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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.




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  • 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.
Reply
  • 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.
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