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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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  • Former Community Member
    0 Former Community Member
    @ AJJewsbury


    Yes chart 3A6 comes in at 10In for a 5 second trip time for a type D MCB.


    However it still does not marry up with what ABB state is actually required to trip their type D MCBs within 5 seconds.


    I found a more detailed time current graph below from ABB specifically for their type D MCBs to 60891-1.


    As you can see the tripping current is around 13In, not 10In as stated in table 41.3 in BS 7671 17th Edition A3.


    Going off the ABB graph below it would seem that table 41.3 in BS 7671 17th Edition A3 has values stated in it that potentially would fail to activate the breaker trip mechanism within the required 5 seconds.

    810741d86b550bfb2bfce068bf70a31b-huge-abb-60898-1-mcb-type-d.png

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  • Former Community Member
    0 Former Community Member
    @ AJJewsbury


    Yes chart 3A6 comes in at 10In for a 5 second trip time for a type D MCB.


    However it still does not marry up with what ABB state is actually required to trip their type D MCBs within 5 seconds.


    I found a more detailed time current graph below from ABB specifically for their type D MCBs to 60891-1.


    As you can see the tripping current is around 13In, not 10In as stated in table 41.3 in BS 7671 17th Edition A3.


    Going off the ABB graph below it would seem that table 41.3 in BS 7671 17th Edition A3 has values stated in it that potentially would fail to activate the breaker trip mechanism within the required 5 seconds.

    810741d86b550bfb2bfce068bf70a31b-huge-abb-60898-1-mcb-type-d.png

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