# Emergency Cutoff Switches- via diyelectriccar.com

Let’s start with this.  If you’re using a switch to handle a specific voltage and current, then match the specs of the switch to the voltage and current.  Period.  End of discussion.  Why is that?  Because switches have to handle two things- the overall load- current – that it can handle, much like a fuse.  If the switch is too small, it will melt under full current.  The second thing is the voltage.  Think of the voltage as the pressure.  If a switch is rated for 12V it may arc when you’re trying to disconnect, say, 72V.  If you’re using it for an emergency disconnect, and it arcs, it may fuse shut as it melts, or simply not quench the arc, both resulting in a failure to disconnect.  Bad.  That \$15 500amp 12V sound-system disconnect from PepBoys is not what I’d bet on for an emergency.

One important note.  With AC current, you have a cycle from positive to negative flow, right?  Through zero.  With DC, you don’t, just a constant flow.  This is why AC will “quench” an arc so much faster and easier than DC current.  …and this is why you need to know the DC volt rating of a switch, not the AC.

You can read my Switches Made Simple post here, for a little more.

So let’s start by stating the goal here.  I want to cut my battery pack out of the circuit, assuming it’s shorting out, crashed, or somehow the motor is under load.  We’re talking 72V at maybe 300amps.  Via DIYelectriccar.com, where I go for badass big information, since most of those boys are running systems that dwarf my motorcycle, in all ways, I found this discussion on high voltage, high amperage cutoff switches…  along with the requisite discussion on the deal I just talked about above.  Here’s what I came up with.

First, the Curtis SD/ED line, specifically the SD150L.  Curtis caters to the forklift/golfcart guys, it’s no surprise they have this pretty well covered.  I can’t find a voltage rating, but you’ve got to assume it’s at least 48V, standard for golf carts.  Tracking down a supplier is a little bit difficult, though, but here’s the Curtis US distributor list, for a start.

Next, I found this little gem, certainly the coolest badass switch, and it says “Emergency Cutoff” all over it – something that’s nice considering the people who may want to find and use a switch like this may not necessarily be you, or even an emergency responder.  This is the EZ-Safe Disconnect, from http://www.rechargecar.com.  It’s a good price, too, at around \$100.  This uses Anderson connectors, and doesn’t make the connection itself, so you’re going by the connector specs, here.  They also look like a pretty new product, so buyer beware.

Finally, there’s the Chinese knockoff of the Curtis, here. from Chennic.  I’d probably go with the Curtis, but the cool thing about this is that it gives us voltage specs- listing it at 80V.  I’d want to confirm it, but that would make me assume that the Curtis will do just fine with 300A at 72V.

I’m going to post a disclaimer here, I know, it’s not something I often do, but this is for a pretty important emergency device so I really need to make sure you understand where my advice is coming from.  I don’t represent or work for any of these companies.  I don’t even have experience with these products, yet, and can in no way attest to their use or appropriateness for the application.  Contact the manufacturers or the distributors and do what they say.  I am in no way responsible for any claims they make or any stupid crap someone may do with the information here.  Especially since my advice is simple: use the correct rated switch for the job you need it to do.