I've mentioned some of the trouble with the "whole-house" main RCD/GFI employed in many homes here, so I thought I'd give you an example to think about from a nearby house. May be a long story, so get comfortable.....

This is a typical U.K. installation for a rural area: Neutral & ground separate at service entrance, grounding to a local rod only, and a 100mA main GFI.

The main panel fuses are 30A range, two 30A rings for general recepts., 15A water heater, & two 5A circuits for lights.

First, they found the GFI tripped every time they turned on the electric kettle. Nothing surprising there, and obviously led me to suspect a faulty element leaking to ground. However, when I tried it there was no problem. I put my megger on the kettle (500V DC is our standard test voltage) and got over 200 megohms.

I started asking if anything else had been on when the breaker kept tripping. After some head-scratching, we realized the water heater had been on before, but wasn't now.

I flicked it on, and nothing tripped. Undeterred, I disconnected the heat-resistant cord to the element & did an insulation test on the heating element anyway. It was an old element, so I was expecting quite a bit of leakage. Result was just over 400K. Certainly not low enough to trip a 100mA GFI, although these elements have a nasty trick of shorting out once hot, so I left it on to warm up while we all went to get a hot drink.

As the kettle was switched on again, off went the GFI.

To cut a long story short (whaddya mean "Too late"?!), I found that with either the water heater or the kettle on alone, no problem. Put both on together, and out went the breaker immediately.

By the way: the water heater is 3kW, the kettle about 2.4kW, and on different circuits remember.

Further experimentation revealed that the GFI also tripped with the kettle and a couple of rings on the stove on. With the kettle unplugged it also tripped when trying to run the stove and the water heater together.

At this point I realized that the kitchen had just been remodeled (yeah, I should've noticed earlier, but it must have been a bad day!) and headed for the main panel armed with screwdriver & megger.

I'll let you all puzzle over the strange foreign wiring a while before I finish the story: What was going on?