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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? 

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  • 0

    am I right in thinking it’s only there to provide short circuit protection rather than overload? 

    The 2nd half of 534.4.10 would seem to agree!

       - Andy.

  • 0 in reply to AJJewsbury

    One hopes that there is almost no current in the MCB or the wiring to it, most of the time. The only times there is would be 

    1) by design for some microseconds during a transient over-voltage when a few kA might flow and the SPD dissipates at peak perhaps hundreds of kW but only for a few microseconds,  and  thinking adiabatically, a wiring copper area of a few mm2 would probably do.

    2) When the SPD finally dies, if it does so in a short circuit sort of way, when the large current will only flow at most for the time to operate the breaker, if the SPD does not fuse first, and to be honest a thinner cable would probably be OK then as well...
    Mike
    PS

    In terms of responding to fast edges for lower inductance,  and best clamping a pair of flat palm braids would be better than twins of  round wire ,but such things are not compatible with normal wiring terminals and methods.


  • 0 in reply to mapj1

    or 3). when a fault (short) occurs on the wiring between the MCB and SPD...

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  • 0 in reply to AJJewsbury

    "or 3)."

    Very true. A case where again, a thinner cable would probably suffice. There is no safe condition where 63 amps flows for very long at all, given the volume of the SPD itself and the absence of cooling air, water or rotating shafts to remove the energy.
    Mike

  • 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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