John Rumm wrote:

http://wiki.diyfaq.org.uk/index.php?...Circuit_faults

1. Wire resistance table: the figures seem rather high - they appear to
be circuit resistances (i.e. resistance of the phase-neutral loop) at 70
deg. C, as you would use for calculating voltage drop, or fault current
calculations. In the context of resistance measurements made "cold" you
need the figures at 20 deg. C, which are all in Table 9A of the OSG (p.
158). Also note spelling of "metre" :-)

You could refine the table by adding two more columns to give
resistances for a single conductor, for the P-N loop (R1+Rn) and for the
P-E loop (R1+R2) of twin & earth cables.

2. Resistance measurements: the method with a 3-pole shorting plug is
invalid unless you disconnect the relevant neutral at the CU. Remember
that N & E are also shorted at the supply end, in the DNO's cut-out for
PME and further back for TN-S, so there are parallel paths which will
invalidate your resistance calculation. I see you do say "one should
always ensure power is turned off at the main switch" but many will
disregard that, taking it only as a safety warning and not realising the
subtle effect it will have on the measurements.

3. Disconnected CPC, ring circuit "circuit safety mostly unaffected":
that's a rather sweeping statement and there will be cases where a
dangerous situation exists, due to inadequate thermal protection for a
CPC. The worst case is probably a broken CPC at one end of a long ring,
with the old-type 2.5 T&E which only has a 1.0 mm^2 CPC and with a 30 A
BS 3036 (rewireable) fuse. With an earth fault near the broken far end
you might find the CPC protects the fuse and the cable catches fire;
even if it doesn't, the disconnection time is likely to exceed 5 s, let
alone 0.4 s, and the touch voltage other metalwork separately earthed or
bonded to the MET will be over 70% of full mains voltage, so protection
against indirect contact is not achieved.

4. Ring continuity tests: the cross-connection method (per OSG, GN3 and
our FAQ) should be mentioned. It's very convenient, having
cross-connected the ends, to go round and measure resistance between L &
N at all sockets. If all is well you should measure the same resistance
value at all points. Ditto for cross-connection of L & E, except that
the resistance will rise slightly toward the point furthest from the
cross-connection, due to the higher resistance of the 1.5 mm^2 CPC.

--
Andy