Ring main IR question from todays EICR

[Ross1]

Hi All,

While carrying out a commercial EICR today, I was doing IR testing on one of the ring mains an was getting 0.00M ohms between L/N - and the earth bar at 250V & 500V (cant distinguish what 2 earth wires belongs to the other 4 cores)

I have continuity on both L & N legs which are acceptable.
Also tested L- E bar and N - E bar both giving me 0.00M ohms
carried out a resistance test for both L & N to E bar, one was 24.4 ohms and the other was 1034 ohms

No RCD protection at all. TN-S system.

Stumped.... any suggestions on coding and where to start looking for this would be greatly appreciated.

 

[HappyHippyDad]

Hi Ross.

I know it may sound obvious but did you have the main switch off when you were testing (not just the MCB) as it is TN-S so will be continuity between N and E (and also L-E unless you were sure everything was unplugged?).

If the main switch was open when you tested then I guess you have an earth fault somewhere (commonly a screw in one of the back boxes clipping the neutral). As this is an EICR I would just be coding it and not thinking too much about solving the fault yet (unless you code it C1).

 

[jackhammerJIM]

as it is TN-S so will be continuity between N and E

Are you sure about that ?

 

[HappyHippyDad]

Are you sure about that ?

Expand please.

 

[jackhammerJIM]

why would there be continuity between N - E on a TNS supply ?

 

[Dave OCD]

I expect he meant TNCS ?

 

[Ross1]

Yes HHD, the main switch was open..both lives out of MCB, both neutrals out of the neutral bar. The earths all still in earth bar. I know the likelihood is that the fault is down between both live conductors and their own cpc. All other ring mains were all >500Mohm apart from another which had a reading of 0.40Mohm.

circuit working fine....although no RCD is present. So I'm thinking a C1 or 2

 

[alanl]

What was the actual measured resistance?
0.00M ohm could mean the actual measurement was 999,999ohms.
I'd be inclined to say there is a load still connected, or a neon or PIR somewhere on the circuit.

 

[Ross1]

Hi Alanl, the resistances are in my original post. This would surely not be the case though as I was doing IR between live/neutral to earth

 

[Richard Burns]

TNS with the main switch closed would still give a N-E resistance of 0.0MΩ, the connection is just further away (at the sub station).

I would say that you have a 52W piece of electronics still connected.
The smoothing capacitors giving you the 24Ω L-E reading as a false reading and the 52W coming from the difference in the N-E resistance to the L-E resistance, so the N-E is 1010Ω higher i.e. 52W

 

[davesparks]

why would there be continuity between N - E on a TNS supply ?

I'll give a clue, it's a TN system! The star point is solidly bonded to earth at the supply transformer or generator giving a negligable impedance between the two.

 

[HappyHippyDad]

why would there be continuity between N - E on a TNS supply ?

See posts 10 and 11.

 

[uksparks]

I would say that you have a 52W piece of electronics still connected.

+ 500v DC = Now potentially damaged, I never blast anything with 500v unless I know 100% thats its clear at 250v and know everything is disconnected, not worth the risk.

 

[davesparks]

I've lost count of the number of appliances I've applied a 500V IR test to, none of them were ever damaged by it!

 

[uksparks]

Alright, neons then!