Protection When Changing CSA

You really do need to know the detail of the incoming protection to allow you to make an informed decision, especially if slow devices are used.

Sometimes a set of 3  "death or glory" fuses at the input even at some rating way above normal operation (800 or 1000A perhaps!!) limits the let-through energy in a way that allows the downstream hardware to be safely  sized much more economically than the 'do not know' case, by setting an absolute upper limit on the i²t.


As an example of what I mean, consider this  fuse

and this explanation of how to use the let-through graphs.

Then stare at this curve where I have highighted the 800A curve,





If we use this fuse fed from a supply of  anything up to about 15kA RMS PSSC, the peak current on the load side could be as high as 2.8 times that, if the fault comes on at the wrong moment in the cycle. But on a higher pssc supply, he curve kinks as the fuse energy limits, so even by the edge of the paper and 100kA supplu side PSSC, the downstream is protected to a level more like twice the peak fault current of the 15kA case, not 8 times.

You really do need to know the detail of the incoming protection to allow you to make an informed decision, especially if slow devices are used.

Sometimes a set of 3  "death or glory" fuses at the input even at some rating way above normal operation (800 or 1000A perhaps!!) limits the let-through energy in a way that allows the downstream hardware to be safely  sized much more economically than the 'do not know' case, by setting an absolute upper limit on the i²t.


As an example of what I mean, consider this  fuse

and this explanation of how to use the let-through graphs.

Then stare at this curve where I have highighted the 800A curve,





If we use this fuse fed from a supply of  anything up to about 15kA RMS PSSC, the peak current on the load side could be as high as 2.8 times that, if the fault comes on at the wrong moment in the cycle. But on a higher pssc supply, he curve kinks as the fuse energy limits, so even by the edge of the paper and 100kA supplu side PSSC, the downstream is protected to a level more like twice the peak fault current of the 15kA case, not 8 times.