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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
    Table 41.3 is giving you the case for both operation between 0.4 seconds and 5 seconds - if you check the edge of the vertical section for Type D you should find 10 x In for 0.4 seconds - this section is vertical so for no change in current, the CB may be operating both magnetically and thermally


    Basically, once you reach 10 x In the CB is going to operate in definite minimum time (around 0.01 seconds) - however, it is only required to operate (for BS 7671 compliance at 0.4 seconds or 5 seconds)


    Regards


    OMS
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  • Former Community Member
    0 Former Community Member
    Table 41.3 is giving you the case for both operation between 0.4 seconds and 5 seconds - if you check the edge of the vertical section for Type D you should find 10 x In for 0.4 seconds - this section is vertical so for no change in current, the CB may be operating both magnetically and thermally


    Basically, once you reach 10 x In the CB is going to operate in definite minimum time (around 0.01 seconds) - however, it is only required to operate (for BS 7671 compliance at 0.4 seconds or 5 seconds)


    Regards


    OMS
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