rcds tripping

[scotteng]

Help!! this one is driving me crazy!!!
Changed a consumer box today for a 17th edition, twin rcd.
Im having stange troubles with the installation.
Firstly the rcds trip, not straight away when the circuits are turned on, but every now and again, sometimes one will trip then the other or sometimes both at once.
Rcd tests passed ok, but and strange but one of the rcds fails on tripping at half fault current when the other rcd is on, when the other is off it tests fine. Changed rcd and same fault!!!

Anyone seen this before? any ideas where to start fault finding?

insulation tests on circuits all good, if there was a shared neutral the rcd would trip instantly which it doesnt!!

this one is making me go crazy, any help would be very helpfull!!

scott

 

[1john]

what is the insulation resistance on the whole installation? Are you certain there are no shared neutrals??

 

[scotteng]

was getting over 500meg for sockets circuits, and used link method for lighting which were all over 200meg @ 250v!!

in the past where ive found shared earths, the rcds would trip instantly it was tunred on, is there any other way of testing for shared earths

 

[LiamPope]

I assume you've already eliminated leaky appliances plugged in to sockets?

 

[scotteng]

yeah unplugged the usual suspects - washer, freezer etc

just so strange that both rcds are tripping!! the house is split into many circuits - sockets utility, sockets kitchen, sockets ring, sockets extension!!

 

[jaysean1]

if both rcds are tripping have you got a live and neutral in the wrong side of the db

 

[1john]

Why are you testing at 250V ??

Shared earths WONT effect a RCD

 

[scotteng]

sorry i ment shared neutrals...been a long day!!

tested at 250v as selv lighting, used link method...is that wrong? all other circuits tested at 500v.

 

[1john]

surely the entire install isn't selv is it??

I am sure that both RCD's would hold if there was a shared neutral until you applied a load somewhere.

I am assuming that you have NOT checked for shared neutrals?? Did you replace an old wylex board by any chance. Is the landing light fed from the downstairs lighting circuit.

 

[scotteng]

thanks for your replies john, i probably havent checked properly for shared neutrals....what tests prove that an installation has shared neutrals?

sorry if im asking dumb questions, still a new sparky and this one has stumped me

cheers

 

[1john]

You are looking for an accessory which has a line taken from one circuit and a neutral from another. Prime candidate in domestic installs is the landing light. The switch will be wired in the othadox style, the feed will be introduced at the ground floor switch, often the ground floor switch also serves the hall and the feed to the hall switch is looped over to the common on the two way switch for the landing, there are only strappers ran to the landing switch and in turn, a single cable from the landing switch to the light, the neutral is then 'borrowed' from the upstairs lighting circuit.

Is there only one landing light?? Is the wiring as i suggested? Are the cables stranded PVC like 3029?

 

[scotteng]

its looking more and more like thats the problem. Ill have a more in depth look tomorrow.

Is there a simple way to test this? using the test equiptment, just thinking of future cu changes!

any suggestions to why it doesnt trip straight away when you turn the light on? or the mcb? would have thought it would with a shared neutral?

cheers again

 

[Adam_151]

what tests prove that an installation has shared neutrals?

Best bet, other than visual inspection of landing light switches, etc, is I've found is to megger the circuits separately L+N together to earth (on 250v first*), circuits with borrowed neutrals should read down as you'll be reading through connected lamps and the neutral earth bond in the cutout/at the tx, to prove its not just low insulation reistance, switch off the DP isolator (assuming SP domestic) or disconnect the neutral from the circuit its borrowed from, from the N block and the IR will go up

If you have an insulation fault, then telling if you have borrowed neutral as well is tricky


*I always megger at 250v first and then only repeat the test at 500v if its read clear

 

[1john]

Is there a simple way to test this? using the test equiptment, just thinking of future cu changes!

any suggestions to why it doesnt trip straight away when you turn the light on? or the mcb? would have thought it would with a shared neutral?

The neutral isnt shared until the light is switched on!

 

[ericmark]

This is the lighting problem

It is easy to miss as it depends on how the two switches as used in one way there is no unbalance the other there is.

However any wrong neutral can cause the problem and the typical result is both RCD's tripping together. But both tripping together does not always mean crossed neutral as any switching can cause spikes enough to trip other RCD. In my house I know to ensure I don't trip second RCD when re-setting one I need to turn off all the MCB's first.

Modern RCD's have improved and some now have warning lights and start to warn of leakage at 15ma and trip at 27 - 30ma rather than anywhere between 15 and 30ma and there are also A and AC types. So today tripping due to spikes is not so much of a problem but mine were fitted in 1992 and a thunder storm can trip them. See here

However some cheap RCD's are still like old type and can trip very easy. I have had problems myself with an old house and had to disconnect some of the spur on spur sockets to get RCD to hold in. Normally I would do insulation test first but with this house I had taken over a job started by another electrician.