fault current/breaker design

[jes25]

I had a job the other day where a cable guy drilled through the 40A ac circuit. This created a L to L fault and it tripped the main rather than the AC breaker....Why?

On a different job I installed a sub panel for a generator and moved some circuits into the sub from the main panel. Both panels are ITE same type of breakers. The circuit on the new breaker now trips even though the old breaker was identical to the new one. Was the old breaker bad? Do breakers trip at slightly different amounts even if they are the same model # and size. BTW the cord and plug load never changed

 

[hurk27]

Re: fault current/breaker design

We have this happen with 100 amp services. If the 40 amp breaker has a higher AIC rating than the 100 amp main the main will trip first.
If the 40 amp breaker has a AIC of 20,000 amps and the 100 amp main only has a AIC of 10,000 amps then yes it would trip the main as it has a lower instantaneous trip point. What is superizing to many is many times they both can have the same AIC rateing and if the circuit is faulted it will trip both breakers. We ran into this when we tried to trip the A/C circuirt when the home owners were not home so we could install the A/C disconect and wire the A/C unit. Boy was that a mastake.
I'm not following the second question.

[ July 15, 2004, 12:51 AM: Message edited by: hurk27 ]

 

[don_resqcapt19]

Re: fault current/breaker design

Wayne,
I don't believe that the AIC has anything to do with the trip times. If you look at the time trip curves for most breakers under 150 amps, you will find that the time for trip under high short circuit and ground fault currents are all within the same range. These breakers cannot be cooridinated for these types of faults.
don

 

[charlie b]

Re: fault current/breaker design

I agree with Don. The AIC rating is a measure of how much fault current the breaker can withstand, before its contact melt with the heat, thereby welding themselves shut, and preventing the breaker from ever opening again.

Clearly, it is desirable to have the breaker closest to the fault trip, and the upstream breakers remain closed, so that a minimal portion of the system loses power. But that is not an easy thing to accomplish, especially for the typical residential or commercial distribution system. You have to select downstream breakers for which the time-current characteristic curves show a very rapid trip, and upstream breakers for which the time-current characteristic curves show a delayed trip. It is a design issue that requires at least the cooperation of the breaker manufacturer, and probably requires the services of an engineer. You need to model the entire distribution system, including all cable sizes and lengths and including the parameters of the service transformer, in order to calculate the fault currents available at all points in the system.

As to your second question, there are a million things that could be different between the old and the new. I would first suggest that you re-inspect the work. Perform a test on the wires that were moved, or the new wires that were installed, to see if there are any shorts to ground. Look inside any junction boxes or outlet boxes that were involved in the change-over, to see if there is anything loose or any exposed wires. I think that 90% of the problems of this nature will be resolved this way.

Was the old breaker bad?

It is possible, though unlikely. You are suggesting that the short circuit or overload condition had existed all along, and that the old breaker did not detect the problem or simply failed to open. If that were true, it would have manifested itself some other way.

Do breakers trip at slightly different amounts even if they are the same model # and size?

Yes. No two things are exactly alike. But here you are suggesting that the actual current was very close to the trip point, that the old breaker decided the current was not enough to trip, and that the new breaker decided the current was enough to trip. Again, it is possible, though unlikely. But even if it were true, it means that there is a problem somewhere.

 

[templdl]

Re: fault current/breaker design

When there is a fault, a bolted fault in particular, it becomes a race as to which one will trip first. It is common for a main breaker in a residence to trip if there is a bolted fault is one of the branch circuits. If you think about it a fault consists of a very high magnitude of current the value of which is based upon the fault current available at the service entrance. If 10ka were available then it is possible that all breakers in series with that fault would see 10ka. If the instantaneous pick-up of the 40at breaker was about 400a and a 100a main was 1000a you would tend to expect the 40a breaker to trip first. However,since both are instantaneous trip, neither having a delay, you would expect that both see 10ka of fault and trip.
Because of the dynamics of the circuit it often is potluck, the one closest to the fault may trip, the main may trip, or both.
It may or may not be a factor but the main normally has more load than the branch involved so the main may have a slight edge magnetically.

 

[jes25]

Re: fault current/breaker design

Thanks for all the replys. It worked out to my advantage that the main tripped because all the pwer was out and I got to charge emergency rates