How refreshing - practical MIs

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Just fitted a Labgear distribution amplifier. Where most equipment MIs just say to use the moulded on plug this has :
Alternatively the plug may be cut off and the power unit wired into a fused connection unit, fitted with an approved 3A fuse to BS1362. The PSUF is of class 2 construction and does not require a protective earth connection.
So far so good. But it goes further ;
If the power unit is not connected to the mains using the fused plug supplied or a fused connection unit, it must be protected by means of a fuse or MCB at the distribution board of rating no more than 6A.

Or reading between the lines - it's OK to connect it to the lighting circuit with a B6 MCB.

I can't recall having come across anything else where the MIs have allowed for anything other than a 3A BS1362 fuse in a plug of FCU. Pity they spoil it by using a blue LED that floodlights the bedroom - some black tape will be getting applied before tonight :rolleyes:
 
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If the power unit is not connected to the mains using the fused plug supplied or a fused connection unit, it must be protected by means of a fuse or MCB at the distribution board of rating no more than 6A.

Or reading between the lines - it's OK to connect it to the lighting circuit with a B6 MCB.

I can't recall having come across anything else where the MIs have allowed for anything other than a 3A BS1362 fuse in a plug of FCU.

It used to be a lot more common to have "a 3A fuse in the plug or a 5A fuse at the distribution board" in the days when 5A sockets were still in common use. They're coming back now of course for plugging in table lamps. I have 4 in my lounge.
 
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I think all this nonsense about 3 amp fuses for fans or anything else you would want to connect to a light circuit is just to cover themselves.

I think they reckon some DIYer is going to fit a 13 amp plug on the end of whatever the appliance may be.
 
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I think all this nonsense about 3 amp fuses for fans or anything else you would want to connect to a light circuit is just to cover themselves.
That's probably right, and I doubt that it even achieves that very effectively/meaningfully.

Anyway, now that Amd3 is published, it seems that one is free to work to that if one wishes, so it is possible to "take the MIs into account" but then decide, on the basis of knowledge and judgement, that the fuse is not required. However, I wonder how many electricians would be uncomfortable to do that through fear that it 'exposed' them ('covering oneself'-wise)?

Kind Regards, John
 
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I still think that if a manufacturer says that his fan needs a 3A fuse then what he is saying is that he has made a product which is so shoddy that it is unsafe to connect it directly to a 6A lighting circuit.

THERE IS NO OTHER RATIONAL EXPLANATION FOR SUCH A REQUIREMENT.
 
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I still think that if a manufacturer says that his fan needs a 3A fuse then what he is saying is that he has made a product which is so shoddy that it is unsafe to connect it directly to a 6A lighting circuit. THERE IS NO OTHER RATIONAL EXPLANATION FOR SUCH A REQUIREMENT.
That would only really be the case if the manufacturer actually knew that the product was 'unsafe' with 6A protection but 'safe' with 3A protection - and it is so unlikely that such would be true that I doubt that it is hardly ever, if ever, the 'rational explanation'. Put another way, if their knew that their product was so shoddy that it was unsafe with 6A protection, it would almost certainly also be unsafe with 3A protection.

I'm not sure that I'd call this one 'rational', either, but I think that the only really credible 'explanation' (given a product which almost certainly would not be significantly less 'safe' with 6A than 3A protection) is that, as has been suggested, the requirement exists because of a belief (which I suspect is largely misguided) that it would in some way protect their corporate backsides.

Kind Regards, John
 
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one reason for fitting a 3 amp fuse is that there is a chance, all be it a small one, that in the event of a fault in the device ( or extractor fan ) the 3 amp will blow before the 6 amp fuse supplying the lights.

Hence the lights may remain on.
 
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I have many times read instructions where it is plain the first section is not specific to the item but a general safety instruction where it refers to voltages or connections which simply don't exist on the item in question.

Extractor fans typically 12W or 0.05 amp have winding that thin they will burn out before even a 1A fuse. Some do have solid state control gear which could fail but even then unlikely it can draw any more than 1A and a BS646 as used in shaver adaptors and clock outlets would be the size required to afford any protection once a 20mm fuse is used there is nothing to stop a 500mA fuse from being fitted.

The best place is in the fan its self manufacturers could include an 80 mA fuse which would actually likely blow should the fan stall before the fan burns out.

The same applies to TV pre-amps and ceiling fans already some shaver transformers include an internal fuse.

