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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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    Well, if you need to always guarantee a 5 second trip time with that  s400 UC ABB breaker, then one should use the maker's curve, and deduce your own lower max Zs than the generic one BS 7671 recommends.


    Equally I note that the only current time points that are actually tested are on page 60, presumably all the trip times for intermediate currents are only an interpolation, so I am not sure how accurate the graph is as a representation of production spreads. May be worth giving ABB a ring if you need an accurate value for some reason.
    Abb UK tech support.

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  • 0
    Well, if you need to always guarantee a 5 second trip time with that  s400 UC ABB breaker, then one should use the maker's curve, and deduce your own lower max Zs than the generic one BS 7671 recommends.


    Equally I note that the only current time points that are actually tested are on page 60, presumably all the trip times for intermediate currents are only an interpolation, so I am not sure how accurate the graph is as a representation of production spreads. May be worth giving ABB a ring if you need an accurate value for some reason.
    Abb UK tech support.

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