On Wed, 6 Dec 2017 12:31:30 -0800 (PST), wrote:

Hi All

I have broken into a ring main to extend it to another room. Essentially cutting the return from one socket and then connecting each side of the extended ring to one of the cut ends.

I test the extended ring and it was sound (ie only continuity between the corresponding pairs of wire). I have just connected it to the main ring and did a quick continuity check. Live is ok but it seems that neutral (blue) and earth now have continuity. Given I tested the extended ring it would appear that the original ring suffered this problem. However if this was the case wouldn't the RCd at the MCU have tripped before.

Any ideas?

I'm not sure this is the case in every installation but I think the
neutral and earth are connected together somewhere (CU?) and further,
the earth is actually supplied via the neutral on the incoming supply
sometimes?

So, as long as you don't have any N to E [1] or L to E leakage current
on your side of the RCD you should be ok?

Or something like that .. I'm sure someone will be along shortly to
give it to you more accurately. ;-)

Cheers, T i m

[1] Often seen when cutting though a cable even with that cct MCB
switched off.