rcd testing query

grousedogtom

Hi , while looking for a fault on socket circuits today which were causing the rcd to trip intermittently i done an rcd tripping time test and discovered the rcd didnt trip at 30mA on 0 degree or 180 , after i found the circuit with neutral to earth short and disconnected it and retested rcd all tripping times etc were fine , is this normal that an rcd tester cant test properly when there is a n to e fault . Also when i pressed the test button on rcd before i found the fault it tripped fine.

M cebee

prob affected it alright

Bruthal

grousedogtom wrote: »

Hi , while looking for a fault on socket circuits today which were causing the rcd to trip intermittently i done an rcd tripping time test and discovered the rcd didnt trip at 30mA on 0 degree or 180 , after i found the circuit with neutral to earth short and disconnected it and retested rcd all tripping times etc were fine , is this normal that an rcd tester cant test properly when there is a n to e fault .

It will affect the test alright with a neutral earth-fault as some of the testers test current which is all supposed to go back on the earth, goes back through the neutral earth fault. So for example if the tester is taking 30ma on the live through the RCD, all this 30ma should go back on the earth and so the RCD has a 30ma inbalance.

With the neutral earth fault, the 30ma test current now comes to the tester on the live, and back on the earth conductor and reaches the earth bar, but some of it now goes down the fault affected earth to the point its connected to the neutral, and so some of the 30ma test current now ends up going back through the RCD neutral, so for example if 10ma heads this way, the RCD now only sees a 20ma difference.

This would be when the tester is used on a different circuit than the directly affected one. If its plugged into the actual affected one the 30ma returning on the earth will split in the circuit at the fault, with the same result, with possible worse test results though.

Also when i pressed the test button on rcd before i found the fault it tripped fine.

The RCD test button would not be affected really. It simply connects a small load within the RCD from the live in to the neutral out, and so it bypasses one of its sensing coils, so tripping it. The test button will work with just the live and neutral connected to the RCD with no outgoing circuits connected to it, as long as the live and neutral are into the same side of the RCD of course.

grousedogtom

As usual , very straight forward explanation. Thanks