Insulation Resistance - Neutral - Earth Reading Issue?

[chrisgc]

Hi all

I am practising with my megger 1553 and when doing a IR test in my ring socket circuit and I put the probes onto Neutral - Earth I get a reading of 7,31 m Ohms.

Is this acceptable?

And also am I doing the test correctly (for the explained problem above):

Its an existing instalation:
- Main Switch OFF
- Circuit Breaker for the Ring Circuit ON
- All socket outlets switched ON
- Probes to N-E
- Press Test button
- Take reading 7,31 M Ohms

NOTE: When testing the N-L & L-E readings are both >299

Am I doing it right or have I got switches in wrong positions etc AND/OR is it best to test the above way (from OSG) or is it better to disconnect L,N & E cables of the individual circuit to be tested and place croc clips on the specific circuit cables and test?

 

[telectrix]

think it's because all your neutrals and earths are commoned up in the CU. test L/E, you are testing 0ne circuit. on N/E, you are testing all circuit in parallel. if you take the neutrals and Earths out of the terminals in the CU, you should get correct readings

 

[chrisgc]

think it's because all your neutrals and earths are commoned up in the CU. test L/E, you are testing 0ne circuit. on N/E, you are testing all circuit in parallel. if you take the neutrals and Earths out of the terminals in the CU, you should get correct readings

Hi telectrix,

I have tested the individual circuit cables by disconnecting them from there terminals and do get a reading for all at >299 m ohms.

So this is fine and yes you were right.

Can I just ask the following;

Is the individual circuit IR test (by taking the neutral and earth of the circuit to be tested on) fine to do when it comes to the point of being assessed by competent person scheme acceptable (only asking as the individual circuit test method on IR testing is different to the method explained in the OSG page 84-85)? OR does the whole CU have to be tested on by leaving cables in the terminals as explained in osg?

Finally on the IR test am I right in saying MAIN SWITCH - off, CIRCUIT BREAKER (to be tested on) - on and all SOCKET SWITCHES - on.

 

[telectrix]

individually do continuity first.then IR. if you are measuring the circuit L/E, measure from the load side of the MCB. MCB OFF. You are not testing the MCB on IR test. then L/N and L/E with them out of terminals. you can do N/E acrosss all circuits, but as you have found, you are reading several circiuts in parallel, so will get a lower reaqding. Main Switch OFF of course

 

[chrisgc]

individually do continuity first.then IR. if you are measuring the circuit L/E, measure from the load side of the MCB. MCB OFF. You are not testing the MCB on IR test. then L/N and L/E with them out of terminals. you can do N/E acrosss all circuits, but as you have found, you are reading several circiuts in parallel, so will get a lower reaqding. Main Switch OFF of course

Thanks for that kind Sir.

I will get back to practising tomorrow as kids home now.

Just want to be able to do the testing like second nature as looking to join competent person scheme next year sometime.

So is doing an Insulation Resistance test on individual circuits (with Neutral, Line and Earth cables disconnected from CU) ok if doing in front of a competent person sceme assessor???

 

[telectrix]

So is doing an Insulation Resistance test on individual circuits (with Neutral, Line and Earth cables disconnected from CU) ok if doing in front of a competent person sceme assessor???

IMO yes. the cert. for the installation requires you to give the IR L/N, L/E and N/E for each individual circuit. you can leave L in the mcb, as with the mcb off, it's not connected to anything. make sure everything is unplugged and sockets off, esp if neons are anywhere. they can give you false readings. on lighting, test with lamps out and switches on

 

[chrisgc]

Thanks for the time and info telectrix.

Much appreciated.

 

[WayneL]

Hi Chris,

Guidance Note 3:

Simple installations that contain no distribution circuits should be tested as a whole.

We were taught that circuits should be terminated in the CU for IR tests as termination afterwards could cause the problems that you are actually testing for.

The test should initially be carried out on the complete installation.

If this produces a reading less than 2 M Ohms, then further investigation may be required (e.g. testing individual circuits.

The reading that you got between N - E is fine and would be recorded on the cert, for each circuit, as >7.31 M Ohms - because, as you are testing in parallel, each individual reading will be bigger than the overall reading.

I think you need to get your head around this prior to any assessment, just in case it's what they are expecting of you.

Some assessors might be ok with you dismantling circuits, others might think you are doing un-necessary testing because you don't understand.

Hope that helps.

 

[chrisgc]

WayneL

Thanks for your answer - now I am confused lol.

