In article 7EaPe.9270$Vg7.2812@trnddc06, says...
I don't know about your tank but my electrical connections tended to corrode
over a few year's time. You say you have had your tank up for 5 years.
Check ALL of the electrical connections for corrosion. Corrosion can cause
"leakage current" which is what will set your GFCI into FREAK out mode and
trip the circuit breaker. It may be working PERFECTLY.

ALSO, make sure there is no moisture buildup anywhere in the electrical
system. Afterall we are talking about a high humidity system that is rather
close to water. Moisture buildup could cause a current LEAK somewhere and
BAM! The GFCI is again working perfectly.

You may find that the GFCI is indeed week, however I'd check the other above
mentioned things as well.

YMMV,

DJay


Thanks DJay-

So far, I've dissassembled all the electrical connetions, made sure they
are clean, and reassembled. Inside the transformer box, the two MH
systems are connected through hardwired connections so unless I want to
unsolder some connections I can't test them individually.

Meanwhile I've also replaced the GFCI outlet itself (because this was an
easy thing to do, relatively), and removed from the circuit a UPS system
supplying backup power to the filter. The UPS lead-acid battery is
shot, and doesn't hold a charge anymore, so it was just taking up room.
I had a concern that the poor state of the battery might have been
introducing some sort of ground leak to the circuit, making it more
sensitive to issues when the MH bulbs fire.

I also changed the older of the two MH bulbs.

Since doing these things, I've had good operation for two days, but
since the problem was intermittent, I'm not sure I've sorted it out yet.
Time will tell! The next thing to do would be replace the caps in the
MH transformer box, but that would cost ~$60 so I'd rather hold off
until I know it's necessary.

Marc