Tripping RCD

[grsebs]

I've got a strange problem: replaced an old fuse box with a Consumer Unit before Christmas. Now a couple of lights (non-RCD) trip the RCD when switched off!? I've checked for cross neutrals, meggered all the cables. The only oddity I can find is that some of the cables appear to have a floating ac voltage (around 20V) even if disconnected at both ends? Weird! Any ideas/suggestions?!

 

[andy]

I've got a strange problem: replaced an old fuse box with a Consumer Unit before Christmas. Now a couple of lights (non-RCD) trip the RCD when switched off!? I've checked for cross neutrals, meggered all the cables. The only oddity I can find is that some of the cables appear to have a floating ac voltage (around 20V) even if disconnected at both ends? Weird! Any ideas/suggestions?!

there nay be a borrowed neutral between the lights and another circuit (i.e sockets). the floating AC will be because of live cables next to it

 

[grsebs]

there nay be a borrowed neutral between the lights and another circuit (i.e sockets). the floating AC will be because of live cables next to it

So I can ignore the floating AC right? Re: the neutrals, I've checked for continuity between the RCD and non-RCD neutral bus bars and there doesn't appear to be any connection... It only trips with one light switch and then only when the earth of a cable running to a second switch (2-way) is connected. Disconnect the switch cable and it stops tripping.

 

[uselessFCUK]

Any fluoro fittings?
What are you detecting 20v between?

 

[grsebs]

Any fluoro fittings??

Just one in the kitchen - but it's on a different circuit and doesn't seem to have any effect: I did try disconnecting it and still had the tripping problem.

What are you detecting 20v between?

Earth and any of the wires in the disconnected cable...

 

[uselessFCUK]

Just for the record, is the rcd a 30mA type, not a 10mA?

 

[plugwash]

a small voltage on isoltated cores near live cores is nothing to worry about.

if the tripping is reproduceable then you could disconnect cuircuits one at a time (remember to disconnect the neutral when doing this) until the rcd stops tripping.

 

[Damocles]

You checked for cross-connection of neutrals. No chance of cross connection of lives? All rings definitely have neutrals in the proper neutral bar, and lives in the same mcb?

 

[mapj1]

Also check for a neutral-earth short if you haven't already - as N and E are at the same voltage that can bypass one of the coils of the RCD, making it 'see' an imbalance.
Sorry if you already know this.
regards M.

 

[sitinduck]

I too have just the same problem as you! two lights in my house trip the rcd one upstairs and one down, I have no idea what to do so i can't help you but from what ive read in other posts this is common. Although no one has yet posted a solution. Lets hope one comes up!

 

[sitinduck]

just found out that if i switch the lights off slowly the rcd trips, but doing it quickly everythings fine (sometimes) its not a consistent thing, does yours do this?

 

[grsebs]

All,

Thanks for your various thoughts and suggestions. To answer some of the questions:-

Useless ****: yep, it's 30mA

plugwash: yes, in fact I've managed to isolate it to a particular segment of one circuit and it is (spasmodically) reproducable (i.e. doesn't trip every time the light is switched off, perhaps every fourth of fifth time). When I completely disconnect the cable to the second light switch its stops tripping but if I just reconnect just the earth it starts tripping again?! The cable is brand new and I've checked each of the conductors for leakage with a megger...

Damocles: yes, all neutrals are definitely in the right bars. Don't think I've checked for crossed lives - are you thinking that a light might have been run off the ring main somewhere? I don't think this is the problem - but worth checking - thanks for the suggestion.

mapj1: yes, checked for N-E shorts already, but thanks all the same!

sitinduck: sorry to hear I'm not alone! The only advice I can offer, having actually solved the same problem with another light (!) is to consider replacing the cable. My cellar light was tripping the RCD as well. The cable was a little old and had no earth, so I changed it and it no longer trips. Unfotunately the other light that does still trip is wired with new cable, so clearly not the same problem!

 

[Damocles]

Sittingduck: It is not common for an rcd to trip on a lighting circuit. McB may trip when a bulb fails, but not RCD. Also it is generally recomended to connect lights to non-RCD section of Cu, if you have a split load box.

grsebs:I was mostly thinking that if you have eliminated the sensible things, try the daft ones. You say the cellar light cable had no earth. Do all the other cables have properly connected earths, i.e. no segments sithout earth still somewhere in the middle?

Now, this lighting circuit is on the RCD or not? Are there any other cables going to this light switch which causes the problem. have you tried another switch?If this is two way switching then maybe the fault is in the one still connected even though it does not appear with the second switch disconnected.

You said that disconnecting or connecting just this one earth cable stops or starts the fault. This rather suggests this earth has to be connected to something else somewhere. What? Any trapped wires anywhere?