Global IR testing

[Baker1988]

Thought i would post this in here as i would probily be ripped apart in the main forum,

Is it not acceptable to do a global ir test on a board with rcd's on the circuit, i did not think there was a problem with it as at college they taught us to do it this way, make sure the board is isolated and all breakers and rcd's are switched on!

As i said this is how we was taught a college and even if you view this video just click the link as it will start you off where he says it, http://youtu.be/U4ef9sAiSfM?t=2m13s even he says it, i am only asking as i dont want to be wrong on something so basic and if i am wrong that is obviously where not working with a spark and only working with agencys has messed me up.

 

[Richard Burns]

Global IR testing is generally OK when testing through RCDs in the circuit, however it is possible to damage them with the 500V IR test, therefore in many cases it is best to avoid IR testing at 500V through an RCD.
You are correct that it is taught that it is OK, but I was told that after many tests through an RCD that occasionally one fails afterward and the risk of this happening means that it is not viable to test through RCDs.

Also on many RCBOs they state in their instructions that the neutral and earth tails should be removed before IR testing which effectively limits you to the circuit under test.

There is the consideration that if you do not test through an RCD you have not checked that the RCD is not potentially showing a fault!

I have never really followed why an RCD that may be capable of handling high currents and voltages cannot manage to cope with 500V dc applied but this does appear to be the case in practice.

 

[happysteve]

There is the consideration that if you do not test through an RCD you have not checked that the RCD is not potentially showing a fault!

I have never really followed why an RCD that may be capable of handling high currents and voltages cannot manage to cope with 500V dc applied but this does appear to be the case in practice.

Gosh, that's a lot of negatives! Took me longer to untangle that, than it did Telectrix's puzzle about volt drops!

My practical knowledge of this is limited, but funnily enough our tutor mentioned IR testing through RCDs last night when we were talking about fault finding. He said that it was older RCDs that can't cope, and that "newer ones will be alright."

As for why it can cope with DC but not AC: not sure really. Might have summat to do with inherent inductance (so with AC the impedance is higher than with DC, meaning less power dissipated) but it can't be huge otherwise RCDs would mess about with your power factor.

Thanks to Baker1988 for posting the original query, always helpful to learn.