Fire-ants are a protected species in Oz? !!!
Had this 'delayed trip' response on a central-heating boiler located in its own boiler room adjacent to the house. The delays in tripping varied from a few seconds to several minutes. Tried everything from new boiler stats, isolation switches etc, spraying the boiler electrics with damp proofer aerosol, ripping out the wiring, the lot. Eventually solved the problem. It was a faulty RCD unit. I'm not saying this is the cause here, but try a new gfci before digging up the cable.
Alan
[This message has been edited by Alan Belson (edited 03-07-2006).]
