Primary amps compared to secondary amps

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O

olly

Senior Member
Location
Berthoud, Colorado
Occupation
Master Electrician
We had a fault in a building I assume phase to phase I dont know any details. It ended up taking 2 fuses on the power companies transformer. It took A phase and B phase. I'm wondering why it didn't take the 200 amp building main fuses instead of the power company's 10 amp fuses. Power company voltage is 13200 phase to phase. secondary voltage is 208 3 phase. If the amperage is 10a at 13200 primary what is it at 208 secondary?
 
Generally in your facility, fusing and breaker fault clearing times should be coordinated so that the protective device CLOSEST to the fault is the one that clears, specifically to AVOID taking down the entire facility. One flaw that happens however is when the utility fails to provide THEIR fuse curves (or the correct ones) for the transformer primary to whomever is doing the coordination study. So that's how that probably happened; your facility likely had a coordination study that provided for the Main in your system to NOT clear in order to give a down stream device time to clear first. But unfortunately that was apparently not coordinated with the utility primary fuse clearing time, so they acted before anything else could. Someone should fix that situation now that you know.
 
Great answer thank you! What is the correct terminology as far as the fuse goes? The fuse needs the correct curve setting?
 
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