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Sub main and TT

Grumpy
I am dismayed that I have to ask but it is so long since I worked in a TT area that I am doubting myself.

I'm running an SWA submain to an outbuilding so instead of the usual switch fuse should I be using a 100mA S RCCB and a 63A mcb?

The armoured passing directly through the outside doesn't necessitate a 30mA RCD does it (as I have been so advised)

Apologies and thanks.

Parents
  • 0
    Is the supply at the origin TT or are you making an island of TT spurred from an TN-s or c-s network ?


    If the latter the armour may be protected by the suppliers earth, and then if you like it that way the RCD can be at the load end, which may be preferred...


    If it is all TT, then yes an upfront RCD is needed to protect the cable regardless.


    100mA delay is a relatively expensive but a good idea, certainly if you need it to discriminate against 30mA instant trip devices further down the line.  An upfront 30mA instant is possible, and cheaper of course but a pain in some situations, as of course it all goes off at once in a fault. (thought some  emergancy light is a possible mitigation)


    Mike
Reply
  • 0
    Is the supply at the origin TT or are you making an island of TT spurred from an TN-s or c-s network ?


    If the latter the armour may be protected by the suppliers earth, and then if you like it that way the RCD can be at the load end, which may be preferred...


    If it is all TT, then yes an upfront RCD is needed to protect the cable regardless.


    100mA delay is a relatively expensive but a good idea, certainly if you need it to discriminate against 30mA instant trip devices further down the line.  An upfront 30mA instant is possible, and cheaper of course but a pain in some situations, as of course it all goes off at once in a fault. (thought some  emergancy light is a possible mitigation)


    Mike
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