Can someone sort this mystery RCD trip out? Please

[LlandrilloSpark]

Can someone offer a solution to this RCD trip?
I've been doing some work at my sister's 112 year old house, it started with me installing a split load board but soon became obvious that the wiring wasn't up to scratch anywhere in the house, I.e two core feeding socket outlets no cpc!
The kitchen extension has it's own ccu with two 15A and one 5A rewirable for power and lights, this is fed via a 10mm t&e with contactum 32A breaker on the rcd side, id say the cable is a good 40 years old, the day after installation the rcd tripped, the kitchen supply was the only circuit on it at the time.
Due to a new kitchen being fitted, i now have been asked to rewire the whole kitchen, I have fitted a new proteus 4 way with rcd main switch, one 6A lighting circuit and two power, the cooker is still on the old board.
When I switched on the mcb in the main board the new rcd main switch tripped straight away, this was without the cable under any loading and all the mcb's in the new ccu switched off.
I have tried the following, I disconnected the bus bar and it still tripped, I but the neutral tail straight into the bar and it held untill I switched on any of the 3 circuits then it popped, I even bypassed the whole rcd putting the neutral tail in the bar and the line into the mcb's, the circuits worked fine and I got good Zs on the sockets, the pre commision tests came back fine on the new kitchen , all insulation resistance over 550 meg ohm.
Can the RCD pick up something down stream?, is it the supply cable?, i was thinking about bringing a new 4mm earth from the met to the new ccu, also should I dissconnect the new circuit neutrals one by one and c if I can pinpoint that way. Or is it back to the main switch and stuff the rcd.*
Help appriciated
Lighting in utility has a small fluerescant fitting on it
Cheers

 

[sparky79]

have you checked that you have not got the out going neutral in the wrong side of the rcd

 

[Spudnik]

Plus, a simple IR test between circuit N conductors to make sure you have nothing borrowed.

Also, did you do an ir Test between N & E?

 

[LlandrilloSpark]

It's a proteus 4 way, the outgoing N from the RCD is a solid copper hook type bar, neutral tail into N on the top of RCD and the hook going the the N bar in the ccu

 

[LlandrilloSpark]

Jason, thanks I will try that on Sunday morning

 

[ian.settle1]

Jason, thanks I will try that on Sunday morning

This is a test you should have done when you first tested

 

[sparky79]

a borrowed neutral from this board and another would give 100% clear readings to earth but still trip the rcd

 

[nickblake]

forgot to mention in my post when you do a continuity test to fault find say the N earth fault the plug top type test lead you need to put the green and blue leads into the live and earth of the tester then change them back when you cary out the ir tests etc

 

[LlandrilloSpark]

Ian thanks for your help, I'm certainly not the most experienced spark in the world and it's not something I thought of , but IR tests between Circuit Neutrals are not a requirment on any book, bs7671, guidance note 3, 2392 or 2391 or anything, I agree it's a test to do now but it's not a recognised pre com test
so we will have to agree to disagree !

 

[ian.settle1]

Ian thanks for your help, I'm certainly not the most experienced spark in the world and it's not something I thought of , but IR tests between Circuit Neutrals are not a requirment on any book, bs7671, guidance note 3, 2392 or 2391 or anything, I agree it's a test to do now but it's not a recognised pre com test
so we will have to agree to disagree !

I was refering to the IR test between neutral and earth not between neutrals

 

[tenheads]

this is a naughty nicked neutral i reckon nicked as in stolen

 

[tenheads]

a borrowed neutral from this board and another would give 100% clear readings to earth but still trip the rcd

 

[LlandrilloSpark]

Ah sorry Ian, all standard IR tests between circuit
conductors came back top banana, will test for borrowed neutrals on Sunday, cheers

 

[LlandrilloSpark]

What a prat!, this fault has taught me two things, never attempt a ridiculously long day, 7.00am-10pm rushing to complete and secondly try to find the obvious faults before the less obvious ones!

Fault was crushed neutral conductor against metal casing of light, N-E, pretty much what we said, I struggled fitting the light latecon Thursday night and obviously trapped it when rushing, I knew I'd struggled but didn't check it, first thing thus morning, clear head, IR tests on lighting circuit....fault found

thanks for the help

 

[nickblake]

One your not a Pratt we all do it , over the years ive taught my self there is always the next day even more now i work for my self ,never rush as what time you save will be spent trying to sort out the faults generated ,thats what is the problem with big companies put the guys under pressure they make mistakes and in the long run it costs more .glad you found the fault

 

[LlandrilloSpark]

Cheers Nick