RCD trips on new CU change

[Newpark7]

Hi all

Replaced 3 old fuseboards today with a Wylex HI CU, customer just bought the property and didn't want a PIR just replace the board, I warned of the possible pitfalls of which he accepted.

On removing old boards did a quick IR test between CPC and phase & neutral joined together on all circuits, all ok apart from Ground Floor lights & Sockets which were 0.38M & 0.12M respectively.

On powering up the new board the RCD trips not surprisingly straight away when the socket MCB is switched on, the lighting circuit only trips when any light switch on that circuit is switched on.

I have temp wired the sockets into a non RCD protected way for tonight because of freezer etc. I appreciate there are faults there but just wondered how you guys would recommend going about testing and locating the faults.

I did quickly connect each leg of the ring separately to the MCB one leg trips straight away, the other trips after approx 5 seconds.

I am going back tomorrow and any help would be great - thanks.

Mark

 

[chris gray]

get ur megger out and start fault finding

 

[Spudnik]

Gotta love them borrowed neutrals!

Do an IR between the neutrals of the affected circuits.

This will at least confirm.

Other than that, could be a trapped neutral behind a switch and/or socket back box.

Whip them all off and try again.

 

[nickblake]

i agree with jason sounds like a shared neutrals check the ground floor and first floor lighting especially the two ways ,if you have two lighting circuits of course disconnect the one neutral stick it in a block and try again see what happens ,switch off all circuits switch on RCD then try each circuit at a time with shared neutrals normally both RCD's will trip !of its just the sockets thats tripping then what id do is put one end of the ring in a terminal ,split the ring in half ,by removing a socket then work back wards down the legs of the ring ,checking each socket as you go ,also do a continuity test between the neutral and earth at the CDU see if you are getting a reading , this will confirm if its a short you can the leave it connected whilst you look for the short trapped neutral etc hope this helps

 

[Hawk81]

I disagree it does not sound like borrowed neutrals on the lighting circuit. The original post says that the rcd trips on the downstairs circuit when any switch on that circuit is switched on. Sounds more like a neutral to earth fault. All you can do is split the circuits (both lighting and socket) down and start testing with the megger.

 

[lost-sole]

Surely if it was a neutral to earth fault the rcd would trip as soon as you reset rcd, if its tripping at a lighting point when light is switched on then theres obviously a live issue which correct me if iam wrong trip the breaker

 

[Spudnik]

In my experience, some borrowed neutrals dont rear their ugly heads until a load is put on affected circuit(s).

 

[Guest123]

A borrowed neutral between an upstairs and downstairs circuit with the joint being at the landing light will only trip BOTH RCD's when the landing light is switched on as the circuit is then completed through the lamp.