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Old 07-14-2014, 12:10 PM   #1
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Odd problem after service connect


I had upgraded my electrical service and during the connect something strange happened. The install was permitted and inspected and I also had an electrician take a look at it before the hookup

There is a 400/320a meter pan outside with two 200a single breaker panels (Eaton) outside, from there it runs through the wall to two 40 space indoor 200a main breaker panels (Square D QO). I had shut the two outdoor breakers off while the Poco was hooking up the new service but all of the interior breakers were left on. When I flipped the two breakers on after the hookup the following happened:

1. One indoor panel had a 30a double breaker dedicated to a dryer receptical, dryer was not plugged in. This breaker burnt out. I was using it earlier that day without issue.
2. The other indoor panel a 30a double breaker run to an AC condensor tripped and then burnt out, then about 5 seconds later the main 200a breaker on the indoor panel tripped. This panel had a 100a breaker running to a sub panel and the 30a breaker, nothing else.

Oddly, both 30a breakers that burnt out were installed on the upper left corner. I replaced the breakers and tried to replicate the problem again but could not get it to repeat. Also, I cannot figure out how the 200a breaker on the indoor panel tripped with only 130a worth of breakers running off of it. I had another electrican come out and look over the setup after this incident and he could find nothing wrong.

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Old 07-14-2014, 05:07 PM   #2
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Odd problem after service connect


At some point one of the hots was not making good contact
this allowed back feeding thru the 240v outlets
this would blow out the breakers on these outlets.

Possibbly one of your main breakers was not seating right.


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Old 07-14-2014, 05:18 PM   #3
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Odd problem after service connect


I will go through and double check the connections again.

For the dryer outlet though, nothing was plugged into it. Just an empty outlet and 2' worth of wire. Why would that breaker have burned out?
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Old 07-15-2014, 12:20 AM   #4
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Odd problem after service connect


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Originally Posted by ccbs1 View Post
I cannot figure out how the 200a breaker on the indoor panel tripped with only 130a worth of breakers running off of it.

This is well known thing to electricians !

Basically there is a lot more current available on the main line side
of your main breaker, so in a very breif event. the main will usually trip first, it is beleived that by the time the fault current has passed thru the mainy joints and switch gear, enough resistance (be it ever so small) is
introduced to slow things down. so the sub main breakers dont have time to react.
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Old 07-15-2014, 07:24 AM   #5
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Odd problem after service connect


There is some tolerance as to how quickly a breaker will trip and also the behavior can vary depending on whether there is an immediate massive overcurrent (such as in a short circuit situation) or a gradually increasing load as in turning on more and more appliances.

In your case the main breaker happened to trip first.

In addition they make breakers (usually not for use as main breakers) that have an extended time before tripping, for motorized circuits such as air conditioners where the starting current is somewhat greater than running current.

A breaker burnout usually means a loose connection where it snaps onto the fin behind it in the panel.

However, with one exception, a breaker will not trip let alone burn out if there is no load (no appliances or lights plugged in and turned on). If the breaker is burning out for no good reason, you probably have a short circuit (maybe a nail penetrating a cable and hitting two wires inside).

The exception is an extreme overvoltage such as from lightning, which can jump through the air between metal parts producing an actual current flow as if there were a continuous metal short circuit.

Backfeeding through breakers in itself won't trip or burn out the breakers. Again there would have to be an overload or a short circuit, the latter possibly due to an incorrect hookup.

Have you measured the voltages between various points including between breaker terminal and neutral and between breaker terminal and ground?
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Last edited by AllanJ; 07-15-2014 at 07:53 AM.
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Old 07-15-2014, 07:25 PM   #6
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Odd problem after service connect


I have a question. How were you using the outlet/breaker on new panel before the utility switched your service over? Fed off old panel?

Last edited by clw1963; 07-15-2014 at 07:41 PM.
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Old 07-15-2014, 07:44 PM   #7
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Odd problem after service connect


Sorry, and were the breakers that burnt up both fed with same phase leg?


Last edited by clw1963; 07-15-2014 at 07:47 PM.
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