If your supply is TN-C-S (PME) the limit is 0.35ohms however the
measurement tools have quite a substantial error when you get down
that far, never mind the test lead contact resistance. It would only
become an issue if the figure rose sufficiently high at the furthest
point on your install to mean a circuit breaker will not disconnect on
a L-E fault within a reasonable time (which is about 0.4sec). Thus
that is not a concern.

There is a Wiki / uk.d-i-y FAQ on RCD tripping. Basically unplug
everything (not just turn off), then see if the RCD trips.

If you have a split-load RCD system you may want to create a dual RCD
setup (RCD1 RCD2) or convert to split load, or convert to a couple of
RCBO thereby (say) keeping lights (up & hall) and one power circuit on
(cooker) if the main RCD trips out.

RCDs will trip on N-E faults - damp socket, screw touching a cable,
trapped cable in a floorboard, water penetration into an external
circuit, or on L-E faults such as failing immersion element, kettle
element, heater element, washing machine or dryer or cooker element,
clothes iron, even microwaves are not unknown.

If you do buy an IR tester from say Ebay to test each cable
individually, you need to ensure any burglar alarm & smoke alarm is
disconnected, along with all other appliances. Neon's will give fake
low IR (eg, 0.67megohm) so they need to be out of circuit (fuse out or
turned off).


JOOI with a PME system where the leccy co supplies the "Earth" thats
just a connection to the neutral of the incoming cable is it not?.

Well it is here..

So if thats providing the "Earth" as such is it that critical if the
resistance to actual "Earth" (being the ground in this instance) is high
at all?.

After all if a cable hits anything thats "earthed" then the fault
current is flowing from Live to wherever that Earth is. Course if thats
connected to the mains incomer earth then thats a full short across the
incoming cable and that should take the fuses to Kingdom come very
quickly.

However if that live should short to some other metal in the gaff then
the resistance of that to "Earth" can either be via a PME system or
wherever that bit of metal is or is not connected to?..

And the fault current can then be lots or rather little?..

So seem to me best protection is via RCD in most all cases..


--
Tony Sayer