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Using BS3871 MCB to protect ne submain

Alan B

Is it ok to connect a new submain to an existing spare 3 phase BS3871 MCB in a light industrial installation. The alternative is to change the entire main DB.

Submain will be clipped direct to wall and steel roof beam. It will feed a new 3 phase DB, probably equipped with two 3 phase 16A MCB's.

My understanding is that BS 60898 replaced BS3871 and the standards are similar, I am sure there are some differences, but do they make a significant impact on safety? In my mind providing they disconnect in the required time everything should be acceptable? Main risks will be after the sub board using modern devices.

Thanks

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    Regulation 531.1.1 might be a sticking point - if the BS 3871 breaker is to provide ADS for the new circuit, it needs to be suitable for isolation.

    BPG2 Issue 3 (page 10) says that not all BS 3871 circuit-breakers are suitable for isolation.

  • 0 in reply to gkenyon
    gkenyon said:
    BPG2 Issue 3 (page 10) says that not all BS 3871 circuit-breakers are suitable for isolation.

    The old push button plug-in Wylex breakers made to retrofit in their rewirable fuse boards are BS387, so unless unplugging them counts as isolation they would not be suitable for isolation, as the buttons cannot be secured.

  • 0 in reply to Sparkingchip
    Sparkingchip said:
    unless unplugging them counts as isolation they would not be suitable for isolation,

    That would be the same approach used for a fuse (of any type) as a ruptured fuse doesn't necessarily provide "isolation" but a removed or withdrawn one (to the standards recognised in BS 7671) does.

    Similarly, products to BS 7288 (withdraw fuse or plug to isolate).

  • 0 in reply to gkenyon

    I would be very wary of using a fuse holder as a means of isolation, yes you may have pulled the fuse and have it in your pocket, but someone could simply move another fuse within the board and reenergise the circuit, as they can move plug-in BS 3871 MCBs in old Wylex fuse boards.

  • 0 in reply to Sparkingchip
    Sparkingchip said:
    I would be very wary of using a fuse holder as a means of isolation,

    Why, there are different approaches to "locking off" that prevent the fuses being re-inserted?

    It just requires the correct lock-off kit used in the right way.

    Yes I agree that just "pulling the fuse" is only part of the job ... but there is stuff out there if you want to prevent re-insertion.

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  • 0 in reply to Sparkingchip
    Sparkingchip said:
    I would be very wary of using a fuse holder as a means of isolation,

    Why, there are different approaches to "locking off" that prevent the fuses being re-inserted?

    It just requires the correct lock-off kit used in the right way.

    Yes I agree that just "pulling the fuse" is only part of the job ... but there is stuff out there if you want to prevent re-insertion.

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