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Operating a shed light with a second FCU

Discussion in 'Electrics UK' started by TheresAlwaysaWay, 16 Sep 2020.

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  1. TheresAlwaysaWay

    TheresAlwaysaWay

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    Greetings and apologies for my basic diagram, but hopefully it serves its explanatory purposes.
    Can I operate the shed lighting using a 3A switched FCU in this way?
    Also do the live and neutral of the load terminals simply connect to the two terminals of the light fitting, and can I use 1.5 sq mm either side of the FCU? Thanks
     

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  3. SUNRAY

    SUNRAY

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    That is a fairly commom arrangement and an excellent drawing.
    Some will grouse about using the FCU as the everyday light switch but I don't believe there is any issue with it and my kitchen under cupboard lights are done this way.

    RCD is required, is your ring circuit currently covered? if not the house FCU can be be a RCD type.

    The 6mm² SWA seems a bit big for a 13A circuit and his is notifiable work, 2.5mm² is usually good up to around 30m or possibly more so do speak with your electrician to do the cable calculations before starting any work.
     
  4. TheresAlwaysaWay

    TheresAlwaysaWay

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    Thank you for your advice, and for the praise of my diagram!
    The final ring circuit, of which the 13A FCU, starts and ends at a RCD on the consumer unit, which was only fitted about 4 years ago, so this should cover the angles?
    The SWA run is about 25 metres.
    I used an online calculator on the Doncaster Cables website and that indicated a CSA of 6 sq mm.
    If 2.5 sq mm is good for up to 30m then maybe I'm buying myself some wiggle room with 6 sq mm?
    Yes I was aware of it being notifiable work.
    It has been suggested that I could call in the company that originally fitted my consumer unit and get them to carry out a health check on my electrical circuits etc and slip this new spur in through the backdoor!
    My only other concern is the earthing situation.
    Does the run need anything extra over and above what is already provided by the house?
    There are earthing rods which look a bit OTT, and cable glands with a banjo, but the SWA starts from an IP rated junction box at the house which is IP rated but plastic.
    Maybe some better grounding is required at the shed due to its remoteness from the houise?
     
  5. AndyPRK

    AndyPRK

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    If you have PME earthing at the incommer then sometimes the shed needs it’s own rod

    6mm cable sounds like a good idea to me
     
  6. TheresAlwaysaWay

    TheresAlwaysaWay

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    Yes good point, but I think I've got some 1.5 sq mm cable knocking about so it saves buying some 1.0!
    Actually I was looking at a 16 watt LED light fitting and using the basic equation and mains voltage, it works out to about 0.7A - peanuts
     
  7. bernardgreen

    bernardgreen

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    Decimal point error. 0.7 Amp and 240 Volt is 168 Watts
     
  8. TheresAlwaysaWay

    TheresAlwaysaWay

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    ah yes so it's 16w / 240v = 0.067A ?
     
  9. bernardgreen

    bernardgreen

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    Yes, 0.067 Amp
     
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  11. TheresAlwaysaWay

    TheresAlwaysaWay

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    I have now completed this work and everything seems ok but I have a lingering doubt over the earthing arrangements in the shed.
    The 6 sq mm swa armour is earthed in a junction box on the outside wall of the house so with a gland kit and banjo it is earthed by connecting it to the earth of the fcu. The cable then travels about 25 metres to the shed and is connected to a metal clad double gang socket. What is the best thing to do with the swa armour inside the metal clad double gang socket? Cut it off and do nothing because it's already earthed back at the house? Connect it to the earth terminal of the socket? Connect it to the earth terminal of the metal back box, which seems pointless as this isn't an earth is it? Connect it to an earthing rod? Or something entirely different?
     
  12. EFLImpudence

    EFLImpudence

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    IF the socket is earthed by one of the SWA conductors then join the armour to it with another gland.

    If not, then:

    It must BE earthed by some method. What have you done?

    Can't tell. Are there any extraneous-conductive-parts in the shed - metal parts earthed by the ground?
     
  13. TheresAlwaysaWay

    TheresAlwaysaWay

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    The shed is just a standalone wooden shed with no metallics in the form of pipework etc.
    I don't have the room to use another gland but if the armor is earthed at the other end, back at the house, then doesn't this give it a safe route to earth? At the moment the armor is terminated to the earth terminal in the metal backbox until I know for sure the route to take and before it's permanently livened up.
     
  14. EFLImpudence

    EFLImpudence

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    Ok.

    For the armour, yes.

    Ok but you said:
    "Connect it to the earth terminal of the metal back box, which seems pointless as this isn't an earth is it?"
    The armour is the earth.

    Is the socket earthed by one of the SWA wires?
     
  15. TheresAlwaysaWay

    TheresAlwaysaWay

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    I've updated and attached my diagram after carrying out the work, and included my question.
    So the swa armour is earthed at the wiska box back at the external wall of the house but I'm unsure how to handle it's termination in the metal clad double gang socket in the shed, if at all?
     

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  16. EFLImpudence

    EFLImpudence

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    Well, as I said - another gland.

    Third attempt - Is the socket earthed by one of the SWA wires?
     
  17. TheresAlwaysaWay

    TheresAlwaysaWay

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    Sorry if I haven't answered your question before now. The socket is earthed via the earth wire of the 3 core swa, which traces back to the earth of the house.
    So if I don't have the room for another gland kit at the shed is there another way? If the armour is earthed at the wiska box why does it need further earthing at the shed?
     
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