Every major electrical system powered from a source capable of starting a fire should have fuses, right? Right. Electric vehicles are no exception. And while a more conservative EV might have a fuse with a barrel the size of a short stack of quarters, the ones I’ll be using are about the same diameter as a soda can.

This is a 700A, 500V Ferraz Shawmut fuse recommended by Cafe Electric for use with the high-voltage, high current system I’ll be running in the Ohmbre. It’s called a “semiconductor fuse” and the element carrying the current is encased in sand, within a fiberglass barrel about five millimeters thick. When a fuse like this blows the sand and fiberglass contain the fairly energetic explosion. Supposedly the sound it makes is something like a shotgun or a grenade. I haven’t heard it, and its an event I hope I never get to experience.

I currently have two of these fuses, to break the pack in two separate places. Given that I have four battery boxes in the current design plan, I may get two more, one for each box. They’re quite expensive though at about $150 each, so it’s not like I’ll have a stack of extras.

In case you’re wondering, these fuses can’t really be counted on for anything except the worst possible electrical scenario — a full short circuit, having the potential to pass as much current as the batteries will dish out. A couple thousand amps in my case. In this dire situation, whether driving or working on the vehicle, these fuses will blow preventing the pack wiring from going incandescent and the batteries from bursting. For other sorts of failures like a runaway motor (if the controller fails in a “full-on” state), it’s a good idea for an EV to have an emergency disconnect device. On a more tepid EV it might be possible to stand on the brakes to load the motor down in hopes of popping the fuses, but even a modest motor will likely out-torque the brakes, and by the time the fuses blow you’ll have run into that tree or pedestrian.

And with a big motor like mine … well, better have another plan.

A final word about fuse ratings, as I’ve had discussions with folks in the past who were confused about what the numbers mean.

• The current rating on the fuse, measured in amps (A), defines how much current the fuse can pass continuously without blowing. Go over that amount, and depending on the design of the fuse, you will only be able to pass that current for a short time. The higher the current above this number, the less time you have.
• The voltage rating, measured in DC volts (VDC) and/or AC volts (VAC), defines the maximum voltage that the fuse will interrupt safely, once the fuse does blow. Go over that amount, and the blown fuse may not completely break the circuit — the current may instead arc across the gap in the blown fuse and continue to flow.
• The interrupt rating (IR), measured in amps (A), defines the maximum current that the fuse will interrupt safely as long as the voltage is within the voltage rating. This number is usually high enough to be irrelevant, in my case measured in thousands of amps (kA).

Bottom line: the current rating is a soft limit especially on a fuse with a delayed response such as these. You can go above it, but not for long. But never ever go beyond the voltage rating, for any reason (ever!). For a 348V pack, which can reach a voltage close to 400V during charge, a 500V fuse is adequate.

Link to today’s part.