Low IR with Multimeter.

[JohnW2]

EFLI & John, are you therefore saying that if it is meggered at 500v there is a distinct probability that the IR will be very low?

Not necessarily (indeed, 'probably not'). The most positive thing we can say is really the other way around - that if the IR measured at 500V were actually 'very low', it's quite possible that a 9V multimeter would give a 'very high' reading.

However, given that the chances are that the resistance of your element would not be low at 500V, that doesn't really help you with your 'gambling decision'!

Kind Regards, John

 

[Astra99]

Test with a Portable Appliance Tester? Surely this will give a conclusive result?

My Seaward PAT500H does an insulation test at 500V and (IIRC) 100mA but the leakage test is at ELV and something like 40mA (I do not have the manual handy, working from memory)

 

[EFLImpudence]

EFLI & John, are you therefore saying that if it is meggered at 500v there is a distinct probability that the IR will be very low?

I think there is but that does not mean your element is no good.

Even my 0.19MΩ is only a leakage of 1.26mA so no one would notice.
(It may well get better with usage)

Tests rather inconclusive from your point of view.


It depends what you want to do.


Just seen PBoDs post - a new element (when/if yours doesn't work) is probably the cheapest.

 

[JohnW2]

My Seaward PAT500H does an insulation test at 500V and (IIRC) 100mA ...

To get 100 mA out of 500V, you'd need a resistance of only 5 kΩ (aka 0.005 M&#937. At the sort of IR values we expect/hope to get, the current should only be a handful of microamps.

Kind Regards, John

 

[conny]

Thanks lads for all the replies.
Think I will get one of the electricians from work to pop over with his test gear next week and do it for me. Cost me a few pints but a lot cheaper than £125!
Pity I'm not in work this week.

Once again thanks for your replies.

 

[EFLImpudence]

To get 100 mA out of 500V, you'd need a resistance of only 5 kΩ (aka 0.005 M&#937. At the sort of IR values we expect/hope to get, the current should only be a handful of microamps.

The 1651/2/3 uses 1mA at all voltages, varying the resistances.

 

[JohnW2]

To get 100 mA out of 500V, you'd need a resistance of only 5 kΩ (aka 0.005 M&#937. At the sort of IR values we expect/hope to get, the current should only be a handful of microamps.

The 1651/2/3 uses 1mA at all voltages, varying the resistances.

1mA is the maximum limit of what current it will allow to flow. However, at 500V, you'd only get that 1mA flowing if the IR were 0.5 MΩ or less. At higher IR's the current would be less - e.g. 0.5 mA at 1 MΩ, 0.05 mA (50 μA) at 10 MΩ, 0.005 mA (5 μA) at 100 MΩ and 0.001 mA (1 μA) at 500 MΩ.

Kind Regards, John

 

[flyingsparks]

I'll check 9V and 500V for you - back soon.

I'm not sure what you had in mind, but I've just done an experiment:

8"x5" card, soaked in tap water then 'dabbed' dry. Measurement across the 8" axis:

1000V - 0.40 MΩ
500V - 0.45 MΩ
250V - 0.45 MΩ
9V multimeter - >20 MΩ
another 9V multimeter - 18.9 MΩ

Kind Regards, John

Interesting!

 

[JohnW2]

Interesting!

Quite so! As I said, "it's a bit of an eye-opener for those who doubt that 'IR' measured at (extra/very!) low voltage (as with multimeters) can be very misleading!"

Kind Regards, John

 

[EFLImpudence]

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• EFLImpudence
• 16 Jul 2014
• 1

 

[JohnW2]

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• EFLImpudence
• 16 Jul 2014
• 1

Sure, but those tabulated figures are just an elementary lesson in Ohm's Law:

50V = 1mA through 50 kΩ
100V = 1 mA through 100 kΩ
250V = 1 mA through 250 kΩ
and, as I said ...
500V = 1mA through 500 kΩ

I'm sure that, somewhere, it is stated that 1 mA is the maximum test current (which makes sense), but I confess that I'm struggling to find it in the manual.

You obviously cannot be suggesting that the test current is 1 mA for all IRs - as Mr Ohm would tell you, to get 1 mA through, say, 100 MΩ would require a test voltage of 100,000 V - so what did you think the situation was? The current which actually flows is, obviously, simply a matter of the test voltage and the IR - again, per Mr Ohm.

Kind Regards, John

 

[Monkeh]

You obviously cannot be suggesting that the test current is 1 mA for all IRs - as Mr Ohm would tell you, to get 1 mA through, say, 100 MΩ would require a test voltage of 100,000 V - so what did you think the situation was?

And of course, the insulation would break down and conduct long before you reached such a voltage.

 

[JohnW2]

You obviously cannot be suggesting that the test current is 1 mA for all IRs - as Mr Ohm would tell you, to get 1 mA through, say, 100 MΩ would require a test voltage of 100,000 V - so what did you think the situation was?

And of course, the insulation would break down and conduct long before you reached such a voltage.

Maybe, but PVC is amazingly good. IIRC, the dielectric strength of PVC is around 60 kV/mm. If that's correct, if one were testing between two conductors which each had about 0.8mm thickness of PVC insulation, it would quite possibly just about tolerate 100 kV!

Kind Regards, John

 

[ban-all-sheds]

See if you can interest the photonic induction nutca^H^H^H^H^H^H guys to have a go at that.

 

[Monkeh]

It only takes the tiniest flaw at those voltages.

He's done this to 1kV rated, 10kV tested insulated screwdrivers already.