I am trying to work to the OSG book (after all the name entails that that's the book to take on-site).

So yes I do agree with you that doing it that way is right.

I haven't booked any assesments up so no worries there - but I want to be able to be totally comfortable with the testing.

SO to recap the way to do it is the following:

Main Switch - OFF
Circuit Breaker - ON (of Circuit being tested - Ring Socket OR should they all be ON)
Socket Outlet Switches (Appliances Removed) - ALL SWITCHED ON
Test between - L/N, L/E & N/E.
Record Results

Is the above correct?
So no disconnection of cables at CU is required?

Hear from you shortly WayneL

My measurements I was getting were fine?

 

[telectrix]

your measurements were ok. testing L/N and L/E tested each circuit individually as there were no parallel paths. your N/E readings are for circuits in parallel as the neutrals and earths were commoned together in the CU. going back to basics, parallel resistors R= R1xR2/R1+R2. eg if you have 2 parallel circuits , 5meg and 10meg, reading would be(5x10) / (5+10) = 50/15 = 3.333meg. so you see why you get a lower reading. the more parallels you have, the lower the total IR reading.

 

[chrisgc]

your measurements were ok. testing L/N and L/E tested each circuit individually as there were no parallel paths. your N/E readings are for circuits in parallel as the neutrals and earths were commoned together in the CU. going back to basics, parallel resistors R= R1xR2/R1+R2. eg if you have 2 parallel circuits , 5meg and 10meg, reading would be(5x10) / (5+10) = 50/15 = 3.333meg. so you see why you get a lower reading. the more parallels you have, the lower the total IR reading.

I think I am more concerned with:

Should I be testing the system as a whole OR should individual circuit be tested?

This is on my own CU (so main supply is obviously connected) so using it as practise for example if adding an extra socket switch for a future assesment.

Would I test the whole cu or the actuall individual circuit.

There seems to be different methods in different books.

OSG seems like the easier method as no disconnecting of cables etc but need to know if this is the right way?

Other books indicate taking CPC or neutral off etc.

This is becoming quite frustrating to be honest as there seems to be no definative answer BUT still interesting learning.

Is it possible you could do a walkthrough and post.

 

[WayneL]

Hi Chris,

If it was me, and taking into consideration the variety of different boards now used (i.e. 16th edition split load, 17th dual rcd etc) the simplest method that would work with them all would be as follows:

Main switch open (off)

RCDs open (off)

MCBs open (off)

Test between earth bar and neutral bar(s)

Test between outgoing side of each MCB and earth bar.

Test between outgoing side of each MCB and neutral bar(s)

Job done

The result of the first test is for all circuits, the result of the second two tests is for each individual circuit.

To test the whole installation at once the only difference would be that you would have all MCBs closed(on), and test between the busbar and neutral and the busbar and earth.
The problem with this is that you have to consider the RCDs and the possibility of damage from the 500v.

Hope that explains it better

Wayne

 

[Des 56]

Chris
A very straightforward and uncomplicated procedure has been desribed and explained by Wayne.
Follow that and you wont go far wrong

The overall reading will be lower when tested individually,but the recomended whole of installation test, can be carried out if some low readings make the individual readings seem suspect,which for initial verification,it would be very unlikely

You could either do the whole installation test or calculate it from the individual readings, if you want to be extra precise for your certificate values
The books dont always reflect the practical on the job situations.you will find that a safe installation can be determined with different methods to what is in the books

 

[chrisgc]

Thanks for the reply.

Just a quicky.

Am I right in saying all switches (sockets or lights) have to be switched off? Unless of course an item cannot be unplugged from that particular socket outlet etc?

 

[WayneL]

Thanks for the reply.

Just a quicky.

Am I right in saying all switches (sockets or lights) have to be switched off? Unless of course an item cannot be unplugged from that particular socket outlet etc?

Hi Chris,

Light switches should be on with lamps removed. If the lamps/loads can't be removed then you would have to leave the switches off and mark as a limitation for the test. (or test L&N to E)
This, of course, wouldn't apply to an Initial Verification.(as you can't have limitations)
Don't forget to switch one of your two-way switches and test again. (if there are any)

I would try to unplug everything from socket outlets, rather than take a chance on damaging any of the customers equipment.
You might not actually damage anything, but if you leave it plugged in and it doesn't work afterwards, you're bound to get the blame

If something really can't be unplugged, you might have to test with L & N connected together to earth. Bearing in mind, this is the only IR requirement on a PIR anyway - it's only 'Initial Verification' that requires full IR testing.