the live feed only. In this situation the neutral is still connected. The
neutral line will have a few volts on it which is a natural feature of the
generation and distribution system. Shorting neutral to earth (whether or
not the live is isolated) will cause a mismatch between the current in the
live side and the neutral side of the supply to the house. This mismatch
(which can only be accounted for by a leak to earth (either direct or via a
higher resistance path, such as you) is what the RCD detects. Yes, it is
very annoying when you are wiring in the loft! You can only get round this
by isolating the live and disconnecting the neutral feed to that circuit as
(anti-spam is as easy as 1-2-3 - not)
live ("phase") wire. E and N are still connected back with all the Es and
all the Ns on the load side of your RCD. The RCD's job is to make sure
that (to within 30mA or whatever it's rating is) all of the current which
goes out its L side is coming back up its N side. When you join the N to
the E, you provide an alternative path back for some of the N current, and
the RCD pops because you're out of balance.
for isolation is less than best practice. You'd do better to switch off
the whole supply, physically remove the L and N conductors from the MCB
and the N busbar respectively - put the bare ends out of harm's way into
a bit of connector block, or for best paranoia into spare ways in the
earth block. Then you can restore power to the rest of the installation,
and work at leisure on the properly-isolated (or even earthed) final circuit
you just disconnected.
The cautious among us when first working on a circuit we haven't
installed ourselves might even measure the voltage from the allegedly
isolated N and L conductors to earth before grounding them, and/or do
the temporary-connection-to-earth through a low-value glass fuse (so
you could see it blow) or an incandescent bulb (so you could see it
glow) Just In Case some eejut has cross-wired the final circuits; but
that's taking caution a bit too far, IMHO. Me, I isolate the Ns and Ls,
put them into free-waving terminal block, and then test AGAIN at the
working position that there is no voltage between earth and the allegedly
isolated Ls and Ns. 'Course, if you use a digital multimeter, you can
be misled by very-low-current induced voltages - connecting a suitable
resistance in parallel with the meter will soon sort out the difference
between a tiny capactively-and-or-inductively coupled current, and a
Thanks to everyone for the explanation.
So... if I was to switch off just the circuit I want to work on, THEN
confirm with a meter that there is no voltage across the N, L and E
conductors... under what circumstances could that still put me at
And, yes, it does happen - it happened to me!
The wazzock was the previous owner who fitted a two-way switch for the
landing light. Clearly unaware of the existence of 3-core +E, he had a bit
of a challenge when it came to wiring it up using 2-core+E. Then he had a
bright idea. "I'll use the live from the upstairs lighting circuit for the
upstairs switch and the one from the downstairs circuit (which was present
in the downstaitrs switch box) for the downstairs switch and connect them
together with the 2-core and earth". Hmmmmm. Muggins comes along and wants
to work on a bedroom light. Pulls fuse, all OK, happy working away. SWMBO,
in all innocence, turns on landing light. Disconnected neutral I am hanging
onto rises to live potential through landing bulb which glows fitfully due
to current through my sweaty fingers, while brain thinks WTF!? Guilty cable
also buried in plaster with no conduit - hooray! Yes, it is wired correctly
now. As they say in the X-files, "trust no-one".
(anti-spam is as easy as 1-2-3 - not)
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