RCD tripping in newly installed CCU in garage..Help!

[timmy123]

I have just finished installing apower and lighting into a detached garage some 15 meters from the customers house.. I have supplied the ccu in the garage via SWA cable. The garage ccu has a 30ma rcd. The supply from the house ccu to the garage ccu is via an mcb protected by a 30ma rcd.
insulation tests and continuity tests indicate no issues with cables however the garage ccu rcd trips everytime any load is connected to the sockets (on a small ring main) or the garage lamp switch is closed. The house ccu rcd is not tripping. I have a feeling that there may be an issue with discrimination on rcds but I am not clear on this area.
I would really appreciate some advice on this issue as the customer is unhappy and is talking about getting another electrician in to sort the problem.

Your help would be greatly appreciated.

Thanks

Timmy 123

 

[Sintra]

You don't need to have an RCD on the sub main to garage as it's SWA.

It sounds like it could be a neutral problem in the garage.
Are your outgoing circuits in the garage connected to the same neutral bar as the 30mA RCD?

 

[pushrod]

As Sintra says - rearrange the CU so that the swa is not on an rcd then you do not have your discrimination problem. If you are not using the armouring as the cpc make sure that it is still connected at the supply end.

 

[timmy123]

Sintra,
Thanks for your comments on SWA and non requirement for rcd in garage. Therer is only oine neutral bar on the garage ccu and it only has the facility for 2 mcbs, so RCD neutral is on same neutral bar an ring main mcb and luminaire mcb. Take your point on possibly neutral problem, but I could not find anything odd on the readings I had today on insulation tests neutral/earth, phase/earth?
Cheers

 

[timmy123]

Thanks pushrod for your help on this one. Also noted your point regarding earthing the SWA armour at house consumer. Have done this.
Cheers and any other comments appreciated

 

[Andyfm]

ramp test the RCD next, to see what its tripping current is

 

[blackiel]

I havent read all your replies,Mate,But you DONT have two RCDs on one circuit,just like Dimmer switches,all to do with balancing and Electronics!!

 

[m4tty]

Balancing and electronics ? WTF Its to do with discrimination

 

[JUD]

I havent read all your replies,Mate,But you DONT have two RCDs on one circuit,just like Dimmer switches,all to do with balancing and Electronics!!

Balancing and electronics ? WTF Its to do with discrimination

Agree with M4tty.

You can have 2 RCDs on one circuit as long as the upstream one is time delayed providing discrimination but in this case there is no need for the one at the house CU as he is using SWA.

ramp test the RCD next, to see what its tripping current is

Also agree. Garage RCD could be too sensitive.

 

[davelerave]

i'm gonna setup business in the Uk
ripping out 30mA rcds that are wired in series

 

[timmy123]

Hi davelerave,
Thanks for your comment and I guess when you are quite new to the job like I am then I am learning the stuff they don't teach you at college as I go along (Like this rcd problem). I appreciate everyone's help and I have certainly learned alot from the comments received from the experienced members. Thanks again

 

[davelerave]

it wasn't aimed at you

seems to be a lot of confusion generally with the requirement to rcd protect buried cables etc.
you have a simple fault in the garage anyhow

as already said either neutrals mixed up or low N-E IR if rcd is ok

 

[blackiel]

Balancing and electronics ? WTF Its to do with discrimination

It all ties in.... WTF!!!

 

[blackiel]

Balancing and electronics ? WTF Its to do with discrimination

just realised,im commenting solely on dimmers there,give it to ya,old chap...

Keep calm.....lifes too good to ruin!!!...WTF!!!

 

[ezzzekiel]

different hymn sheet