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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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    Just a thought - if manufacturer's data isn't available - BS EN 60898 has maximum permitted energy let-though figures for various types of MCB and short circuit levels - e.g. 35,000 A²s for a  ≤16A B-type at 6kA or 84,000 for a similar C type at 10kA - which might be somewhat lower than the results of your calculations.


    I'm not sure if BS 3871 had any similar figures, or whether, assuming that BS EN 60898 MCBs are fundamentally similar technology to BS 3871 ones (the later incarnations at least) and so should have similar characteristics, has any merit.


      - Andy.

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  • 0
    Just a thought - if manufacturer's data isn't available - BS EN 60898 has maximum permitted energy let-though figures for various types of MCB and short circuit levels - e.g. 35,000 A²s for a  ≤16A B-type at 6kA or 84,000 for a similar C type at 10kA - which might be somewhat lower than the results of your calculations.


    I'm not sure if BS 3871 had any similar figures, or whether, assuming that BS EN 60898 MCBs are fundamentally similar technology to BS 3871 ones (the later incarnations at least) and so should have similar characteristics, has any merit.


      - Andy.

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