Fault(s) when changing a 3036 to RCD CCU

[Jon Bridges]

Yesterday I was changing a board and encountered the following faults when I livened it up;

RCD 2 ok, RCD 1 with the upstairs lights and down stairs ring on, both MCB,s tripped the RCD. I found one desk light I'd missed on initial testing and took it off the ring. RCD now ok. Assuming the reason for the upstairs lights being a borrowed neutral I put put both the upstairs lights and downstairs lights on the same mcb and neutral bar on RCD 2 and all ok.

Happy that I'd worked out the faults, a borrowed neutral and faulty desk lamp I went upstairs to live test and when I turned on the bathroom light it tripped out the other RCD (RCD 1), not the one it was on (RCD2). I turned every MCB off on RCD 1 and tried again, even with nothing on it it tripped.

Then I did an RCD test on the downstairs ring (RCD1) and for x1 I got >400ms. I changed the RCD incase it was faulty but the replacement did the same.

My Ze was 0.2
PFC was 0.76
Insulation resistance was ok on the rings, but the lighting was about 1 doing a soft test at 250v (there are a few dimmers etc)
oh, and I also got an open circuit on end to end neutral on the downstairs ring

Anybody got any ideas why the other RCD would trip when you turn on the bathroom lights, I really dont know where to start.

 

[philip962]

Is the neutral in the ccu on the correct rcd side had a fault before where an mcb was tripping the other rcd

 

[Deleted member 9648]

IR test each circuit L-E and N-E separtely and disconnected from the db one circuit at a time....also IR test each circuit while disconnected from N to the other Neutrals still connected which will indicate any borrowed neutrals between circuits.....Double check each N is identified to the corresponding live and connected to the correct N bar....Double check the N links from the RCD's to the bars are correctly connected

 

[Jon Bridges]

I checked each neutral was connected to its corresponding mcb, so dont think this is the cause. Wirepuller, how will a seperate insulation resistance test between N and L and N and CPC tell me if there is a borrowed neutral, and what reading should I look out for?

***Just re-read, do it between neutrals - great idea

 

[ajdeath]

I would probably turn everything off apart from lighting circuit discconnect the neutrul for that circuit and turn bathroom light on. if it trips then its either a borrowed neutrul or earth problem.

 

[Jon Bridges]

But an earth problem wouldnt explain the >400ms rcd reading, assuming the 2 things are related

 

[Deleted member 9648]

Excessive RCD trip times are invariably down to a load still in circuit....make sure there are no loads and it will trip within the required time.

 

[pushrod]

oh, and I also got an open circuit on end to end neutral on the downstairs ring

Anybody got any ideas why the other RCD would trip when you turn on the bathroom lights, I really dont know where to start.

Possibly just a poor termination at a socket, check neutral continuity with a wandering lead to find it.

Its a good idea to do all the testing before you actually change the CU so you don't get all these unpleasant surprises at the end of the job

edit might be worth checking in the loft for connected loads if it is not your place eg tv aerial amplifier, spurred sockets, lights etc.

 

[Jon Bridges]

Its a good idea to do all the testing before you actually change the CU so you don't get all these unpleasant surprises at the end of the job

Not getting prickly Pushrod but I did all the dead tests and only really got an open circuit on the neutral and low ins res on the lights. Neither of which made me think I was in for the faults I found. I didn't just lash it up and hope for the best.

Thanks for your other suggestions though!

 

[oldtimer]

You say there is 2 light circuits are they in T+E or are they singles ? Also if you can take the RCD out of circuit so that you can confirm that the circuits are correct.

The thing is it a shared neutral or has someone commoned the neutral in the field ?

Must admit I have never come across a shared neutral is this an english thing ?

 

[pushrod]

Not getting prickly Pushrod but I did all the dead tests and only really got an open circuit on the neutral and low ins res on the lights. Neither of which made me think I was in for the faults I found. I didn't just lash it up and hope for the best.

Thanks for your other suggestions though!

Apologies if it sounded smug or like a smart a*** , not meant that way

 

[pushrod]

just a term to describe the pre RCD practice of taking the neutral back to the CU on a different circuit to save a bit of cable. Quite common on the stairway lights in older properties.