Rcd do not trip at x1

[Joe100w]

hi,

I am testing a circuit that goes to the garden with armoured cable. The RCD will not trip at x 1 (gets over 400ms) but trips fine on x5. The other circuits seems to be fine with the RCD test (30 mA). Using megger mft 1552. Any reason?

When I move that circuit to the other RCD (30 mA) it trips fine within the time.

My puzzle why will the garden circuit not trip at x 1. What could have been wrong.

Thanks.

 

[ColJack]

distance of run? dodgy earth? dodgy neutral?

what are the Zs readings for the garden circuit?

 

[securespark]

It is incorrect to test an RCD from a final circuit.

Test it in isolation.


What was the Zs at that point before you started work?

 

[Joe100w]

Zs is 1.32, Protected by type B MCB rated at 16A. Wiring all 3 is 2.5 mm and the armoured also is connected to Earth.

Insulation resistance >299 on all three.

Could not get a trip on 1 RCD with several attemps, but same circuit was fine when swapped over to the other RCD

 

[securespark]

Test the RCD in isolation. If it fails, swap it out.

 

[Joe100w]

Securespart

Might sound silly, how do I test Rcd in isolation.

 

[ColJack]

take the cables out of it and put the test leads on the load side of it...

 

[securespark]

Or if it is difficult to take the cables out of the outgoing terminals, switch off all the MCB's attached to that RCD to isolate the live terminal, then either disconnect all the neutrals from the busbar attached to that RCD or pull out (if it is flexible) the neutral feed to that busbar.