AIC rating on output of a double conversion UPS

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mshields

mshields

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If the fault current at the input of a UPS is, for example 50kAIC, what is it on the output, neglecting the maintenance bypass. I realize you can't neglect the maintenance bypass in reality and that the available fault current at the output distribution would need to be based on that worst case scenario, but what would happen if you had a bolted fault supplied only by the UPS?
 
It would be limited by the maximum output rating of the UPS, usually a (relatively) small percentage increase for a few seconds before the UPS would clamp it or shut down. Common is 200-300% for 3 seconds, so nowhere NEAR the Available Fault Current (AFC) on the line side (unless you had a UPS capable of 16,000A continuous output).

But again, if there is a bypass (and there usually is) then that is irrelevant. Bypasses on UPS are not always just for "maintenance" either, they are quite often used for when the LOAD requires more current than the UPS is capable of delivering, so it is basically available all of the time.

And just to be accurate, a UPS would not have an "AIC" rating. AIC is "Amperes Interrupting Capacity", a term used for protective devices like circuit breakers and fuses. A UPS would have a "withstand" capability, or now maybe an SCCR (Short Circuit Current Rating), indicating it's ability to SURVIVE the AFC until the interrupting device clears the fault.
 
Understood about the withstand vs interrupting. When you say bypass, are you referring to the static bypass if there is a fault downstream of the UPS, would the UPS automatically go to static bypass and in so doing essentially become a zero impedance path for the fault current?
 
There are online and standby UPS's, and some that can work both ways depending on how you set them up. A standby UPS only inverts the DC power to AC when the utility power is off, the rest of the time there is a bypass from input to output of the UPS.
 
If the current is going through the semiconductors, IGBTs or SCRs, into a bolted uncontrolled fault, they will blow open or apart like a glass plate hitting the floor. Which is why you find them paired with expensive fast fuses. They are about as fragile as the fuse, just much more costly.

When the IGBT is controlled, they commonly have a fault clearing mode and a spec I have seen for large Lieberts was 200% for two or three seconds like Jraef said. It will try to deliver or clear the fault, but only for a short time, and the semiconductors are natively current limiting, up to the failure point. There is a low limit on availability of charge carriers in the semiconductor.

The static bypass could be SRCs, semiconductors, to close in parallel in synch. Then the hard switch. the maintenance bypass, closes in parallel, to take everything else out and isolated. The maintenance bypass is a big breaker usually and will pass typical fault current before it opens. The maintenance bypass will not blow apart in pieces like the output IGBTs will.
 
A downstream fault would be sensed by the double conversion UPS and it would transfer to static bypass to try to assist in clearing the fault and you end up with the worst case fault current on the output, which is via the static bypass.
 
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