>299 Mohms ... or not

[Casterbridge]

You press the button, and there is always anticipation - will that needle smoothly run across the screen and promptly exclaim ">299Mohms" or will it start off on its journey across the screen, looking like it will easily keep going, biut ... then ... stumble somewhat to (e.g.) 180, 185 or 190, not giving the ultimate satisfaction that you crave?

So, while a high reading like that appears to be OK in respect of safety, does it really say something is not quite right, for example if you've installed a new completely board and all the circuits are new (and assuming that everything is off and alll sensitive equipment removed)?

 

[PrenticeBoyofDerry]

If you have installed a new circuit, then your IR reading should measure to infinity on your meter.
if not something is amiss!

 

[JohnW2]

You press the button, and there is always anticipation - will that needle smoothly run across the screen and promptly exclaim ">299Mohms" or will it start off on its journey across the screen, looking like it will easily keep going, biut ... then ... stumble somewhat to (e.g.) 180, 185 or 190, not giving the ultimate satisfaction that you crave? .... So, while a high reading like that appears to be OK in respect of safety, does it really say something is not quite right, for example if you've installed a new completely board and all the circuits are new (and assuming that everything is off and alll sensitive equipment removed)?

I sympathise (and empathise) with what you're saying about that feeling of satisfaction, but don't forget that ludicrousl low IR measurements (sub 10M&#937 are officially regarded as 'acceptable'.

In some senses, it's actually more reassuring to get a 'measurable', but very high, answer, since >299MΩ (or >499MΩ or whatever) can result from one not having connected one's meter satisfactorily! It takes remarkably little (a little moisture, dust, or insect parts!) to get these 'measurable, but very high' readings. On a brand new circuit, one would probably expect to get 'unmeasurably high' readings, but if there are any pre-existing components in the circuit, then that might not be the case.

Kind Regards, John

 

[EFLImpudence]

If Casterbridge is doing a 'global' IR test then the results are fine.

 

[westie101]

Try using a 5kV unit on a couple of km of large size 11kV cable, the reading never stops rising as the cable charges up.
A bit of overhead line and you would be surprised how low a reading can be energised

 

[JohnW2]

If Casterbridge is doing a 'global' IR test then the results are fine.

I must say that I would have said that readings >150MΩ were probably 'fine' for any IR test (even if only of a single circuit) - although, as I said, I'd be surprised if the IR were measurable if the entire circuit were brand new.

Kind Regards, John

 

[JohnW2]

Try using a 5kV unit on a couple of km of large size 11kV cable, the reading never stops rising as the cable charges up.

Is that what you do? I would have expected/guessed that (for 'obvious' reasons), as with LV work, one would undertake an IR test using a DC voltage at least as high as the peak of the operating voltage.

Kind Regards, John

 

[westie101]

The other test would see that (a new laid cable would see 13.6kV L-E if I recall supplied from a +/- 25kV pressure test set. On an older 3 core we would test all 3 to earth at -13.6kV and then core to core at 26kV all for 15 minutes), but as we now use XLPE cables at 11kV there is a restriction regarding maximum DC voltage. XLPE should be tested using VLF AC

There are also time restrictions if we are testing into certain switchgear when parts of that gear are still live (1 minute maximum).

All in all a small briefcase sized battery operated 5kV Megger is handier to carry than a DC pressure test set needing 2 to carry it and an LV supply. The Megger is permitted so we all use them.

 

[JohnW2]

... but as we now use XLPE cables at 11kV there is a restriction regarding maximum DC voltage. XLPE should be tested using VLF AC ...

That's interesting - I was not aware of that.

Kind Regards, John

 

[Risteard]

If Casterbridge is doing a 'global' IR test then the results are fine.

I must say that I would have said that readings >150MΩ were probably 'fine' for any IR test (even if only of a single circuit) - although, as I said, I'd be surprised if the IR were measurable if the entire circuit were brand new.

Kind Regards, John

Generally speaking circuits should not be tested independently of each other as the requirement is to ensure that the insulation resistance of the installation (or distribution circuit with all final circuits connected for larger installations) is greater than the figures quoted. If circuits are tested individually then one would need to calculate the effect of the parallel resistances in order to ascertain the insulation resistance value.

 

[JohnW2]

Generally speaking circuits should not be tested independently of each other as the requirement is to ensure that the insulation resistance of the installation (or distribution circuit with all final circuits connected for larger installations) is greater than the figures quoted. If circuits are tested individually then one would need to calculate the effect of the parallel resistances in order to ascertain the insulation resistance value.

That's true in terms of confirming compliance with some arbitrary 'requirement' for a whole installation (or sub-installation) - and it obviously makes sense, where possible, to first undertake a 'global IR test', since if that reveals a very high result (and I mean 'very high', not merely 'higher than the required minimum'), then no further testing is required.

However, if the global figure is anything but 'very high' then I, for one, would want, as far as possible, to test individual circuits in order to identify/locate (and, if appropriate, rectify) the cause of the 'less than very high' global measurement. Others may perhaps be comfortable to 'look no further' provided that the global IR is above the 'required minimum'.

Kind Regards, John