For some reason my newsfeed is being slow today, so this post will be
threaded in the wrong place as I have just grabbed the text from google
and pasted a reply somewhere close to the right place)
Terry Pinnell wrote:
I trust that the socket you were testing is not fed from the lighting
circuit, and so is in fact powered from the right hand #3?
Yes, sorry, you're right, my mistake - and I'm glad you're being more
careful with your wording than I am! The garage is fed from #3 on the
*RCD* side. [ As per my earlier full listing yesterday, which I got
right that time ;-) ]
Good, on the right tracks then.
OK, but this is where I'm still obviously being dense. Given a normal,
properly working setup, can you please summarise what *should* happen
and which switches in the main CU should open for each of the
following events:
1. A device on the LH side (non RCD) develops a short.
My assumption: it opens that MCB and the main LH switch.)
As per your follow up post, the MCB would open, the main switch will
stay put.
2. Leakage to earth (say 30 mA) occurs in one of those non-RCD
circuits.
My assumption: nothing obvious happens.
Correct.
3. Leakage to earth occurs (say 30 mA) in an RCD circuit, such as if I
accidentally touched a live wire while standing on damp ground.
My assumption: I'd jump a bit but within a few ms the main RCD would
trip, so all power to *all* RCD protected circuits would be lost, the
Correct.
individual MCB for that circuit would open, so I could identify it,
Alas no. A MCB will only open on an overcurrent. This needs to be either
a "fault current" or an "overload current".
See for an explanation:
http://wiki.diyfaq.org.uk/index.php?...ault_Cu rrent
but power would remain on for the rest of the non-RCD circuits.
Correct. The big advantage here si you are not plunged into darkness
should you get a trip. Historically they used to use a whole house RCD
which would result in just this effect. What they found was it reduced
electric shock injury, but then *caused* more tripping and falling injuries.
4. A device on the RH side (RCD) develops a short.
My assumption: it opens that MCB and the main RCD switch, but power
would remain on for the rest of the non-RCD circuits.
Depends. If the short is Live to Neutral then the MCB will open, but not
the RCD. If it was live to earth then chances are one or both would open.
The fact that the MCB opened would suggest that the immersion is faulty
or there is a wiring error / damage on its circuit. These things don't
last forever so a faulty element would be my first guess. As a first
step I would suggest you need to isolate it from its supply and test it.
OK, that will be done next.
A simple multimeter test will be a good start. Stick it on its highest
ohms range and check it looks like an open circuit when you measure
between live and earth and neutral and earth. Stick it on its lowest
ohms range and check it is about right for the power of the element. So
a 3kW element at 240V should have a resistance of 240^2/3000 = 19.2
ohms. If it is significantly less then that is your problem. Note that
it is possible that it may look ok on a multimeter, but then give poor
readings at mains voltage - you would need more sophisticated test
equipment to test this however.
--
Cheers,
John.
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