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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
    OK - I think we are getting at cross purposes here, chaps


    The so called instantaneous tripping time of the MCB is 0.1 seconds (100ms)  - it may operate faster, but we don't know how fast


    The definite minimum time to operate is 0.01 seconds (10ms) - it cannot operate any faster


    For the region in between, the only accurate assessment is the energy let through determined by testing and published by the manufacturer (ie the I2t characteristic)


    For an adiabatic assessment, provided you have enough current to drive the operation of the MCB (and that must be true in this case as the concern is of in increased fault level being applied to the MCB) then if you assess the system based on definite minimum time to operate then you have the probable worst case energy let through in the absence of manufacturers data.


    the derived 90,000A2s from above is quite modest - any conductor above 2.0mm2 will handle that OK


    Regards


    OMS




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  • Former Community Member
    0 Former Community Member
    OK - I think we are getting at cross purposes here, chaps


    The so called instantaneous tripping time of the MCB is 0.1 seconds (100ms)  - it may operate faster, but we don't know how fast


    The definite minimum time to operate is 0.01 seconds (10ms) - it cannot operate any faster


    For the region in between, the only accurate assessment is the energy let through determined by testing and published by the manufacturer (ie the I2t characteristic)


    For an adiabatic assessment, provided you have enough current to drive the operation of the MCB (and that must be true in this case as the concern is of in increased fault level being applied to the MCB) then if you assess the system based on definite minimum time to operate then you have the probable worst case energy let through in the absence of manufacturers data.


    the derived 90,000A2s from above is quite modest - any conductor above 2.0mm2 will handle that OK


    Regards


    OMS




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