Would Neutral - Earth Ins Res of 5.0 Trip a breaker

[Pattyb]

I wanted my (3) old fuse boards replaced with a modern consumer unit. A local electrician came and said he would replace it but he wanted to do an inspection first as problems not found on old fuse boards would become apparant on these new consumer units. This sounded reasonable to me. He came today and said that he was getting a reading of 5.0 between neutral and earth on the insulation resistance test and that this was low enough to trip a rcd on a new consumer unit.
He said he would have to come back and investigate which could involve a lot of carpet lifting etc. I wasn't sure if he was taking the mick or not so I did some googling. It seems to me that the regs allow 2.0 so if this is the case why would a reading of 5.0 trip the rcd?

 

[londonboy]

He doesnt sound like hes taking the mick, infact he sounds like a concienscious electrician.

2M Ω is the lowest permissible insulation resistance.

5M Ω is close, given that generally circuits are tested and found in excess of 200M Ω.

I'd be tempted to do as he's done, and recommend finding the problem.

Generally its damp/rodent damage: exterior lights, water heater, damp wall, junction in damp ground, shed/garage, transformers, fans, etc you get the idea. He will lift the floorboards as a last resort, I'd imagine he'd find the fault without lifting any (unless theres junctions etc hidden everywhere).

Stick with him.

 

[Taylortwocities]

Did he say 5 ohms, 5 kilohms or 5 megohms?

There is a big difference.

EDIT: reding your post he obviously said megohms. I agree, he is doing the right thing (not many do).
He does not want to remove your fuseboards and install a new one and then say to you "whoops, it doesn't work". And leave you in the cold/dark with your freezer defrosting until he can find the problem.

Much better to fix the fault and then do the changeover.

 

[Pattyb]

Thanks for your replies, I am assuming it is Megohms as he said the other two were readings were 255 and 250 (which ties in with what londonboy posted). So although it is within the regs it is still low enough to warrant investigating which is fair enough.

 

[Sherbertlion]

I'd imagine he'd find the fault

What "fault"?

5M ohm is well above the limit set out in the regs, and depending on how this test was carried out (whole installation / individual circuit) as well as many other factors seems like a perfectly reasonable result (remember it's 5 million ohms!)


A reading of 5M ohm will not trouble a 30mA RCD.

 

[RF Lighting]

But it's a pretty good indication that all is not well.

Granted as things are the new board would work, but I'd want to find out what is dragging the IR reading so low, partly for my own peace of mind, and partly because these sort of readings tend to get worse until it's popping the newly fitted RCD.

You look incompetent to the customer if this happens, even though as far as the regs are concerned you haven't really done anything wrong.

 

[Sherbertlion]

But it's a pretty good indication that all is not well.

Granted as things are the new board would work, but I'd want to find out what is dragging the IR reading so low, partly for my own peace of mind...

True, and I agree, I would also want to know. I suppose it depends on how much money the customer has and how busy the electrician is?!

Essentially, if the situation is made clear to the customer then they at least have the choice as to whether to write a blank cheque or whether to proceed with the board change and maybe just monitor the situation with the knowledge that they may have to lift a few boards in the future. If it is a potentially dangerous fault then at least the new RCD will clear the fault eh.

 

[riveralt]

He came today and said that he was getting a reading of 5.0 between neutral and earth on the insulation resistance test and that this was low enough to trip a rcd on a new consumer unit.

Has he done a board insulation resistance test?

He said he would have to come back and investigate which could involve a lot of carpet lifting etc. I wasn't sure if he was taking the mick or not so I did some googling. It seems to me that the regs allow 2.0 so if this is the case why would a reading of 5.0 trip the rcd?

Not sure why he didn't test the individual circuits at that time to find out which one (s) were causing the problem - then he can decide whether to uproot carpets etc.

 

[holmslaw]

..

 

[Sherbertlion]

Apart from shebertlion, I agree with all the advice given.

I haven't given any advice.

 

[Pattyb]

Not sure why he didn't test the individual circuits at that time to find out which one (s) were causing the problem - then he can decide whether to uproot carpets etc.

Sorry I didn't make this clear from the start but he did say it was the main ring which had the low reading.

Thanks for your advice, it seems like he is doing the right thing then, though he hasn't given me a price yet.

 

[sparkydude]

When he did his Test, did he make you unplug everything on the circuit or did he do that ?? if neither then its more likeley to be a faulty appliance rather than a cable/wiring fault.


Nick

 

[PrinceofDarkness]

I see nothing wrong with Sherbert's post. He's quite correct in saying that a 5 Meg reading won't upset an RCD. Neither will a reading of 1 meg do so.

Such low readings are always a matter of concern, even when they are within the permitted tolerances. But the cause is often easy to find and to remedy. That reading is not sufficiently low enough to prevent the fitting of a replacement CU with RCD(S) - and it's certainly not bad enough to start ripping up carpets and floors!


Lucia.

 

[Pattyb]

Just thought I would inform you of the outcome. He came back and we agreed he would remove all the sockets and check them and replace some older unswithched ones. Having done this he got a reading of 44 (megaohms?). He seemed happier with this so we agreed he change the consumer unit. This was done and he charged only a little more than originally quoted for.