Was taking down strip light in the garage conversion tonight, in order to
Turned off lighting circuit MCB. Then disconnected light, checking with
electric neon test screwdriver that all was dead. As I prepared to remove
the light, I must have connected together the neutral and earth - and the
whole garage RCD tripped.
Is that normal? Frightened the life out of me! :-)
Quite normal proves the system works.
Others on group will probably site technicalities but simply the breaker
monitors difference between earth potential and line/neutral.
Can be a pain on PME systems IIR.
The MCB only switches live (some would say a failing of the UK
way of doing things.)
If you are not on a PME supply - and clearly you are not - there
can be a small potential between N and E (usually only a volt or
two) but there is sufficient current when shorted to earth for it
to trip the RCD.
A PME supply has local earthing, i.e. the neutral and earth are
locally bonding, hence it is unlikely that there would be enough
potential present to cause in excess of 30mA to be drawn.
On the other hand if it is not PME and the local earth is at the
substation some distance away then there could be enough induced
potential present for that level of current to be drawn.
In old houses where the supply cables were a bit on the thin side, the
large currents coming back down the neutral could give enough voltage
drop to have a significant current down the earth lead if joined to the
neutral some way away from the sub station. Years ago a friend of mine
lived in an old house and could feel a small voltage from the neutral
when working on his wiring.
In the case of an RCD tripping with a PME supply it is not the neutral
having a higher potential than the earth that causes the trip. It is the
alternative path to earth the short circuit created that allowed enough of
the current from the other circuits using the RCD to pass to earth creating
an imbalance in the RCD.
But being pedantic the only reason that current will take the
alternative root down the earth wire is because there must be a
potential difference (all be it fairly small). In other words no
potential difference no current flow.
Sorry - just resurrecting this thread. I managed to trip everything out
again yesterday, whilst painting. To get into some awkward corners, I
removed some sockets, leaving the lighting on. But in order to get the power
back on again, I re-connected the socket again, rather gingerly, whilst the
paint was still wet, intending to leave it loose, but connected. As I
fumbled around (why the hell did I put the socket there?), I must have stuck
my finger between the neutral and earth. Momentarily, it felt similar to
sticking a PP3 on your tongue, and the RCD covering the whole CU went out
PDQ. I guess that's normal - but thought I'd check. When I use neon
screwdriver on neutral, there is no glow.
Beginning to wish there were seperate RCDs now! :-)
Just re-read my message, and it sounds as though I got a tingle on my
tongue. What I meant was I felt a similar tingle in my finger.
Having cleared that up, does your response still apply? Is there anything I
can do to check this out? It was all professionally installed for me.
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