Series Rating Testing

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ajb5102

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When series rating testing, a main breaker (65 kAIC) upstream from a branch breaker (10kAIC), the circuit is calibrated for the 65k AFC prior to the test beginning. After set-up, and when the fault is initiated, will the branch breaker interrupt all 65kA; or is the AFC (at the moment of interrupt, at the location of the branch) reduced dramatically by the impedance of the breakers and conductors (from the power supply to the main, and from the main to the branch, and back to the power supply)?
 
The amount of current interrupted by the downstream device is not reported as it is immaterial to the series rating.
But, it is not uncommon for a downstream device to finish clearing the fault before the upstream device begins to open.

As soon arcing occurs the fault current can fall off significantly.
 
It makes sense that the branch opens first, as the fault is right at the branch, and the added impedance to the circuit due to the inclusion of the conductors and breakers, causes the AFC to be less than 65k at the branch (... I have thought to myself - the AFC at the branch must be less than 10kA, otherwise the branch would fail to successfully interrupt)
 
ajb5102 said:
It makes sense that the branch opens first, as the fault is right at the branch, and the added impedance to the circuit due to the inclusion of the conductors and breakers, causes the AFC to be less than 65k at the branch (... I have thought to myself - the AFC at the branch must be less than 10kA, otherwise the branch would fail to successfully interrupt)
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Just because a device is listed at a specific AIC by UL does not mean it will immediately fail spectacularly at a higher value. For example, UL might list at 10kA and at 22kA but if a breaker could actually safely interrupt 21kA it would be very easy to get a 22/10kAIC series rating.
 
David Castor said:
I'm sure Jim is correct, but I'd never assume the downstream device will trip first. Also, it's quite possible for both breakers to trip.
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Thank you. Not assuming the downstream will trip first. Just that it can happen. Ideally, the upstream trips too, so it's not simply a block of copper creating impedance, and the branch doesn't do all the work.
 
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