The 25+ "Burning Up and Blown Fuses"

We ended our last blog entry talking about overloaded circuits and that they can cause components to over heat and we used the term "burning up"...

#12. When you say “burning up” do you actually mean burn?

Unfortunately, in a few extreme cases, yes. And that can happen to any of the components of a circuit that is overloaded - the switch, the wire, a connection - any component. What we hope is that part of the circuit design process also includes the right sized fuse so it burns (literally melts) first. That opens the circuit, stops the electrical flow and saves the rest of the circuit and the day. That’s why fuses, of the correct size and positioning, are an essential safety item.

We have only personally witnessed a couple of cases of near disaster - one was a Jeep CJ that was billowing smoke from under the dash as the guy was driving down the street! If I was him, the seat would have needed cleaning in addition to the electrical problem to be fixed. 

Another was in our own street rod when it was still original. The previous owner (ya, it’s always the previous owners fault) had coiled up a wire that he was no longer using behind the dash. Unknown to me, it was hot and not fused. On our first major trip, the wire rubbed through the insulation and started arching to ground. The telltale result was molten insulation dripping onto the carpeting creating a crater hole (don’t worry, it was still the old stuff). Very lucky it didn’t drip on either my or my wife’s feet! That would have been the end of our street rodding before it even got started.

#13. I thought that blown fuses just happened, you know, an annoyance thing. You mean that it happens for a reason?

Oh, boy, yes! Fuses are a sacrificial devise intended to protect the rest of the circuit that they feed. To do that, they are sized, by amperage, to be able to allow enough amperage for the load to do its job, but...and this is a big BUT...if the amperage goes higher than the designed limit, they melt internally shutting off the circuit. Fuses are usually chosen at about 25% higher than the designed amperage draw of the circuit - that allows a little leave way - but is still low enough to protect against most other problems.

Steve Watson, Watson's StreetWorks    http://www.watsons-streetworks.com