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Hi, I'm Mike G and I'll be assisting you. I suggest disconnecting the photo cell to see if the circuit still trips. Replacing the cell tells me there was a problem previously. Let me know what happens.
To trip the breaker I suspect that you have a bad ballast. Open the circuit Midway to eliminate the good ones. Continue breaking the power until you find the bad one. If they are not electronic, you may see a tar like substance leaking. That would be the bad one. In many commercial installations an inline fuse is wired into the fixture to avoid troubleshooting that many fixtures.
Very well, I'll ipt out.
Each one of the fixtures has a ballast(transformer) inside. Disconnect each fixtures one at a time until you find the defective one. By splitting the circuit in half you maybe able to lessen your troubleshooting time unless there is more than one bad ballast. Some of the older ballast have tar in them that will smell when they go bad.