Testing Questions - help.

[chrisgc]

Following the On site guide for IR test and still getting used to testing was wondering why the following:

Readings:

L-N = 566.9 Mohms (Pass)
L-E = 475.9 Mohms (Pass)
N-E = 0.060 Mohms (FAIL)


Yet when I disconnect the earth from the earth bar I get this reading:

N-E = 352.2 Mohm (pass)

1) Why is it that doing it the way which is in the OSG (ie. earth still connected to bar) gets the fail reading on bold above?

2) Is it best to do it as per the OSG or to completely disconnect both Line, Neautral and Earth completely from CU?

3) Also When testing cooker circuit (and it indicates in OSG to test at 'each point') do I test at the back of the cooker control unit OR at the very end of the disconnected cable (the very end of cable which terminates into the rear of cooker?

 

[Guitarist]

What does having neutral and earth connected, and getting that reading tell you?

 

[telectrix]

could it possibly be PME?

 

[keniff]

Testing with the cpc connected to the met will them throw up any ir faults between pipe work also.....

 

[chrisgc]

Any advice as to what could possibly cause that reading - as still trying to learn and best way is to get problems like this as can learn from them?

Also any answers on the other questions in my original post?

 

[chrisgc]

Anyone???

 

[Deleted member 9648]

It is a requirement to IR test with the cpc connected to the means of earthing since the 17th edition.....the reason being that if there is a fault to an earth potential independant of the electrical earthing system on that circuit it may not show up with the cpc disconnected. You clearly need to investigate your N-E result.

@ no 3.....I assume you mean continuity testing?......if the cooker is hard wired test at the cooker terminations,the furthest point on the circuit.

 

[Strima]

Following the On site guide for IR test and still getting used to testing was wondering why the following:

Readings:

L-N = 566.9 Mohms (Pass)
L-E = 475.9 Mohms (Pass)
N-E = 0.060 Mohms (FAIL)


Yet when I disconnect the earth from the earth bar I get this reading:

N-E = 352.2 Mohm (pass)

1) Why is it that doing it the way which is in the OSG (ie. earth still connected to bar) gets the fail reading on bold above? Most obvious answer quoted from Telectrix below.

2) Is it best to do it as per the OSG or to completely disconnect both Line, Neautral and Earth completely from CU? If you get good readings using the first method then all is good, any bad readings you then need to split down the circuits. First method saves a lot of time.

3) Also When testing cooker circuit (and it indicates in OSG to test at 'each point') do I test at the back of the cooker control unit OR at the very end of the disconnected cable (the very end of cable which terminates into the rear of cooker? Full length of circuit that is part of the installation.

The OSG is just a guide, a very helpful one, but if every method was put into it you'd need a bigger van. Testing boils down to common sense and interpreting the results you get, if you have a good understanding of how a circuit works and how it is installed with others it makes things a lot easier.

could it possibly be PME?

 

[ackbarthestar]

could it possibly be PME?

I'm not sure that 60k ohms represents a N-E link.

However I would say that if the IR values are below 2.0M ohms (1.0M ohms) then further investigation would be required. Which from your results, is precisely what has been done.

Most electronic filtering in certain electronic appliances will produce these types of values.

 

[Deleted member 9648]

Strima......it is now a requirement to IR test with the cpc connected to earth,testing with the cable completely disconnected is not acceptable.....612.3.1

 

[Strima]

Strima......it is now a requirement to IR test with the cpc connected to earth,testing with the cable completely disconnected is not acceptable.....612.3.1

I should really get my head into the BGB, cheers for the heads up. This wasn't even mentioned on my 2391 when it should have been.

 

[chrisgc]

With regard to the fail on the N-E result on my IR test - whats the best way to breakdown the circuits to try and pin point the circuit causing the low reading?

Would it be to disconnect each neutral OR earth in turn then to do IR test? Then when I get to the circuit with the fault my tester will then be passing the N-E reading?

 

[HandySparks]

With regard to the fail on the N-E result on my IR test - whats the best way to breakdown the circuits to try and pin point the circuit causing the low reading?

Would it be to disconnect each neutral OR earth in turn then to do IR test? Then when I get to the circuit with the fault my tester will then be passing the N-E reading?

