Main switch short circuit capacity.

No, given DNO max PFC for such an installation. I do not expect the REC 4 to be switching at fault levels and will be feeding two 3 phase boards fitted with 10k protective devices. Tails less than 2m in length. Not impossible introduce a fault during maintenance, but the same could be expected of the main switch in each DB if only isolating from there.


Many thanks all.

Yes I see.

Thanks it's been a useful tutorial.

The line up the middle of the graph is the prospective short circuit current line


For the RMS values (which is principally what you are concerned with here) then start on the bottom axis at say 25kA, follow that up to the 100A fuse line - then turn left and continue until you hit the prospective line, then follow that down to the bottom axis and read off the cut of value (in kA RMS) - it should be around 5 or 6 kA


For the peak values repeat the exercise starting at the vertical axis  and head right to the fuse line


The much maligned fuse is actually an incredibly useful bit of kit when it comes to managing high fault levels, without having to engineer the Bismark


Regards


OMS

Yes, useful info. Thanks.

Actually having checked, OMS's answer is (of course, silly me, why do I check?) the one you want. I am indeed doing as I usually do, when I do anything I don't do very often without the text book in front of me, and dropping random factors of root two, pi, -1 etc.


The line that is not a fuse line, and slopes up steeper, is to allow us to convert  between pk-pk and  RMS values (in the ratio of 1RMS = 2.5 and a bit  pk-pk) , and this line too needs to be extended to allow you to convert what a REC 4 could stand back to RMS again. (This is useful because at short breaking times the waveform is not really a steady state sine wave, being much less than a cycle long, so a single sided pulse with some ringing but we need an RMS equivalent to estimate the heating. )

It does not alter the conclusions, luckily for us, that the the REC 4 is fine on any supply you can safely protect with a  100A BS1361: 1971 fuse, just the margin of safety is rather greater than I first said, by about 2.5 to 1.. (I did warn you it would rely a bit on other people checking my workings ? )

Actually this means you could probably use 6kA MCBs after the fuse and REC4 arrangement if you wanted to.

As an aside the NOZED and DIAZED bottle fuses are used on the continent as incomer fuses to have similar dramatic damage limiting effects see this data sheet  here on  page 4 and nearby  The 'death or glory' fuse may be old hat but it is jolly handy thing to remember. Also why bypassing them with bits of coathanger wire can be such a bad idea.

Hi Mike


It might help to think of the RMS value of current being the value of current that can be safely broken by the device (so for example a rated 50kA 3 second switchboard that can and will open safely the design RMS value)


Then think of the peak current as being the safe making current (if you were unlucky enough to close into a fault) - so a board that may be rated at 100kA peak


The relationship used for all practicable purposes is a factor of ranging from 1.5 through to 2.5 for design conditions. Alternatively you may use a value of 2 x square root of the RMS value squared (the so called doubling effect) if you are in the typical range of transformer fed LV switchboards at around 50kA prospective levels (eg a parallel pair of 1.5MVA Transformers feeding into A and B switchboards with bus sections closed)


Note the 2.5 factor is only used for very large fault levels - there is a range of multipliers depending on fault levels - these range from say 1.5 for faults less than 5kA to a factor of 2 for faults at say 50kA(as above), to a factor of 2.5 for faults exceeding 80kA - these are further defined in switchgear protection standards such as BS EN 60947-1 (the so called "n" factor)


Regards


OMS