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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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    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.

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  • 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.

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