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In the UK is your Domestic dwelling correctly protected by the SPD fitted in the CU/DB

Sergio Fernandez

BS7671 regulation 534.4.5, mandates that SPDs be protected against overcurrent via an MCB or Fuse
I think there are some other regulations which also state MCB/fuse protection is a requirement like


BS7671 reg 534.4.4.1.5
BS7671 reg 533.2.1
4BS7671 reg 34.5.1
BS7671 reg 712.534.101

This in my opinion goes against claims like the following and probably needs to be made clearer to the manufacturers, I would also suggest that BS7671 and GN (Guidance Notes) make this more clear for the Designer and Installer.
proteusswitchgear.com/.../

There are other manufactures also stating "No requirement for MCB" Proteus just happened to be the first my internet search engine came up with.


As always please be polite and respectful in this purely academic debate.

Come on everybody let’s help inspire the future.

  • in reply to AJJewsbury
    AJJewsbury said:
    what are the chances of the MCB tripping and leaving the installation unprotected from subsequent surges, unbeknown to the householder?

    That's why the SPD is supposed to have a window, or other means of indicating "end of life".

  • I would like to know the real risk of not having an SPD. I was persuaded by this forum (and the 18th Ed) to fit one in my new installation and it did not break the bank, but we managed for almost three decades without one. So if the event is only very remotely likely, it really does not matter whether you have an OCPD or not.

    FWIW, mine has a C63 MCB and a red/green indicator lamp.

  • in reply to Chris Pearson

    I don't know how you would qualtify the risk for any given house.  But the SPD is there to reduce the risk of a power surge (e.g. a lighning strike on a nearby power line) from frying your most expensive gadgets.

  • in reply to gkenyon

    On the simple wylex/crabtree types, that indicator window is usually held up against a spring, by the bit of fusewire (*)that evaporates when the SPD goes short circuit, then the spring pushes the coloured flag. Expensive ones also operate a microswitch to alert the building controllers to send a man out with a new cartridge.
    Mike

    * those do have an internal fuse of sorts. Not sure its breaking ability.

  • in reply to Simon Barker
    Simon Barker said:
    I don't know how you would qualtify the risk for any given house.

    We did in a rather approximate way in the 17th Ed.

  • in reply to gkenyon

    That's why the SPD is supposed to have a window, or other means of indicating "end of life".

    Ah, no, I've not explained myself well enough. What I meant was the MCB potentially nuisance tripping even through the SPD remains serviceable - after all we're expecting the SPD to divert possibly several kA while the magnetic element of the MCB is designed to trip at say anything above 160A. Granted surges are generally very short lived (the test waveforms are in microseconds rather than milliseconds) but can we be sure that real world surges will be as neat or individual MCBs not overly sensitive? The main danger is thermal, so a protective device that lacks the instantaneous element (e.g. a fuse or "blob of solder") rather than conventional MCB feels like it might have an advantage to me.

       - Andy.

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