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BS 3871 Miniature Circuit Breakers Let Through Energy

MG
Currently involved in a project where I need to do the energy let through calculations for existing circuit breakers to prove that the increased fault current even though within the breaking capacity of the MCB, damage will not occur to the final circuit cables.


In a nutshell, the following formula must be true I2t<k2S2 . In BS 60898 MCBs the I2t is provided by the manufacturers as these are energy limiting devices.


As these happen to be BS 3871 and in the absence of such data can I get your recommendations? Use a definite trip time of 10ms and plug that in the equation?


Thanks

Mike
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  • Former Community Member
    0 Former Community Member

    mapj1:


    IF you put in the maximum breaking time of 10ms you will come up with a very large minimum cable requirement,and may well (incorrectly) conclude the cable is not protected.

     




    Hi Mike

    I think that should be the minimum time ie the definite minimum time to operate (DMT) of the MCB is generally assumed to be 0.01 seconds provided the fault current is above the minimum value to give disconnection within 0.4 to 5 seconds - and also below the breaking capacity of the MCB (which may make a demand on further upstream protection providing a cut off characteristic)


    Regards


    OMS






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  • Former Community Member
    0 Former Community Member

    mapj1:


    IF you put in the maximum breaking time of 10ms you will come up with a very large minimum cable requirement,and may well (incorrectly) conclude the cable is not protected.

     




    Hi Mike

    I think that should be the minimum time ie the definite minimum time to operate (DMT) of the MCB is generally assumed to be 0.01 seconds provided the fault current is above the minimum value to give disconnection within 0.4 to 5 seconds - and also below the breaking capacity of the MCB (which may make a demand on further upstream protection providing a cut off characteristic)


    Regards


    OMS






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