On 8 Jul, 17:39, John Rumm wrote:
Terry Pinnell wrote:
The house suddenly lost power a couple of days ago and on checking the
circuit breaker box I found the one controlling the immersion heater
had tripped. But the immersion heater is not switched on (it hardly
ever is), so how I'm puzzled how a leak could be detected by the
circuit breaker?

Reading between the lines, you have a RCD that acts as the main switch
on you consumer unit, plus a dedicated circuit for the immersion heater
protected by a (probably) 16A MCB. Does that sound about right?

It sounds as if something caused not only the MCB to open (i.e.
overcurrent or fault current) and also the RCD (leakage current).

lightning, or a widespread power failure, but the fridge and freezer
would be a major issue.

Indeed. Immersions are a known cause of RCD sensitisation, and hence
should not ideally be fed from a RCD protected circuit (TT supplies
excepted).

The immersion heater cable from the heater coil on top of the hot
water tank in the airing cupboard goes directly to the switch on the
wall, with no intervening mains plug/socket. So presumably, to get an
ohm reading on this unit to test for leakage, I have to first switch
off at the breaker box, remove the switch panel in the airing
cupboard, unscrew a connection, and work from there?

Any practical advice would be much appreciated please.

Firstly to get some background on how these things work and the possible
causes of trips etc, you can find most of what you need to know he

http://wiki.diyfaq.org.uk/index.php?title=RCD

See the section on nuisance trips in particular.

Your particular fault is a little interesting since the MCB opened as
well. This suggests we are not looking for just a high leakage problem
in isolation (in which case the RCD would have tripped but not the MCB).

It may be you do have high leakage and a sensitised RCD as well, but you
could have a faulty immersion as well. It may be the element in it is
both thinning and drawing excess current.

You immersion switch may only be a single pole device (hence not
preventing RCD trips caused by neutral to earth leakage).

If in doubt disconnect the wires to the immersion heater prior to your
holls!

As a longer term solution look at moving to a split load consumer unit.

--
Cheers,

John.

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There is always the possibility that someone has "tapped into" the
immersion cable somewhere out of sight. Did anything unexplainedly
stop working?