Another neutral-earth fault???

[multimapper]

But to trip the MCB there has to be an overload, The MCB trips when you turn the immersion on. Are you sure that nothing was done by the electrician to any of the wiring associated with the immersion. Did he change or inspect or tighten terminals in the switch for the immersion heater or replace any cables.

Does the flexible cable from the heater go direct to the switch or does it go to an outlet next to the tank and the switch is located elsewhere ?

Could a screw or nail have been driven through a cable about the same time the CU was being changed ?

No, I was there all the time and he didn't do anything to the wiring except to disconnect the supply to the faulty element after the fault occurred.

As the element definitely seems to be faulty (in another post just now I've compared it with other elements) I am now convinced that the fault is solely in the element. What I can't understand is why the MCB breaks unless when mains voltage is applied to the element something breaks down and conducts heavily.

Thanks

 

[bernardgreen]

It is quite likely that chemical actions inside the element will create some complex " semi conductor " ( zener diode ) type effects that break down to a short circuit when 230 volts is applied to the element.

But I would still inspect the flexible cable along its length for damage in case there is an internal short in the cable that dis-appeared when the cable was disconnected and flexed.

 

[multimapper]

But I would still inspect the flexible cable along its length for damage in case there is an internal short in the cable that dis-appeared when the cable was disconnected and flexed.

Thanks - have done, can't see anything and it's only about 50cm long. It's never been touched from new, only I would have done so and I haven't. There definitely is something wrong with the off-peak element. I could test the off-peak wiring by connecting it to the boost element, but I think I'll only do that if there are still problems after changing the element.

Thanks again

 

[ericmark]

It is a cheap meter. The water has not been heated. I measured directly across the element terminals, not through the stat.

Then with such a high reading it must be US.

However there may be a second fault as it should not have tripped MCB it should only have tripped the RCD.

If you turn off immersion heater local to heater does the MCB still trip?
no, it doesn't

I would therefore say fault local to heater so not something electrician has done.

I will guess when you change the heater you will clear any second fault anyway so likely once changed all will be OK.