Confused ...IR readings

[Dannyg8810]

anyone shed some light?

100m run of 4mm nyy cable

New cable and getting 135meg ohm when testing surely this should be >999 on martindale mft

Checked cable and no damage(visual anyway)

I know it's well above minimum permitted but still

 

[davesparks]

Is the cable completely isolated, or are any conductors connected to a load?
How long was the test voltage applied for before taking the reading?

 

[Dannyg8810]

Is the cable completely isolated, or are any conductors connected to a load?
How long was the test voltage applied for before taking the reading?

Not long just usually press test and within seconds it gives me reading it's the et4000 if that helps

 

[Dannyg8810]

Is the cable completely isolated, or are any conductors connected to a load?
How long was the test voltage applied for before taking the reading?

Nothing connected just cable either side
I got 365 from the socket before this

 

[westward10]

Sounds like damp or contamination.

 

[Dannyg8810]

Sounds like damp or contamination.

It's the same give or take between all conductors ,would the reading of 135 be considered acceptable on a new installation cert? So for so many questions I'm fairly new to testing so these results threw me a bit and just filling cert out now

 

[Dannyg8810]

Sounds like damp or contamination.

Last bit of info for you

I had a 3m length left and tested that and got >999

 

[westward10]

Yes it is acceptable but for a new cable of only 100m I would expect above 200 meg.

 

[westward10]

So you are testing this with the conductors at both ends free and nothing connected although strictly speaking the cpc should be connected to earth.

 

[Dannyg8810]

Yes it is acceptable but for a new cable of only 100m I would expect above 200 meg.

At a test voltage of 250 I'm getting >200

 

[davesparks]

Not long just usually press test and within seconds it gives me reading it's the et4000 if that helps

If you carry out IR testing like that then you have to expect to get readings like this on long cables.
The cable forms a capacitor and you need to apply the test voltage for long enough to fully charge this capacitor, otherwise you will not be measuring the resistance of the insulation but will just get a slightly random number.

re-test the cable, but this time make sure you allow long enough to overcome the effect of the capacitance of the cable.

Don't forget to safely discharge the stored energy in the cable.

 

[davesparks]

Sounds like damp or contamination.

Id say it sounds more like the capacitance of the cable since it is brand new.

 

[Ian1981]

Compression fault in an accessory perhaps?
Cables crushed/squashed in too tightly

 

[Des 56]

It can be worthwhile doing a longer test if a reading is not acceptable to you
Try doing the test for a minute or two to see if the value rises
There are a couple of reasons for doing so,I have copied and pasted some info for you

Capacitance:
The capacitance charging current necessary to charge the capacitance of the insulation being tested. This is a transient current which starts relatively high and falls exponentially towards a value close to zero once the circuit being tested is charged electrically. After a few seconds or tenths of seconds, this current becomes negligible compared with the current to be measured.

Absorption:
The absorption current, corresponding to the additional energy necessary for the molecules of the insulating material to reorient themselves under the effect of the electrical field applied. This current falls much more slowly than the capacitance charging current, sometimes requiring several minutes to reach a value close to zero. •
Leakage current
The leakage current or conduction current. This current characterises the quality of the insulation and is stable over time.

 

[Dannyg8810]

It can be worthwhile doing a longer test if a reading is not acceptable to you
Try doing the test for a minute or two to see if the value rises
There are a couple of reasons for doing so,I have copied and pasted some info for you

Capacitance:
The capacitance charging current necessary to charge the capacitance of the insulation being tested. This is a transient current which starts relatively high and falls exponentially towards a value close to zero once the circuit being tested is charged electrically. After a few seconds or tenths of seconds, this current becomes negligible compared with the current to be measured.

Absorption:
The absorption current, corresponding to the additional energy necessary for the molecules of the insulating material to reorient themselves under the effect of the electrical field applied. This current falls much more slowly than the capacitance charging current, sometimes requiring several minutes to reach a value close to zero. •
Leakage current
The leakage current or conduction current. This current characterises the quality of the insulation and is stable over time.

So due to the length off cable this can take longer to get the expected result while testing ?

All other tests fine and compliant as expected

Really helpful and useful this forum

Thanks guys

Anymore info help on this still welcome

 

[davesparks]

So due to the length off cable this can take longer to get the expected result while testing ?

Yes, which is why you continue the test until the reading stabilises or a suitable length of time has elapsed.
I have always used 2minutes for a 500V test.
I think modern higher voltage IR testers perform a timed test (amongst other functions) automatically.

 

[Dannyg8810]

Yes, which is why you continue the test until the reading stabilises or a suitable length of time has elapsed.
I have always used 2minutes for a 500V test.
I think modern higher voltage IR testers perform a timed test (amongst other functions) automatically.

From the socket I tested with the plug from the tester and got 365 and like I've said 135 from the board so wasn't making sense to me

But I now know where I've gone wrong thank you

 

[Toneyz]

It's the same give or take between all conductors ,would the reading of 135 be considered acceptable on a new installation cert? So for so many questions I'm fairly new to testing so these results threw me a bit and just filling cert out now

The good thing is as you say you're fairly new to testing you are questioning the values you get'' is this right, should I be getting this reading ,why have I got this reading.

 

[Dannyg8810]

The good thing is as you say you're fairly new to testing you are questioning the values you get'' is this right, should I be getting this reading ,why have I got this reading.

Yes always learning and being making sure everything I do is correct ,

 

[soms]

So you are testing this with the conductors at both ends free and nothing connected although strictly speaking the cpc should be connected to earth.

This is interesting, I wasn't previously aware of this. There is no explicit mention of it in the OSG, which many trainees/apprentices use as their quick reference/refresher to the testing procedures.
Looking into it, I see it is explained in GN3 as being introduced with the 17th Edition referring reg. 612.3.1.

Don't forget to safely discharge the stored energy in the cable.

Out of curiosity, how would this be done?

 

[Octopus]

^^ you should always do IR tests on a cable with the cpc connected to the cu, along with the other circuits.....

 

[Hellmooth]

If you carry out IR testing like that then you have to expect to get readings like this on long cables.
The cable forms a capacitor and you need to apply the test voltage for long enough to fully charge this capacitor, otherwise you will not be measuring the resistance of the insulation but will just get a slightly random number.

re-test the cable, but this time make sure you allow long enough to overcome the effect of the capacitance of the cable.

Don't forget to safely discharge the stored energy in the cable.

Is this why my Megger will give a low reading when button is first pressed but increases pretty rapidly as button is held down?