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3 pin single socket of lighting circuit - fused spur ok?

Discussion in 'Electrics UK' started by jonwestuk, 2 Jan 2015.

  1. jonwestuk

    jonwestuk

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    Hi,

    I want to add 2 wireless rear speakers to my 5.1 surround system. I have found a piece of kit that sends/receives the audio signal, and powers the rear speakers. The unit uses 1 amp at 24v so about 24watts of power.

    Now I have a concealed cubby in the rear of my living room ceiling which houses an LED strip power supply and in there is the lighting circuit. I would therefore like to add a 3amp fused spur to a single regular 3 pin plug to power the speaker receivers.

    Having overseen and helped designed the lighting circuits with my electrician a while back I know that all the cable is 1.5mm covering the living, dining and kitchen only and being received by a 6amp RCBO. That RCBO does cover 2 floors, but each floor has its own 1.5mm radial lighting circuit. All the lights are bulb-less LED spots and there is one kitchen extractor running off the circuit (which will move to the ring when I redo the kitchen) - so the draw on the circuit at peak usage is still very low.

    I am therefore 99.9% happy that the draw from the speakers is low enough to keep well within the tolerance of the wire, and that both the 3amp fused spur and 6amp RCBO are sufficient to protect the cabling from someone in the future plugging something large into the socket (which is unlikely as it will be hidden in a ceiling void :). To me this seems similar to adding a 3 pin socket to the lighting circuit for a TV amp in the attic.

    Are there any other issues that I need to consider? I am really just looking for that additional 0.1% confidence that I have not forgotten anything.

    Thanks,

    Jon

    PS - I also believe that this is non-notifiable as it is a fused spur to a radial circuit.
     
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  3. ericmark

    ericmark

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    Although it is common to fit 13A sockets to power a TV pre-amp that does not make it right. Using a mast head type they can be fed with DC from where the TV is to power the amp.

    The same applies to any other amplifier speaker cables can carry DC as well as the signal to power an amplifier 100 volt DC was very common.

    It may be safe and easy to do but the fact is some one could at some time put something else into the socket.

    There is a big difference between being safe and complying with regulations but the regulations have been made with the whole idea of having a safer electrical system.

    If you look at the fusing curves for even a 1A fuse and a 6A B type MCB they do cross. So a fault could very easy cause the RCBO to trip before the fuse blows plunging you into darkness.

    So to me it's an unnecessary risk.
     
  4. winston1

    winston1

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    So you want to add a fused spur feeding a 13a socket on the lighting circuit. The answer is no. Nor should you have a 13a socket on the lighting circuit in the loft.
    Fused spurs are pointless on a lighting circuit anyway as the 6a RCBO adequately protects the cables.

    You may not think anyone would plug in a large load, well they might.
    It happened to me when I moved into my prievous house. On moving in day I plugged the kettle into the only 13a socket in the garage and immediately took out the downstairs lights. The prievous owner had passed away so I couldn't ask, maybe he only used it for a battery charger.
     
  5. Nozzle

    Nozzle

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    Why not fit a 3amp or 5amp lighting socket/plug set and then just change the plug on your gear?

    Nozzle
     
  6. jonwestuk

    jonwestuk

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    Thanks for your response. I am interested in home electronics and therefore want to understand it better.

    My current understanding is that the fuse on a fused spur is designed to protect the upstream cable from exceeding its limit of power draw. In this case 1.5mm cable will be used, that is rated for 15 amps so a 3 amp fuse should be plenty.

    I believe the RCBO is designed for two things:
    1 - to protect the radial circuit from exceeding its power draw. With 1.5 mm cable rated for 15 amps then a 6amp fuse should protect it very well.
    2 - to protect the circuit from appliance faults such as power surges or short circuits etc.

    Is the above wrong or are there additional things to consider?

    If that is right, then by design the RCBO should trip first before the fused spur as it is much more sensitive to changes in current. Therefore this is not really a risk, but instead by design.

    The risk of being plunged in to darkness is just the same as if one of my LED's developed a fault or if a bulb blew. I am not sure how adding this appliance increases that risk in any greater way than if I added an additional light? Both could develop a fault, the risk of such faults increases the more appliances are on the circuit.

