" wrote:

The 2 events may be unconnected. The MCB on your immersion may have
tripped some time ago - and went unnoticed until some time later when
the whole house lost power and you checked the consumer unit.

Is there any other event that you could associate with your RCD
tripping?

(I'm assuming you've got a whole house RCD - is that correct?)

Thanks for those prompt and helpful replies. I'm responding to the
requests for more info:

The unit is a modern Crabtree 'modular' control unit, and carries a
label showing it has a sensitivity of 30mA.

It has two sections, and I made the following summary from the
electrician's hand-written notes when he installed it a year or so
ago:

CIRCUIT BREAKERS:
-----------------
1. Upstairs lights: 2 shaver-sockets in bathrooms; outside security
light (from loft extension)

2. Downstairs lights: hall, kitchen ceiling & under cupboard,
downstairs toilet (and fan), cupboard under stairs (and alarm)

3. Downstairs lights: lounge, dining room

4. Transformer by board (This is an add-on of my own, as part of a
gadget to detect when the front door bell was pressed, to buzz a
garden extension. Not a possible culprit IMO.)

5. Spare

RCD PROTECTED CIRCUITS:
-----------------------

1. Immersion heater. (This is the one that tripped.)

2. Sockets (radial): My office. Too many items to list here.

3. Sockets (ring): Downstairs except kitchen (5); Garage (1); Upstairs
except main bedroom (6)

4. Sockets (ring): Main bedroom including spurs (1): Hall (1); Kitchen
(6), Utility (3), upstairs bedroom

5. Cooker and 1 socket by cooker switch.

I also have two other RCD units
- In garage mains socket, protecting garden and workshop circuits
- In lounge mains socket, protecting garden pond submersed pump

After the failure I thought *all* power was lost. But now I can't be
100% sure whether I had the lights on, and I don't recall checking
them. Certainly things like fridge, microwave, etc were off.

The only switch I reset in the Crabtree unit was the RCD one
controlling the immersion heater.

I haven't unscrewed the immersion heater wall switch yet, so can't
confirm whether it's single or double pole. But doesn't the fact that
this RCD was tripped mean that it *must* be single pole, with a leak?

Hope this helps further diagnosis.

I'm going to have to do some more careful reading, starting with the
link John suggested, as I don't really understand the basics, and
hence some of the advice given. Meanwhile, beginning to regret
switching from my old fuse-based unit a year or so ago! At least I
understood that ;-)

--
Terry, West Sussex, UK