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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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    Thanks for your responses guys, I tried to be fast in the first post and did not provide enough information. The fault current is in excess of X times of multiples quoted for the type of MCB. So as per the regulations a trip time less than 0.1s (instantaneous) should be proved with energy let through. I have contacted Dorman Smith which appear to be the manufacturers of these MCB with no positive response - picture attached. They did not have any data. 


    There are about 100 MCBs of the same make and manufacturer so it would be of benefit if there is a way around the lack of data. I have not looked into the fuse providing backup but I will, discrimmination is a must in this installation. Would not want to loose the whole board with a blown fuse.


    Indeed as Mike mentioned 10ms gives a very large number (for 3kA and 10ms -->> energy let through is 90000).


    My only hope is now if someone has any brochures stored somewhere with let through tables/graphs. Otherwise I will be replacing them


    Thanks,

    Mike
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  • 0
    Thanks for your responses guys, I tried to be fast in the first post and did not provide enough information. The fault current is in excess of X times of multiples quoted for the type of MCB. So as per the regulations a trip time less than 0.1s (instantaneous) should be proved with energy let through. I have contacted Dorman Smith which appear to be the manufacturers of these MCB with no positive response - picture attached. They did not have any data. 


    There are about 100 MCBs of the same make and manufacturer so it would be of benefit if there is a way around the lack of data. I have not looked into the fuse providing backup but I will, discrimmination is a must in this installation. Would not want to loose the whole board with a blown fuse.


    Indeed as Mike mentioned 10ms gives a very large number (for 3kA and 10ms -->> energy let through is 90000).


    My only hope is now if someone has any brochures stored somewhere with let through tables/graphs. Otherwise I will be replacing them


    Thanks,

    Mike
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