Likelihood of nuisance RCD tripping @ 0.4 megohms

[WellBonded]

Hi,
I understand that these things are hard (if not impossible) to be predict, but I just wondered if I could get your best guesses.

If a domestic installation has an unplugged insulation resistance of 0.4 megohms on all ring circuits, what is the likelihood/likely frequency of nuisance RCD tripping on a newly installed consumer unit?

Any educated guesses would be much appreciated.

 

[Octopus]

If its been tested that low, properly, then it should be investigated.........

 

[Darkwood]

If the ring is clear of all loads (often readings can reflect something thats still connected and this may be a spur on top of a cupboard, the gas cooker igniter, boiler etc).. then a reading of 0.4 Mohms is not acceptable and would have to be addressed before the board is changed, considering this could be a damaged cable or contaminated joints etc you should prioritise safety of the install as oppose to thinking of leaving it as is.

What was the reading between ?
Who took the readings and are they competent in doing the full set of Insulation Tests?

 

[Leesparkykent]

A low resistance between line and neutral conductors, or from live conductors to earth, will result in a leakage current. This current could cause deterioration of the insulation. The resistance between poles or to earth must never be less than half of one meg ohm for the usual supply voltages.

 

[frontier219]

Hi,
I understand that these things are hard (if not impossible) to be predict, but I just wondered if I could get your best guesses.

If a domestic installation has an unplugged insulation resistance of 0.4 megohms on all ring circuits, what is the likelihood/likely frequency of nuisance RCD tripping on a newly installed consumer unit?

Any educated guesses would be much appreciated.

Your tagline says 'DIY Member' so I'm not convinced you should be asking questions which would appear to be pertinent to changing your own CU. Can I ask what you are planning to do with this information?

If you have an IR reading of 0.4MOhms, you should be investigating that, or much better still, getting a qualified Electrician in to test your existing installation.

 

[davesparks]

How many ring circuits are there?

You say you have the same result on all ring circuits but it is unlikely that multiple circuits will have faults of identical resistance. Unless there are just 2 ring circuits in which case it suggests there may be another reasonably common fault present.

 

[Octopus]

I think the op needs to come clean. Just read the previous posts.

 

[WellBonded]

Hi. Yes, I'm not a qualified electrician, and I certainly wouldn't be doing any work myself.

The installation needs a full rewire as far as I am aware, but that is not possible for at least a few months, so I want to know if a new CU could be installed in the interim.

The guy that tested was qualified, and he said it "might" trip with a new CU, hence why I am asking you guys for your guesses.

There are four circuits, two lighting and two sockets. He recorded them all as 0.4 megohms.

He reckoned that, given the age of the installation (40 years), the most likely reason was age related breakdown of the insulation on all circuits.

 

[frontier219]

Hi. Yes, I'm not a qualified electrician, and I certainly wouldn't be doing any work myself.

The installation needs a full rewire as far as I am aware, but that is not possible for at least a few months, so I want to know if a new CU could be installed in the interim.

The guy that tested was qualified, and he said it "might" trip with a new CU, hence why I am asking you guys for your guesses.

There are four circuits, two lighting and two sockets. He recorded them all as 0.4 megohms.

He reckoned that, given the age of the installation (40 years), the most likely reason was age related breakdown of the insulation on all circuits.

Then he didn't do his job properly. If he got readings of 0.4MOhms bunch testing the DB (testing all circuits together) then he should've split them and tested individually. I would suggest a reading of 0.4 means that they are very likely servicable, however, without seeing it, the rest of the installation may not. You should get him back to test properly or get someone else in IMO.

 

[Des 56]

If the installation is past its last legs,why on earth would you want a new distribution board fitted ?

If it is in poor condition,there most certainly would be earthing and bonding issues to address before wasting money on a shiny box of tricks

Identifying and rectifying the problems that exist and doing the above rather than just changing a board would probably make the installation safer even if not quite so pretty
Oh add an Rcd by all means,they are known to provide additional safety on occasion

 

[amlu]

if its not a big place install an rcbo loaded board. if something trips then it wont trip any other circuits.
leave couple of spare ways to be used for new circuits after the place will get rewired.

 

[WellBonded]

If the installation is past its last legs,why on earth would you want a new distribution board fitted ?

If it is in poor condition,there most certainly would be earthing and bonding issues to address before wasting money on a shiny box of tricks

Identifying and rectifying the problems that exist and doing the above rather than just changing a board would probably make the installation safer even if not quite so pretty
Oh add an Rcd by all means,they are known to provide additional safety on occasion

Thanks. I have had earth bonding added to the gas and water mains, and just saw a new CU as the way to add RCDs to increase the safety as much as possible, in lieu of eventually doing the rewire.

Why that, rather than investigation and fixing issues? Basically because I have previously advised that the low resistance is most likely ageing of the whole installation, and that a re-wire would be cheaper than investigation and fixing/partial re-wiring.

 

[frontier219]

Thanks. I have had earth bonding added to the gas and water mains, and just saw a new CU as the way to add RCDs to increase the safety as much as possible, in lieu of eventually doing the rewire.

Why that, rather than investigation and fixing issues? Basically because I have previously advised that the low resistance is most likely ageing of the whole installation, and that a re-wire would be cheaper than investigation and fixing/partial re-wiring.

So what would you do if the RCD then began to trip? I would recommend getting two or three quotes for a re-wire and biting the bullet as soon as possible, rather than wasting time and money on patching over the cracks.

 

[Des 56]

his 0.4 M ohm insulation res value is still a mystery to the viewers

You have not answered the questions regarding the above
I would be a little surprised,if that value (whatever it happened to be between)as mentioned earlier by frontier 219 was not investigated at the time of test
I still think and the regulations also think, you need to sort out the existing problems before alteration or addition

 

[Charlie_]

How many circuits does the installation have?
You can't just do an overall IR test of the installation and claim it needs a rewire based on a reading of .4M
You may for example have 5 circuits that have an IR of 2M each which would give you an overall IR of .4M