new hob circuit tripping rcd - giving me headache

[cairns]

Hi all, have taken over a 2nd fix right at the end, just turned power on, hob circuit is tripping rcd in split load board, the polarity is correct, have done continuity on l-n-e getting 99.9kohms and 299mohms insulation resistance on the hob supply cable, however when I test between n and the earth bar with the hob circuit earth removed am getting 99.9k ohms and 299mohms, when I do live and earth bar with the hob earth removed am getting 4.1kohms and 0.01mohms on insulation resistance.

I would've thought with this low insulation resistance reading it would get a lower continuity reading wouldn't it? Any people with will help me with there knowledge I would be most appreciative.

Thanks

 

[Pete999]

Diss all the conductors from the CU L, N and CPC relating to that circuit, measure r1, r2 and rn this with the hob switched off, then carry out an IR test and see what the readings are, your post is a little disjointed.

 

[garnhamr]

yeah what pete said, test the circuit completely disconnected first. i'f i'm reading your post right, you are saying theres very low IR between the live of the hob cable to another earth so i'd disconnect the hob circuit fully (including at the hob) and one by one test between hob live and all other circuits earth cables and then the bonding earths.

 

[Darkwood]

New hobs cookers are common place for been faulty with earth leakage, ensure your wiring tests ok, ensure its connected as instructed then ring manufacturers up and get a replacement.... when Id did domestic I changed 4 faulty new hobs over a several jobs ....cheap build foreign crap usually.

 

[cairns]

Hi there, yeah it trips straight away, have disconnected the load on the hob circuit, insulation resistance tested the hob cable with it disconnected and have got greater than 299mohms on the insulation resistance between l-n-e and 99kohms on a continuity test between l-n-e.
What I don't understand is how can i get 4kohms when i test between the hob circuit live and the earth bar but then 0.01mohms on an insulation resistance between live and earth bar, thats with the earth of the hob circuit and live and neutral disconnected.

Cheers for your help guys

 

[Darkwood]

Hi there, yeah it trips straight away, have disconnected the load on the hob circuit, insulation resistance tested the hob cable with it disconnected and have got greater than 299mohms on the insulation resistance between l-n-e and 99kohms on a continuity test between l-n-e.
What I don't understand is how can i get 4kohms when i test between the hob circuit live and the earth bar but then 0.01mohms on an insulation resistance between live and earth bar, thats with the earth of the hob circuit and live and neutral disconnected.

Cheers for your help guys

Are you saying you have done an insulation test on the hob itself or am I misreading that?

 

[cairns]

no not the hob, the hob supply cable, the hob has been disconnected all the time. its tripping with no hob connected.

 

[garnhamr]

i don't knoe but doing IR uses 500v normally so maybe that makes the short more aparent than a continuity test. Not sure i would care about all that though and just see why theres 0.01mohms.
I'm thinking a metal back box has been screwed into the live of the hob cable or something.

 

[Darkwood]

no not the hob, the hob supply cable, the hob has been disconnected all the time. its tripping with no hob connected.

If its tripping with no hob connected then you have a fault possibly with the cable or a connection point ... have you checked the cover screws on the cooker switch isn't partially into the L insulation giving a high impedance fault...

You have your test results so follow them disconnect the cable between the cooker switch and outlet and narrow the fault down.... this is routine test procedure tbh.