I've got a serious problem, potentially deadly, and I'm wondering where to start. Occasionally, say 8 times over the last year year, to no predicable pattern, my wall-mounted boiler sends what can best be described as a 240v lightning bolt (with a flash + a bang) down the wall to the floor. In my kitchen I have a Ferroli modena 102 combi boiler mounted high on the wall, with through-wall flue to the outside, and with visible wall-mounted copper feed and flow pipes to the floor. The boiler was new installed 6 years ago, to replace an older boiler, so much of the older wiring and pipe work was re-used because it was already in place. 99.9% of the time the boiler works great, controlled by the boiler mounted timer for central heating in mornings and evenings, to switch off at 23:00hrs. Now and again, randomly usually in the middle of the night when heating is timed off, there's is a visible arc of electricity from the boiler down the wall-mounted copper pipe, accompanied by a bang, and the fuse-box trips off. The boiler has it's own dedicated trip switch. I have been in the kitchen and seen this flash only twice, and it is very scary. But the incident has happened several times, as I hear the bang and find the fuse-box tripped more often. The boiler usually seems to be timed off when this happens, but maybe not always. After it happens, usually when I reset the trip, everything works perfectly again as if nothing has happened. There are no burn/scorch marks where the flash appears, and no smell of ozone or burning. Last time, when I resent the trip immediately after it happened, the trip fused/tripped again so the problem was still active. I waited a few minutes, and it reset no problem, and the boiler started up again as if nothing had happened. I've had two electricians (not boiler specialists) to look at it, and testing the house wiring and found nothing wrong. I've considered: condensation inside the boiler - but tonight it happened when 5C degrees outside, not so cold. Maybe it is mice eating the cables, but how come it resets ok back to normal - and why does it arc from the boiler down the water pipes? Maybe there is gas leakage and some spurious spark is igniting it, but no smell of gas, and that wouldn't trip the fuse. I need to fix this urgently. Can anyone suggest the problem, or how I can track what is happening by leaving voltage/current monitoring equipment or boiler diagnostic equipment connected to get data to work it out? Hopefully you experts will have experienced the same problem before? I hope so! Otherwise I'll have to rip out all the house wiring, and the plumbing, and the boiler, and replace all with new.