RCD in series?

Les Player · Mar 31, 2012

I was in discussion with electrician fitting some new electricsand he noted that there was a spur off to an external garage from the CU. Therewas an existing RCD fitted at both ends (in the CU and garage via 10m shieldedcable), which he said was not allowed- but could not offer an explanation as towhy. Any explanation here?

Richard Burns · Mar 31, 2012

Interesting one that. The easy answer is that the wiring regs state that there must be discrimination between Protective devices in a cascade.

With two 30mA RCDs in series you will not be able to tell which one will go first as they will have slightly different characteristics.

I feel that this is not a problem in a domestic situation since the loss of power is not critical.
You could also say that in the case of one RCD failing the other will protect the end circuit.
I have installed series RCDs at the customers insistence despite my telling them that it was pointless.

Still he is correct it is "not allowed"

malcolmsanford · Mar 31, 2012

If the RCDs in question were both just normal types ie 30-500mA, in the event of a fault there would be unlikely discrimination between them, so either or both would trip which would constitute non division of installation.

In certain circumstances this would be unacceptable, if you have a copy of the regulations have a look at section 531 on RCD and 531.2.9 in particular.