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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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  • 0

    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



    Interesting, very interesting. So the graphs aren't actually defined in BS EN 60898 then? (and furthermore 10x Idn doesn't guarantee 5s disconnection time for devices over 32A or under 10A). I wonder where the charts in BS 7671 came from then?


       - Andy.
Reply
  • 0

    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



    Interesting, very interesting. So the graphs aren't actually defined in BS EN 60898 then? (and furthermore 10x Idn doesn't guarantee 5s disconnection time for devices over 32A or under 10A). I wonder where the charts in BS 7671 came from then?


       - Andy.
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