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Topic Title: Megger 1720
Topic Summary: IR testing
Created On: 11 December 2012 08:11 PM
Status: Read Only
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 11 December 2012 08:11 PM
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Evening all, I wonder if I could have the benefit of your wisdom. I have noticed that when IR testing sometimes the display runs smoothly round and then displays >999Mohm but other times it may firstly display say, 10.8Mohm or 365Mohm before counting up until >999 shows. This was on an entirely new lighting circuit which I had broken back down into individual sections between the (loop in) switches. I rang Megger who said that this was typically a result of capacitance on the circuit but with plain t&e cable of runs of a few metres this doesn't seem very plausable. The new board has two RCCB both giving very similar 1x 5x and ramp tests. The right hand one would not hold on the above lighting circuit (which is why I had eventually disassembled it) but the left hand one would (which is why I've managed to get back home). Could you explain what I'm seeing and is the >999 just as valid if the meter has had to think about it for a bit as when it is an instant conclusion. Many thanks.

Only dead fish go with the flow. Be a salmon!
 12 December 2012 10:05 AM
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Sounds a little odd - even insulation test as low as a meg or two shouldn't upset a 30mA RCD in the least (1 meg resistance = 0.23mA at 230V) - presumably both RCDs held for the 1/2x test (15mA).

What's different between the test & live conditions? Was anything disconnected for the test (fluorescent fittings for example)? Or face plates removed?

- Andy.
 12 December 2012 05:26 PM
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I was reading an information sheet about rcd's recently, I think it was from Beama, and it mentioned that rcd's can have an affect on insulation readings. Why is this?

Edited: 13 December 2012 at 09:48 PM by leckie
 12 December 2012 05:55 PM
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When you IR test a cable, there is a charging effect (effectively a capacitor) and an absorbtion current although on short runs of T&E I agree it should be imperceptible over time.

The RCD electronics (ie the op amp etc) will influence IR readings (often significantly)



Let the wind blow you, across a big floor.
 12 December 2012 07:29 PM
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Grumpy, (me too but hanging in there)

Since changing to a megger I have noticed the sliding scale much more than I used to with the fluke. I don't think you need to worry about it simply because the megger eventually settles on approximately the same reading, on a circuit in my own home, to that which the fluke went instantly.

It takes a little longer and I don't especially like having to keep my finger on the button, but it will settle.

If the sliding scale is slow to rise and doesn't settle, then I think you are probably boiling off moisture in a Norm-stylee. MIMS is a good candidate for this.

I'd be inclined to get used to it, as I have done since the change, and not worry too much. But you are quite right, it does do exactly what you describe.

 12 December 2012 09:26 PM
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After reading he beama rcd info I set up four different make rcd's on a bench and did a ir test. None read a thing between line and neutral.

Should I ir test a resistor then connect it to the output of an rcd and see if there is any difference? I want to see the change in action. Any suggestions? When you look at a diagram of an rcd it just looks like an in and an out with a imbalance sensor coil operated when the test button is pressed.

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