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RCD testing (RCDs in series)

Paul Brewerton

Afternoon folks,

What is the best way to test RCDs when you have 2 in series? For example a distribution board supplies a cabin via an RCBO. The cabin has its own consumer unit with an RCD ,main switch.

When RCD testing, the RCBO trips first everytime, therefore we can only test the CU RCD via its button. 

The circuits downstream are protected via the RCBO perfectly fine, so there is no issue, however I would like to be able to prove the CU RCD as well.

Any ideas?

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    This query probably should move to the wiring forum, but mean while, here is one way.

    To test the downstream RCD without firing the first one, instead of returning the fault/ test  current to earth (CPC) return it to the upstream neutral - the first RCD then sees no L-N imbalance, as the test current goes out via its live and back via its neutral, while the downstream one sees the test current leave its live and not come back, just as if it went to earth.

    So the green lead of your tester to the incoming neutral on the 2nd RCD, while blue goes to the outbound side.

    Please check PN polarity before doing this test.  

    PS the self test resistors inside RCDs are usually wired this way too, but ‘it trips in an unknown time’ is not a rigorous test.

    Mike.

     

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  • 0

    This query probably should move to the wiring forum, but mean while, here is one way.

    To test the downstream RCD without firing the first one, instead of returning the fault/ test  current to earth (CPC) return it to the upstream neutral - the first RCD then sees no L-N imbalance, as the test current goes out via its live and back via its neutral, while the downstream one sees the test current leave its live and not come back, just as if it went to earth.

    So the green lead of your tester to the incoming neutral on the 2nd RCD, while blue goes to the outbound side.

    Please check PN polarity before doing this test.  

    PS the self test resistors inside RCDs are usually wired this way too, but ‘it trips in an unknown time’ is not a rigorous test.

    Mike.

     

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