Blown bulb trips RCD even when removed? Ask questions and find answers to many subjects relating to electrics and electrical work

[Pioden]

Hi,
One of our light bulbs blew the other day, and tripped the RCD for the house. Bulb was removed, but before we could replace it (no spares) we found that activating the light switch would trip the RCD again. No bulb = no circuit (I hope) but still the RCD trips. Surely that's not normal?!

Can anyone suggest what the problem is? Dangerous? Been in this house for 17 years and not had any electrical problems until this one. House wiring is 1980s/90s at a guess.

[Mr White]

An RCD will trip if there is an imbalance of current flow. Since you have removed the lamp that failed, but the RCD still trips when you operate the switch, this suggests that the fault that caused the RCD to trip the first time is still present, as for what the fault is, from this end of a computer no one can say for sure, I would however suggest that as it was a light you have a look at where the lamp connects (You do not say much about the lamp holder.) and look for chaffed cable, I suspect the fitting is metal? It could even be water has got into the light or its connections.

[ericmark]

I remember returning home from working abroad to be told son over flowed the bath, and now dinning room lights trip the RCD.

Found bulb had cracked and on cooling had sucked in water, and was still full of water a month latter, changed bulb and all was OK, but since no earth to suspended bulb holder, it should not have tripped the RCD, but it did.

Often we find things which don't seem to make sense, but unless the bulb has an earth connection to it, then it should not trip a RCD, could trip a MCB or RCBO, but should not trip a RCD.

The main problem is when RCD's came out in the 80's and 90's there were no RCBO's, so one RCD protected many circuits, theory we should have checked no more than 9 mA leaking under normal use, however back then my clamp on ammeter would only measure down to 0.1 amp, (100 mA) so it was a case of trust to the lord.

Today I can measure 0.001 amp with clamp on, even measure DC, but not in the 90's when I first fitted 30 mA RCD's. Think around 1992 I fitted them to my house. And over the years I would get a batch of trips, then OK again for years, and never did find out why.

Some times I have found cause, spider in a socket, etc. Lucky not rodent damage, this house some times get bats, but don't think they damage cables.

We have two meters we use, the clamp on if not tripping tells us how close to tripping, it uses the AC so better for that, and also the insulation tester, this uses DC so can miss things, but normally 500 volt, although often can test at 250 volt as well, but either way, not the sort of meter the DIY man has.

So you can look for spiders, insects, and rodents, or water ingress, but after that no worth buying the test equipment, more cost effective to get an electrician.