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Main switch short circuit capacity.

wessex
Some guidance please.

Three phase TN-S supply from a private 800 kVA transformer. Distribution via a 240mm2 4 core SWA approx 20m in length from a transformer mounted feed pillar and protected by 315A fuses. At the intake position 100A BS 1361 type II fuses are fitted. I was looking to install a REC 4 main switch down stream from which would be three phase distribution by 10kA rated MCB's. My question is about the suitability of the main switch and its short circuit capacity at it's point of installation with regard to prospective fault current at this point which will be greater than 16kA or do the BS 1361 afford adequate protection for the switch ? Your thoughts please.
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    switch is tested up to 16k




    Amps RMS I presume. In which case there is no issue at all on any circuit  protected by that 100A fuse.

    Go back to that PDF of fuse data I linked in above, and up the side find the 16kA disconnection level and look sideways until you hit the 100A curve. You will see that you run out of curve first  !! . But for fun extrapolate the lines o to the right a bit  Then look down for the pre-fuse prospective fault level that corresponds, and see that it would be off the end of the scale at over 50kA . So absolutely any supply for which a 100A fuse is not going to explode, is also good for a REC4.

    Which is good to know actually, thankyou for finding that out.


    Note that your 2m of 25mm2  tails contribute as much to the volt drop as your 20m of 240mm2, namely sod all in normal operation, but enough to tame that PSSC still further.

     


Reply
  • 0

    switch is tested up to 16k




    Amps RMS I presume. In which case there is no issue at all on any circuit  protected by that 100A fuse.

    Go back to that PDF of fuse data I linked in above, and up the side find the 16kA disconnection level and look sideways until you hit the 100A curve. You will see that you run out of curve first  !! . But for fun extrapolate the lines o to the right a bit  Then look down for the pre-fuse prospective fault level that corresponds, and see that it would be off the end of the scale at over 50kA . So absolutely any supply for which a 100A fuse is not going to explode, is also good for a REC4.

    Which is good to know actually, thankyou for finding that out.


    Note that your 2m of 25mm2  tails contribute as much to the volt drop as your 20m of 240mm2, namely sod all in normal operation, but enough to tame that PSSC still further.

     


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