ES550 tripping mains circuit breaker

[Anonymous user]

I had a new mains consumer unit put in yesterday after years of living in the darks ages with old wire fuses. The electrician noticed that the live to neutral (or earth) resistance on one of the domestic circuits was quite low (about 1.1 Megohm) and warned this could potentially trip a circuit breaker.  We tracked down the problem and it was my APC ES550 (BE550-UK model purchased in January 2011).  Sure enough last night I turned on a bedside light with an energy saver bulb which is on the same circuit and a mains circuit breaker tripped. I'm guessing the APC is contributing to the sensitivity of the MCB, though i could be wrong on this. It seems unlikely an energy-saver CFL bulb on its own could trip the circuit.

Could it be a degrading battery that's causing the low-ish live-neutral/live-earth resistance? The APC has been plugged in since Jan 2011 which is 32 months. I realise it might be getting close to battery replacement time anyway but don't really want to replace it it's not going to fix the issue. Is this a common problem?  I'm fairly concerned that it could be causing potential safety issues on the mains circuit...

Thanks!

[BillP · Accepted solution]

How did you track this down to the UPS initially? What else is plugged into the UPS? A highly resistive circuit indicates there as a lot of load connected to it so it doesn't sound like the UPS itself but maybe another piece of equipment connected to it possibly.