rcd trips on a dead circuit?????

chrisreid186 · Jan 20, 2011

hi all

my parents just bought a new place and the electrics are dodgy to say the least,
one circuit in-particular is giving me a headache, okay here's the set up

The supply is TNS and the consumer unit has one 30mA rcd covering half the circuits,
the circuit that is causing problems is a 20amp mcb on what looks like to be a ring
or maybe a lollipop,

my brother was complaining that when he plugged the play-station in it tripped the rcd, so i thought it was polarity problem,plugged socket and see in and it showed ok then i isolated the circuit and had a look behind the socket, all connections were tight and cpc sleeved,fixed the faceplate to socket and plugged the play-station in as soon as the plug was connected to the socket the rcd tripped even though the mcb was still in the off position???

i think this is an accumulation of neutral to earth current a bit like when you snip neutral and earth and the rcd goes on a dead circuit,

going back this weekend to do insulation test ,earth leakage and rcd ramp test

also i noticed that one leg of the ring was in aluminium and the other half copper

anyone came across this fault before? and would a play-station plugged into a circuit with the mcb off contribute to accumulation of leakage

forgot to say this also happens when the xbox or hoover is connected

any advice appreciated

SimpleSimon · Jan 20, 2011

Does it trip with just the playstation plugged in or other appliances at thr same time?
Have u tried disconnecting all appliances and leaving the playstation in?
Have u tried it in a different socket?

simpson93 · Jan 20, 2011

hi there chris does it trip straight away when the playstation is on or does it take a while?

nickblake · Jan 20, 2011

sounds like a split load board check to see if the neutrals are in the correct neutral bar , does it trip in any other socket or just the one , if its a 20 amp mcb protecting the circuit it may well be 2 radials , check the N to E switch of board main isolator off and switch off all the MCB and to a continuity between the neutrals and earth see if you get a reading , if not do an IR test , it does sound to me like a trapped earth / or neutral in the wrong position

chrisreid186 · Jan 21, 2011

when the play-station is plugged in it trips straight away,this also happens on other socket outlets when a load is connected. i applied my voltmeter to both line ends of the suspected ring and got continuity so don't think its a radial
Thinking about it i reckon nickblake's on the money with the neutrals being on the wrong bar,just like borrowed neutrals on a lighting circuit,
this would also explain why the socket and see picked nothing up

simpson93 · Jan 21, 2011

in my vast electrical knowledge(not)
if you did a test p-n and p-cpc at the socket
wont that verify that you have the right cables in the terminals?

nickblake · Jan 21, 2011

A socket and see will only give you basic socket connections ,it will need further investigation by some one very experianced in electrical fault finding , just caryy out all the tests you would normally do on a ring circuit being carefull not to blow anything up with the IR tests , if you are in any doubt call an experianced regestered electrician

miner69er · Jan 21, 2011

what we have here is a lovely situation that i come across all too often whilst carrying out PIR's. there is a short from neutral to CPC, an insulation resistance test will confirm this between N and CPC and chances are you will find a nick on the neutral cable in the back of a socket where it is touching the backbox, or maybe mr & mrs miggins put a piccy up above a socket and the nail has done the deed (check for small holes in line with sockets verticaly and horizontaly) The reason it only happens when you plug something in, is there is no power on the neutral side so the rcd is in balance, as soon as power goes through the plug the neutral is energised and you have your imbalance so rcd trips. (sorry for the simplistic desciption it's been a long week!!) oh by the way the rcd will still trip even though the mcb is in the off postion as you have only isolated the line, the neutral and cpc are still connected to the to the bus bars and through the rcd. good luck finding the cause

Wiredspark · Jan 21, 2011

Most likely the neutrals are in the unprotected side of the board. Came across this a few times.

SMB · Jan 21, 2011

miner69er said:
what we have here is a lovely situation that i come across all too often whilst carrying out PIR's. there is a short from neutral to CPC, an insulation resistance test will confirm this between N and CPC and chances are you will find a nick on the neutral cable in the back of a socket where it is touching the backbox, or maybe mr & mrs miggins put a piccy up above a socket and the nail has done the deed (check for small holes in line with sockets verticaly and horizontaly) The reason it only happens when you plug something in, is there is no power on the neutral side so the rcd is in balance, as soon as power goes through the plug the neutral is energised and you have your imbalance so rcd trips. (sorry for the simplistic desciption it's been a long week!!) oh by the way the rcd will still trip even though the mcb is in the off postion as you have only isolated the line, the neutral and cpc are still connected to the to the bus bars and through the rcd. good luck finding the cause

I don't quite see how that works. What about the other appliances that are already plugged into the circuit?

But, im a village idiot.

miner69er · Jan 22, 2011

SMB (or village idiot)

if you read the question, he doesn't say anything about any other appliance being connected to the circuit, not untill his 2nd post. but when he does (even in another socket) it trips. therefore it is a circuit problem, not an appliance.
not avin a go just explaining in a nice parental voice. not that my kids would listen!! that's why we all end up on here isn't it?