Terry Pinnell wrote:

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.)

Just this one, or the RCD as well? If you lost power elsewhere then it
must have been the RCD as well.

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

As long as these are not fed from an exiting RCD protected circuit then
you can eliminate these from consideration for this problem.

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.

This bit does not make sense.... You said above the MCB for the
immersion opened. But your description suggests the RCD should have
tripped as well. However you only reset the RCD - presumably leaving the
immersion off at the MCB. Is this right?

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?

Not necessarily. If you read the bits on RCD sensitisation in the link I
gave before, you can see that big transient loads switching in and out
can trip a RCD if it is already close to its tripping point.

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 ;-)

Well on the plus side, this one has a better chance of keeping you alive
when something goes wrong in a big way ;-)

Shifting the immersion over to the non RCD side would also be a good
move. It may not cure your problem, but it will stop the immersion
contributing to it.


--
Cheers,

John.

/================================================== ===============\
| Internode Ltd - http://www.internode.co.uk |
|-----------------------------------------------------------------|
| John Rumm - john(at)internode(dot)co(dot)uk |
\================================================= ================/