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Does a spur always need a fused unit?

Discussion in 'Electrics UK' started by Minime46, 24 May 2021.

  1. Minime46

    Minime46

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    Hi all,

    I'm going to be putting in a double socket to power a POE switch. I am looking to take a spur from a socket on the main circuit.

    My question is do you have to, first feed to a fused spur unit before the socket. The socket is going to be in a lockable box which will be labelled 'for cctv switch only'. Unsure if regs need the fused part??? The plug for the poe switch will have a 3amp fuse in it.

    Hope someone can shed some light

    Cheers
     
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  3. bernardgreen

    bernardgreen

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    If you can be sure that the plug for the POE will have a 3 amp fuse then there is no need for fused spur.

    My concern is increasing numbers of "13 Amp" plugs are in reality Switched Mode Power Supply modules providing ELV power to the equipment with little if any fuse protection in the "plug" If ( when ) the module fails then the only protection is the over current device in the consumer unit which could be a 32 Amp MCB.
     
  4. ericmark

    ericmark

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    One can have an unfused spur (2.5 mm²) from a 32A ring final but that spur must only feed on single or double socket, the same applies with a 32A radial when using 2.5 mm² and the radial is 4 mm² or 6 mm² but you can't take a spur from a spur when unfused.
     
  5. EFLImpudence

    EFLImpudence

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    No.

    It needs a fuse if you have to limit the current in the spur because the cable used is too small for the overload protection device (fuse/breaker) on the main circuit.
     
  6. JohnW2

    JohnW2

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    Is that not a rather silly comment, even for the ultra-cautious, given that I imagine that the great majority of plug-in SMPSUs ('wallwarts') such as you mention usually are plugged into sockets 'only protected by a 32A MCB' (sockets "on" a 32A ring circuit or 32A radial circuit)?

    Or are you suggesting (I've never heard anyone suggest this one before!) that, contrary to 'normal practice', such things should only be plugged into sockets which are "fused spurs"?!

    Kind Regards, John.
     
  7. JohnW2

    JohnW2

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    Indeed.

    Of course, it (virtually) never needs a fuse anything like as small as 13A, but we're stuck with the fact that, if using an FCU, 13A is the highest rating of BS1362 fuse available. If the cable in question were 2.5 mm² ('Method C'), fed from a "32A circuit", then one could (electrically speaking) use a 25A MCB in a suitable enclosure, rather than an FCU.

    Kind Regards, John
     
  8. bernardgreen

    bernardgreen

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    No I do not think it is "rather silly".......

    The concept of the UK 13 Amp plug with a fuse in the plug is totally ignored in a lot of wall wart power supply "plugs"

    230 Volts across 20mm of conductive debris ( charred PCB, soot and molten plastic casing material ) can conduct far more than 3 Amps and while it is conducting the 32 Amp MCB will not trip.

    If a socket is intended for very low current wall warts then the insertion of a 3 Amp limit into the supply to that socket will offer better protection than the 32 Amp limit the consumer unit offers
     
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  10. JohnW2

    JohnW2

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    Fair enough. So, as I asked, does this mean that (even though I've never heard anyone suggest this one before!) you are suggesting ...
    ... since to suggest that the concern only exists if the device is plugged into an unfused spur, but not if it is plugged into a socket "on" a 32A ring or radial circuit, would, surely, be 'silly', wouldn't it?

    Kind Regards, John
     
  11. bernardgreen

    bernardgreen

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    To be blunt putting a lump of electronics, a wall wart, directly onto a 32 Amp supply is not low risk. But the wall wart is attractive to the mass of the population so the dubious concept of the wall wart can be easily sold to the public
     
  12. winston1

    winston1

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    I’m pretty sure wall warts are internally protected. After all they are used on 16 amps circuits the world over which could also be devastating. But I don’t hear of fires being caused by them here or overseas.
     
  13. JohnW2

    JohnW2

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    I'm not arguing with your concerns about having such devices plugged into a supply which has only 32A protection.

    What I am asking is why this should be any different when a socket is on a 'spur', rather than part of the main 32A circuit. If, as I would hope/imagine you agree that there is no difference, then I am trying to get you to 'admit' that you're saying that it is your view that these devices should not be 'allowed to be' plugged into a socket "on" a 32A ring or radial circuit.

    If you did 'admit' that, then I would have to ask how you would hope to be able to advise, let alone 'compel', people to only plug these things into sockets which were on 'fused spurs', given that the great majority of users will not have a clue as to which of their sockets is supplied in a particular way.

    In fact, given that few people would contemplate 'tighter' than 13A protection for sockets which were on a 'fused spur', I'm not sure that would be much better than 32A protection in terms of the sour of theoretical issues which concern you, would it?

    The only really solution would be for 'wallwarts' to have (appropriately rated) fuses within them. However, that is not the case, so we have to advise people in terms of "how things are at present" (and are not going to change any time soon)

    Kind Regards, John
     
  14. JohnW2

    JohnW2

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    The majority, if not all, are (albeit often fairly crudely) - but bernard seems to be unsatisfied by the extent of that protection.
     
  15. JohnW2

    JohnW2

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    As I've been saying, I am not disputing your concerns, but am trying to find out what you (realistically) expect to be done about it.

    Talking about this issue (as you did) as if it were only a problem with 'unfused spurs' will be, I fear, plain confusing to most readers, since it is a general concern which applies to the devices, regardless of what socket they are plugged into.

    Kind Regards, John
     
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