Poor Insulation resistance?

[Bazultra]

I am currently Insulation testing a new install and all circuits have been greater that 299 meg ohms apart from 2 circuits where I get greater than 30.6 meg. On a new install surely this is not correct. Nothing is plugged in and no electronics are on the circuit. Both circuits are socket circuits . I thought that the fact it was above 2 meg ohms that the circuits would hold in but the protective devices blew so I know something is a miss. The 2nd time I switched the breakers on the circuit breakers hold. I am not prepared to leave them turned on as I know something is not quite right here. Any ideas or has anyone else had a problem like this. I have done all visual checks.

 

[Pete999]

" the protective devices blew" an explanation RCD CB?

 

[Bazultra]

" the protective devices blew" an explanation RCD CB?

The RCBO went bang when I tried to turn it on but the 2nd time it was ok. The circuit still comes in at greater than 39 meg ohms on a new instalI. Surely damp couldnt cause this

 

[telectrix]

short circuit L-N. blew off when the RCBO went out. check behind all sockets.

 

[Pete999]

What Tel said

 

[dingledong]

What Pete said

 

[Bazultra]

What Tel said

Already done all visual checks. What would a reading of 30.8 meg ohms on one circuit live - earth on one circuit and the same but Neutral to earth on the other circuit possibly indicate?

 

[Bazultra]

Could I be looking at damage dont to cables in walls or damp?

 

[gutterball]

split into smaller sections and test until you can narrow down where the fault is.

 

[Pete999]

Don't want to put you down Baz but your profile describes you as an Electrician, you should be able to sort this yourself, it's not going to be rocket science

 

[Bazultra]

Don't want to put you down Baz but your profile describes you as an Electrician, you should be able to sort this yourself, it's not going to be rocket science

I am trying to get to the bottom of a problem and get some advice which is what this section is for. I was trying to find out if anyone else has experienced this. If you cant help Pete or dont want to help then dont bother to reply with aload of mumbo jumbo like you just have do one

 

[Engineer54]

If you had 30 to 40 M ohms between all conductors on these two socket circuits, nothing is going to go bang. Not with an insulation level of 30 to 40 million ohms between any of the conductors. maybe think along the lines of faulty RCBO.

Oh, and damp can well be the reason for the lower than expected IR results on these two socket circuits!!

 

[Pete999]

I am trying to get to the bottom of a problem and get some advice which is what this section is for. I was trying to find out if anyone else has experienced this. If you cant help Pete or dont want to help then dont bother to reply with aload of mumbo jumbo like you just have do one

Mumbo Jumbo???? you have been given good advice, I can't add any more.

 

[Bazultra]

Thankyou very much engineer. The only strange thing about this is that these are the only 2 circuits in the whole building that I have had these low readings on and both of then blew! Both circuits were both disconnected from Db for IR test before energizing. Seems a coincidence

 

[Octopus]

Two socket circuits. Are you sure you haven't mixed up the tails?

 

[Bazultra]

Two socket circuits. Are you sure you haven't mixed up the tails?

Yes I have done a complete test of the whole install. All tests have been carried out to BS7671. 2 circuits with low meg ohms readings surely indicates damage of somekind thats not visual. Having said that engineer is saying it shouldnt blow the RCBO.

 

[dave274]

Have you started breaking the ring down to establish where the low reading is coming from or are you assuming someone is going to give you the answer you're looking for. Get the tester out and find the fault and put it right.

 

[Bazultra]

Have you started breaking the ring down to establish where the low reading is coming from or are you assuming someone is going to give you the answer you're looking for. Get the tester out and find the fault and put it right.

Read the pists above I have done eveything and know exactly where the 30Meg ohns is but its not accessable to get into the wall!

 

[Octopus]

Yes I have done a complete test of the whole install. All tests have been carried out to BS7671. 2 circuits with low meg ohms readings surely indicates damage of somekind thats not visual. Having said that engineer is saying it shouldnt blow the RCBO.

Which obviously you should realise

 

[dave274]

well if its tripping the rcbo you need to dig it out and sort it. don't just hope it wont trip again. if its a new install then its better to do it now before its decorated.

 

[7029 dave]

30meg, new install you know what to do, rip the faulty piece out and replace.

 

[Engineer54]

If you are getting a value of 30 to 40 Mohm, i doubt if it'll be actual damage to the cable, far more likely the cable is being compressed/squashed somewhere or other....

 

[PEG]

Few days old but...had a job a few weeks ago,lady away,daughter looking after house,32A MCB goes,fish die,freezer down,blah blah. Reset,lasts a day,resets again,another week,trips one night,wont let her reset,sparks etc (her words),morning after,resets ok. Only RCD is on stand alone DB shower/cooker,ALL other sockets off this one MCB,a proper hotch-potch of additions,part ring,part spurs and add-ons,recently added kitchen circuits,conservatory,garden lights etc...a real pain. IR at board similar to OP's,splitting as could only "splitting the difference" and not revealing any specific areas...getting ready for the big dissection and thought how hot the weather had been,appeared to be blowing at end of day when cooling,had a wild notion maybe conservatory getting biggest temperature change...so 4 double SSO in there,pull faces off,third one has big plasma shadow,end of 3.5mm screw rounded and melted,small burn on looped line. Big amount of luck,but i'll take that. So,long winded way of saying,a big overload,can be intermittent,without showing up between faults,as a catastrophic IR fail. In other words,you could still have a fair problem to find. Good luck.