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shed supply

Eclipse
Guys,

Just some advice please! 

my son has installed a new 6m/m 3C SWA supply to his new wooden shed,( no extranous parts) its going to be terminated into a 4 way consumer unit (RCD main swt) and used to power & lighting in the shed and some lighting in the garden,

question is:

The source end in the house is a twin RCD board, so using a spare way on either side will give the supply cable RCD protection, but what about the RCD in the shed board? is it advisable to keep both? 



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    Alan Capon:




    Eclipse:

    . . . the incomng supply is tnc-s, so initially i was thinking of spliting the incoming tails through blocks and installing a single enclosed MCB which the SWA would terminate to, this then leaving the RCD at the shed end, correct or not?




    Correct. You don’t really need to protect the SWA with an RCD as long as you can operate the protective device from the far end in the event of a short to the armour. As John said, you need to insulate the armour and take just the phase and neutral into the consumer unit in the shed. The earth would then come from a local earth rod, ideally less than 200 ohms. 


    Regards,


    Alan. 


     




    Although, the consumer's earth electrode to form a TT system must also be separated (below ground) from buried metalwork connected to the PME earthing system:


    (a) To ensure it really isn't connected to the PME earthing system

    (b) To avoid HV fault transfer voltages if the PME earthing system is connected to a Global Earthing System

    (c) To help reduce the risk of touch/step potentials between the two earthing systems.


    Some guidance recommends this separation is as much as 10 m; this is based on Figure 16 in BS 7430; some DNO's may require a lesser separation distance, but it would be of the order of metres, often  at least 2 m.


    And then there's BS 7671 - there must be no possibility of exposed-conductive-parts (this time above ground) of anything connected to the TT earthing system being simultaneously-accessible with (within 2.5 m of) any exposed-conductive-parts or exposed-conductive-parts connected to the PME earthing system.


    Hence, in a small curtilage property, especially where there are no extraneous-conductive-parts associated with the outbuilding (which would require minimum 10 sq mm main bonding to the MET in the house), it could well be safer to do as John originally said, and simply connect the shed to the property's PME earth.

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  • 0

    Alan Capon:




    Eclipse:

    . . . the incomng supply is tnc-s, so initially i was thinking of spliting the incoming tails through blocks and installing a single enclosed MCB which the SWA would terminate to, this then leaving the RCD at the shed end, correct or not?




    Correct. You don’t really need to protect the SWA with an RCD as long as you can operate the protective device from the far end in the event of a short to the armour. As John said, you need to insulate the armour and take just the phase and neutral into the consumer unit in the shed. The earth would then come from a local earth rod, ideally less than 200 ohms. 


    Regards,


    Alan. 


     




    Although, the consumer's earth electrode to form a TT system must also be separated (below ground) from buried metalwork connected to the PME earthing system:


    (a) To ensure it really isn't connected to the PME earthing system

    (b) To avoid HV fault transfer voltages if the PME earthing system is connected to a Global Earthing System

    (c) To help reduce the risk of touch/step potentials between the two earthing systems.


    Some guidance recommends this separation is as much as 10 m; this is based on Figure 16 in BS 7430; some DNO's may require a lesser separation distance, but it would be of the order of metres, often  at least 2 m.


    And then there's BS 7671 - there must be no possibility of exposed-conductive-parts (this time above ground) of anything connected to the TT earthing system being simultaneously-accessible with (within 2.5 m of) any exposed-conductive-parts or exposed-conductive-parts connected to the PME earthing system.


    Hence, in a small curtilage property, especially where there are no extraneous-conductive-parts associated with the outbuilding (which would require minimum 10 sq mm main bonding to the MET in the house), it could well be safer to do as John originally said, and simply connect the shed to the property's PME earth.

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