RCD test button not working

[GLENNSPARK]

I maybe having a brain fart here (always possible before my second morning coffee...lol!!) but what external influences are going to affect the operation of the internal test circuit button??

its hard neutral-earth faults eng.....

causes some RCDs to not work at all....even on manual test..

 

[Lucien Nunes]

If the test button circuit goes from supply L to load N, a downstream N-E fault can stop it working by reducing the test current returning to the supply N through the sensing transformer. But if the test was from load L to supply N, the fraction of test current going to earth at the fault would pass through the transformer twice in the same direction, possibly making it test OK even if it had reduced sensitivity. This makes me think that all RCDs have the test circuit the first way round, but I can't say I've spent enough time looking at the schematic on the side to notice.

 

[PEG]

Nice explanation,Lucien. I have stripped a fair few for fun,and most standard items seem configured as you first suggest,with the test circuit coil,that simulates the imbalance,linked from supply line to load neutral. A N-E fault downstream would divert returning current from sensing coil and not trip. There's no way round it,my faith in test buttons has evaporated,and in future,i am amending my quarterly press of the button to include mandatory disconnecting of out going L and N. Some of my less able clients are going to struggle...

 

[Geoffsd]

If this is the case then wouldn't the RCD already have tripped in normal use?

 

[appspark]

Hey guys i've got an update for you. There was a ring main with a N-E fault on the same RCD. had to put in a new leg, guessing the door had a fixing through the cable, couldnt pull it at all, site agent wasn't too happy, flats were basically being commissioned. When i removed the ring from the neutral and earth bars the rcd would trip on test. The lighting circuit was functioning properly now aswell.
so in conclusion the N-E fault on the ring main was stopping the rcd from functioning properly.

 

[dazza1981]

Hey guys i've got an update for you. There was a ring main with a N-E fault on the same RCD. had to put in a new leg, guessing the door had a fixing through the cable, couldnt pull it at all, site agent wasn't too happy, flats were basically being commissioned. When i removed the ring from the neutral and earth bars the rcd would trip on test. The lighting circuit was functioning properly now aswell.
so in conclusion the N-E fault on the ring main was stopping the rcd from functioning properly.

Appspark i congratulate you on not only tracing the fault but also letting us know what the problem was. Not many do !!!

 

[appspark]

i probably spent a good 8 hours in total tracing the fault and getting my head around it, then another 2 to replace the faulty leg. definitely learned a few things along the way, i actually had the same fault on another board, loose earth at a socket shorting to neutral, much easier to rectify.

 

[dazza1981]

i probably spent a good 8 hours in total tracing the fault and getting my head around it, then another 2 to replace the faulty leg. definitely learned a few things along the way, i actually had the same fault on another board, loose earth at a socket shorting to neutral, much easier to rectify.

I know what you mean i had N/E fault the other week in a big cottage and it took me best part of a day to find it and rectify it...... i swear i must have done 1000 laps of that house lol

 

[jaypp]

Appspark i congratulate you on not only tracing the fault but also letting us know what the problem was. Not many do !!!

Indeed.

 

[Midwest]

posssible N-E fault. remove all load from RCD the try the test button again. then get mft on it and do a proper test.if it then works, you have a fault on a circuit.

I've had same situation, found neutral earth fault on radial on a rewire doing IR test. Put that circuit to one side to get the rest of the house powered up, except I mixed it up with another radial and actually connected it up! Went on to do remaining tests, but Tester would not do ZS nor RCD tests on that side of the CU. Test button also wouldn't work. Went back to the beginning and found my mistake.