Thoughts on low r2?

[timhoward]

Domestic house, ring final circuit on an rcbo, end to end tests: r1 = 0.95 ohms, rn = 0.98 ohms, r2 = 1.02 ohms.
It's rare a reading is too suspiciously good!

Sane R1+R2 of around 0.52 for most of house except kitchen area where readings increase wildly suggesting several spurs off spurs so will be looking into this.
IR tests L+N -> E at 500v was fine.
So far I've only seen ordinary T+E on this circuit, no metal conduit / trunking etc.

I'm slightly struggling to come up with reason for disproportionately low r2 when compared with r1 and rn.
There is no continuity between cpc for either leg of this circuit and the earth bar, all other circuits connected.

I did wonder if r2 is in fact correct and r1 and rn are high due to poor connections, tomorrows job is pulling it apart.
I'm struggling to imagine a bridge in the circuit that is only bridging the cpc.
Any other ideas folks?

 

[Dartlec]

high r1/rn sounds plausible, though if so it's probably at the same socket, if they are both higher by more or less the right amount?

You've ruled out bonding at a boiler or similar by the looks of it. Only other reason I could see is if a separate earth was run for some part of it for whatever reason (maybe to remedy a fault)

Any likelihood of hidden junction boxes?

 

[catdog1121]

disconnect all the sockets then from one leg connect line to cpc and go find the first socket, once found note the line to cpc value, now connect line to neutral. as cpc should be 1.66 times higher you can work out if it is correct. obviously line and neutral both being the same size they should be the same.

So if line to neutral is say 0.22 then both line and neutral should be 0.11
So line to cpc should be 0.29. aka line 0.11 * 1.66 = 0.18 (cpc) so 0.18 + 0.11 = 0.29

If that checks out reconnect that socket and go find the second socket and keep going, when you get to a socket with three connections just connect 2 and follow that till you get to then end then come back and connect and follow that till the end, you will find any spurs off spurs.

There can be hidden jb's so even when you find the socket, check the others.. It can be time consuming at first but as you reduce the number of sockets it gets quicker and you will know what sockets connect to where.

 

[timhoward]

Thanks guys. Yeah that kind of testing regime was my plan for tomorrow.
Boiler and cooker have own radial circuits. Now wondering at what point in the day I unplugged dishwasher (and if it's touching a water pipe)
We'll see!