Rcd fused spur bathroom Fan

[nrg]

Hi All

I have a customer with a single rcd board, protecting the sockets and the garage only. They wanted a fan installing in the bathroom which needs to be rcd protected. I was going to pit and RCBO on the board for the upstairs light but crabtree have discontinued them. So I though I would install a RCD Fused spur.

I split the bathroom lighting circuit in the rose and did the following.

Ran a L N SL from the rose to the rcd using 3 core
Connected L N to rcd and SL to fan isolator switch
Connected L N from RCD to fan isolator
Connected bathroom light switch to L SL of isolator
Connected L N SL to fan

Basically what happens is the rcd unit trips out as soon as I turn the bathroom light switch on. If I remove the neutral from the rcd direct to the fan isolator everything works fine, but obviously the rcd is then useless.

I suspect a neutral issue but all lights have the same neutral. Any ideas?

Thanks In advance

 

[Deleted member 26818]

BS7671 only specifically refers to circuits requiring RCD protection in relation to special locations.
Other references refer to cables concealed in walls or socket-outlets.
RCDs only detect earth faults downstream from where they are placed.
So if as you suggest you were to place an RCD FCU on a ring main to spur off into a bathroom. Any earth fault on the ring main, or indeed in any appliance connected to that ring main would be able to be imported into the bathroom without causing the RCD to operate.

 

[SMB]

Thanks for the reply Spin, but im afraid I don't really understand. Sorry.

From my way of thinking anything fed from the RCD into that bathroom (regardless of what circuit its fed from) would be controlled via that outlet...ie it would be RCD protected.

Sorry to be thick.

 

[Deleted member 26818]

Yes anything after the RCD would be protected.
That isn't what BS7671 requires for special locations.
That aside whilst the RCD is protecting equipment downstream, it won't protect equipment upstream and it won't protect against voltage on the CPC from entering the bathroom.

 

[SMB]

Mmmm. I think I need to do some more reading then.

Thanks for the input.




Yours, rather confused,

SMB

 

[RogertheBodger]

Yes anything after the RCD would be protected.
That isn't what BS7671 requires for special locations.
That aside whilst the RCD is protecting equipment downstream, it won't protect equipment upstream and it won't protect against voltage on the CPC from entering the bathroom.

Thats an interesting debate.
Wouldn't it be true that any non RCD circuit could import potential via the cpc into the bathroom, all circuit cpcs are connected together.

 

[SMB]

I think I must be missing something here.

 

[SMB]

So if as you suggest you were to place an RCD FCU on a ring main to spur off into a bathroom. Any earth fault on the ring main, or indeed in any appliance connected to that ring main would be able to be imported into the bathroom without causing the RCD to operate.

Im so sorry mate but im sure this can't be right.

 

[telectrix]

surely, an earth fault on the ring final would trip the 32A MCB of that circuit, thus removing the potential from entering the bathroom circuit.

 

[SMB]

Okay, ill try to explain what im thinking.....

If any circuit is fed from a RCD unit then the out going circuit from that unit will be covered from that RCD (thus comforming with BS7671 within an alteration to an electrical installation in a special location).

If im wrong with this, and what Spin has said earlier is correct, then first thing tommorow morning im going to go out and buy a cement mixer.

 

[telectrix]

what he is saying is that an earth fault on the ring main will be transferred to the cpc on the bathroom, and the RCD will not see this as a fault. and get an orange one ( cement mixer, that is ) and watch which horses you bet on

 

[SMB]

and watch which horses you bet on

......or who installs my underfloor heating.


I still don't get it though.


The MCB would trip.



I'll shut up now.

 

[telectrix]

i said the MCB would trip.

 

[SMB]

I know you did.



I promise to shut up this time..........laugh.

 

[Deleted member 26818]

Thats an interesting debate.
Wouldn't it be true that any non RCD circuit could import potential via the cpc into the bathroom, all circuit cpcs are connected together.

Yes it could very well do so.
Something that should be considered when designing any installation that includes a special location.
I believe the main concern is the availabillity of earth fault paths and the impedance of those paths.
What for instance would be the earth fault path for a fault on the ring feeding the bathroom, if the CPC of the ring was disconnected for whatever reason from the earth bar at the CU?
Would that earth fault path be available for a similar CPC failure for other circuits that are not otherwise connected to the bathroom?

Yes the MCB should trip with just about any fault, so why then are we required to provide RCD protection at all?

 

[SMB]

Whoever tested the circuit was not competent?