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Type D MCB 5 Second Tripping Current

Former Community Member
Former Community Member
In BS 7671 17th Edition A3 table 41.3 for type D MCBs gives a 5 second tripping current as 230 x 0.95 / (10In).


When cross referencing time current graphs for type D MCBs I do not get a value of 10In for a 5 second tripping time.


For example see ABB time current graph below giving a current of around 16In for a 5 second trip time.


What is the reason for the difference in values when comparing type D MCB time current charts and table 41.3 in BS 7671 17th Edition A3 for 5 second trip times?

3e6afca19b11a5e876e17f88e8258582-huge-type-d.png
f5deabd80c6b55bd27c69795fb1b7c94-huge-20190722_083103.jpg

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  • 0
    Which is rather surprising.

    It is perhaps telling that the Schneider Data  indicates that their breakers would be well within the spec.

    Given the figures in the BS are lifted from the BS EN 60898 for the breaker, they should match or be better, and it would be interesting to see if ABB claim full complaince to the breaker standard, or if it is a printing error on their chart - do you have the breaker no to go with that chart please. Equally in practice there are very few situations where breaking in 6 seconds instead of 5 will matter - it cannot be safety of life against shock or it would be 0.4 or 0.2 seconds we would be looking, so it is lower risk.
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  • 0
    Which is rather surprising.

    It is perhaps telling that the Schneider Data  indicates that their breakers would be well within the spec.

    Given the figures in the BS are lifted from the BS EN 60898 for the breaker, they should match or be better, and it would be interesting to see if ABB claim full complaince to the breaker standard, or if it is a printing error on their chart - do you have the breaker no to go with that chart please. Equally in practice there are very few situations where breaking in 6 seconds instead of 5 will matter - it cannot be safety of life against shock or it would be 0.4 or 0.2 seconds we would be looking, so it is lower risk.
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