John Rumm wrote:

Terry Pinnell wrote:

Also, I realise that "the RCD controls electricity to all the MCBs on
the RCD side of the unit. It does not just control the immersion
heater." (That of course is why everything I mentioned lost power.)
But think I was correct in my recollection that "The only switch I
reset in the Crabtree unit was the RCD one."

Still thing there's some confusion here but if you only reset the
'RCD' controlling the immersion heater then something is wrong
internally to the CU.

Can you amplify on that please? Why would it imply that?

If you meant the main RCD had tripped

No, just the immersion switch on the RCD side, the one I labeled #1 in
the photo I posted.

So, let's get this straight:

The MCB in position 1 on your CU (just that and nothing else) tripped.
In doing so it cut power not only to the immersion heater, but also to
your computer and some other appliances?

To restore normal operation you reset that single MCB for the immersion
heater, and did not need to touch the RCD itself?

If this is the case then you have a serious wiring error that needs
immediate attention.

Although that's exactly what I recall happened, I'm now beginning to
doubt it even myself! I could swear I didn't have to reset the RCD
switch itself as well. But, as you and others have said, that doesn't
seem to make sense.

The following may be unrelated, but I'd appreciate it if you and
others would consider the following additional information to see if
it offers any new clues.

Earlier this morning I decided to do some methodical leakage tests. My
intention was to introduce a leakage from the live connection of the
circuit under test to a convenient earth, and observe what tripped.
That would at least confirm that I understood what behaviour I was
*supposed* to get from my Crabtree StarBreaker CU. Maybe it would also
reproduce that bizarre behaviour I described. I used a couple of
robust 15k resistors in parallel, theoretically giving me a leakage
around 32 mA (in my UK 240 V circuits).

I decided to start with a circuit which also had an extra RCD unit
over and above the main CU. This is a double socket RCD in the garage,
fed from #3 on the Main Switch (non-RCD) side of the CU. These power
all sorts of stuff in my garage and shed/workshop. As expected,
momentarily connecting this leakage current immediately tripped the
garage RCD. However, to my surprise, maybe 1-2 seconds later (with the
leakage removed), the *main* switch on the CU was tripped. So my PC
and various household devices and clocks went down anyway, despite my
intention to avoid that ;-)

I'd not have expected this to happen, so could someone offer a
possible explanation please? Does it offer more insight into the odd
behaviour already reported? Or totally unrelated?

In case it helps, the electrician's scrawled installation notes record
trip times of 18/19 ms for all the RCD circuits, and 7 ms for the
external garage RCD unit. There are various other columns like 'Loop
Impedance', 'CPC-CPC ohms', 'Ph-Ph ohms', 'R1+R2 or R2 ohms'; they
mean nothing to me but please let me know if they could help the
diagnosis.

---------

Pondering this over a coffee just before hitting Send, I'm wondering
if perhaps some sort of inductive effect might be responsible? Amongst
other things, I have an ancient large DC power supply in my
shed/workshop (30A), permanently connected to provide automatic garden
lighting at dusk. Could the sudden removal of power have had the
effect observed? If so, my starting choice was poor, and I'll try
another circuit. Or maybe disconnect the two plugs from the garage RCD
unit.

--
Terry, West Sussex, UK