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Selection of MCB for SPD protection

Julien Leblanc
Hello,

I need to install a type 1+2 SPD into the main panel of a small business building, which needs lightning protection.

The main breaker in the main panel is a 4 poles 40A MCB.

I was looking for a SPD and I found this: Schneider A9L16634.

In the technical specification sheet it says "Associated circuit breaker NG125N 80A curve C - Icu 15 kA".

I was wondering why the associated breaker needs to have such a high current (80A)? It seems i cannot ensure the selectivity of the breakers.

Could anyone explain what can happen if i use a smaller breaker, for example a 32A one for the protection of the SPD?


Thank you.




  • 0
    I think you could use a 16 or 32 amp breaker with out any problem as an SPD doesn't consume that much energy except when it's clamping a surge I think I'm right about that but let's wait for our clever members to answer
  • Former Community Member
    0 Former Community Member

    Julien Leblanc:

    Hello,

    I need to install a type 1+2 SPD into the main panel of a small business building, which needs lightning protection.

    The main breaker in the main panel is a 4 poles 40A MCB.

    I was looking for a SPD and I found this: Schneider A9L16634.

    In the technical specification sheet it says "Associated circuit breaker NG125N 80A curve C - Icu 15 kA".

    I was wondering why the associated breaker needs to have such a high current (80A)? It seems i cannot ensure the selectivity of the breakers.

    Could anyone explain what can happen if i use a smaller breaker, for example a 32A one for the protection of the SPD?


    Thank you.




     




    Hi julien, have you considered the A9L15693?

  • 0

    Hi julien, have you considered the A9L15693?



    That model is type 2, whereas the OP requires a type 1+2.


    The thing about the type 1 is that that its purpose is to (very temporarily) equipotentially bond the conductors of the electrical system to the LPS. So when there's a direct (or near direct) lightning strike, there won't be a huge PD between say a water pipe and a nearby electrical cable, which could result in a flashover and maybe a fire starting. Any protection it might provide to downstream equipment is incidental: that's the type 2's job. In theory the lightning strike should be of such short duration that neither the SPD's CPD nor the main CPD should trip. I would speculate however, that having a CPD at a lower rating than the 80A suggested by Schneider might cause premature disconnection of the SPD. As for discrimination, that doesn't really matter: it only really becomes an issue when the SPD has failed in a way that provides a short circuit, presumably after a particularly massive strike, and you probably don't care too much whether it takes out the main power at the same time, as long as the primary job of avoiding a fire has been satisfied.

  • 0

    I was wondering why the associated breaker needs to have such a high current (80A)?



    I'd agree with ww - a lower rated MCB risks opening just when you want the SPD to remain in circuit to do its stuff. Note that it's no so much it's long duration current carrying capacity (80A) but it's reaction to very short duration but very high current pluses that matters (so it's not so much its 80A rating as 80A type C).


    The main breaker in the main panel is a 4 poles 40A MCB.

    :

    It seems i cannot ensure the selectivity of the breakers.



    Could thing be arranged so that the SPD and its OPD are connected upstream of the existing 40A MCB? (even if that means fitting an extra isolator upstream of the lot).


      - Andy.
  • 0
    On a similar note, I have a 'Fusebox' branded 10 way CU to fit. The unit comes complete with pre-assembled SPD protection built in.

    The interesting thing is that there is no protective device - the SPD tails are wired directly into the bottom of the main switch.
  • 0

    whjohnson:

    The interesting thing is that there is no protective device - the SPD tails are wired directly into the bottom of the main switch.




    Sounds like a similar arrangement to the Wylex ones. These have a built-in (presumably) spring-loaded fuse which when blown causes the window to turn red: because of the fuse, it doesn't require its own breaker as long as the upstream CPD is <125A

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