IR Testing

[fattony]

Im completing a EICR for an empty shop which is over 3 floors, TNCS 250Amp TP+N supply which feeds a MCCB board, there is a MCCB per floor plus the lift, the tails into the MCCB board are in galv trunking and the supply up to the floors are wired in SWA.

We wanted to test the insulation resistance at same potential on one of the boards, (Single Phase) with the main switch off, the incomming earth disconnected and a link between the neutral bar and line we tested but this returned a 0.00M Ohm reading.

Thinking the mistake being not opening the neutral link at the MCCB board we went down to the MCCB board, (basement) we isolated the supply and opened the neutral link and had the incomming earth disconnected for Ze, we measured continuity between the neutral bar and the earth bar in the MCCB board and measured 0.18 Ohm, obviously a insulation resistance test would just return a 0.00M Ohm reading so we needed to find out what was causing the continuity, we disconnected the bonding to the water and gas but we're still measuring 0.18 Ohm.

Have I missed something stupidly obvious, there is obviously a connection between neutral and earth somewhere but I havent found it as yet

 

[ezzzekiel]

pme supply with neutral to bar

 

[needasparks]

Would have thought the only logical way is disconect each neutral in turn may shed light on it.
Do you think any of the lighting has been cross conected at all from common areas to a floor area, possibly emergency lighting ?
I know that sounds odd but additions do get made over the years or possibly a cable crashed against a backbox ?

 

[fattony]

Would have thought the only logical way is disconect each neutral in turn may shed light on it.

Will look into that next week, it may be the only way forward

Do you think any of the lighting has been cross conected at all from common areas to a floor area, possibly emergency lighting ?
I know that sounds odd but additions do get made over the years or possibly a cable crashed against a backbox ?

Possibly we do have a last man switch, the emergncy lighting is wired onto the last man switch contactor which isnt correct

 

[needasparks]

Possibly we do have a last man switch, the emergncy lighting is wired onto the last man switch contactor which isnt correct

Be interesting to see where this switch gets its feed live/neutral for the contactor ?
Don't think i can get my head round this switch and the emergency's though.
Let us know how you get on !

 

[fattony]

I meant the emergency supply is on the same mcb which feeds the contractor coil so pretty useless unless there is a power cut, can you code that or just advise it

 

[needasparks]

Think i have you correct, not advisable setup. does the contactor control more than one circuit ? if so a fault on primary feed circuit will give a blackout on all circuits. Thats why always best to have a dedicated circuit for a contactor or timer. Would be contrary to BS5266. Not the best setup so I would mention it but your only testing the installation so would be an observation without a code.
Somethings ringing in the 7671 but I'm sure one of the book worms will advise better.

edit
Just read that again. Is the contactor making or breaking ?

 

[fattony]

edit
Just read that again. Is the contactor making or breaking ?

Making I think but need to double check to be 100%

 

[oldtimer]

I had this chased my tail for 2 hours then I looked at one of those 4 way extention leads plugged into the circuit and said to myself na but unplugged it anyway went back to DB IR was >299

 

[fattony]

I had this chased my tail for 2 hours then I looked at one of those 4 way extention leads plugged into the circuit and said to myself na but unplugged it anyway went back to DB IR was >299

I guess the way forward is to disconnect neutrals until it clears then investigate that floor, never simple!

 

[GLENNSPARK]

I had this chased my tail for 2 hours then I looked at one of those 4 way extention leads plugged into the circuit and said to myself na but unplugged it anyway went back to DB IR was >299

yep never count out stuff like that..even if new.....we had it the other week on a job in harrogate.....IR reading was crap between line-neutral on a kitchen ring.....dropped all the outlets off and tested point to point......the only thing left was a brand new IP 2 gang outlet just outside the back door.....took it off and tested between the line and neutral terminations on it......yep you guessed it.......

 

[needasparks]

I had this chased my tail for 2 hours then I looked at one of those 4 way extention leads plugged into the circuit and said to myself na but unplugged it anyway went back to DB IR was >299

Your bang on with that. I went to an rcd fault and it was just one of them, the type with the surge filter in it.