Terry Pinnell wrote:

This puzzle now has another twist. 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. 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 an unimportant 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. 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 despite my cunning plan ;-)

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?

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 shortly after hitting Send, I realised
that I was wrong to imply the devices on this circuit were
insignificant. I left all the devices in place when I applied the
leakage. 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. I'm wondering if perhaps some sort of
inductive effect might be responsible? Could the sudden removal of
power from that and other lower-rated transformers 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