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.