I think I'd leave all the earths connected, open all the breakers/pull all fuses, then for each final circuit disconnect the neutral at the board and IR test the circuit L+N to E. This should identify which circuit has the N to E fault and you can then break down the offending circuit to home in on the fault.


By the way, you need to be a bit clearer when asking questions. For example: Is this a new installation? When you say "I disconnect the earth from the earth bar", do you mean the earthing conductor? Are the main protective bonding conductors also connected to this earth bar or to another main earthing terminal?

 

[chrisgc]

The Failed N-E IR test reading I was getting was because the outbuilding has a small consumer unit inside powering lights and switches.

The main switch of this outbuilding CU was on/closed but when I opened the switch (turned main switch off) the N-E reading was fine.

This is an existing installation that I am practising testing on.

Also one question with regards to IR testing and OSG:

In the OSG (red edition) it says - main switch off .... switches and circuit breakers closed.

Yet figure 10.6 on page 84 clearly shows that the circuit breakers are open/off??

Why is this?

 

[Richard Burns]

Because they do not proof read the book carefully!

For the low N-E IR value if the remote CU was supplied from a way in the main CU then the open switch resolving the problem means there is a fault in the outbuilding, if it supplied by a split from the main supply then, so long as the CU at which you were testing had its main switch off, this should not have had any effect.

 

[chrisgc]

1) So should the Circuit Breakers be in the off/open position or the on/closed?


2) The remote (outbuilding) CU comes from a 'way' in the main CU (located in the house), and turning main switch off on the remote CU makes the IR test (which was being done on a cooker circuit) a PASS.

If what you say means that there is a fault in the outbuilding are you able to suggest some possible things I can do to narrow down the possible problem?

 

[Strima]

Test the individual circuits on the second DB, without seeing it there's no way of diagnosing a fault.

Description of circuits, fixing methods, attached equipment etc would be a start.

 

[chrisgc]

Am going to have a look at it again tomorrow. I know the person and am using the CU to get used to testing so have permission to probe away.

1) When testing on the remote cu do i also have the main switch off on the main Cu in the house as well??

2) And also (opposite of above), when testing on the main cu in house should remote cu main switch be off?

3) And to clarify should circuit breakers be in the off/open position or on/closed position?

 

[Guitarist]

The Failed N-E IR test reading I was getting was because the outbuilding has a small consumer unit inside powering lights and switches.

The main switch of this outbuilding CU was on/closed but when I opened the switch (turned main switch off) the N-E reading was fine.

This is an existing installation that I am practising testing on.

Also one question with regards to IR testing and OSG:

In the OSG (red edition) it says - main switch off .... switches and circuit breakers closed.

Yet figure 10.6 on page 84 clearly shows that the circuit breakers are open/off??

Why is this?

Time to get the latest OSG methinks

 

[chrisgc]

Time to get the latest OSG methinks

I know lol - have they updated their picture then on the IR test or is it still the same as the red one?

 

[Guitarist]

I know lol - have they updated their picture then on the IR test or is it still the same as the red one?

It's not always about pictures, you do realise?

 

[spark 68]

In the OSG (red edition) it says - main switch off .... switches and circuit breakers closed.

Yet figure 10.6 on page 84 clearly shows that the circuit breakers are open/off??

Why is this?

I just had a quick look at the old red OSG, the first test with the mains switch off and all circuit breakers on is a global test between live conductors (L-N if SP) but not including the tails, the picture fig 10.6 is testing individual circuits between live conductors, ie. main switch off and circuit breakers off, but in fairness this is not explained in the text.

 

[Richard Burns]

Am going to have a look at it again tomorrow. I know the person and am using the CU to get used to testing so have permission to probe away.

1) When testing on the remote cu do i also have the main switch off on the main Cu in the house as well??

2) And also (opposite of above), when testing on the main cu in house should remote cu main switch be off?

3) And to clarify should circuit breakers be in the off/open position or on/closed position?

1+2) If you are a doing a global test of the whole installation then you are testing the remote CU as well so the remote CU switch would be ON. If you are trying to break things down then you can leave the remote switch off and test the main CU installation and then with the remote switch still off test the remote CU installation.
Think about what you need, the power must be off and the electrical connections around the whole installation must be connected.

3) For a global test of an installation all the circuit breakers should be on and the main switch off.

 

[chrisgc]

Thanks for the help guys.

