1) It mmight not be obvious to someone that supply is from the upstairs ring rather than the downstairs lighting. Not withstanding that there ought to be a service isolator for the fan.
2) If the fan is the sort that requires both permenent and switched live supplies (such as timed overrun models) then you will have also to supply switched live from two seperate circuits. Which not only is contrary to the regs but poses hazards for maintainers and might even make two circuts have common live connections through the fan timer circuits.
It wasn't that which caused the problem, it was the suggestion that for a run-on fan you should wire the "permanent live" from such a spur, but take the "switched live" from the (e.g. bathroom) light switch. In this instance you have connected the fan across two circuits which
*should* be separate for all the reasons already mentioned (refer to my previous reply).
If the fan was, say, a stand-alone fan controlled by its own switch or humidistat for example, then there's no problem connecting it to the sockets circuit. In fact, especially for large fans in large kitchens for example, there are positive benefits.
I'm not sure you missed anything actually in the exam, except the very small but vital point of keeping circuits separate. I took mine earlier this year too, and don't think that was mentioned, but then a lot of other stuff wasn't mentioned either.
This is the kind of point which to many people seems so blindingly obvious as not to need labouring (oh boy, after last night, do I know about labour!) but which to the person on autopilot can quite easily lead to a major safety issue. Again, for clarification of the possible safety issues I refer the reader to my previous reply on the current version of this thread :-)
I certainly didn't mean it to be wired that way, and you are absolutely right about me being locked up if that's what I suggested..... ;)
What I was thinking was that the fan would be standalone and not controlled by the light switch. In our current house for example our bathrooms have a fan which is controlled explicitly by a switch outside of the bathroom, and I assume that this would be wired to the ring main (switching the lights on and off has no effect on the fan).
I need to take more care about expressing myself..... ;)
Yes, sorry, I dived into a thread in which, for some reason, most of the articles weren't visible. Mind you IME of fans with switched live terminals, which is confined to one model (Vent Axia VA100H), the current drawn via the SL is minuscule (a few hundred uA). SL just feeds some electronics which drives the gate of a triac in the main fan motor cct, so the motor current is always taken from the permanant live terminal. Other designs may be quite different
I had the same problem which is why I went off to Google - they seemed to have the lot. Clara and Demon don't share a newsfeed, do they?
That is the way I'd design it, and isn't really a problem. The problem is that, whatever the actual current drawn, power from (as suggested) two otherwise completely separate circuits is present on the same circuit board, probably in adjacent tracks from the connecting block. There is no guarantee that these tracks have 500V worth of insuation between them. Likewise there is no guarantee that any switching device (a triac is certainly a possibility) will not connect the "switch" and "load" sides of itself in a failure mode, or for that matter under normal conditions.
If such a connection absolutely must be made than the most obvious way I can think of to do it would be some kind of relay with two operating coils, one for "switch" and the other held on via a timer from "load". I doubt any fan manufacturer would do this when they can get away with a triac and an RC timer :-)
Well, we haven't actually heard from Mark Evans. ISTM that PoP knew what he was talking about, didn't quite write it down clearly enough and was then selectively quoted by Mark...
Thanks! All home the same day and settling into routines again... not much more sleep though :-/
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