RCD fails half test

[stilllearning]

Hi, what would be the possible causes of an RCD failing the "half" test?

Tested a 30ma RCD today after installing a new wylex split load board. It trips at 24ms on 1X test; 8ms on 5x test but fails the half test. There are 3 rings and 4 lighting circuits on the CU, I tested the RCD on each circuit with the others switched off but got the same result on everyone.

I switched off all the breakers and tested the RCD at the CU, it failed the half test once but then passed a second time. I then switched the breakers on, tested across the RCD and it failed just the half test again.

I then tested the ring circuits and they passed but checked them again later and they failed!!

Is it just a duff RCD?

Thanks

 

[Spark123]

You need to test RCDs in isolation i.e. no circuits connected. Can you do a ramp test? The "half I" test is to test that it won't nuisance trip.

 

[sparkybird]

Hmm, can you try another rcd in it's place? It sounds like it might be a tad sensitive, although the 8ms trip at 5X is not the fastest I've seen

SB

Not that quick at typing! Yes, he's right. Test at the RCD too with all circs off, rather than at a socket.

 

[stilllearning]

Thanks for the quick replies.

Yes, I tested it in isolation, it failed the first time and then passed the second time I tried. Sorry, should have mentioned, I did a ramp test it came out at 15ma on one cycle and 19ma on another.

Thanks

 

[Spark123]

It is a bit close to the mark, probably worth swopping it for another one.

 

[stilllearning]

Thanks alot, will swap it and see what happens. Just out of interest COULD there be another reason? I've not come across one that fails on half test before

Thanks

 

[RF Lighting]

some leakage present on the installation.

Your 15mA leakage for the 1/2 trip combined with the (for examples sake) 20mA present on the installation would cause the RCD to trip, but not under normal non test circumstances.

 

[Spark123]

Have come across a few before which fail half trip tests.

 

[stilllearning]

Thank. The leakage that RF talks about, is that more likely to be leakage from an appliance or from somwhere in the installation itself?

 

[Spark123]

Could be either, but as you are meant to test the RCD in isolation that shouldn't matter! Shouldn't be from a new installation wiring if you have done your IR testing!

 

[stilllearning]

Thanks. Sorry to sound thick but are you saying that you shouldn't test an RCD from a socket outlet but just at the board?

 

[Spark123]

You can test an RCD at the board, if there's nothing else on the protected circuits which can affect the results then there isn't a reason why you shouldn't be able to test from a socket outlet tho.

 

[RF Lighting]

Yes, with nothing connected to the load terminals (apart from your tester)