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Installation Resistance Readings..


micky23english

Question

I'm wondering if you can give me some guidance regarding installation resistance readings?? So based on college learning when doing this test I was under the impression the correct reading for no faults would be 299 while 999 for a fault on the circuit but a question I have is why would I get the readings of either 195 M ohms and 0.75 M ohms ?? While I completely understand the insulation resistance test these reading have some what confuse me. Any guidance would be appreciated.

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23 answers to this question

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My Metrel's display max's out at >1000M. So if i see any less than this I personally investigate.

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My Metrel's display max's out at >1000M. So if i see any less than this I personally investigate.

So you have a reading of say 75 meg ohms you would investigate?

Madnesd

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That is overkill.

At the most I would bring the time to the next recommended inspection down but would not investigate further unless it was under 1Mohm or close to it.

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AFAIK

Anything under 1Mohms is a fail

Anything under 2Mohms is a pass but should be investigated

Anything over 2Mohms is a pass... End of

Ideally on a new circuit you should get a reading higher than your meter can measure, however when doing a PIR you often get lower readings which are acceptable

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AFAIK

Anything under 1Mohms is a fail

Anything under 2Mohms is a pass but should be investigated

Anything over 2Mohms is a pass... End of

Ideally on a new circuit you should get a reading higher than your meter can measure, however when doing a PIR you often get lower readings which are acceptable

It's never that clear cut.

 

In an old house that may well be damp, you can probably take readings of a few M :ohms  as normal, particularly if all circuits are showing some leakage.

 

But yesterday, testing a 10 year old house, all circuits tested fine, almost infinite leakage readings, but hob tested about 2M :ohms  I thought that warranted investigation and quickly found a pinched cable in a back box.  when something sticks out so obviously "wrong" even if technically a "pass" I would still rather investigate.

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It's never that clear cut.

 

In an old house that may well be damp, you can probably take readings of a few M :ohms  as normal, particularly if all circuits are showing some leakage.

 

But yesterday, testing a 10 year old house, all circuits tested fine, almost infinite leakage readings, but hob tested about 2M :ohms  I thought that warranted investigation and quickly found a pinched cable in a back box.  when something sticks out so obviously "wrong" even if technically a "pass" I would still rather investigate.

 

Really - I'm hoping you had the clients permission to spend their money!

 

I was testing a house yesterday - the L-N on the socket circuit was 0.18 m ohms - until I removed the RCD socket, and then it was 200 meg ohms!

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Really - I'm hoping you had the clients permission to spend their money!

 

 

My experience is a landlord would rather have an extra half hour labour and an EICR that says "satisfactory" than an EICR saying unsatisfactory and an estimate for extra labour to go and investigate what was almost certainly going to be a simple fault.

 

Arguably you have to remove a certain number of accessories anyway as part of an EICR so I decided to have a look inside this one and there waa my problem 2 birds, one stone.

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Really - I'm hoping you had the clients permission to spend their money!

I was testing a house yesterday - the L-N on the socket circuit was 0.18 m ohms - until I removed the RCD socket, and then it was 200 meg ohms!

should that not have been done prior to testing???

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should that not have been done prior to testing???

Hum.... Customer not aware of what is on which circuit, so unplugged what it thought was everything and thus a quick 250v IR test gave the low reading and lead me from the downstairs circuit to this outlet upstairs!

Do you locate all outlets every time you are testing an old installation?

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Wise move going 250 in the first place...so many do not understand what an IR test can do "damage wise"

I have been quite close to a petrol pump installation that was IRd at 1000V....and it was fuelled as well

It is now refered to as ....."do,you remember that whooooomopfffff test?.....what a mess"

It was a long time ago, but spectacular

Just be careful,out there......attrib. Hill,Street Blues. c. 1980s

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Hum.... Customer not aware of what is on which circuit, so unplugged what it thought was everything and thus a quick 250v IR test gave the low reading and lead me from the downstairs circuit to this outlet upstairs!

Do you locate all outlets every time you are testing an old installation?

No of course I don't sometimes it's nearly impossible due to the nature of my work.

RCD sockets usually stick out as they would have been installed fairly recently and they are usually massive! but if it's buried under a pile o crap then your not gonna get at it.

my point was you were having a pop at PD for putting minor issues right on an EICR then you say you had to go socket hunting.