I dont mind running into problems testing as best way to learn how to find faults. Having this forum gives you more confidence and makes you realise that you're always learning.

Thanks again.​

 

[spark 68]

So in reference to my earlier post:

1) When testing on the remote CU do i also have the main switch off on the main CU in the house as well??

2) And also (opposite of above), when testing on the main CU in house should remote CU main switch be off?

3) And to clarify should circuit breakers be in the off/open position or on/closed position?

​

1) you need to isolate the remote CU from being powered, as the IR test is a dead test, this depends on how the remote CU is powered eg. by an MCB from the main CU ?, if it is powered via an MCB in the main CU then this needs to be off/open.

2) depends on if you are testing both cu's together

3) this depends if you are testing all the circuits together (global) or indvidually. If all MCB's are on then all of the L conductors will be connected together via the L comb/busbar this is global, if they are in the off position you will need to test each one at the out going side of each MCB.

 

[chrisgc]

1) you need to isolate the remote CU from being powered, as the IR test is a dead test, this depends on how the remote CU is powered eg. by an MCB from the main CU ?, if it is powered via an MCB in the main CU then this needs to be off/open.

2) depends on if you are testing both cu's together

3) this depends if you are testing all the circuits together (global) or indvidually. If all MCB's are on then all of the L conductors will be connected together via the L comb/busbar this is global, if they are in the off position you will need to test each one at the out going side of each MCB.

With regard to the above answer in red:

So doing an IR test on main cu in house AND with the remote cu main switch on - this was why I was getting a fail on my ir test between n-e?

As in post number 15 indicates that there is a fault in the outbuilding but you have indicated that the outbuilding cu main switch should be in the off position anyway (which when it is off clears the fail on the ir test)???

 

[spark 68]

If it is any consolation Chrisgc, the same mistake in the diagrams is carried over in the new GN3, ie. it shows testing an individual circuit but showing all of the MCB's on when they should be off.

 

[chrisgc]

If it is any consolation Chrisgc, the same mistake in the diagrams is carried over in the new GN3, ie. it shows testing an individual circuit but showing all of the MCB's on when they should be off.

OSG (red) page 84 figure 10.6 - Are the MCB's off?? It looks like they are off - are they??

But description on page 83 states circuit breakers closed (on)?

 

[spark 68]

With regard to the above answer in red:

So doing an IR test on main cu in house AND with the remote cu main switch on - this was why I was getting a fail on my ir test between n-e?

As in post number 15 indicates that there is a fault in the outbuilding but you have indicated that the outbuilding cu main switch should be in the off position anyway (which when it is off clears the fail on the ir test)???

Richard answered this further back, You would then break the installation down further, turn off the remote CU main switch, turn all MCB's off in the remote CU, and then test each outgoing side of each MCB in the remote CU and then you will be able to identify which circuit is giving the faulty reading.
It is a question of elimination.

BTW on an existing installation it is usually better to link your L-N and test to earth in case a load is still connected, it is also wise if you are going to test between L-N to test at 250V first (soft test), in case you damage something such as central heating controllers, alarm panels, aerial amplifiers and the like which may still be connected.

 

[chrisgc]

Richard answered this further back, You would then break the installation down further, turn off the remote CU main switch, turn all MCB's off in the remote CU, and then test each outgoing side of each MCB in the remote CU and then you will be able to identify which circuit is giving the faulty reading.
It is a question of elimination.

BTW on an existing installation it is usually better to link your L-N and test to earth in case a load is still connected, it is also wise if you are going to test between L-N to test at 250V first (soft test), in case you damage something such as central heating controllers, alarm panels, aerial amplifiers and the like which may still be connected.

Linking L-N and test to earth - Is this perfectly acceptable if and when assessed by an assessor for competent person schemes? (long way off yet).

 

[spark 68]

OSG (red) page 84 figure 10.6 - Are the MCB's off?? It looks like they are off - are they??

But description on page 83 states circuit breakers closed (on)?

Yes they are off lol, but it does not match the text in the red OSG, that drawing should be in the new GN3 for testing indvidual circuits, or at least the labelling.

 

[spark 68]

Linking L-N and test to earth - Is this perfectly acceptable if and when assessed by an assessor for competent person schemes? (long way off yet).

Yes perfectly acceptable on an existing installation, on a new build or rewire and the like you should do the L-N test on the cables before the accessories are fitted.

