Post by charlesPost by John RummPost by Graham.On Thu, 18 Dec 2014 18:30:50 +0000 (UTC), "A.Lee"
Post by A.LeeSnip - fan directly fed from 32A circuit.
Post by charlesUnfortunately I can't quote regulations that were superceded in 1981 to a
contractor who did the work in 2014. But, thanks
What do the Fan Instructions say?
They are what will determine whether you need a fused spur, or fed via
the 32A circuit.
You already have an isolator - the circuit breaker.
The fan is unlikely to need overload protection, unless called for by
the Manufacturers Instructions.
Of course, it would be good practice to locally switch/fuse it for
maintenance purposes, but it is not required if the Manufacturers
Instructions do not require it - most do BTW.
Good grief Alan, it's a kitchen fan, not a wind-tunnel at BAE. No way
can he rely just on a 32A MCB.
To be fair, it depends on the circumstance. If you can argue there is no
way for the fan to cause an overload of its supply cable, then you can
dispense with overload protection altogether. Then the remaining
requirement is for fault protection. Say it were connected via 1.5mm^2
T&E, then that would be adequately fault protected by a B32 MCB.
whereas the internal wiring that I could see was probably only 0.75mm^2.
True but not particularly relevant.
This is why you need to keep the concepts of fault and overload
protection separate in your mind. Usually they are both provided by the
same protective device, but not always - the responsibilities can be
separated.
Fault protection must always be present at the origin of the circuit.
Overload protection can be elsewhere, or possibly not present at all and
provided simply by design options limiting or eliminating the
possibility of overload.
A common example would be the 3A drop flex to a pendent fitting on a
lighting circuit. That circuit may be protected by a B6 MCB, which will
not provide overload protection to the drop flex. However the single
lamp holder on the end, and the choice of lamps available does impose a
limit. So additional protection for sustained overload is not needed.
However it would be possible to cause a short circuit at the end of the
drop flex, and that must be accounted for. The way you work out whether
a wire has adequate fault protection is to use the adiabatic calculation:
http://wiki.diyfaq.org.uk/index.php?title=Calculating_A_Cable_Size#Adiabatic_Check
You start by working out the likely fault current - this will be
inversely proportional to the the total circuit impedance at the point
of the fault. The impedance simply being the sum of the resistances of
the cable runs, and the supply and possibly earth impedances. Once you
have this and the k factor for the cable in question you can work out
the minimum cross sectional area of cable required to survive long
enough to clear the fault.
So for example, take your nominal 0.75mm^2 cables in the fan. Say the
loop impedance was 1 ohm at the point of fitting. That gives a potential
fault current of 230A, which will open the B32 MCB in 0.1 secs or less
(i.e. on the magnetic part of its response). If your wires are PVC
insulated we can take K at 115. So that gives us a minimum wire size of:
sqrt( 230^2 x 0.1 ) / 115 = 0.63mm^2
So even if someone sticks a 6" nail through the fan, and manages to
short a couple of those wires, we are still ok ;-)
Now for the avoidance of doubt:
I am not suggesting here that directly connecting fans to 32A ring
circuits is a good idea. In most cases I would expect the manufacturers
instructions (which take precedence) to impose lower limit protection
anyway. Even if they don't it would (IMHO) still be good practice to
include a switched FCU for applications like this.
--
Cheers,
John.
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