    When you talk about safety, could you please expand on the point as this is really the thing I care about most. If the cabling is protected by the fuse in the spur, and faults are protected by the RCBO what additional risks am I bringing to the the house? Why would adding the appliance to this circuit be any greater, baring in mind the protection in place for the wire, be worse than onto a ring?

    In fact could one not argue that it is safer, as the appliance is protected by an additional 3amp fuse and a lower amp RCBO. On my ring it would only be protected by a 32amp RCBO.

    ----------------
    Sorry if I come across as argumentative. I learn best through challenging and asking questions.

    Jon
     
  7. jonwestuk

    jonwestuk

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    good point - unfortunately the power adaptor is a regular 3 pin. I guess I could put a lighting socket and then get an adaptor for the power supply. Or I could just label the socket as being low current only?
     
  8. Nozzle

    Nozzle

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    Those power adaptors with built-in plugs are the work of the devil. You could get a power adaptor where there is a HV side and LV side plug, more like a laptop kind. The spec of your power supply needs to be matched, in power, volts, polarity and plug size.
     
  9. jonwestuk

    jonwestuk

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    so it seems like the main issue that I have is the risk of someone putting a high load current into the hidden plug socket. As it is hidden in the floor/ceiling cavity and there are plenty of sockets in the rooms both above and below that risk is low. If I put in a lighting socket rather than a regular 3 pin and use an adaptor for my appliance and additionally label the socket as being for low current only that mitigates the issue.

    Electrically, so long as I use the correct socket type the rest is fine?
     
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  11. jonwestuk

    jonwestuk

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    Blimey, in which case I am sure to be condemned to eternal damnation! Every appliance seems to come with them these days and I am definitely not going to replace them all :)
     
  12. sparkwright

    sparkwright

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    One has to wonder if fitting a socket (possibly a single extension lead socket with a short 1.5 mm² 3 core flex) labelled speakers only, and fitted in an adaptable box with a lid on it would be REASONABLE.

    It would be difficult for anyone else to plug anything else in.

    I think common sense has to be applied sometimes, and this way a different 'transformer' with a lead on it can be put in place later, or not as the case could be.

    Getting a different transformer would of course be the best answer, with it's lead wired into the fused spur, or a 5 amp plug/5 amp socket, or even 5 amp plug/5 amp socket/fused spur if you wanted to.
     
  13. deadshort

    deadshort

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    I would label the socket :LOL:

    The wiring is protected at 6a what's the issue ?

    Regards,

    DS
     
  14. flameport

    flameport

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    Fit the socket and plug the amp in.
    Label it if you want.
    The 3A FCU could be fitted but is not required.

    If someone did plug some high powered thing in there, a few lights would go off. Hardly the end of the world.

    Nothing will melt, set on fire or cause total destruction of the house.
    Not notifiable either.
     
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  15. JohnW2

    JohnW2

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    I agree totally. IMO, there are times when common sense should be allowed to prevail.

    Kind Regards, John
     
  16. JohnW2

    JohnW2

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    deleted - quoted instead of edited, yet again!
     
  17. JohnW2

    JohnW2

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    As you will have seen, I've just suggested that you simply exercise common sense and do as you have suggested. However, since you have expressed interest in understanding ... the primary purpose of any fuse (or MCB or RCBO) is to protect the cable downstream of that device from excessive current, whether that is due to excess loads being applied to the circuit or to faults in the loads or wiring. Your 'upstream' 1.5mm² cable is already more than adequately protected by the 6A RCBO. That 6A RCBO would also give adequate protection to any cable that you could seriously contemplate for wiring the new socket, so the 3A fuse would really serve no purpose. Furthermore, in the case of an excessive current flow (from any cause), there is absolutely no guarantee that a 3A fuse would blow before a 6A RCBO.

    There really are no electrical safety issues (in what you propose) that I can think of - any risks being the consequence of 'being plunged into darkness'. The 'risk' of someone in the future plugging a large load into your 13A socket on a 6A circuit is not really any different from someone plugging in, say, 4 x 13A loads into a 32A sockets circuit.

    Kind Regards, John
     
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