Trying to match up a switch?  If you’re trying to figure out what switch rating to use for which purpose, you really don’t have to look past one simple thing.  The manufacturer rating.

For some reason I thought there was some way you could take a switch rating and calculate it out to see if it could handle different currents and voltages.  I had a cutoff switch that is rated 12V DC and 300 amps.  Is that going to be OK for 72V DC and 300 amps?

There’s no telling…  no real way to be sure.  Thanks to this thread, I think I finally get it.  Nigel: “No, you just need to consult the manufacturers ratings. Essentially there are two problems, one is the current handling capacity of the switch – too much current through the contacts will make them overheat, leading to failure of the switch. Secondly is the breaking capacity of the switch, as the contacts open current will arc across the gap, burning the contacts – too much current and the arc won’t stop – this is why switches generally have a much higher AC rating than DC.”

(note: AC goes through 0V for every cycle, right?  That tends to “quench” the arc.  …and something to keep in mind here- we’re talking mostly about the “breaking” capacity, that is, the ability of the contacts to break the current.  That”s when you start the arc, and that’s also when you run the danger of not being able to stop the arc- thus, not being able to stop the flow.  Very scary.

I’m back to the drawing board.  I’m going to wire my contactor on/off circuit to a relay that can handle 72V at at least 10 amps.  Something like this, from here. For some really interesting information on what a relay does, how it works, and how to wire it up, check this page describing the Bosch automotive relay.

As far as my main 72V pack cutoff goes, over at Cloud Electric they have a bunch of high-amp switches that should work fine- although none of them look like they’re rated at 72VDC.

Wait- this just in: over at EV Parts you got you a whole page of over-72V switches there…  nice!

If a switch seems like it has a huge current rating, but a low voltage rating, can you use it?  Not recommended.  The current rating has little to do with the arc-handling ability of the switch, and if it could safely handle higher voltage, you can be sure the manufacturer would say so.

So there you have it.  Bottom line?  Read the label, and don’t cheat.

Or this will happen.