Consumer unit problem

Rat? Snake? Giant spider?

The main isolator was presumably not just a switch then, and was a RCD or similar...?

The smell would presumably be the clue. If it was a fishy /urine type smell then it would indicate a loose connection or other high resistance joint that was heating and causing the thermosetting plastic (probably urea formaldehyde) to breakdown and smell.

Alternatively it was something that had crawled into the CU and decided to exit this world as a part of a circuit forming significant earth leakage - frazzeled iguana anyone?

He lives next door to a prison where they execute people in an electric chair. They couldn't get enough juice for the process so decided to 'borrow' some of your son's. The smell, burning flesh of course. Do I win a prize?

Clearly an ex-animal in the CU (otherwise not worth posting probably :o) ).

Hmm, my bid is a snake of some sort.

Yes you're right the electrician fished a slightly cooked gecko (small lizard) out of the innards of the consumer unit.

I'ts surprising that such a small thing can trip a 100A breaker.

It probably tripped a 30mA or 100mA RCD.

So I was close with iguana then ;-)