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

 

[frontier219]

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

This also depends how many cores are in each OCPD, and how many spurs are present. Could be absolutely nothing wrong with these cables - or equally as likely IMO, one leg of one circuit bringing the whole installation down.

 

[Leesparkykent]

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

If he got a global IR of 0.4 M ohm then it should of been investigated further anyway. When tested the individual IR results of each circuit should of been recorded on the EIC.

 

[Darkwood]

I'm confused here, you have paid for testing of the install and the said 'claimed' electrician has only done global tests on insulation resistance, had any competent Electrician got those results they would have tested at an individual circuit level to identify the circuit.

A you now inform us every circuit has this reading then its possible that because he is incapable of doing a full testing of the installation that you paid him to do that he has probably left a load connected without realising it.

Finally and repeating others here, I would be surprised if an install of only 40yrs was in that bad a condition that the insulation was breaking down, it will most likely be in pvc and still in good form, although I cannot say this for sure because many factors may bring an installation to a premature end.... and hangs a lot on the fact it was installed well too.

Get 'claimed' Electrician back and remind him if you have paid for an Inspection report on only 4 circuits then you expect to have it to have more depth than a global test that says all circuits are at 0.4Mohms...

No Electrician would recommend fitting a board until the said reading had been fully investigated and corrected if need be.

Lastly 0.4Mohms in itself will not cause any tripping issues but its not an exact science as the tester generates a DC test voltage of minimal current and this does not mimic the the real life loaded circuit scenario, it just gives a pointer of possible issues. Ohms Law the most basic Electrical calculation tells you the leakage that would flow, had said Electrician used it he could have informed you with a better answer than he spouted out.

 

[Charlie_]

By the sounds of things it would seem to me that the sparky in question doesn't know what he's doing or he is trying to pull the red and black wool over your eyes

 

[Octopus]

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.

Don't get him back.

Get another sparky to do individual tests on the circuits, it ain't rocket science, nor will it take long with only 4 circuits...... then, and only then, will you know the "real" issues.

 

[Pete999]

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.

So he has done a global IR test, come up with a reading of 0.4 Mohm and not gone any further? sack him then because he hasn't carried out the test correctly

 

[Deleted member 72666]

Don't get him back.

Get another sparky to do individual tests on the circuits, it ain't rocket science, nor will it take long with only 4 circuits...... then, and only then, will you know the "real" issues.

Best advice so far. Get a 'proper' spark to look at this!

 

[WellBonded]

Thanks all.

Each circuit is listed in his report, with the 0.4 value recorded for each circuit (neutral to earth). The line to earth readings are actually different (ranging between 0.45 and 0.7).

When he did the readings the electric oven and the fridge could not be unplugged, and I don't believe that he took the lightbulbs out.

I don't think he was after extra work, because I think that he only does testing (and occasional small jobs) , and definitely doesn't do re-wires.

 

[Darkwood]

Thanks all.

Each circuit is listed in his report, with the 0.4 value recorded for each circuit (neutral to earth). The line to earth readings are actually different (ranging between 0.45 and 0.7).

When he did the readings the electric oven and the fridge could not be unplugged, and I don't believe that he took the lightbulbs out.

I don't think he was after extra work, because I think that he only does testing (and occasional small jobs) , and definitely doesn't do re-wires.

Hate to ask how much he charged for his services ... sounds like a cowboy..

When you say could not be unplugged I assume he isolated the oven at the double pole switch but wondering if he switch the fridge off and left plug in ...

If they were left switched on then he may have sent high voltages into your fridge and oven, this could have damaged the electronics on any timers, displays etc... if you cannot un-plug or isolate it, you don't test it , I have never come across a utility I could not unplug and yes some were a pain to get in and out but it is part of the job... those readings are more likely to do with the fact you have things still plugged in and if he knew what he was doing he would not have sort excuses to leave them plugged... he should have tested at 500v this is to cover peak voltage of the standard AC supply sign wave and this test voltage can damage appliances especially if housing electronics..so if your oven timer or display packs up in the near future you know now you have just paid a bloke to damage your property.

I'd be asking a few questions raised here to this 'Electrician' because I never heard such a stream of excuses for not testing 4 circuits properly!

 

[markc123]

Thanks all.

Each circuit is listed in his report, with the 0.4 value recorded for each circuit (neutral to earth). The line to earth readings are actually different (ranging between 0.45 and 0.7).

I'm really really confused here, so bare with me ...

OP, when you got the electrician in, what did you actually ask him to do? An EICR ?

Most standard cert's don't have a column for N-E on EICR the electrician should either note this down himself somewhere, or if necessary record the reading in further remarks...granted 0.4, 0.45 and 0.7 and whatever else you've got are all **** poor results, BUT - if it says unsatisfactory on the front of the certificate then im failing too see what the electrician has done wrong. He hasn't taken a global IR if you've got different readings for each circuit ....

Unsatisfactory, and then it's up to you to get someone to come and fault find and put everything right. That's if you've got him to just do a report.


^^ i agree with what darkwood said aswell ... surely the cooker and fridge were isolated from a FCU somewhere?

 

[WellBonded]

Hi Again,
A few clarifications.

Yes, it was an EICR.

He couldn't get to the socket for the fridge, or the isolator for the oven (fridge very hard to move and isolator hidden in very weird place - I found it later, by chance).

He said that he ran the test at lower voltage to protect plugged in equipment.

I've no complaint about his work, because he was just hired for report, I just want to understand the impact on rcd.

I will be doing a full rewire, but cannot do so for a few months, so just want to know if it is workable to get rcd added in the interim. I dont mind that I might end up paying more for rcd now and then rewire eventually.