Circuit diagram for Bathroom ventilation fans

[canteachnow]

I want to install a new ventilation fan in the bathroom - probably a humidistat type as the room is generally cold - but I want to be able to "isolate" the fan ie stop the automatic stat effect if need be. I already have a three core cable installed from the light fitting to the fan and do not want to add or change this arrangement. I think Manrose offer a pull cord arrangement as an "overide" but does this simply turn on the fan when the stat is off or do what I want and disable the fan ??
Does anyone know how this operates or functions ? If I had a circuit or wiring diagram I could probably see how this works, does anyone have any advise on operation/solution/details of wiring ??
Many Thanks.

[ericmark]

Bathroom fans come in many types from those costing hundreds of pounds with heat exchangers built in to the simple type at less than £20.
Many are switched by the lights and this would seem best method where a WC is included in the room.
When used with light trigger there are two common methods of connection. One is to use a triple pole isolator and connect to lighting supply this means the overload protection is the same as that for lights normally 6 amp.
However some require a protection device of 3 amp and in this case the light switch is changed for a double pole so although same switch the circuits are completely separate.
In both cases a timer ensures a run on once lights are turned off.
The type often used in bedrooms with shower or in some cases also bathrooms without a WC is the humidity sensor although with bedroom there is not real alternative with bathrooms because the humidity type seem to run on a lot longer than the timer type they are not so popular.
There is also a type designed to run 24/7 with a lot lower exchange rate and with more expensive types a heat exchanger which takes the hot air it expels in the bathroom to pre-heat the replacement air normally feed into a down-stairs room ensuring fresh air without heat loss but looking at £400 to £600 for these.
As a general diagram you can look at http://www.ericpalmer.fsnet.co.uk/Images5/LIGHTS.JPG this shows one example of most lighting options in general use including bathroom.
One problem today is all items in a bathroom now need RCD protection and which type is selected may be easier or harder to protect with a RCD the changing of light switch method to duel pole allows a duel pole isolator instead of triple pole to be used which in turn means a RCD FCU can be used.
Because of Part P of course in a bathroom it has to be right otherwise LABC or registered electrician will not be able to sign off the job and although you may want to select the type I would also confirm with who ever is signing the work off they will accept the type you propose. They may not accept a humidity type if there is a WC in same room.
Does that help? If not please ask.
Eric