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Tripping 600A Main

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We went to a service call yesterday at an old tire shop. They lost utility power for about 8 hours a few days ago during some bad weather. Now, they are continually tripping their 600A main. Before their outage, they never had this problem.

After a little troubleshooting, we narrowed it down to an air compressor. The air compressor is turned on/off via a 100A disc. fused at 80 amps. This 100A disc. is fed from a 200A breaker in a subpanel, and the 600A main they are tripping located in an exterior panel. The 200A breaker is not tripping, nor are they blowing the 80A fuses.

Here's the other thing, the 600A main does not trip upon initial start up of the air compressor. It does trip however after the initial tank of air has bled out enough that the compressor kicks on to refill.

The starter on the air compressor seems to be brand new. Has the 600A main just gone bad?

Thanks for any help.
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