Does anyone have a list of expected Insulation Resistance test results?

[HappyHippyDad]

I would like to get a better understanding of what the specific IR test results could indicate. I'm talking more about the below 1MΩ area. I'm talking about.. wet plaster, flooding (sitting in water), degraded insulation etc etc, I realise it wouldn't give the answer but it may help in narrowing the fault down.

Has anyone made their own list already and if so it would be great if you could post it up?

I started my list today and it has one entry.. 'both conductors sitting in pool of water' 0.003MΩ!


Just as an after thought I have a result which I realise is deemed as adequate (4.45MΩ), but this was between the earth bar and the neutral bar with ALL conductors removed. Is this out of the ordinary? It was an old rewireable sub main board and wasn't full of dirt inside. They had had a recent flood which had been by the CU but that was 2 weeks before and it was completely dry.

Cheers guys!

 

[Octopus]

One thing to search for when you get a low IR reading is a RCD device on the circuit.

They always give really low IR readings.
neons
fans
lamps

 

[telectrix]

wet apprentices give a reading of about 750 - 800Ω

 

[Marvo]

wet apprentices give a reading of about 750 - 800Ω

At 10KV I've seen apprentices as low as 500 ohms even before they wet themselves. Please note you should remove the iphone out of the apprentices pocket when testing at voltages >250V

 

[Marvo]

A common cause of low insulation readings is heater elements. The mica powder they're filled with is hygroscopic and when it becomes even slightly damp you'll see it on an IR test. Sometimes they can be 'repaired' by oven baking them in an oven. They can absorb moisture over time just from the surrounding air especially if they're not used very often. When we get deliveries of element tubes we have to batch bake them for 24 hrs before we test them.

 

[tazz]

How long is a piece of string, every situation will be different, you can only give a aprox readings. example, pure water is a insulator, but due to impurities, it conducts.....so southern hard water, will conduct better then northern soft water

 

[Octopus]

How long is a piece of string, every situation will be different, you can only give a aprox readings. example, pure water is a insulator, but due to impurities, it conducts.....so southern hard water, will conduct better then northern soft water

We've got a water softener - does that make me a northerner?

 

[tazz]

We've got a water softener - does that make me a northerner?

Definately ...!!

 

[Marvo]

As Tazz says it's the impurities and salt in the moisture that allows it to conduct so you're never going to get hard and fast figures or ranges of figures for insulation faults. Also with an IR test you're seeing the cumulative effect of numerous insulation faults or areas of leakage, some of them will be inconsequential and not worth finding but the larger faults obviously would be in need of localising. When we used to have analogue Mega testers (with a needle rather than digital display) you could get an idea of the severity of moisture faults by the way the reading would trend over time but this isn't as easy to see with a digital tester.

You'd be better off just making a list of things that are likely to cause a poor reading such as neon indicator lamps as previously mentioned..

 

[telectrix]

We've got a water softener - does that make me a northerner?

nah. it makes you a southern softie. :smilielol5::smilielol5::smilielol5:

 

[ipf]

nah. it makes you a southern softie. :smilielol5::smilielol5::smilielol5:

Beat me to it.................Scaaase sod.

 

[Marvo]

You do get some interesting ones where there's a polarized insulation fault due to mould or corrosion. IR testers use a DC test voltage so these types of faults would only be visible if you reverse the test probes to test in both directions.

If you want to do some homework Happyhippydad then see if you can find out why IR testers use a DC test voltage rather than an AC voltage.

 

[shanky887614]

You do get some interesting ones where there's a polarized insulation fault due to mould or corrosion. IR testers use a DC test voltage so these types of faults would only be visible if you reverse the test probes to test in both directions.

If you want to do some homework Happyhippydad then see if you can find out why IR testers use a DC test voltage rather than an AC voltage.

quite interesting

www.openelectrical.org/wiki/index.php?title=Insulation_Resistance_Test

 

[Lucien Nunes]

Just as an after thought I have a result which I realise is deemed as adequate (4.45MΩ), but this was between the earth bar and the neutral bar with ALL conductors removed. Is this out of the ordinary?

Yes. I would expect >500MΩ for the insulation itself, but you have to be sure that high humidity in the building after the flood wasn't causing surface leakage. A quick waft with a hairdryer can differentiate between that and genuine insulation failure.

And that is one of the problems with generalising about insulation tests: nothing encountered in the everyday life varies over such a wide range as resistivity. A tiny smear of condensation can have a resistance lower than a square yard of porcelain so a seemingly irrelevant or invisible factor can dwarf an obvious one.

Experience is the key, watching how things vary with time and voltage as mentioned above, e.g. to distinguish between dielectric absorption, surface leakage and bulk resistivity. Even after 15 years of re-testing our own installations I still sometimes find something of interest in the readings.

 

[Marvo]

quite interesting

Insulation Resistance Test - Open Electrical

From your linked page;

One of the advantages of the IR test is its non-destructive nature. DC voltages do not cause harmful and/or cumulative effects on insulation materials and provided the voltage is below the breakdown voltage of the insulation, does not deteriorate the insulation.

Okay there's one reason, see if you can think of another.

 

[shanky887614]

From your linked page;


Okay there's one reason, see if you can think of another.

the idea of an ir test is to put 500v etc at a fixed load to check the insulation.

it would be less reliable with ac because of the sign wave bassically the pushing and pulling means its not a constant load like dc

 

[Marvo]

the idea of an ir test is to put 500v etc at a fixed load to check the insulation.

it would be less reliable with ac because of the sign wave bassically the pushing and pulling means its not a constant load like dc

Not less reliable because AC tests just use the RMS values or the 'heating value' of the AC waveform.

The test would be less accurate using AC rather than DC. I'll leave you to find out why.

 

[davesparks]

the idea of an ir test is to put 500v etc at a fixed load to check the insulation.

it would be less reliable with ac because of the sign wave bassically the pushing and pulling means its not a constant load like dc

Neither of them is a load, a load is the thing you apply the potential to.

 

[shanky887614]

Not less reliable because AC tests just use the RMS values or the 'heating value' of the AC waveform.

The test would be less accurate using AC rather than DC. I'll leave you to find out why.

bassically using an ac tester it cant distinguish between

The desensitized ac dielectric tester cannot tell the difference between 5 and 15 mA. Consider what happens if the circuit under test has capacitance between the conductors that causes 5 mA to flow in normal conditions during the test. A device under test (DUT) with faulty insulation that allows 300% of the normal amount of current to flow (15 mA) would still be considered an acceptable test result by the desensitized ac dielectric tester.

www.gigavac.com/apps/dielectric.html.edu

 

[Marvo]

Yep. That's pretty much it. The capacitance between wires that run parallel over any distance would appear as a short circuit or at least a partial short to an AC test so you'd no longer be testing the dielectric or insulation between them, you'd just see the capacitance.

It's interesting to think though that an earth leakage device would also see any losses due to capacitance between live and earth. So this means firstly that an IR test is not an accurate indication as to whether an RCD will trip and secondly if you want to measure leakage current you must do it as a live test with an e/leakage clamp around L+N simultaneously rather than as a dead test with an IR tester between L+N shorted together and earth.