What causes low IR readings?

[ppelec100]

I’m currently at the second fix stage of a rewire and before all the flooring goes down I’ve decided to do r1+r2 readings to make sure everything is continuous and polarity is correct. I also decided to do insulation resistance tests and have got lower than expected readings.

On the downstairs lighting circuit I got a reading of around 200-300Mohm. After breaking the lighting circuit down I found the reading was caused by the switch cable in the utility room however I can’t get to the entire length to inspect it. I might replace this one as it is a quick run.

I went to test a few radials circuits and they were perfect >1049Mohm. Went to do the upstairs ring and I was getting about 400Mohm Between neutral and earth.

Downstairs ring between live and earth I was getting around 600Mohm then it will climb to about 1000Mohm.

I know these readings are satisfactory as I’ve looked in the guidance note and it says minimum insulation resistance is 1Mohm but on a new install anything less than 20Mohm needs investigating.

I’ve read moisture is the main culprit and can be caused by damp from plastering, but it’s been plastered about 2 months ago.

There are no loads or sensitive equipment on the lighting circuits. On both rings there is a fused spur with no neon however I did forget to take the fuse out but that couldn’t cause these readings could it?

What else can cause readings to be lowered to these numbers?

I know guidance note 3 says less than 20Mohm is needs investigating but would you investigate if you got these readings or is it just over the top and a waste of time?

 

[timhoward]

You've mentioned what I was going to mention - plaster, damp and moisture.
Even though we've had nice weather it takes a very long time for things to dry out.
You've also mentioned that the readings climb, and this suggests your test is drying it out.

It could even be that the cables have been stored somewhere damp before you fitted them.
I wouldn't waste any time looking further into those readings myself; on some houses I work on I'd give anything for those numbers!

Only other idea - were the fused spurs feeding anything with heating elements, and were the spurs switched on during the tests?

 

[7029 dave]

The results should be off scale new cable.when you say fused spur did you isolate them turn them off. You could be testing with loads still on.

 

[ppelec100]

You've mentioned what I was going to mention - plaster, damp and moisture.
Even though we've had nice weather it takes a very long time for things to dry out.
You've also mentioned that the readings climb, and this suggests your test is drying it out.

It could even be that the cables have been stored somewhere damp before you fitted them.
I wouldn't waste any time looking further into those readings myself; on some houses I work on I'd give anything for those numbers!

Only other idea - were the fused spurs feeding anything with heating elements, and were the spurs switched on during the tests?

Hmm you might be right about the moisture on this job as the house has been stripped apart. Theres been no heating since January. Only temporary sockets which the plasters used to power some little heaters to dry the plaster out with the windows open. The cables were also stored in the house when installing which was in January.

Both fused spurs are feeding a single socket which has nothing plugged into at the moment. One is unswitched and the other is switched but I cannot remember if it was switched on will need to do the test again tomorrow. The fuse has been left in both spurs but I can’t imagine this would affect the IR readings?

I’m just a bit baffled as on the switch cable the IR reading was only reading low between the live feeding the switch and earth. And on the rings, one ring the low reading was between L and E the other ring was between N and E as opposed to both L and E and N and E

 

[ppelec100]

The results should be off scale new cable.when you say fused spur did you isolate them turn them off. You could be testing with loads still on.

When you say off scale do you mean greater >999Mohm or what ever the figure is depending on the MFT being used?

one of the fused spurs cannot be isolated as its unswitched, I should have taken the live out the load side and put it in the supply side to bypass the fuse. And the other fused spur is switched but I did not turn it off and not sure if it was on or off to be honest.

There are no actual loads such as tv, washing machine etc plugged in at the moment

 

[7029 dave]

When you say off scale do you mean greater >999Mohm or what ever the figure is depending on the MFT being used?

one of the fused spurs cannot be isolated as its unswitched, I should have taken the live out the load side and put it in the supply side to bypass the fuse. And the other fused spur is switched but I did not turn it off and not sure if it was on or off to be honest.

There are no actual loads such as tv, washing machine etc plugged in at the moment

Yes

 

[ppelec100]

Yes

The switched cable I cannot investigate unless I cause damage to the ceiling instead of doing that I’m thinking of just replacing the cable it will be way easier.

But the rings, do you think that the 13A fuse in the spurs would cause the lower IR readings? There are no other loads on these circuits. There are no neons on the spurs

 

[westward10]

They are not low readings don't worry about it.

 

[ppelec100]

They are not low readings don't worry about it.

Yeah you’re right. I’m over thinking it. Thank you for the reassurance.

Out of curiosity what would cause these less than expected >1049Mohm readings on new cables/rewire?

 

[7029 dave]

Yeah you’re right. I’m over thinking it. Thank you for the reassurance.

Out of curiosity what would cause these less than expected >1049Mohm readings on new cables/rewire?

You could test a length of cable which you can access end to end and take a reading just to see what reading you get. That's what I would do to satisfy me.

 

[westward10]

May not be the cables could be an accessory that has been contaminated.

 

[7029 dave]

May not be the cables could be an accessory that has been contaminated.

True and easier. If I installed or renewed cabling I would expect off scale readings.

 

[ppelec100]

True and easier. If I installed or renewed cabling I would expect of scale readings.

Yeah same this is why it’s irritating me. I was expecting everything off the charts

 

[ppelec100]

May not be the cables could be an accessory that has been contaminated.

the switch cable which I was getting about 200-300Mohm between the permanent live and Earth I disconnected from the accessories and still got the same reading which kinda bugs me.

Being contaminated for the ring circuits is a possibility as they have been filling around the sockets and sanding etc

 

[timhoward]

Put it this way; if you had used a Megger 15xx, the ring circuit readings would have off the scale as it only goes up to 299M at 500v.
I've spent all day getting a circuit from 0.00M to 5M before now and gone home pretty happy!