RCD not tripping

cdisney · Jul 27, 2015

Hi all

I'm stubbed on a new installation which I tested today. It is a M2 split board with a TT earthing system. All test results were fine until I attempted RCD tests. The test button works on both RCD's and the RCD main switch. The main switch is 0.1A and the two RCD's is 0.03A. But they both don't trip when 1x and 5x is attempted on my Fluke. It is a small installation so I checked everything and there is screws etc through anything and the polarity of the RCD is fine.
I haven't changed the RCDs yet as wondering if anyone else has come across this and may know anything else I could check.

Thanks

davesparks · Jul 27, 2015

Are you testing the RCDs at the load terminals with outgoing cables disconnected, or with the tester connected to a circuit?

What is the Ra?

cdisney · Jul 27, 2015

davesparks said:
Are you testing the RCDs at the load terminals with outgoing cables disconnected, or with the tester connected to a circuit?

What is the Ra?

Tester is connected to a circuit

cdisney · Jul 27, 2015

This is the other query as the figure I got was 412 ohms which I believed was to high (200 ohms)but my boss stated it was fine and within 1665 ohms which is acceptable.

davesparks · Jul 27, 2015

How many rods and how deep?

I don't know the soil conditions where you are so can't comment on that value without a bit more information. If you've achieved that with multiple long rods then it will probably be stable, or if it's just one little rod then it is unacceptable.

That is a very high Ra but it should not prevent the RCD operating.
Test the RCDs with the outgoing conductors disconnected and see how it goes.

NDG Elecs · Jul 27, 2015

I have had a couple of instances with Ra's around 500-600 ohms where the RCD would trip, but not in the permitted times. Both these installs were upgraded to PME and the trip times were then satisfactory. Agree with DS; if it's one four foot rod only then it will likely be unstable and fluctuate markedly.