:C

just spraying...

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The INSULATION resistance test is essentially testing the quality of the conductor insulation, be it PVC, rubber or even magnesium oxide.

The higher readings are what is desired >250Mohms >500Mohms or infinity depending on how you tester works.

Low readings are in indication of a fault or the insulation breaking down. BS7671 defines this as <1Mohm but in reality a low reading should be investigated.

Low readings are usually caused by damage to cables, cables poorly installed in accessories, faulty accessories, damp and flora n fauna.

A reading of 175Mohms is fine but could be damp a reading of 0.75Mohms could be a cable physically damaged.

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Thats great thank you @ Blue duck that's hit the nail on the head for me so a reading lower than 1 m ohm ie 9.99 would indicate fault etc where as 299 is perfectly fine I think the readings them selfs of 175 an 0.75 just confused me as im used to either 299 and 999 on meggas meters. Thank you

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Both readings could indicate a fault. It is just that BS7671 stipulate that sub 1Mohm is unsatisfactory where as a 299 may indicate the beginnings of deterioration and/or a loose connection amongst many other things.

Sub 1Mohm would warrent an 'unsatisfactory' EICR but a 299 reading may not.

A perfect result would always be infinite.

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However, infinite insulation resistance is theoretical only, it can never exist in real life.

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As Sidey says , the human brain cannot grasp the concept of infinity .     It was said that the universe was infinite as in , it has no end , but then , how can it have an end ?   If you travelled past the furthest planet in the universe  you would still be somewhere , your craft would become the last object in the universe but as you left that planet behind you , where would you be going ?       To infinity and beyond !!! 

 

Infinity used to be used when the resistance was greater than the range of the meter used .

 

You don't say if you were measuring between L-E   L-N or N-E

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I think also reading your post again you say 'the correct reading for no faults would be 299'. This is not correct. I think you may be getting mixed up with what is written in the box on the test sheet for the insulation resistance test. Some meters will only measure upto 299Mohm so that is what you should put in the box if using that kind of tester. Other testers may go upto 500Mohm so in this case a 500 will be put in the box.

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I was once testing the IR on a lighting circuit I'd just been through doing various minor faults. L-N, L-E, N-E were all ZERO Mohms...........

 

Panicking now I took a deep breath thinking WTF had I done, nailed, pinched or screwed. 

 

It was just after I started using WAGO block clamp terminals. It was late and the last place I'd been was in the loft. The lamp batten holder was smashed to pieces so not having another I decided on the tried and tested method to "make safe" of fitting a bit of connector strip wrapped in tape and I'd replace the next day..............

 

The slow realisation that 222-413 is NOT the same as same as a 3-way terminal strip!

 

But then sometimes I'm a C***!  :lol:

 

Still, if ever I needed a reminder of the benefits of dead testing!

 

(As an aside I was never a big fan of that Windows Minnellium................)

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When an apprentice we were rushing to finish a rewire

I was putting the landing floor and carpet down

Spark,was fitting the switches

Switched back on....little testing then

Light not working

Mate takes switch off with power on

Neon lit up when he touched unused side of a twin we had canibalised as a singles

Shite....cable nailed

Carpet up,floor,up

No,nails

WTF

Neon lit on induced voltage onto unused core

Lesson learnt

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If you get into the habit of copying the result reading of the test accurately them you'll never have this confusion. Record the answer as per your testeter not what may generic for another test meter?

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If you get into the habit of copying the result reading of the test accurately them you'll never have this confusion. Record the answer as per your testeter not what may generic for another test meter?

If you get into the habit of copying the result reading of the test accurately them you'll never have this confusion. Record the answer as per your testeter not what may generic for another test meter?

That is exactly what you should do. I never used to. I always put 299 but never really knew why

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This is the problem in many instances. People can point and test write a reading without knowing why or what it means or translates to mean. Perhaps if people took more time to learn the translation then we'd get better understanding and greater electricians???

But then what do I know!

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My Metrel, when doing auto L-N, L-E and N-E records values up to 30 meg - so thats what I put on the certs.

 

If I set if to do those individual readings the results often come back with 200 meg!

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