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Max Zs for MCB and disconnection times

Apostolos

Good afternoon

A quick and maybe somewhat silly question regarding the table 41.3 of BS7671 where it gives the max zs permitted for MCBs

We can see that the fuses have separate tables for 0.4s and 5s disconnection time max Zs

However MCBs have one table only that gives the maz Zs for both 0.4s and 5s

Why dont MCBs have one table with max zs for 0.4s disconnection and one table with max Zs for 5s disconnection?

So for exampe a C32 has max zs 0.68Ω

How can this be both for 0.4s and for 5s disconnection time?

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    Apostolos said:
    How can this be both for 0.4s and for 5s disconnection time?

    Because the mcb contains both a thermal and a magnetic element. Basically, the device is designed so that the magnetic part ensures 0.1 s disconnection time occurs at either before, or roughly the same current as, the time in which (worst case) the thermal characteristic of the device would operate.

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  • 0
    Apostolos said:
    How can this be both for 0.4s and for 5s disconnection time?

    Because the mcb contains both a thermal and a magnetic element. Basically, the device is designed so that the magnetic part ensures 0.1 s disconnection time occurs at either before, or roughly the same current as, the time in which (worst case) the thermal characteristic of the device would operate.

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