New Consummer unit with one rcd that won't set

[milllerr]

Before |am told off for doing something illegal, I am a qualified electrician (albeit 25 years ago). I've installed a new stacked (12 x 2) MK consumer unit in my house. A part p qualified friend of my will test the system and certify.

I have a 100 mains switch, which feeds some RCBO's, and then two further RCD and associated circuits. However the rcd on the same rail as the RCBO's will not set and keeps tripping, whereas the rcd on the unit below is fine. I've also tried the circuits on the ok rcd, and they work fine. I've swapped over the rcd and still get the same fault. I'm at a loss to understand what is going wrong. Do rcbo's cause ghost tripping and can't be mixed with rcd's?

 

[Atilla]

Before |am told off for doing something illegal, I am a qualified electrician (albeit 25 years ago). I've installed a new stacked (12 x 2) MK consumer unit in my house. A part p qualified friend of my will test the system and certify.

I have a 100 mains switch, which feeds some RCBO's, and then two further RCD and associated circuits. However the rcd on the same rail as the RCBO's will not set and keeps tripping, whereas the rcd on the unit below is fine. I've also tried the circuits on the ok rcd, and they work fine. I've swapped over the rcd and still get the same fault. I'm at a loss to understand what is going wrong. Do rcbo's cause ghost tripping and can't be mixed with rcd's?

were, were a qualified electrician - now your just dangerous.

 

[sparkiedave]

could have a borrowed neutral somewhere, but this would've shown up during the testing of course, so it can't be that can it!!

 

[GaryMo]

You have RCBO's fed from an RCD? I understand that's what you've done and if so, why?
There's no benefit to doing that so you should either use RCBO's fed from a main switch (not RCD) or MCB's fed from an RCD.

Borrowed neutral or crossed neutrals sounds like a reasonable start.

 

[milllerr]

Genuine thanks GaryMo. I have a 100amp main switch, which feeds the rcbo's via the busbar then mk split load cables that feed the two rcd's. It's the first in these two rcd’s, which won't set. I've connected the rcbo's neutrals to one neutral bar which go's on to feed the two rcd's. I can't understand how one rcd works and the other doesn't.

For the wider audience, I served a four year apprenticeship as a qualified electrician, I didn't pay £**** for a four week course, and then spent 15 years in the industry working in domestic and in large scale indusrty projects. Some of the electricians I worked with, I wouldn't pay to sweep my drive. I am not dangerous; I take a pride in my workmanship (unlike the electricians who wired my house using nails to fix flush boxes and no sheathing to protect the cables from the plaster). It is very easy to criticise especially if you don't know the answer to the problem.

 

[electronicsuk]

Genuine thanks GaryMo. I have a 100amp main switch, which feeds the rcbo's via the busbar then mk split load cables that feed the two rcd's. It's the first in these two rcd’s, which won't set. I've connected the rcbo's neutrals to one neutral bar which go's on to feed the two rcd's. I can't understand how one rcd works and the other doesn't.

So the CU has three neutral bars in total, correct? One which is fed direct from the outgoing N of the 100A isolator, which then in turn provides neutral for the RCBOs and two RCDs, each of which then have their own neutral bar also.

Does the RCD trip as soon as you try to turn it on? What about if all the outgoing MCBs fed via that RCD are isolated?

Could you perhaps take a high resolution picture of the CU so that we can take a look to see if there are any problems with miswiring that you might not have spotted yourself?

I am not dangerous; I take a pride in my workmanship (unlike the electricians who wired my house using nails to fix flush boxes and no sheathing to protect the cables from the plaster).

There is no requirement for capping to be installed over cables plastered in a wall, nor is there anything in the regs that prevents nails being used to secure a backbox. It may be a little lazy and not the best workmanship, but it's far from dangerous.

