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Difference between fused spur and multiple extension leads?

Discussion in 'Electrics UK' started by pbryd, 16 Oct 2019.

  1. pbryd

    pbryd

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    You're allowed to add multiple double sockets to a spur as long the cable is protected with a 13amp FCU.

    What is the difference between an FCU'd spur and running multiple extension leads which have 13amp fuses in the plugs?

    Are multiple extension leads frowned upon because people have a habit of plugging in too many high amp appliances?
     
  2. DetlefSchmitz

    DetlefSchmitz

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    What do you mean by multiple extension leads? Who do you hear frowning on them?
     
  3. pbryd

    pbryd

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    Daisy chaining them together
     
  4. EFLImpudence

    EFLImpudence

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    There is no difference. They are all protected by the 13A fuse in the plug of the first extension lead.
     
  5. AndyPRK

    AndyPRK

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    the bad points could be that they are cheaply made

    and ones on reels can have thin cable.


    I guess it adapters without fuses is the biggest worry
     
  6. OwainDIYer

    OwainDIYer

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    Earth fault impedance, among other things.

    As well as protecting the cable with a 13A FCU you also have to have permanent sockets comply with earth fault impedance and disconnection times and all the other safety requirements.

    Chained extensions can have thin cable and high resistance contacts, increasing the earth fault impedance to the point where protective devices wouldn't operate quickly enough to protect the extension lead or the user.

    In the workplace there are guidelines on the maximum length and minimum conductor size for extension leads and they should not be daisy chained.
     
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  7. Risteard

    Risteard

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    Extension leads should not be used permanently, and should be fully unwound. You shouldn't link multiple ones together. The impedance as mentioned could be excessive, also they can pose a tripping hazard.

    Adequate socket-outlets should be provided, and this indeed is a requirement of BS7671.
     
  8. skenk

    skenk

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    Provided the sockets are protected by RCD (which they should be) the earth loop impedance is not an issue. Assuming a correctly designed 32A socket circuit the fuse in the plug will deal with short circuit conditions for fairly long lengths of extension cable.
     
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  10. DetlefSchmitz

    DetlefSchmitz

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    I don't see the connection between multiple drum type cable extensions (attempting to reach distant location), and providing sufficient sockets at one location.
     
  11. aptsys

    aptsys

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    The main risk with daisy chained extension leads is all of the additional potential failure points. There is nothing inherently dangerous about the electrical configuration.

    According to whom?
     
  12. ericmark

    ericmark

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    The problem with daisy chain extension leads is every 13A fuse has some resistance, and this builds up, so you get excessive volt drop. In real terms the only domestic items which has a problem with excessive volt drop is the fridge/freezer, they can burn out the overloads if there is an excessive volt drop, they will be some other odd items, but not many.

    So TV, DVD, Sky box, VCR, Bluray, hard drive, and radio all plugged into a single socket outlet is not going to cause a problem, but washing machine and tumble dryer in same outlet then yes possible problem, a fuse will get hot, that's how they work, so a tumble drier which runs for maybe 1.5 hours will likely cause the fuse to warm up the plug and any extra could cause damage due to that heat.

    In years gone by the only protective device was the fuse, and for a fuse to rupture within the stated time the earth loop impedance with a 13A fuse should be 2.42Ω or less, adding too many extension leads could mean that figure is exceeded, however today with RCD protection there is not really that much of a problem, as although one should not rely on the RCD, it is unlikely that the 4 second disconnection time is exceeded, it will likely disconnect in under 40 millisecond due to the RCD.
     
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  13. skenk

    skenk

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    posted in wrong thread by mistake..
     
  14. JohnW2

    JohnW2

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    I don't know the figures off the top of my head, but is that really a significant issue?

    Kind Regards, John
     
  15. ericmark

    ericmark

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    As far as I am aware about the only house hold item which is likely to be damaged due to volt drop is the fridge/freezer. The overload which switches the unit off should is stall due to trying to start before pressure has dropped fail if used too often, and with a volt drop this can happen.

    So you will see it states with most do not use an extension lead. The problem is as well as the resistance of the lead, there is also a resistance in the fuse, 13A I think 0.04Ω which is not a lot, but I have seen a 1 to 3 adaptor in the plug feeding three 4 way extension leads in the main for audio/vision so very little load but then one off that to chest freezer so 3 fuses plus length of lead so easy to get to 1Ω = 13 volt at 13 amp. Add this to the volt drop at the outlet and now it has reached the point where a freezer may fail.

    Where I had the problem was Algeria, we had often a 220 volt supply and were using British equipment rated at 240 volt so 20 volt before we even started, we has some Dutch and some British electricians and the Dutch would set output to 220 and British to 240 and it was noted not as many AC units failed on the camps with British electricians in charge.

    Fluorescent lights also very voltage sensitive but don't tend to be plugged in, and radio transceivers, problem with latter often only on transmit so user may be unaware of the problem. I have had problems with a shrink rap machine, but that was not domestic. In the main the switch mode power supply has removed the problem with volt drop, my fridge/freezer is three phase powered by an inverter (built in) so today modern fridge/freezers don't have the same problem, however instructions still say don't use an extension lead.

    In theory every 13A socket on a ring final should have an loop impedance of 1.44Ω or better, radials could be 2.87Ω and a fused spur 2.42Ω (13 amp) and 16.4Ω (3 amp) however how often does a person adding a socket actually check the loop impedance, should clearly do it every time with fixed wiring but with portable extension leads rare. When PAT tested there is a pass limit, can't remember off hand, but nothing stopping a device in a device, and nearly every plug in item has a fuse, time clock, energy monitor, all add to the resistance.
     
  16. JohnW2

    JohnW2

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    That may or may not be true, but it wasn't my point.

    My point/question was that, if one did have a voltage-sensitive load, and remembering that UK supply voltage is allowed to be as low as 216.2V, do you really believe that the voltage drop across a handful of 13A fuses in series could have any effect worth even thinking about?

    Does anyone know the resistance (or, alternatively, the voltage across it) of a 13A BS1362 fuse when carrying 13A (hence fairly hot)?

    [The total length of several extension leads in series could potentially become a problem, but you were talking about the fuses, not the cable length].

    Kind Regards, John
     
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