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SPD - cable selection

J P

Hi all, just out of curiosity, trying to establish if I’m completly

overthinking this. There is an existing type 1/2 SPD installed on a main mccb panel. It’s fed from a 63A mccb and is approx 30cm away and goes into a separate enclosure. 

the cable size is 10mm with a 16mm earth. Iv been wondering regarding the 63A mccb, but am I right in thinking it’s only there to provide short circuit protection rather than overload? 

  • 0

    In the new Hager single-phase consumer unit there is not any circuit protection for the SPD  and there are not any cables connecting the SPD.

    Line and neutral to the SPD are connected into the main switch using busbars and the SPD is earthed to the consumer unit metalwork though a metal clip pop riveted to the Din rail.

  • 0 in reply to Sparkingchip

    So, if the MCB is there to protect the cables, in their absence, no fault protection is required.

  • 0 in reply to Chris Pearson

    Hager did not connect the SPDs in single-phase consumer units when they did use cables, the connection cables were terminated into the main switch.

  • 0 in reply to AJJewsbury
    AJJewsbury said:
    or 3). when a fault (short) occurs on the wiring between the MCB and SPD...

    But the MCB won't help if there's a dead short in the wiring between that main switch and the MCB.

    If you don't have an MCB, there won't be a short between the MCB and the SPD.  Problem solved.

  • 0 in reply to Simon Barker

    Well yes, in any CU the line busbar could come loose and drop on to the enclosure.

    Where the SPD is in its own little home next to the CU and the cables pass through a grommet (or grommets) I suppose that the chance of a fault is higher, but still remarkably remote. If there is concern, why not use double-insulated cables like meter tails?

    All this seems like shaving pennies off the cost, but that is the name of the game!

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