As I said some items don't like 500V up 'em, and it is possible to damage vunerable equipment, and on a job if you break it you have to fix it/replace it, and some central heating controller panels and the like are bloody expensive.

 

[chrisgc]

Yes perfectly acceptable on an existing installation, on a new build or rewire and the like you should do the L-N test on the cables before the accessories are fitted.

As I said some items don't like 500V up 'em, and it is possible to damage vunerable equipment, and on a job if you break it you have to fix it/replace it, and some central heating controller panels and the like are bloody expensive.

Totally understand about doing the 250v IR test first.

Is it not wise for electricians to have a brief disclaimer form for when assessing or testing installations (for IR test), in case something is blown due to voltage being put through?

Just a thought really as we sign forms for many things in life like operations, dental treatment etc for experts in their field to operate yet if anything goes wrong they have you sign disclaimer beforehand to cover themselves.

Would it not be an idea for electricians too and prudent in the likely hood that something was damaged by mistake?

 

[spark 68]

Totally understand about doing the 250v IR test first.

Is it not wise for electricians to have a brief disclaimer form for when assessing or testing installations (for IR test), in case something is blown due to voltage being put through?

Just a thought really as we sign forms for many things in life like operations, dental treatment etc for experts in their field to operate yet if anything goes wrong they have you sign disclaimer beforehand to cover themselves.

Would it not be an idea for electricians too and prudent in the likely hood that something was damaged by mistake?

By linking the L to N you remove the risk all together, the other option is to double check that everything is disconnected by testing at 250V first, and only if that is clear then do the test at 500V, you can also link/bypass any vunerable equipment.
Again it is better to test @ 250V first in case you have missed something.

I would only bother with the L-N test if it was a newly installed circuit/s that I know exactly what is what, but on a CU change and an EICR etc. I just do the L+N to E test, as I said this is an acceptable method..

Sometimes Neon indicators on shower switches and the like give false readings on the L-N test.

 

[spark 68]

I have just re-read your earlier posts and you say you have a N-E fault at the remote CU, as the MCB's are single pole devices turning these on or off in the remote CU won't isolate or narrow your fault down.

I would start by opening the main switch at the remote CU then test between N and E bars, if your fault is now clear then it may be a supply cable fault, if it it is still reading faulty then you will have to do as some of the other posters have said much further back in the thread, and start disconnecting the Neutrals from the N bar in the remote CU, and test each one to earth, this will at least narrow it down to one circuit, unless you are really unlucky and have more than one fault lol.

Edit: do you have any RCD's or RCBO's in the remote CU ?

 

[ackbarthestar]

Five pages of distance learning and there's still confusion.

The answer was in the first couple of replies.....

 

[chrisgc]

I have just re-read your earlier posts and you say you have a N-E fault at the remote CU, as the MCB's are single pole devices turning these on or off in the remote CU won't isolate or narrow your fault down.

I would start by opening the main switch at the remote CU then test between N and E bars, if your fault is now clear then it may be a supply cable fault, if it it is still reading faulty then you will have to do as some of the other posters have said much further back in the thread, and start disconnecting the Neutrals from the N bar in the remote CU, and test each one to earth, this will at least narrow it down to one circuit, unless you are really unlucky and have more than one fault lol.

Edit: do you have any RCD's or RCBO's in the remote CU ?

There are no RCD's or RCBO's on the remote CU.

 

[Guitarist]

Just wondering Chris, have you been taught fault-finding techniques at all, or are you learning "as you go" ? It's very important to really understand the mechanics of your system before "probing away".

 

[chrisgc]

Just wondering Chris, have you been taught fault-finding techniques at all, or are you learning "as you go" ? It's very important to really understand the mechanics of your system before "probing away".

Am probing only on one CU (of which I know the person and he is not in the property as of yet. So getting used to the tester I have. Was used to a megger tester a few years ago (not for long though).

I have a metrel tester at the moment. So getting used to it.

 

[Guitarist]

Am probing only on one CU (of which I know the person and he is not in the property as of yet. So getting used to the tester I have. Was used to a megger tester a few years ago (not for long though).

I have a metrel tester at the moment. So getting used to it.

I can appreciate the fact that some testers take a bit of getting used to. That is a bit different to understanding how to test, and what results to expect. Just wondered what your training entailed.