External RCD trips Consumer unit RCD

[Charles Paterson]

I have an external RCD unit to protect outside lights. This trips intermittently, when it does it also trips the RCD in the consumer unit. If I test the external RCD with the trip switch, it trips and also causes the RCD in the consumer unit to trip. I have tested the circuit and the tests are ok. Any ideas why the consumer unit RCD trips as well, both are 30mA.

 

[Leesparkykent]

I have an external RCD unit to protect outside lights. This trips intermittently, when it does it also trips the RCD in the consumer unit. If I test the external RCD with the trip switch, it trips and also causes the RCD in the consumer unit to trip. I have tested the circuit and the tests are ok. Any ideas why the consumer unit RCD trips as well, both are 30mA.

Because there is no discrimination/selectivity. The upfront RCD should be time delayed/S type if the installation method permits.

 

[westward10]

Why is there a separate rcd for the external lights.

 

[Pete999]

I have an external RCD unit to protect outside lights. This trips intermittently, when it does it also trips the RCD in the consumer unit. If I test the external RCD with the trip switch, it trips and also causes the RCD in the consumer unit to trip. I have tested the circuit and the tests are ok. Any ideas why the consumer unit RCD trips as well, both are 30mA.

Are you an Electrician?

 

[Octopus]

This is why having 2 x 30mA RCD's in series is NOT a good idea...

 

[Rpa07]

Sort the problem that's causing the trip in the first place then the lack of discrimination won't be such an issue -
Hands up for outside floodlight
Hands up for pond feed
I need to drink less coffee!