RV Electricity – Don’t fix a blown fuse like this!

By Mike Sokol

Dear Readers,
I just saw this rather disturbing picture of a couple of bullets used to replace blown fuses in a vehicle, and thought I should post it here along with several other DON’Ts. But first, let’s review what a fuse does and why it’s important to the safety of your vehicle. (Click any image to enlarge.)

Fuses (and their close cousins, circuit breakers) are there to protect your wiring from a meltdown, which could result in a fire. And because the wiring in your RV isn’t required to be run in a metallic conduit (like it is in a commercial or industrial building), that fire could start right inside of the flammable walls of your RV, which would be very bad indeed.

So fuses are sized to protect a wire from passing so much current that it overheats and begins to melt the insulation. That’s really their main job, and they do it quite well.

Available in a variety of shapes and sizes, all fuses work pretty much the same way, with a short piece of small-gauge wire that will melt and open up the circuit before the insulation on the wire melts. Thus, the most important parameter about a fuse is its AMP rating, which is the amount of current it can pass without blowing.

So the First Rule is this: NEVER replace a fuse with one that has a higher amperage rating.

The reason that a fuse blew in the first place was probably due to an excessive amount of current flowing in the wire. There are too many possibilities as to the cause of that excessive current for this short article, but don’t tempt fate and plug in a higher-amp-rated fuse.

Bad things will likely happen. Just know that every wire gauge has a maximum current that it’s rated for before it can overheat and melt. And once the insulation melts it can make contact with all kinds of things it shouldn’t touch. That’s when short circuits happen!

Second Rule: Fuses come in fast-blow and slow-blow versions which are not interchangeable.

A fast-blow fuse will operate nearly instantaneously (sometimes within one line-cycle) to offer protection for electronic devices. They’re sometimes added in series with circuit breakers on big motors, with the fuses sized to protect the motor winding, and the breakers sized to protect the wires. These look like a single piece of wire which can be as thin as a hair for small amperage ratings.

A slow-blow fuse is typically selected to protect circuits that have high inrush currents, such as motors and high current power supplies that have a big “bump” of current on startup. This type of fuse will have some sort of spring-loaded contact sitting in a puddle of solidified solder, which will turn molten with enough temp and open up. Or they’ll have a winding of wire around some sort of ceramic core to provide a little heat sink for a few seconds of 200% overload, allowing the motor or power supply to properly start up without a nuisance trip.

Third Rule: NEVER replace a fuse with a bolt, aluminum foil or even a .22-caliber bullet.

That’s right, people do strange things, and while I’ve seen a few screwdriver blades stuck into large fuse holders, I’ve NEVER seen a bullet used as a fuse … until now. Yikes!

So go get a box of appropriate replacement fuses for your RV or tow vehicle, and don’t let me see you wrapping foil around a dead fuse, or sticking a bolt (or bullet) in its place.

Let’s play safe out there….

Mike Sokol is an electrical and professional sound expert with 40+ years in the industry. His excellent book RV Electrical Safety is available at Amazon.com. For more info on Mike’s qualifications as an electrical expert, click here.