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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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    Mike,

    Very well put, but to further complicate the issue the let through energy for a circuit breaker can potentially be greater with a smaller fault current. Normally the maximum let through energy can be calculated from I²t where I is the maximum fault current and t is the opening time of the contacts (in this case I assume the 10ms of the OP). However if the maximum fault current is not much greater than the instantaneous (i.e. short circuit) trip time of the CB then a slightly lower current which remains below the instantaneous trip time, keeping within the overload part of the characteristics curve with a much longer trip time, can end up with a higher I²t since the trip time may be in hundreds of ms, if not seconds. 

    I suspect that this latter condition will not be a problem in this case as this is for a specific fault current below the maximum that is already within the scope of the existing installation, so it will be sufficient to do a calculation for the increased let through energy with the new short circuit current.

    Alasdair
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  • 0
    Mike,

    Very well put, but to further complicate the issue the let through energy for a circuit breaker can potentially be greater with a smaller fault current. Normally the maximum let through energy can be calculated from I²t where I is the maximum fault current and t is the opening time of the contacts (in this case I assume the 10ms of the OP). However if the maximum fault current is not much greater than the instantaneous (i.e. short circuit) trip time of the CB then a slightly lower current which remains below the instantaneous trip time, keeping within the overload part of the characteristics curve with a much longer trip time, can end up with a higher I²t since the trip time may be in hundreds of ms, if not seconds. 

    I suspect that this latter condition will not be a problem in this case as this is for a specific fault current below the maximum that is already within the scope of the existing installation, so it will be sufficient to do a calculation for the increased let through energy with the new short circuit current.

    Alasdair
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