BS 1361 cartridge fuse curves for example (from TLC website)

Now, there curves are the ‘all fire’ lines, at this amp-time all fuses of that rating will always have blown clear, including slowest fuse off the production line, operating at coldest ambient . (at the fast end it becomes a near constant I2t, at the slow end a near infinite time, this curve is the interesting curved bit in between)

You need to consider pre-arc - the I2T below which we can be totally sure it will never open.

Between the two is ‘may operate’ and it is a wide band..

 datasheet shows similar graph to the above, but also tables a value of pre-arc for 60A to be 9,100 (so 300A for 0.1 seconds, or maybe a touch under100A for a second) and the let through to be 25,000 - compare that with the fast end of the 60A curve above, 0.1 second, and 900A - looks like I2t is more like 81,0000. This is why when you cascade fuses, to make sure the larger one does not blow at the same time as the small one it needs to be ~ 3 times the rating. In reality anything over the pre-arc level ‘might just’ fire the fuse if it is one of the friskier ones in the production spread.

Mike.

BS 1361 cartridge fuse curves for example (from TLC website)

Now, there curves are the ‘all fire’ lines, at this amp-time all fuses of that rating will always have blown clear, including slowest fuse off the production line, operating at coldest ambient . (at the fast end it becomes a near constant I2t, at the slow end a near infinite time, this curve is the interesting curved bit in between)

You need to consider pre-arc - the I2T below which we can be totally sure it will never open.

Between the two is ‘may operate’ and it is a wide band..

 datasheet shows similar graph to the above, but also tables a value of pre-arc for 60A to be 9,100 (so 300A for 0.1 seconds, or maybe a touch under100A for a second) and the let through to be 25,000 - compare that with the fast end of the 60A curve above, 0.1 second, and 900A - looks like I2t is more like 81,0000. This is why when you cascade fuses, to make sure the larger one does not blow at the same time as the small one it needs to be ~ 3 times the rating. In reality anything over the pre-arc level ‘might just’ fire the fuse if it is one of the friskier ones in the production spread.

Mike.