ECN Forum Forums Technical Discussion Electrical Theory and Applications Using OCPD at low voltages

This past summer, I rewired a 33' 1970 Chris Craft pleasure boat that had 2 small (factory installed) Square D breaker panels. Each panel had about about eight 15A breakers. Each breaker fed a 12 volt load. I kept the panels in place for posterity sake and installed fuses. I always wanted to dead short one of those breakers at 12V just to see if it worked... Never did though.

"Just for fun, a Bussmann 7.2kV 25A fuse has a resistance of 0.039 ohms. Take this fuse and misapply it in a 12V application, and the results would be _safe_, but you couldn't really claim that you could ignore the resistance of the fuse *grin* Though this does go back to the original question of applying higher voltage OCPD to lower voltage circuits."

This is why there is a difference between, for example, a 3AG automotive fuse (32v) and a 3AG line-voltage fuse (250v).

Look at the fusible element in an automotive fuse: the element is narrow for a small section, for lower voltage drop.

Conversely, a line-voltage fuse is narrow for the entire length, better suited to break a higher voltage with less arcing.

Just FYI, in the past, there have been some electronic type devices that are fused in the transformer secondary (high voltage). One piece of Collins radio equipment has a fuse rated at 3000 volts and whatever current it is supposed to pass. It's about 6 inches long. I've never made much sense of the different voltage ratings on fuses, other than the increased size when you get to voltage ratings like the one in the Collins gear. There is probably some difference in the resistance of the various voltage/current ratings, but I'd suspect they're all very low values.

Fuses can fail partially, but it is rare. I did have a fuse go high resistance on my boat. The VHF radio worked just fine on receive, but the voltage collapsed on transmit. That's a very obscure failure, but it can happen.