"John Borman" wrote
| After having rewired a lighting circuit using the loop in method,
| (Upstairs of which works fine) downstairs seems to trip a MCB.
| When all light switches are switched off the MCB is fine but as
| soon as one is switched on its trips. ...
| Ok I have checked all ceiling roses and all seem fine, but how do
| I go about testing the circuit to establish where the short is.
The circuit should have been tested with a megger before being connected ot
the mains. Disconnect and make safe the circuit cable at the CU. With all
switches closed (ie on) (i.e. you will have to repeat the test for each
permutation of two-way switches) and all lamps removed, megger the circuit
(i.e. L-N, L-E, N-E all open circuit). This should tell you whether you are
looking for an L-N or an L-E fault. N-E would trip an RCD but not MCB unless
you have live and neutral reversed.
If it fails the initial meggering, which it should as it's faulty, go to the
mid point of the circuit, which should be fairly easy on a radial lighting
circuit, break into the circuit by separating the loop-in and loop-out and
test each half separately (again repeating for each permutation of two-way
switches). Repeating ad infinitum/nauseum will eventually get you down to a
An alternative and possibly quicker method is to open (i.e. off) all
switches and remove all lamps, then megger the switched live (L1) against N
and E at each switchplate. E will be present at each switchplate through the
circuit protective conductor, but for the N (if it is an L-N fault) you will
need a long lead that you can wander round the house with, connected to
circuit N at some point