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!

 

[davesparks]

Neons just glow and leave you with a puzzled expression on your face, or laughing at the poor apprentice trying to work out what they've done wrong.

 

[jackhammerJIM]

See posts 10 and 11.

Apologies happyhippydad i must engage my brain before writing

 

[UKMeterman]

Ok
1. 500V is the standard test voltage for pat testing, if anything can be damaged by 500V current limited to 1ma then it shouldn't be plugged in as it would fail the type testing
2. The output of a megger is current limited to about 1mA so it can't power anything.

 

[Lucien Nunes]

The smoothing capacitors giving you the 24Ω L-E reading as a false reading

Not sure I get your meaning here. No capacitor (except a failed one) should affect a reading on a DC insulation test, although suppressors legitimately create AC leakage when energised. Smoothing capacitors would never be connected in any way to E as they are part of the current-using equipment. Did you mean varistors? They might give an unduly low reading L-E at 500V but not at 250, and would look open-circuit to the continuity test.

 

[Richard Burns]

Perhaps I should have said centre earthed filter capacitors and the like that cause low readings of resistance on power supplies.