said:
The fault current reached the trip curve of the main breaker.



Ninja'd!

eta: OP, the length of the wire did contribute, but only insofar as to get caught between the deadfront and panel and create basically a bolted fault within a foot or so of the branch circuit circuit breaker.
Click to expand...

I thought the loop may have created some sort of high impedance condition that was over my head.
 
2Broke2Sleep said:
I thought the loop may have created some sort of high impedance condition that was over my head.
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If it created a high impedance it would have lessened the amount of current in the fault. Placing a steel core (a nail or bolt would be sufficient) within the loop would likely have made it a much more effective "inductor" and would possibly limit current enough to not trip the main.
 
luckylerado said:
Were the main / feeders circuit breakers set up properly? I cannot tell you how many times I see an electrical sub pay thousands of dollars for a coordination study only to leave all of the breakers at their factory settings and never even wonder what all those little dials and dip switches are for.

May or may not have anything to do with your situation but I see it all the time.
Click to expand...
It is almost impossible to have selective coordination where you are using standard molded case breakers with a rating 400 amps or less and there is a high available fault current. The instantaneous trip bands over lap and either or both can trip where you have this condition.
 
don_resqcapt19 said:
It is almost impossible to have selective coordination where you are using standard molded case breakers with a rating 400 amps or less and there is a high available fault current. The instantaneous trip bands over lap and either or both can trip where you have this condition.
Click to expand...

I am curious to know if the instantaneous dial setting on the main is set on min.
 
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