Rules and Guidelines
Want to participate in the discussions? Please read our Community Rules and Guidelines. Need some help? Visit Support and Answers.

  • State Not Answered
  • Locked Locked
  • Replies 13 replies
  • Subscribers 32 subscribers
  • Views 5041 views
  • Users 0 members are here
Options
You may also be interested in
This discussion is locked.
You cannot post a reply to this discussion. If you have a question start a new discussion

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
  • 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.

  • 0
    All,


    Thank you for your responses, I thought I attached the picture (I did, it did not appear on the post though so will try again now). Indeed usually the BS EN 60898 MCBs fitted in the UK are class 3 and 10kA Icu rated, I have attached the 60898 extract I hope it appears.


    The devices are the BS 3871 loadlimiter white case MCBs all 9kA. Dorman Smith have no data so I suppose the standard did not require the manufacturers at that point in time to test and produce such data.


    The source of supply is a 1.5 MVA transformer as the project proposes so there are some fault current levels to handle. This assessment method (energy let through) stated at the beginning of the forum was agreed as part of the engineering meetings.


    Also if you take a MEM 2, C32, 10kA Icu, circuit breaker experiencing all 10kA under fault, then as per the manufacturer's datasheet the let through value is limited to 43000A2s which is less than what the standard quotes. This emphasizes the need for manufacturers datasheets, the standard is very onerous for some reason.


    Thanks for your responses and your time. Looks like i will be replacing some MCBs

    7b0cfc9818b76a224ad282d9315e7440-huge-untitled.jpg
    5a7d9630acd6dd3a66392c9dd286a85c-huge-capture.jpg

  • 0
    I am surprised DS cannot rustle up a data sheet for the load limiter 9kA   those are the modern ones, just before the new standard came out.

Join us to get the best from IET EngX.

Joining EngX lets you personalise your experience so you stay up to date on the topics that interest you, plus you’ll be able to make connections who are looking to collaborate, exchange ideas and more.

Join us Not now

Community Rules & Guidelines