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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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    That looks rather like a mis print, and is a dead copy of the same advice for C types.

    However, note that it will be less than 20 times because at 5 seconds we might be tripping on the thermal part for D type, though not always for C type, where even at 10 times, the magnets kick in.


    In the end the maker's data trumps the generic appendix to the regs in this case.
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  • 0
    That looks rather like a mis print, and is a dead copy of the same advice for C types.

    However, note that it will be less than 20 times because at 5 seconds we might be tripping on the thermal part for D type, though not always for C type, where even at 10 times, the magnets kick in.


    In the end the maker's data trumps the generic appendix to the regs in this case.
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