Shared Neutral?

[andyww]

Hi All

Recently installed a supply to a new consumer unit for an outbiulding. All tests fine. The outbuilding c/u is a 5 way rcd mainswitch Wylex and has it's own TT earth, the armour on the SWA is earthed to the main house c/u. The main house c/u is a dual rcd 17th edition 10 way Hager, and the distribution circuit for the outbuilding is on the same rcd as the shower in the main house. When the shower is turned on the RCD's in the main house and the outbuilding trip. I called the NICEIC helpline and they said it was due to a 'shared neutral' problem and that I needed to place the MCB for the dist circuit on a non rcd protected circuit. Problem is, the Hager only has rcd circuits and is not as adaptable as a Wylex for example.
One idea I had was to put the MCB next to the main switch and move the first RCD along, and take a supply from the mainswitch to the MCB ( currently there are two cables form the mainswitch, one to each rcd )- can anyone see any problem with adding another straight into the mcb? - other question then is, the neutral is still 'shared' so will this alleviate the problem?

 

[T&Earth]

Hi Andy

Had trouble with lighting circuits with shared netuals, but not showers etc! Try swapping the shower over to the other RCD. Perhaps theres a fault on the shower itself, were IR readings ok? not sure what eles to suggest.

Steve

 

[Crackedwolf]

I agree with T&E,
Seperate the 2 supplies garage and shower
Soft test the shower with the shower in the on position ( live,neutral & cpc disconnected at the consumer unit........then ir test at the CU with nuetral and live connected together in one clip between the cpc on the other)
Try that first!

 

[andyww]

thanks chaps. All IR tests were fine - will try putting the shower on the other RCD

 

[sparkeymonkey]

You would need to move the neutral to a non RCD protected position also.
What happens when you turn on lights or use an appliance in the outbuilding? Do the RCD's trip? If so, could you have inadvertently wired the outbuildings neutral to the wrong neutral bar in the house CU...?
Does the shower trip the RCD when outbuilding is disconnected (phase & neutral disconnected)? If so, the shower is the problem but im guessing as this problem started after an alteration, the new wiring is the place to look.
Im with Steve, re-check the IR's on all of the outbuilding cables.

 

[Crackedwolf]

Other thing to check is where you have connected the outbuilding neutral to, is it on the same bar as the shower or a different one.
You could also disconnect at the outbuilding incoming neutral and earth switch off and see if you still get problems.

 

[Crackedwolf]

just seen monkey reply

 

[sparkeymonkey]

Thats a simpler way of putting what I was trying to get at crackedwolf, cheers!

 

[pushrod]

Other thing to check is where you have connected the outbuilding neutral to, is it on the same bar as the shower or a different one.
You could also disconnect at the outbuilding incoming neutral and earth switch off and see if you still get problems.

Would this cause both rcds at the house CU to trip or does the supply side one just go first?



Andy
Very intrigued as to why the rcd in the outbuilding trips - is the armouring in the swa connected to the earth in the outbuilding?

Are there effectively 2 RCDs, of the same value, in series to the supply of the outbuilding?

 

[sparkeymonkey]

Pushrod,
If the neutral is connected to another point other than that on the house rcd neutral then both rcd's will trip because they both see the neutral creating an imbalance in the system. Its nothing to do with an earth fault its simply the phase/neutral currents being out of balance because the current in the wrongly placed neutral is not passing back through the rcd it is supplied from.
Thats if this problem is even caused by this situation..... How are you doing at solving the problem Andy? What have you found?

 

[pushrod]

Pushrod,
If the neutral is connected to another point other than that on the house rcd neutral then both rcd's will trip because they both see the neutral creating an imbalance in the system. Its nothing to do with an earth fault its simply the phase/neutral currents being out of balance because the current in the wrongly placed neutral is not passing back through the rcd it is supplied from.
?

That's what i thought - from the initial description it is difficult to tell whether he is saying that one or two rcds are tripping in the house CU . If it is just one then that is ruling out the neutral connected on the wrong bar??

 

[sparkeymonkey]

Yup that would be right. He says RCD's so im guessing thats the case... Otherwise if it is only one in the house that is tripping then its probable its something in the outbuilding cct. Its all getting a bit ambiguous! Lets hope we get an update on the culprit when the problem is solved

 

[andyww]

Hi All

Thanks for all the input. I haven't had a chance to get back to take a look yet (I've isolated the circuit ) Both RCD's are tripping as I said - I don't think the neutral is in the wrong neutral bar but will double check. I'm going to IR test the outbuilding supply and all circuits and if they are all ok (which they were on testing at installation) I will move the circuit across to the other RCD ( sepearte from the shower) and see if that makes any difference.
If not, the only other resolution i can see is to replace the main c/u with a high integrity board and have the outbuilding supply circuit non rcd protected in the main house.

Hoping to get there tomorrow but stacked out with other work at the moment

Will let you know what happens

 

[terminator]

So, what happened? I've waited 3 years for your answer.:39:

 

[Outspoken]

So, what happened? I've waited 3 years for your answer.:39:

He disconnected the outbuilding neutral and was holding it when they tried the shower, the RCD didn't trip, but it blew the bejesus out of his pacemaker...hence the silence!