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principles of selectivity between upstream fuses protecting a parallel conductor distrib circuit and the downstream fuses

psychicwarrior

Goodly morrow

Academic scenario:   a final circuit is protected by a 32A fuse/mcb and it is supplied by a distrib circuit that uses two parallel line conductors each protected by two 32A fuses

For selectivity,  if there is an overcurrent (overload or fault) on the final circuit near to the fuse,  can one 'simply' consider the distrib circuit fuses as one fuse (each summed as such)  ?

As there might not be 'selectivity' graphs for a 'summed' fuse/mcb arrangement,  what would one do ...  best ask the manufacturer ?

Thanks for any help in understanding, for my lacking brain.

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  • 0

    I have to admit to not having read App. 10 previously. Why does the same principle not apply to ring final circuits?

  • 0 in reply to Chris Pearson

    Why does the same principle not apply to ring final circuits?

    I suppose the same principles are applied for fault protection - as the cables are about 2/3rds the rating of the protective device, the protective single device is adequate for fault protection. Overload protection is a slight modification of the 'equally divided' rule, to not worse than two-thirds, which the designer is meant to ensure by layout etc. - so the underlying theory is fairly similar.  We have had discussions in the past about using separate overload protection in each leg, for situations where it's difficult for the designer to be reasonably confident the loads can be spread evenly.

        - Andy.

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  • 0 in reply to Chris Pearson

    Why does the same principle not apply to ring final circuits?

    I suppose the same principles are applied for fault protection - as the cables are about 2/3rds the rating of the protective device, the protective single device is adequate for fault protection. Overload protection is a slight modification of the 'equally divided' rule, to not worse than two-thirds, which the designer is meant to ensure by layout etc. - so the underlying theory is fairly similar.  We have had discussions in the past about using separate overload protection in each leg, for situations where it's difficult for the designer to be reasonably confident the loads can be spread evenly.

        - Andy.

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