Low IR with Multimeter.

[conny]

Simple qiestion.
We are storing our electric double oven cooker in the garage whilst in rented property.
Just over a week ago while we were away from home there was a freak thunderstorm and the garage got flooded to a depth of approximately 2 inches.
As the cooker has a warming oven close to the floor I was worried it may have touched some of the wiring going to the warmer. It doesn't appear to have but I connected my multimeter across the live side of the heating element, (which sits below the base of the warming oven), and the earth terminal. I got a reading of 30M Ohms
As this is using a 9v multimeter could I expect a lower reading if I was to use a 500v megger to test the IR? Is 30M Ohms a safe reading for a cooker in V. Good condition or would you expect higher? It is not connected to any supply, merely in storage in an otherwise dry garage.

Multimeter is a Wavetek DM25XL from RS Components a number of years ago but has always been reliable.

 

[PrenticeBoyofDerry]

30Mohms, is an acceptable reading. I wouldn't expect any issues with it regarding earth leakage.

 

[conny]

Thanks PBoD. I would gladly accept that reading from a megger but this is using a 9v multimeter. Was wondering if using a 500v megger would bring the reading down due to the higher voltage.

 

[JohnW2]

Thanks PBoD. I would gladly accept that reading from a megger but this is using a 9v multimeter. Was wondering if using a 500v megger would bring the reading down due to the higher voltage.

It is, in theory, quite possible that one could get a much lower resistance measurement with 500V than with 9V (you're lucky to have even that much - many multimeters use only 1.5V or 3V!), even though I think it would be fairly unusual.

Unfortunately, you can only really be certain all is OK if you check the IR at a voltage of at least 250V, ideally 500V. Even 250V would not usually be considered adequate in the context of an 'electrical inspection'.

Kind Regards, John

 

[westie101]

So using ohms law work piut the leakage current based on your results, low isn't it!

 

[conny]

The reasoning behind the original question is that the insurance company have said ......."If it fails an electrical test they will cover the cost of the replacement."......... However, I have rung three electricians to ask if they will come out and test it with one quoting £125 plus a call out charge and the other two saying they wouldn't bother with such a small job.
If I knew for certain that using a 500v megger would drag the IR down further then I could borrow one from work and do it myself. Unfortunately I work over 20 miles away and am on holiday at the moment. Don't want to make a 40 mile round trip if someone could indicate that it would be worth the trouble.

Thanks guys.

 

[JohnW2]

So using ohms law work piut the leakage current based on your results, low isn't it!

Indeed, but Mr Ohm doesn't take account of the possibility of resistance varying with voltage!

Kind Regards, John

 

[JohnW2]

The reasoning behind the original question is that the insurance company have said ......."If it fails an electrical test they will cover the cost of the replacement."......... However, I have rung three electricians to ask if they will come out and test it with one quoting £125 plus a call out charge and the other two saying they wouldn't bother with such a small job. ... If I knew for certain that using a 500v megger would drag the IR down further then I could borrow one from work and do it myself. Unfortunately I work over 20 miles away and am on holiday at the moment. Don't want to make a 40 mile round trip if someone could indicate that it would be worth the trouble.

Unfortunately, you're talking about gambling, not the certainty to which you refer. I would say that it's pretty unlikely that the IR would be appreciably lower at 500V, but it's certainly not impossible - and it could theoretically be a lot lower (low enough to need replacing). I'm afraid you therefore have to decide what gambling you want to do!

Kind Regards, John

 

[conny]

Thanks John. As I am not normally a gambler I think I will find someone reliable to test it properly. Certainly not going to take the chance of thinking it is alright only to wire it in at a later date, see that it doesn't trip the protective device, (mcb or fuse), and then someone gets a shock from it.

 

[EFLImpudence]

Answering the other parts of the question:

30MΩ is very good and completely safe.
Even if it has got wet, it will dry.

Do you want a new cooker?
If you have to buy a new element it's no big deal. £20 - £40?



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

 

[JohnW2]

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

 

[EFLImpudence]

Yes, an old oven element -

Two very good multimeters, Fluke 116 and 117 - both 23MΩ

Fluke 1652 multifunction IR @ 500V - 0.19MΩ (yes 0.19)

Very similar results.



Also a storage heater element (are they better made?).

1652 - 150MΩ
Multimeters - Over range of 40MΩ

 

[JohnW2]

Yes, an old oven element -
Two very good multimeters, Fluke 116 and 117 - both 23MΩ
Fluke 1652 multifunction IR @ 500V - 0.19MΩ (yes 0.19)
Very similar results.

Indeed, very similar to my results (in 'big picture' terms) and 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

 

[conny]

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?

 

[PrenticeBoyofDerry]

You could always hire an insulation resistance tester, will cost a lot less than £125.00. or even buy one of ebay.