I think that this is one for ohms law

[gpz900jim]

Hi, I was wondering if anyone can explain to me how to work this out...... When taking an insulation resistance reading beteeen line and cpc, at what reading in megaohms might you expect the 30 milliamp leakage threshold to be reached that would trip the rcd? Its just a theroretical question, its not to do with any work that im on. Its just that I had a reading yesterday of around 465 megahoms testing some existing wiring. I know thats fine as the rcd was holding in ok, I was just wondering how low that reading can go before tripping will occour ? Cheers

 

[andyb]

R=V/I
Therefore R=230/30ma
So R = 7.67K ohms

Don't forget that most 30ma RCDs will trip at less than 30ma, also the voltage is often higher than 230v.

 

[Dic Penderyn]

Hi, I was wondering if anyone can explain to me how to work this out...... When taking an insulation resistance reading beteeen line and cpc, at what reading in megaohms might you expect the 30 milliamp leakage threshold to be reached that would trip the rcd?


<2 Mega ohms suggests a latent fault!

 

[gpz900jim]

Ah, so thats how you do it. So what would anyone who reads this ,their opinion be of the minimum reading you would accept in terms of megaohms so you could be reasonably sure that the rcd would not trip?

 

[gpz900jim]

just to be clear, this is a theroretical question, of course the powers off, and ive carried out my safe isolation procedure, ive locked off the main switch, ive switched off my van outside the house and im wearing my trousers

 

[malcolmsanford]

As the accepted value for an IR test in table 61 for a 230/400 volt circuit would be 1Mohm and as in GN3 advises that if it's below 2Mohm you should investigate, so for me I would accpt anything over 2Mohm.

 

[telectrix]

gpz, you've got your answer. . the IR test is not to determine the trip threshold of the rcd. although it would take around 7.67.ohms to trip the rcd, the IR test is to determine the state of the insulation between conductors , including the itegrity of accessories. as malcolm says, a reading of less than 2 meg. would indicate a potential breakdown of insulation which, left , could in time decrease to a value to cause arcing between L/N/E.

 

[gpz900jim]

excellent, thanks for that, clears another one of those foggy areas in my mind

 

[chiplard]

The IR test should be done at 500V so the theoretical minimum resistance figure is 16.667 Kohms for 30mA to flow

This is 120 times lower than the recommended 2Mohm.