short circuit ratings on typical disconnect switches

[tricard]

Hi everyone,

I have a question that is nagging me about short circuit ratings of LV fused disconnect switches. On many low voltage disconnect switches two short circuit ratings will be given. One will be given as 5 or 10kA with the statement "when installed fuses are Class H or Class K"; the second will usually be huge like 100kA or 200kA with a statement "when equipped with class J or RK current limiting fuses". Now, I understand why the higher rating is shown when proper current limiting fuses are installed, but this doesn't seem to take into consideration the possibility of a short circuit fault on the line-side of these installed fuses.

Let's say somehow a short circuit/arc was initiated on the line side of a disconnect switch installed with class J fuses, which I have seen happen in the past. As far as I am concerned, it doesn't matter what the rating of that fuse is, because the fuse will never see the fault current if the fault is on the line side. So, how is it that the ratings for these switches is based on the fuse installed in that switch and not based on the fuse installed in the upstream protection? Does anyone have a good explanation for this? As an example, see the link to a Schneider/SquareD heavy duty safety switch catalogue where this is true for almost all the switches in there (copy and paste the link for it to work):

http://www.global-download.schneide...C43C330E1B9126E852577890057F11A/$File/de2.pdf

 

[dpc]

The short circuit rating is a withstand rating for a through-fault on the load side of the disconnect.

If the fault is on the line side of the disconnect, the switch's rating is not relevant. The switch has no rating for a line side fault.

 

[tricard]

Thanks dpc. I suppose it would not be relevant since like the fuse, current will not actually go through the switch in the case of line side faults. Is there any generally accepted practice with regard to line side faults or is this something that is typically neglected?

 

[dpc]

It's not neglected - the upstream protective device must be able to clear the fault. But the disconnect switch withstand rating is based on a fault external to the switch. If the fault is in the switch, it is already damaged, so the withstand rating is not a consideration.

Withstand ratings and interrupting ratings only apply to faults external to the device.