The problem with the BS1362 fuse is most people carry 13 and 3 amp only and although they are available as 1, 2, 3, 5, 7, 10 and 13 amp you need to order them from RS, Farnell, or TLC rather than pick up in local store. The same applies to 20mm fuse but one is forced to get the 20mm fuse one either has a box of assorted sizes or nothing and with latter you pop in to local radio shop and buy correct size rather than stick in a 3A.

I loved the clock outlet
Clock_plug-outlet_1.jpg
they looked neat and were ideal for fans. This picture shows a
Clock_plug-outlet_4.jpg
shows a BS 1362 fuse but most I had used a BS646 fuse and these are available in smaller sizes like 80 mA.

I went to try and buy one and could not believe the price. Better picture of clock connector here with a BS 646 2A fuse. My attempts to find any for sale failed.
 
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That would only really be the case if the manufacturer actually knew that the product was 'unsafe' with 6A protection but 'safe' with 3A protection - and it is so unlikely that such would be true that I doubt that it is hardly ever, if ever, the 'rational explanation'.
I'm kind of with BAS on this - except that rather than "knowing it's unsafe", they simply can't be bothered doing any engineering to make a "safe" product. Hence without thinking they just slap a "3A fuse requirement" on it and "job done". I think it says as much about the quality of the engineering as BAS's suggestion that they know it's "unsafe" on a 6A circuit.

Still, at least we can ignore the requirement now ;)
 
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That would only really be the case if the manufacturer actually knew that the product was 'unsafe' with 6A protection but 'safe' with 3A protection - and it is so unlikely that such would be true that I doubt that it is hardly ever, if ever, the 'rational explanation'.
I'm kind of with BAS on this - except that rather than "knowing it's unsafe", they simply can't be bothered doing any engineering to make a "safe" product. Hence without thinking they just slap a "3A fuse requirement" on it and "job done". I think it says as much about the quality of the engineering as BAS's suggestion that they know it's "unsafe" on a 6A circuit.

Still, at least we can ignore the requirement now ;)
I am with JohnW2 on this so many times we see the same preamble to products it is quite clear no effort has been made work out what is required the same disclaimer is printed with a whole group of products even to the extent of showing how to fit a plug with a battery produce.
 
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That would only really be the case if the manufacturer actually knew that the product was 'unsafe' with 6A protection but 'safe' with 3A protection - and it is so unlikely that such would be true that I doubt that it is hardly ever, if ever, the 'rational explanation'.
I'm kind of with BAS on this - except that rather than "knowing it's unsafe", they simply can't be bothered doing any engineering to make a "safe" product. Hence without thinking they just slap a "3A fuse requirement" on it and "job done".
As I've said, not only don't I believe that they do it because they "know it is unsafe" with 6A protection but nor do I believe, in the vast majority of cases, that the product is "unsafe" with 6A protection. Rather in sort-of the way that you have suggested, I suspect that they simply stick on the '3A fuse requirement' without any real consideration of whether it is necessary, in this belief that it somehow 'covers them'! Let's face it, if the product were a similar one which was connected by a BS1363 plug/socket, no-one would be criticising them for requiring a 3A fuse in the plug, suggesting that it ought to be manufactured to be safe enough with a 5A, 10A or whatever fuse.
I think it says as much about the quality of the engineering as BAS's suggestion that they know it's "unsafe" on a 6A circuit.
As above, at least in the case of 'reputable manufacturers', I do very much doubt that they have "not bothered to make it a 'safe' product". As I've suggested, even if one set out deliberately to do it, I think it would be pretty hard to make a product which was 'safe' with 3A protection but 'unsafe' with 6A protection.
Still, at least we can ignore the requirement now ;)
Not 'ignore' - you still have to 'take it into account'. As I wrote yesterday, I do wonder how many electricians will, in practice, feel comfortable to use their judgement to 'over-ride' what is suggested by MIs.

Kind Regards, John
 
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I think they reckon some DIYer is going to fit a 13 amp plug on the end of whatever the appliance may be.

Its a good point. It can be how DIYers add things, a long length of flex to a socket. (as they are not confident with playing with the house electrics) And in this case pointing out a 3A fuse rather than 13A is useful.
 
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I think they reckon some DIYer is going to fit a 13 amp plug on the end of whatever the appliance may be.
Its a good point. It can be how DIYers add things, a long length of flex to a socket. (as they are not confident with playing with the house electrics) And in this case pointing out a 3A fuse rather than 13A is useful.
If that were their only concern, they should perhaps say so.

Mind you, I think most of us believe that things like these fans would be (certainly should be) 'safe' with 6A protection, and I'm not at all convinced that they would actually be significantly less 'safe' with 13A protection (which would afford adequate true 'fault' protection), would they?

Kind Regards, John
 
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