Molded Case Breakers, how many trips are too many? Jim D?

[zog]

I don't see it that way. Let's say you have a 480V 400A MCCB with a 30kA AIC.

When UL refers to a rated AIC fault they are talking about something close to that 30kA as a fault that it is designed and tested to interupt twice.

When you do the INST trip test you are appling a controlled current right at the INST pick up. So for that 400A breaker you are only injecting something in the range of 4x-10x of the frame size. 1600A-4000A for that 400A breaker. Nowhere near the AIC rating. And, the test voltage is only a few volts, not 480V, so even less energy being interupted.

1600A-4000A at around 5V is not the same as 30,000A at 480V as this breaker could possible see during an inservice short circuit event.

 

[rbalex]

I don't see it that way. Let's say you have a 480V 400A MCCB with a 30kA AIC.

When UL refers to a rated AIC fault they are talking about something close to that 30kA as a fault that it is designed and tested to interupt twice.

When you do the INST trip test you are appling a controlled current right at the INST pick up. So for that 400A breaker you are only injecting something in the range of 4x-10x of the frame size. 1600A-4000A for that 400A breaker. Nowhere near the AIC rating. And, the test voltage is only a few volts, not 480V, so even less energy being interupted.

1600A-4000A at around 5V is not the same as 30,000A at 480V as this breaker could possible see during an inservice short circuit event.

If you are referring to the latest (2009) AB 4, I agree with you; NEMA has definitely toned down the ?inspection? requirements. However, they are still basically telling you to take the breaker out of service, dismounting it if possible, to do all the recommended inspection steps. There still aren?t any instructions to simply ?reset? and move on.

I realize the cost of the breakers is often prohibitive to have a bunch of spares; again however, in my primary ?continuous process? background, it is usually far less expensive to simply ?replace? and move on ? after you have determined what caused the fault in the first place, of course.

BTW the test voltage is pretty much irrelevant, only the current interrupted matters ? it?s an i^2t effect.

 

[zog]

[
BTW the test voltage is pretty much irrelevant, only the current interrupted matters ? it?s an i^2t effect.

I have witnessed air breaker tests at KEMA at 480V at different fault levels and done thousands of primary injection tests at the same currents but reduced voltages and they are very different. The energy in the arcs being interupted is not comparable so the damage to the contact structure will be different, depending on voltage.