short circuit ratings on typical disconnect switches
short circuit ratings on typical disconnect switches
(OP)
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.glo bal-downlo ad.schneid er-electri c.com/8525 7689000007 EE/all/DC4 3C330E1B91 26E8525778 90057F11A/$File/de2.pdf
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






RE: short circuit ratings on typical disconnect switches
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.
RE: short circuit ratings on typical disconnect switches
RE: short circuit ratings on typical disconnect switches
Withstand ratings and interrupting ratings only apply to faults external to the device.