RCD Tripping instantly

[who took my fluke?]

Guys

Just witnessed something weird and was wondering could you help. Recently replaced a CU in my brothers house that he is renovating. I undertook the rcd tests using an alphatec tester and it tripped in the required times (and not at 1/2) so i was happy that all was ok..... until today! i just connected his cooker and turned it on and the rcd tripped immediately, then so did the fridge. Both on seprate circuits, fridge on the kitchen ring main and the cooker on a seperate feed (40 A, 10mm cable). However.... when i plugged a cd player in to the socket on the cu and other sockets in the kitchen, it worked ok without tripping the rcd. Any ideas???? im well stuck

 

[JohnD]

tell us about the neutral bars.

presumably the CD player has no earth connection to its plug but the other things do?

 

[Spark123]

TN-CS?
Possibly a N-E short on a circuit somewhere?

 

[luminaire]

Sounds like a neutral - earth fault. Why is your cooker on the RCD side is it because you have a 13A socket? If not and assuming you installed a split-load, put it on the non-RCD side.

Anyway, you could start by checking your connections, you may have trapped a neutral.

If you don't find it you will have to try IR test on both circuits.

 

[davy_owen_88]

You use CU twice in your post, and I'm assuming the first instance that you mean a consumer unit and in the second the cooker control unit?

As JohnD has touched on, are you sure you haven't got the neutrals in the wrong bar?

Since the RCD tripped immediately have you got a loose connection causing a L-E/N=E fault?

Have you sleeve the earth conductors?

What were the IR readings like?

On another note, are you competent and registered to do this notifiable work?

 

[who took my fluke?]

thanks for the quick reply.

It is on the RCD side because of the socket which is on the coooker connection unit and if i plug the cd player into it it works fine so i dont think ive got the neutrals mixed up at the board?? Good point about the earth not being connected, must admit didnt check. How would a trapped neutral cause two seperate circuits to trip out??

 

[JohnD]

the CD player will have a very low current load. I don't know if it will be as low as 0.03 amps, but if it is, it would not trip the RCD even if you had used the wrong neutral bar.

 

[who took my fluke?]

re: registered. not part p but have done the 2391 and bs7671 so like to think i know what im doing! just not seen a fault like this before.

The IR readings for both circuits were ok when the work was first carried out. All earths are sleeved.

Yes it is TNCS, Reading was 0.44 ohms

Where would you suggest starting to look at trapped connections?

Thanks again

 

[JohnD]

Another common prob would be that there is a background leakage of e.g. 15mA, and the slight additional leakage typical of an electric cooker would push it over the edge.

I suppose you can't PAT the fridge and cooker?

 

[who took my fluke?]

both fridge and cooker are out of the box brand new!

would a trapped N- E on one circuit affect others on it??

 

[davy_owen_88]

Start by checking all the connections you have made today.

Is there a possibility that you have put a nail/screw through a concealed cable?

 

[luminaire]

How would a trapped neutral cause two seperate circuits to trip out??

Could be two trapped neutrals when screwing up units.
Just a thought and worth checking.

 

[who took my fluke?]

sorry, wasnt being sarcastic, genuinely didnt know if a trapped neutral would cause other circuits!

i have changed a socket face today as well as finish the cookerpoint so a possibilty.

 

[Spark123]

A N-E fault on any of the RCD protected circuits will create a parallel path across the RCD which causes an imbalance, it may only happen with high current appliances as the impedance of the fault is probably a lot higher than the impedance of the normal path of the RCD neutral. However, when a fault exists you only need 30mA through the fault to trip the RCD.
What are all the IR test results like?

 

[Softus]

both fridge and cooker are out of the box brand new!

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