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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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    curse the short duration edit window. or I would go back and reword to be less ambiguous. So its a second post.

    To be clear, the 10ms is the limit of the   curves  no longer in the current BS 7671 show that as the bottom of the graph, these  curves are the maximum time it might take, at a particular fault current.

    The problem of the way the graph is drawn  does not help us to work out how it may behave when when driven harder - when  not only are we in the fast magnetic region, but we are saturating the magnet core so the force stops increasing much and the contacts are moving as fast as they can, all we can say is that we would be below the 10ms line, but we cannot say by how much. Also a time to reach an open circuit is not the full story, as once an arc is established there is an effective series resistance (a complex and current dependent one, that is almost a constant voltage but not quite) that starts to affect the let through.
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    curse the short duration edit window. or I would go back and reword to be less ambiguous. So its a second post.

    To be clear, the 10ms is the limit of the   curves  no longer in the current BS 7671 show that as the bottom of the graph, these  curves are the maximum time it might take, at a particular fault current.

    The problem of the way the graph is drawn  does not help us to work out how it may behave when when driven harder - when  not only are we in the fast magnetic region, but we are saturating the magnet core so the force stops increasing much and the contacts are moving as fast as they can, all we can say is that we would be below the 10ms line, but we cannot say by how much. Also a time to reach an open circuit is not the full story, as once an arc is established there is an effective series resistance (a complex and current dependent one, that is almost a constant voltage but not quite) that starts to affect the let through.
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