> Can I presume that a GF(C)I (or whatever you call them there-- 
> Residual Current Device?) is not fitted to the water heater?

Only a resettable 16A circuit breaker in the fuse box

> If the insulation is damaged, the 'hot' wire will conduct electricity
> to the ground on your hot water heater. It will bubble like crazy

The electrician did ask if power bills had been unusually high lately

So any defect in the insulation will make a hot spot that spreads
as metal is eaten

Eventually, I suppose then, enough of the wire conducts to trip the
circuit breaker. AIUI water is a poor conductor compared with
wire. So if the portion of the element closest to the active terminal
(where the potential difference is greatest and where the corrosion
starts) was more electrically insulated from the water this might delay
corrosion

Also, for a few days before the element finally opened, I'd noticed
what I thought were air locks in the hot water pipe. The HW taps
sputtered before good flow. I presume that this was due to the
bubbling you mention. If this is gaseous steam inside the tank it
might be a pressure or condensation effect at the taps


Thanks for all comments/explanations

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