RCD tripping over 40ms on 5x

[Russell mac]

On testing RCD tripping times all was ok till 5x then it failed

 

[binky]

so replace it! 

 

[Sidney]

What's the rating of the RCD?

 

[kerching]

A) have you got test meter set correctly for type of breaker?

B) is it calibrated

assuming the answer to the above two questions is YES and you are testing at correct point then swap it

simples

 

[binky]

other option is to test at the RCD in the board, and not through a cct - sometimes makes a difference.

 

[UNG]

On testing RCD tripping times all was ok till 5x then it failed

Out of interest did you ramp test the RCD. I've had a few RCD problems in the last few years where the current needed to trip the device exceeded the rated value

other option is to test at the RCD in the board, and not through a cct - sometimes makes a difference.

Any from past experience proves it is faulty although the tests at the board suggest otherwise

 

[binky]

Any from past experience proves it is faulty although the tests at the board suggest otherwise

Tend to agree, but testing the RCD in isolation to ccts can give a different result, it's  subject that has cropped up on here several times before.

 

[Murdoch]

Before I set off to but another RCD, I would be tempted to disconnect all the outgoing neutrals ........... and test the RCD again at the CU

 

[Andy™]

other option is to test at the RCD in the board, and not through a cct - sometimes makes a difference.

not acceptable to say its passed by disconnecting the circuit. you want it to work when the circuit is connected, i.e if someone gets a shock, not for it to work only in 'ideal' conditions with various bits disconnected to make it 'pass'

 

[binky]

ok, so if an RCD covers serveral ccts, which cct do you pick?  Personally I think it's buggered and needs replacing.

 

[Sharpend]

not acceptable to say its passed by disconnecting the circuit. you want it to work when the circuit is connected, i.e if someone gets a shock, not for it to work only in 'ideal' conditions with various bits disconnected to make it 'pass'

You would think so, however the IET tech gurus advised me that You are testing the RCD therefore it should be tested without circuits attached as they may prevent it from tripping, this the rcd isn’t at fault but the fault lies within the circuit. 

 

[Doc Hudson]

Have a read of the RCD testing procedures in GN3 and On Site Guide. If it fails whilst doing the prescribed tests in the recommended method then replace it. Many electricians don’t always follow the BS7671 guidance.  

Doc H

 

[Sharpend]

But this is the grey area, if the rcd works when tested on its own then it isn’t faulty, so why would you change it? 

 

[NozSpark]

not acceptable to say its passed by disconnecting the circuit. you want it to work when the circuit is connected, i.e if someone gets a shock, not for it to work only in 'ideal' conditions with various bits disconnected to make it 'pass'

The RCD tests should be tested in isolation.... the continuity and insulation tests verify the circuit 

 

[Andy™]

The RCD tests should be tested in isolation.... the continuity and insulation tests verify the circuit 

if there is something with the circuit thats stopping a safety device from functioning as intended then do what you want, but i wont be ignoring it 'because the book says to test in isolation'. simple fact is, if the RCD isnt capable of working when a circuit is connected then its not fit for purpose

 

[Murdoch]

if there is something with the circuit that's stopping a safety device from functioning as intended then do what you want, but i wont be ignoring it 'because the book says to test in isolation'. simple fact is, if the RCD isn't capable of working when a circuit is connected then its not fit for purpose

Not 100% sure I agree with that.

The installing or testing spark has no control over what is going to be connected to the sockets so they can't be held accountable for the RCD not working at a later stage with multiple appliances connected.

 

[Doc Hudson]

if there is something with the circuit thats stopping a safety device from functioning as intended then do what you want, but i wont be ignoring it 'because the book says to test in isolation'. simple fact is, if the RCD isnt capable of working when a circuit is connected then its not fit for purpose

The test currents applied during RCD testing can be adversely affected by a circuits wiring or connected load. Real world loads are generally higher than RCD tester allowable currents. Erroneous results can be encountered if the RCD is not tested as near to source as possible with all loads disconnected as per BS7671. Of course this does not negate the need to test the circuit in isolation as well.  ie Ins Res results can also have erroneous results if certain loads or RCDs are still connected. 

Doc H

 

[UNG]

if there is something with the circuit thats stopping a safety device from functioning as intended then do what you want, but i wont be ignoring it 'because the book says to test in isolation'. simple fact is, if the RCD isnt capable of working when a circuit is connected then its not fit for purpose

Totally agree if it fails a low level current test on the circuit then how reliable is it going to be with a real fault

The test currents applied during RCD testing can be adversely affected by a circuits wiring or connected load. Real world loads are generally higher than RCD tester allowable currents. Erroneous results can be encountered if the RCD is not tested as near to source as possible with all loads disconnected as per BS7671. Of course this does not negate the need to test the circuit in isolation as well.  ie Ins Res results can also have erroneous results if certain loads or RCDs are still connected. 

Doc H

You are making an assumption that the fault current may be higher without any guarantee it will be. Anything above 30mA is sufficient to cause serious harm or death, the current situation as extolled by the IET for the testing of RCD's may as well tell us not to bother with RCD's as they may not operate when really needed

The only erroneous problem I have ever found with an RCD tripping at source and not tripping from a test on the circuit was the fact that on ramp test the RCD consistently tripped with a current exceeding it rated value. This was not an isolated case and some were brand new boards and some only a few months old that tested OK at install but not when doing other work some months later

Due to the blame game culture the UK has adopted I always ramp test any RCD I work on for piece of mind even though it is not a testing requirement to do so. The number of times I have found faulty RCD's is backing this approach up, I look at every job as a potential legal case these days if something god forbid happens then I know I have done everything I could do to mitigate any prosecution for negligence

 

[kerching]

Totally agree if it fails a low level current test on the circuit then how reliable is it going to be with a real fault

You are making an assumption that the fault current may be higher without any guarantee it will be. Anything above 30mA is sufficient to cause serious harm or death, the current situation as extolled by the IET for the testing of RCD's may as well tell us not to bother with RCD's as they may not operate when really needed

The only erroneous problem I have ever found with an RCD tripping at source and not tripping from a test on the circuit was the fact that on ramp test the RCD consistently tripped with a current exceeding it rated value. This was not an isolated case and some were brand new boards and some only a few months old that tested OK at install but not when doing other work some months later

Due to the blame game culture the UK has adopted I always ramp test any RCD I work on for piece of mind even though it is not a testing requirement to do so. The number of times I have found faulty RCD's is backing this approach up, I look at every job as a potential legal case these days if something god forbid happens then I know I have done everything I could do to mitigate any prosecution for negligence

I think a certain Mr @Sidewinder may be interested in your findings

 

[Sidewinder]

You would think so, however the IET tech gurus advised me that You are testing the RCD therefore it should be tested without circuits attached as they may prevent it from tripping, this the rcd isn’t at fault but the fault lies within the circuit. 

Sharpend is correct, an RCD is tested without any loads connected to comply with BS 7671.