What trips an RCD when all other tests are OK??

[sparkybird]

OK peeps - here's one for you! This week I was working with my boss, upgrading the board and doing a rewire - but not the kitchen as the client didn't want 'disturbance'.

First step was to test out the existing installation - which we did and it was all tickety boo. Kitchen tested out as a ring.

However, when we connected up the new board, the RCD kept tripping on the kitchen circuit. Further investigation lead us to think that the kitchen was not a ring, but semi ring, JB and spurs. (Didn't ever find it - must have been buried in the wall!)

Two questions
1. If all the spurs out of the JB were of a similar length, would this give you the R1 plus R2/4 readings indicative of a ring (ie all vvv similar?)
2. Why did the RCD trip when it all tested out OK? We could only assume the JB was responsible for this, but surely the Insulation Resistance or Continiuty Tests would have thrown something up?

Anyway, we ended up putting in a new ring - the customer was a bit cheesed off, but had no choice.

Hope someone can shed some light on this, as I'm baffled!

Cheers

SB

 

[andy]

OK peeps - here's one for you! This week I was working with my boss, upgrading the board and doing a rewire - but not the kitchen as the client didn't want 'disturbance'.

First step was to test out the existing installation - which we did and it was all tickety boo. Kitchen tested out as a ring.

However, when we connected up the new board, the RCD kept tripping on the kitchen circuit. Further investigation lead us to think that the kitchen was not a ring, but semi ring, JB and spurs. (Didn't ever find it - must have been buried in the wall!)

Two questions
1. If all the spurs out of the JB were of a similar length, would this give you the R1 plus R2/4 readings indicative of a ring (ie all vvv similar?)
2. Why did the RCD trip when it all tested out OK? We could only assume the JB was responsible for this, but surely the Insulation Resistance or Continiuty Tests would have thrown something up?

Anyway, we ended up putting in a new ring - the customer was a bit cheesed off, but had no choice.

Hope someone can shed some light on this, as I'm baffled!

Cheers

SB

could be a faulty appliance?

if insulation tests are OK then there isnt any reason for the RCD to trip unless something is plugged in.

 

[sparkybird]

No - all appliances were unplugged, fused spurs disconnected etc as they have to be when doing the tests and no one plugged anything back in before we put on the new board.

One thought we did have was if there was an old socket or fused spur with a 'load' on it (eg an old plug??) that was hidden/buried somewhere - but seems like a long shot. The RCD was fine with the new ring and all appliances plugs back in.....

??

 

[ebee]

No - all appliances were unplugged, fused spurs disconnected etc as they have to be when doing the tests and no one plugged anything back in before we put on the new board.

One thought we did have was if there was an old socket or fused spur with a 'load' on it (eg an old plug??) that was hidden/buried somewhere - but seems like a long shot. The RCD was fine with the new ring and all appliances plugs back in.....

??

You say the readings were Ok.
Can you tell us the actual readings?
If you dis r1/r2 did you see one or two T & E at each socket?
Intruiging to say the least

 

[HDRW]

For an RCD to trip there must be current flowing, but you say it tripped with nothing plugged into the kitchen ring. The insulation tests should rule out leakage between conductors and Earth...

Could there have been a borrowed Neutral somewhere?

It would have been interesting to test for current flow in the cables under the conditions that the RCD tripped, but that's not a standard test, it would be a bit of a lash-up to do, and your boss may not have been happy with the idea. Shame - we may never know!

Cheers,

Howard

 

[sparkybird]

ebee - haven't got the readings, but all were within regs. We did end up opening all the kitchen sockets and established that it was a jb/spur job not a ring.

HDRW - weird, eh? Not sure about borrowed neutral - I thought this was only for lighting circuits and this was power..... can you elaborate?
We would have liked to investigate further, but we were out of time on the job and having to keep it quick as the client wasn't too pleased about having her kitchen rewired as an extra!

Maybe it's just one of those spooky things

Cheers all
SB

 

[Spark123]

Not sure about borrowed neutral - I thought this was only for lighting circuits and this was power..... can you elaborate?

Borrowed neutrals are not just confined to lighting circuits, imo borrowed neutrals happen more often on lighting circuits due to cables with only live conductors i.e. in switch wires.
With this in mind, when an uninstructed diyer is installing a light and realises there is no neutral in their switch, instead of wiring back up to the correct lighting circuit neutral, they may wrongly aim for the nearest / easiest neutral wether it be a socket or another lighting circuit.

 

[sparkybird]

Hmm - so if this is the case then something in the flat might not now work? I guess when the client phones us, we'll know what it is?

 

[andy]

Hmm - so if this is the case then something in the flat might not now work? I guess when the client phones us, we'll know what it is?

not your fault if the outside light now does not work because they bodged it and used the ring main for a neutral

 

[Adam_151]

though if it was a borrowed neutral, those old ring cables that you probably cut back as far as poss. will be capable of giving a shock...

 

[andy]

though if it was a borrowed neutral, those old ring cables that you probably cut back as far as poss. will be capable of giving a shock...

possibly... but then it was bodged in the first place. also, the light should have been re-wired if it was on the lighting circuit. only the kitchen ring was gonna be left

 

[sparkybird]

you lot must be psychic - there was an outside light - however, it was supplied through a FFS off the old ring, so we obviously added it to the new ring and i did test it!
SB