RCD test button not working

[appspark]

Hi all, was testing some lighting circuits today(nice easy friday job), over a few different boards, ze, r1+r2, zs coming up normal. putting in the lamps, first one blew the bulb straight away, oh well i though stupid you for leaving the switch on(even though i'd done 4 flats previously with no probs), switch off, new bulb in, turn on and fine.
Move to bedroom, switch off this time, put in bulb, flick switch, rcd trips, pull up rcd again both lights on fine, turn bedroom switch on and off a few times and pop goes the rcd. hmm, so i open up the switch and rose again to make sure all is in order, seems good.
go back to board to reset rcd, think, oh il just test it incase the contacts are playing up and the test button doesn't work. its a split way board, 63a main switch with 2 30mA rcd, left side wont trip when test button is pushed but the right side seems fine, double checked all connections, can Zs of far side of both rcd's(same reading).
anyone any idea what would cause this or is it just a gubbed rcd?
Thanks for the help.

 

[telectrix]

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.

 

[camerabloke]

Deleted

 

[Pete999]

Pedantic Pete's on the warpath Mate no such thing as r1+r2, what sort of tests did you carry out on both RCDs?

 

[appspark]

tel, tried test button with no load and no difference.
Have just thought though, in one the boards(most likely this one), there was a N-E short on a ring main(found during IR test), wasn't needing it on but noticed when i tried to save taking the earths and neutrals out of their terminals. this could be the problem then, thanks.
pete, on both lighting joined line and cpc at boards and went round each rose and took highest reading. then insulation resistance tested each individually), then zs at rose which had highest reading before. may have just solved it when i've though about tels reply above.

 

[telectrix]

and just to muddy the water, the fault could be on the other RCD as well.

 

[Engineer54]

and just to muddy the water, the fault could be on the other RCD as well.

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??

 

[Guitarist]

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??

My thoughts exactly. As long as you have a line and neutral correctly connected, then the test button is only there to test the mechanism by producing an internal imbalance. Faulty RCD seems likely here.

 

[telectrix]

other week i went to a house where both RCDs on a MK CU would not trip either on test or with the button. found an unused 2.5mm radial with a fault. once this had been dissed, both worked perfectly: buttons, x1, x5, and ramp test. don't ask me why or how, but that's what i found.

 

[Richard Burns]

If there is a neutral earth fault in a circuit then when the RCD test button is pressed the current that should flow from the test button, to out going neutral, though the coil and then via the incoming neutral, can flow instead along the final circuit neutral through the fault and to earth bypassing the coil in the RCD.
If the relative resistances of the two possible paths are right there then may not be enough current to trip the RCD.

 

[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.