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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
    OK - probably worth a bit of clarification

    Instantaneous tripping current - For Type D  - Above 10 In up to and including 20 In


    For test requirements (with the MCB cold, and again for Type D), for test currents of 10 In then:
    0,1 s < t < 4 s

    (for In ≤ 32 A)
    0,1 s < t < 8 s

    (for In > 32 A)

    Where the MCB is rated less than 10 A then  t < 8 s is permissible


    For test currents of 20 In then t < 0,1 s


    The standard requires the tripping characteristic of circuit-breakers to be contained within the zone defined by the parameters above


    Regards


    OMS





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  • Former Community Member
    0 Former Community Member
    OK - probably worth a bit of clarification

    Instantaneous tripping current - For Type D  - Above 10 In up to and including 20 In


    For test requirements (with the MCB cold, and again for Type D), for test currents of 10 In then:
    0,1 s < t < 4 s

    (for In ≤ 32 A)
    0,1 s < t < 8 s

    (for In > 32 A)

    Where the MCB is rated less than 10 A then  t < 8 s is permissible


    For test currents of 20 In then t < 0,1 s


    The standard requires the tripping characteristic of circuit-breakers to be contained within the zone defined by the parameters above


    Regards


    OMS





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