old safety switches for DC

[dscheckman]

Hi, I have a couple of old, fused, knife type safety switches I'd like to use in my hydro system.[500 watt output, smartdrive alternator rectified to dc, 48 volt battery bank, apc matrix 5000 ups as invertor]  One uses cartridge type fuses, the other screw in type fuses. I'm aware that I need to use DC fuses. One of the safty switches only has AC ratings, If I'm able to find dc fuses that fit it, is it safe to use? Should I lower the Volt/amp rating by some factor for Dc? Are there AC vs. Dc issues regarding the knife switch and fuse holders as well as the fuses themselves? Does anyone know of  a source for screw in type fuses with DC ratings? Thanks for any help,

David

[maker of toys]

while I don't have a source for DC rated 'Edison base' fuses, check the Bussman selection chart to see if there's something you could adapt.  try:

http://www.cooperet.com/library/products/Fuse%20Selection%20Guide.pdf

I'm sure Littlefuse has a similar guide.

if the switches in question are open knife switches, then they should be usable for DC with little or no trouble.  Knife switches will burn up relatively fast if used as an interuption device, however. . . . especially on 48V dc, as the arc from DC voltages over 36V (nominal) is not inherently self-quenching.  Put a decent sized capacitor across the switch to help with that.  OR put the switches on the AC side of the system?

and I personally would not use open switches on anthing over 24 V. . . 50V is the low edge of what is considered in the industry as 'potentially lethal' voltage;  i.e. it has sufficient voltage to drive enough current to stop a human heart.

your milage may vary, of course, depending on your risk tolerance and such factors.

-Dan