Shower extractor/light unit wiring problem

[elbowgrease]

Can anyone help? I have tried wiring up my combined shower extractor/light as per the attached diagram but the RCD (not the MCB) on the distribution board feeding the shower continually trips when the light is connected. If the light is disconnected the fan runs perfectly.

I have done all of the following to check things out:
Turning off all the other MCBs on the same board as the shower MCB. It still trips.
Disconnecting the fan and connecting the light only. It still trips.
Wiring the light direct to the shower isolator. the light works and does not trip the RCD.
Wiring the light directly to each of the FCUs in turn. The light works and does not trip the RCD when wired to shower side or the MCB on the lighting circuit (the lighting circuit is on a different distribution board).
Checking the earth connections do not cross from the shower circuit to the lighting circuit.
Disconnecting earth wires into the two FCUs (plastic drywall boxes in use). Still trips.
Changing the wiring arrangement to ensure that a cable fault is not causing the problem. It still trips.
Connecting in a different lamp (normal 240v incandescent) rather than the transformer fed LV halogen. It still trips.
Wiring the halogen on the permanent live/neutral off the fan isolator. The light works and does not trip the RCD. Obviously it is on all the time since it is not on the switched live but hey, you cant have everything.
Have I missed something obvious?
My only solutions are to take the permanent live off the FEED side of shower isolator and a switched live off the LOAD side but this is not good as it bypasses the switch. Alternatively I could put in a relay to electrically isolate the shower and lighting circuits. Anybody else suggest anything?

 

[pipme]

I am just an interested onlooker.
I think you are about to recieve a mild chastising from the experts !! Be prepared.
Look at the light - Borrowed neutral !!
What does the RCD detect twixt live and neutral, balanced or not with light on ?

Good luck !!

 

[plugwash]

mild chatising?! you mean major grilling

lets see:

1: you've connected from the output of the shower isolator to other powered equipment which could cause the shower to become live even when its isolator is off

2: you've connected between cuircuits which could cause dangers for someone working on those cuircuits in future

3: you've run a single insulated core between two enclosures

the reason you got away with it without the rcd tripping with just the fan connected is that fans draw a negligable current down the switched live wire. once you put the light in you were drawing significant current though a loop which only went through one side of the rcd and this caused it to trip.

is your shower isolator a pull cord inside the bathroom or a wall switch outside

if the latter then you could replace it with a triple pole (or TP+N if thats all you can get) isolator and dedicate one pole to the lighting/fan control

if the former then your only real option is to use a contactor

 

[breezer]

i would be inclined to take it all out and do it properly

 

[plugwash]

the problem is there is no easy way to do this properly

most people who ask about this give up and have thier fan controlled by something else

 

[Adam_151]

the problem is there is no easy way to do this properly

most people who ask about this give up and have thier fan controlled by something else

I thought ban all sheds had worked out how to do it, with the shower isolater also switching a contacter, which switches a live from the lighting circuit?

 

[plugwash]

i said no easy way

 

[pipme]

Pain in the butt !!
If you connect to act with the light having timer, then kids in and out of bathroom, fan is on far more than necessary and becomes an annoyance.
If you put it (timer type) on it's own pull cord you need one (pricey?) with a 'tell tale' visual and/or neon- Otherwise if fan running has it been switched off by pull cord or not ? ... Could be good reason for SFCU on landing .. interrupt / reinstate power to fan, if it stops ok, if not, pull cord, then switch SFCU off/on .... Nah too much bother !!
How well does the humidistat type perform ? Expelair have extractors which are, apparently, adjustable for time delay, room size, humidistat action .. are they excellent .. at a price ?
As an experiment I opened the top light of bath room window, used normal 16" table top cooling fan, on landing well away from damp bathroom, airflow aimed through bathroom door, 4 minutes and no condensation on tiled wall above window .. outside frosty at the time. Quite obviously not an option, but pressurising the room with an open window seemed very efficient.
P

 

[elbowgrease]

Not being the sort to quit, the first response from PIPME prompted some thoughts and I got it working by linking the light into the FCU fed from the shower supply.

I take the point about cross connecting two different circuits although the single core is within a dual box holding an FCU and the fan isolator.

Having located the solution from BAN ALL SHEDS referred to I have an alternative solution. This does not involve ripping out and doing it again properly as helpfully suggested by one respondant. By replacing one FCU with a normally open relay I think I can achieve the same result. The only question is whether I can get a relay that will fit in the confines of one half of an Appleby dual dry lining mounting box and permit the fitting of a single gang blanking plate. Anyone think this is possible?

 

[plugwash]

yes the relay you would want is a contactor with both mains coil and contacts

your soloution is fine if you can get approproate size cables into all the terminations (6mm is just about ok for a 45A cuircuit and you should manage it with that but its almost certianly a squeeze

remember to mount the contactor in a suitable enclosure

 

[plugwash]

http://www.tlc-direct.co.uk/Products/MK6220.html
http://www.tlc-direct.co.uk/Products/MK5502.html
http://www.tlc-direct.co.uk/Products/MK5544.html

 

[breezer]

NO, NO there is one very dangerous thing

there is a 2.5 cable connected to the shower cable, cant do that it may cause a fire.

as i said best do it again, you will not be able to get 2, 6 or 10mm shower cables into the "shower isolator switch"

 

[plugwash]

hadn't noticed he had changed that bit

though im not entirely sure your description of a short 2.5mm spur off a 45A cuircuit as very dangerous is exactly accurate either

its probablly best to stick with 6mm for the run to the relay/contactor if you can fit it in though even if its not stricly nessacery

 

[pipme]

What about protecting the relay and it's wiring ?
45 amp MCB ??? 2.5² T&E = Potential problem ???

Sorry chaps beat me to it !!
P

 

[breezer]

its probablly best to stick with 6mm for the run to the relay/contactor

"probably best" is a gross understatement, you MUST since when a fault occurs which will go first the 6mm cable or the 2.5 mm cable, not rocket science is it? the 2. 5 will heat up and cause a FIRE

but you physically will not be able to get all the cables in the terminals as i said.

so also as i said, sorry do it properly, rather than wait for it to go wrong and catch fire