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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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    Dunno OMS, my calculations show different. 

    Assuming short circuit of 3kA,, 70°C PVC phase cables (k=115), 2.5mm² CSA, trip time of 10ms this works out to: 90000 A²s (I²t) and 82656 A²s (k²s²) so the equation is false.


    Besides there are 1.5mm² lighting cables which definitely are not going to be amended as part of the project.
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  • 0
    Dunno OMS, my calculations show different. 

    Assuming short circuit of 3kA,, 70°C PVC phase cables (k=115), 2.5mm² CSA, trip time of 10ms this works out to: 90000 A²s (I²t) and 82656 A²s (k²s²) so the equation is false.


    Besides there are 1.5mm² lighting cables which definitely are not going to be amended as part of the project.
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