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

 

[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?

 

[telectrix]

i see you logic there , spin, can't disagree with you on that. fair few ifs though. wonder what malcolm's thoughts are on this one.

 

[Deleted member 26818]

Fault protection and additional protection should never be required whith a sound and fault free installation.
However faults do occur, vermin eat cable insulation, vibration causes terminations to loosen, carpet fitters bang nails through cables etc.
It used to be that only certain fixed equipment in bathrooms required RCD protection, now it is all circuits of the bathroom, not the equipment, not part of the circuit or just alittle bit of the circuit in the bathroom.

 

[telectrix]

i totally agree with you there. always allow for the unforseen. i've always erred on the side of caution, did a CU job last month. was no RCD whatever. existing buried cable (SWA) been in for donkeys, feeding floodlights on his horse paddock. was concerned if would be problems if this was RCD protected, no, after seeing what happened at newberry, i'm bloody glad i wired it to the RCD.

 

[RogertheBodger]

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?

Good questions, In a hypothetical situation, where cpc's are disconnected, the earth path would have to be to whatever earthed metal is around, maybe there wouldn't be one, perhaps you are making a case for all circuits to be RCD protected, as they are in other countries, but if the cpc's are disconnected even that wouldn't make a difference.

 

[Chr!s]

Spins correct, when any fault to earth exists, all extraneous conductive parts connected to the met and exposed conductive parts will see a rise in potential,itsw the duration which is the issue. In the sixteenth, they used to have a reg regarding mixed disconnection times, .4 and 5 secs, where there was a possibility of a 5 sec fault appearing on a circuit which required .4 disconnection, it was dropped i believe due to harmonization.

Hence why one of the conditions along with a rcd for omitting supplementary bonding is by ensuring extraneous conductive parts within the location are effectively connected to the met.

 

[nrg]

Very confused now!

 

[nrg]

Very confused now!

 

[RogertheBodger]

Sorry NRG, went off on our own there for a while, forgot about you, the majority opinion seems to be, fit your bathroom RCD, make sure all electrical equipment in the bathroom is supplied through it, and make sure all your cpc's are connected and continuous.

 

[telectrix]

serves you right for coming into the asylum of electrical confusion

 

[nrg]

Yes my own fault! So basically run my incoming live and neutral to the rcd unit and then back to the ceiling rose and connect as you would normally to Fan Isolator and onto fan.