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SPDs in domestic DBs - no OCPD required.

alanblaby

Following a few debates here, and in other places about whether over-current protection is required for SPDs, BEAMA issued a statement in January saying, extra OCPDs are not always required for SPDs. Full details in the link below. 

As an aside, I do check the BEAMA site semi-regularly, but didnt spot this one until I had an email from Dehn yesterday.

Beama SPD guidance

  • in reply to AJJewsbury
    AJJewsbury said:
    Isolation - the main switch is still there, before the SPD - so little advantage there

    What about BS 7671 Regulation 314.1 inconvenience?

  • So what is/was the intention?

    For SPD to contain their own suitable over-current protection (which deliberately omits the magnetic operation that MCBs have which can trip very quickly on very high currents - which is just what you don't want for SPDs) - if that can be accomplished with a blob of solder holding together a couple of spring contacts, so be it.


    What about BS 7671 Regulation 314.1 inconvenience?

    Well most of the time you won't need to isolate - the SPDs have a plug & socket arrangement. For other occasions it's a matter of balancing benefit against cost - we don't after all supply every socket and light fitting from its own dedicated overcurrent device. If you were fussed, a DP switch disconnector rather than a MCB might be a better choice.

       - Andy.

  • in reply to Sergio Fernandez

    my personal engineering judgement goes along the lines of - if there is a switch or MCB in series with the surge arrester, when is it turned off ? When the SPD has failed short circuit, and its internal fuse wire or similar trip has not operated, or by mistake when the householder turns off the wrong thing, and does not notice.

    I can see an argument for no dedicated breaker, rather like the argument for not putting fire/ smoke alarms on an MCB that may be turned off and stay off, but better with say the lights, as those might actually be noticed if not working.

    The blowing of the company fuse will in all but the silliest of situations be a red herring - the SPD element will fail short, but something in series with it will then open promptly, and so long as the designers have made the enclosure to contain the bits that's all fine. The company fuse will also go for shorts to the live bus bar or stuck MCBs, but that is equally very rare and we dont worry about it.
    I'd expect the din mounted SPD holder to have a similar life span to a cheap MCB or RCD, .and changing it to be a similar board-open operation, while the SPD itself is intended to be unplugged as an unskilled like for like replacement. Hardest  part seems to be getting a replacement module without the din rail mount.

    Seems reasonable enough to me.

    Mike

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