 

[sparkwright]

are you certain the neutral wires are in their corresponding terminals?

if so, on the circuits served by the tripping rcd;
disconnect one live from one mcb AND its corresponding neutral.
restore power.
see if rcd trips.
if it does re-connect wires.
disconnect next live and corresponding neutral.
and so on until you can identify which individual circuit (if any) causes tripping.

sometimes a neutral can be snagged against an earthed metal back box, to name one of many possible faults.

 

[milllerr]

Genuine thanks GaryMo. I have a 100amp main switch, which feeds the rcbo's via the busbar then mk split load cables that feed the two rcd's. It's the first in these two rcd’s, which won't set. I've connected the rcbo's neutrals to one neutral bar which go's on to feed the two rcd's. I can't understand how one rcd works and the other doesn't.

So the CU has three neutral bars in total, correct? One which is fed direct from the outgoing N of the 100A isolator, which then in turn provides neutral for the RCBOs and two RCDs, each of which then have their own neutral bar also.

' Yes correct'

Does the RCD trip as soon as you try to turn it on? What about if all the outgoing MCBs fed via that RCD are isolated?

'Yes it trips straight away. I've isolated everything, and tried a circuit that works okay in the other rcd, but this rcd trips straight away. I've swapped rcds and the fault still materialises.

Could you perhaps take a high resolution picture of the CU so that we can take a look to see if there are any problems with miswiring that you might not have spotted yourself?

I am not dangerous; I take a pride in my workmanship (unlike the electricians who wired my house using nails to fix flush boxes and no sheathing to protect the cables from the plaster).

There is no requirement for capping to be installed over cables plastered in a wall, nor is there anything in the regs that prevents nails being used to secure a backbox. It may be a little lazy and not the best workmanship, but it's far from dangerous.

Agreed, poor workmanship, but that just shows the level of competence.

 

[nozspark]

Do you have a seperate neutral bar for that RCD don't you???

 

[sparkwright]

are the rcbos totally separate from the rcd(s)?

they should be.

 

[scousespark]

Scary or what.

Your other post had a load of questions regarding what your electrician wanted to do and how you could do things to help him. Now you say you have done it and got it wrong.

An electrician cannot certify your work for BC. Get a real electrician to come and sort your job out.

 

[Chri5]

Isolate and disconnect are different. Have you physically removed the suspect neutrals ?

As other posters suggest it very, very, very likely a neutral issue.

Please re-read the advice given.

Would you please check and confirm:-

CU has 3 x neutral bars- 1= direct circuits (non RCD) such as RCBO's
2= ONLY neutrals from MCB's in that RCD's rail 3= ONLY neutrals from MCB's in that RCD's rail

Frequently shared neutrals from landing switches will cause the problem you mentioned. Proof can only be confirmed by removing neutrals at the relevant neutral bar.

It could just as easily be you accidently putting a circuit on say RCD1 and having misread the neutral bar lable- it does happen !

Sparkrights advice is sound, follow it

are you certain the neutral wires are in their corresponding terminals?

if so, on the circuits served by the tripping rcd;
disconnect one live from one mcb AND its corresponding neutral.
restore power.
see if rcd trips.
if it does re-connect wires.
disconnect next live and corresponding neutral.
and so on until you can identify which individual circuit (if any) causes tripping.

 

[RF Lighting]

Have you connected the supply and load tails the right way round?

 

[securespark]

Have you tested both RCD's to confirm their healthy operation?

If so, what are the maximum 1x and 5x tests and ramp reading for both units?

If you have confirmed correct operation of the RCD (and thus eliminated the unit itself from the equation), then the fault MUST be elsewhere.

You will doubtless know that the recommended way to test an RCD is in isolation, so if you have not tested the RCD's yet, disconnect ALL outgoing wiring from that RCD then test the RCD on its own.

Then, if you get a good result, you'll have to run through some dead tests on the wiring you have just disconnected to look for the cause of the SC.

What vintage is your test equipment?

 

[1john]

What are the insulation resistance readings from the 'live' conductors to earth?

you either have a fault on one or more circuits or shared neutrals, or you have got a line feed going in one side of the RCD and the neutral feed coming in the oposite side of the RCD.