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Max Zs for adjustable MCCB's

Gareth 1

When determining the max permitted Zs for an MCCB with variable settings.

Do you have to apply the Zs value for the maximum possible settings, regadless of actual settings.

OR

Is it permissible to apply the Zs value for the actual settings, despite the fact that these could be increased at a later date.

Many Thanks

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  • 0

    Is it permissible to apply the Zs value for the actual settings, despite the fact that these could be increased at a later date.

    That's always been a risk - rewireable fuses could have the wrong fuse wire fitted, many cartridge fuses of different ratings are the same physical size (e.g. a 20A BS 1361 is the same size as a 15A one, and of course 13A fuses may be replaced by 13A in FCUs and the like). Correct labels are considered sufficient.

    I agree BS 7671 doesn't spell that out for ADS, but for overcurrent protection it does make it clear that the "rated current or current setting of the protective device" (my emphasis) is what counts (433.1.1) so that might also be reassuring if you expect consistency.

        - Andy.

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  • 0

    Is it permissible to apply the Zs value for the actual settings, despite the fact that these could be increased at a later date.

    That's always been a risk - rewireable fuses could have the wrong fuse wire fitted, many cartridge fuses of different ratings are the same physical size (e.g. a 20A BS 1361 is the same size as a 15A one, and of course 13A fuses may be replaced by 13A in FCUs and the like). Correct labels are considered sufficient.

    I agree BS 7671 doesn't spell that out for ADS, but for overcurrent protection it does make it clear that the "rated current or current setting of the protective device" (my emphasis) is what counts (433.1.1) so that might also be reassuring if you expect consistency.

        - Andy.

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