Re: Circuit breaker trip puzzle

"Terry Pinnell" <terrypinDELETE@xxxxxxxxxxxxxxxxxxx> wrote in message
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

Well, that's certainly strange to me, and like you, not what I would have
expected. I suspect that you are now getting beyond my level of expertise
with this, so if I catch up with him in the next day or two, I will ask my
next door neighbour, who is a fully UK qualified electrician, and talks
about this sort of stuff all the time.

Terry, West Sussex, UK