Extractor fan need 3 amp fuse

[ebee]

I've already commented on that, and my answer (in terms of 'electrical common sense') was essentially 'no' - on the basis that the only two situations I could think of that would/could cause overcurrent in the S/L feed (a 'new path' from the S/L terminal 'appearing', or the input resistor "failing S/C") would both seem to be 'incredibly improbable'.

I would base it on some at least not being sure what is driving the SL in the circuit so far as the fan is concerned so might assume thec same rules as for the Perm L , which at least is logical on its simplest level so to assume the same aplies for both I would not criticise.

You will not believe this (I didn`t so I asked him to repeat it just incase I missheard, no one else picked him up either), a tutor stated that on SELV timed overun the fan control had two transformers inside.
When I enquired why two transformers not one he replied that that one transformer was supplying 12v for the perm supply and the other one for the timed overrun part of it.
He seemed a bit lost when I retorted, take a mains fan with timed overun (one supply) and remove the fan, then replace it with a 12v transformer then connect a 12v fan to it "how many transformers now?" .
I am not kidding.

 

[JohnW2]

I would base it on some at least not being sure what is driving the SL in the circuit so far as the fan is concerned so might assume thec same rules as for the Perm L , which at least is logical on its simplest level so to assume the same aplies for both I would not criticise.

Could you perhaps re-word that a little, in a way that enables even me to understand exactly what you ae saying? Thanks,

You will not believe this (I didn`t so I asked him to repeat it just incase I missheard, no one else picked him up either), a tutor stated that on SELV timed overun the fan control had two transformers inside. .... When I enquired why two transformers not one he replied that that one transformer was supplying 12v for the perm supply and the other one for the timed overrun part of it. .... He seemed a bit lost when I retorted, take a mains fan with timed overun (one supply) and remove the fan, then replace it with a 12v transformer then connect a 12v fan to it "how many transformers now?" . ...I am not kidding.

I suppose that we are all sometimes guilty of 'missing the obvious'or 'not seeing the wood for the trees'

Without wishing to spoil your story, if one did exactly as you describe (with the 'mains-powered timer module' still in situ, and being used, one would then have LV inside what was meant to be an ELV item. What one presumably should do (still with just one transformer) is to have just a simple 12V non-timer fan, fed from (in a separate enclosure) something like the standard mains-powered timer module, followed by the (single) transformer?

I suppose it's slightly like those who, without thinking too much (enough?), expect an 'integrated' mains-powered PIR+LED light fitting to have a 'switched' LV output, which they can use to feed other lights (or use for 'PR override,but applying 230V to it), as was usually the case with the equivalent using LV lamps/bulbs. However, since the unit obviously has to convert 230V AC to DC ELV for the electronics, it really makes little sense to then use the (DC ELV) output of those electronics to switch (with relay or triac) the AC LV supply to yet another LV-DC ELV converter in order to run the LED(s).

Kind Regards, John

 

[ebee]

Yes John,
in reverse order,

it was a thought experiment of the simplest kind to explain the basic principle of only having one 12v transformer to convert from LV to ELV (in this case it would be the SELV version of ELV) .

Of course in a real life situation the SELV transformer would be mounted away from the actual fan motor but all the other principles are the same to describe how the 12v fan is powered using just one transformer not two.

In this instance I mentioned no one else picked him up on his statement and I thought "What? have I miss heard or missunderstood what he just said?" so I asked him " Sir, please could you repeat what you said about needing two transformers in the timer box to run one 12v fan?" and he did repeat it.

In the first instance ref fusing for timed over run.
Any Electrician etc would appreciate that any circuit would probably require an overcurrent protective device of some type.
If a manufacturer informs them or indeeds suggests that an additional/alternative device is required, in this case a 3A fuse, then without any knowledge of exactly how the electronics operates the timed over run it could be reasonable for them to make an assumption that both the perm L and the switched L should be similarly fused for similar reasons .
A humble electrician might not have enough knowledge of electronics to make assumptions.
If that were, in real life, true then logically it would be preferred that both L conductors share the same fuse.

Indeed we could pretty much protect against overload and short circuit with one fuse connected to the N conductor but we avoid that for sound reasons in all circuits.
Of course that OPD would not protect against an earth fault either but in the fan appliction it might not be actually needed for such.

Therefore we might decide to put a bathroom light and fan on its own dedicated circuit from the consumer unit.
Very often we would actually decide to have it as part of a larger circuit servicing lights in additional locations too.
If that is the case and we decide to do as the manufacturer requires and fuse down to 3A then that, by definition in part 2 of BS7671, is a circuit in its own right, although many of us would probably refer to it as a sub circuit.

The most elegant way is to put in a SFCU for the bathroom part of the lighting circuit then a 3 pole isolator just for the fan.
That way the SFCU and fuse energises both the light and the fan and the 3 pole switch operates only the fan itself.