Earth to neutral fault

[Cider]

Having just changed the first c/u in 20 years for my part p inspection i have encountered a fault which has to be sorted.
Ive replaced an old wylex fused 6 way unit and installed a split load board.
System comprises of lights up, lights down, immersion, cooker, shower and ring.
PME system with 6mm earth to gas and main earth at pme.
There was another earth leaving the old board which didnt seem to go anywhere as tests between old c/u earth terminals and pipes before i started gave very poor readings.
New 16mm to pme, new 10mm to gas and cold mains at stop tap.
Several accessories need replacing.
Now I have continuity between ring RN and R2 on both legs of ring and continuity between RN and main earth im guessing because i have installed a new earth to the cold mains.
When I carried out a resistance test on R1 R2 and RN I got even readings on R1 and RN but R2 was more then double. The cpc is only 1mm.
I then carried out a test on entire ring on R2 and worked out the cable route and the readings jump by .20 ohms between 3 accessories in the middle of the ring where a spur has been connected.
All the wiring is original but it seems a few accessories were done in spurs (heating and garage to name a few)
If the fault is just at an acessorary (plate screw in contact with RN) i can understand the continuity between RN and R2, however there is continuity between RN and main earth terminal even with the 2 legs of the ring completely disconected from the c/u.
The only explanation for this I can think of is a damaged cable in direct contact with a pipe (now correctly earthed) however the R1 and RN readings were identical within .01 and I would have thought this would have shown up a differantly if there was contact of N to earth.

Tomorrow (sunday) I shall inspect all accessories but am worried it may be a fault under the floor.

I hope this makes sence and someone can give me some advice.


Many thanks

 

[johnboy6083]

Have you disconnected all loads on the circuits? If not then you will get continuity from live to neutral through the load. Also are you disconnecting all cables from the board? Say you test between the lives of all circuits, but there are loads connected, then this will give you false readings between the circuits, as the neutrals are commoned.

 

[Cider]

Thanks johnboy

I have disconnected all cables on the 2 ends of the ring.
Im getting continuity between N and earth on these ends and continuity between N and main earth terminal.
I should not get any continuity on either of these with the ends fully disconnected from consomer unit ?
All other circuits are still connected, tested and working ok.

 

[paul]

try turning the db. main switch off. turn off all mcbs. disconnect all neutrals at db. you can now retest at db and see if you have any links to other circuits or bonding wires

 

[truckster]

It sounds like you may have an interconnection between circuits on the neutral. As suggested open the main switch, if this clears the N-E fault you will need to test between the neutrals.

The jump in the R2 reading could be a loose cpc connection where the cpc's are sleeved together but one is not fully in the terminal. The higher reading compared to the R1 and RN is due to the cpc being proportionately smaller, i.e. 2.5/1.5 T&E the cpc resistant is 1.6 times the others.

 

[Cider]

Thanks everyone.

I had to replace the heating spur and a socket to solve the problem.
Found loads wrong, quite scary stuff.
Double socket with no earth connected in lounge.
Melted wires in immersion spur.
Garage socket earths completly worn out.
Every other downstairs socket is a spur (from original wire on new build).
The old main earth to cold mains was not connected.
Bathroom light switched on neutral.
Light fittings with burnt wires in bulb holders.
New electric shower is 9kw on a 6mm (only recently replaced by a plumber) so that needs sorting.

Glad I wasnt doing it for a paying punter on a price!
Learnt loads but all the hard way.

My main concern is the R2 on the ring is 1.08 as apposed to R1 of .47 and Rn .46
The conductors L & N are 2.5 mm but the cpc is very small, say 1mm! ???

 

[paul]

no problem R1 at 2.5 mm R2 at 1mm = 5to 2 ratio as does 1.08 to 0.46 approx usually a good guide. with rcd covering circuit will be fine
6mm can feed 9kw on 40amp mcb depending on other considerations ie if in insulation etc

 

[TPES]

Thanks everyone.

I had to replace the heating spur and a socket to solve the problem.
Found loads wrong, quite scary stuff.
Double socket with no earth connected in lounge.
Melted wires in immersion spur.
Garage socket earths completly worn out.
Every other downstairs socket is a spur (from original wire on new build).
The old main earth to cold mains was not connected.
Bathroom light switched on neutral.
Light fittings with burnt wires in bulb holders.
New electric shower is 9kw on a 6mm (only recently replaced by a plumber) so that needs sorting.

Glad I wasnt doing it for a paying punter on a price!
Learnt loads but all the hard way.

My main concern is the R2 on the ring is 1.08 as apposed to R1 of .47 and Rn .46
The conductors L & N are 2.5 mm but the cpc is very small, say 1mm! ???

What were you expecting the r2 to be?

 

[Cider]

I was going on 1.67 times higher as on site guide says for 2.5 / 1.5 so was expecting it to be something under 1
I could not find a guide as to what to expect to be for a small cpc but sure its in 7671 somewhere.

One problem I encountered was identifying conductor size.
Is there a way of measurment?

Thanks

 

[Rikki Barrett]

I know a BT Engineer and he showed me a little key-ring they get given to identify conductor sizes, had half circle cut outs along one edge. Could be an option for you.