Headlights and Fuses

Phred

Phred

Senior Member
The occasional discussion of the electrical "weak link" at the Model A headlight connection has got me thinking. I am asking the Barn if my logic is good or flawed.

Most of our cars have a single fuse at the starter (let's assume a 25a fuse).

If a headlight connection were to short out, the main (and only) fuse blows and the entire car shuts down. This could be unnerving, especially at night.

(1) Assume a single 20a fuse is installed inline between the alternator/generator and the two high beam/low beam wires. and a 25a fuse at the starter. In the case of a headlight short, would the 20a fuse blow first, protecting the 25a fuse at the starter, and allow the car to continue to run?

(2) Assume a single 10a fuse is installed inline between the alternator/generator and each of the two high beam/low beam wires. and a 25a fuse at the starter. In the case of a headlight short, would one of the 10a fuses blow first, protecting the 25a fuse at the starter and the other high or low beam circuit, and allow the car to continue to run?

(3) Assume a single 25a fuse is installed inline between the alternator/generator and the two high beam/low beam wires. and a 25a fuse at the starter. In the case of a headlight short, is there any guarantee that the 25a headlight fuse would blow first leaving the 25a fuse at the starter protected and the engine running?

Of course, I understand that a "better" headlight connection greatly reduces the chances of a headlight short and any fuses blowing.

Thank you in advance for any reasoned responses to (1), (2), or (3).

Peace, Phred
 
Phred said:
If a headlight connection were to short out, the main (and only) fuse blows and the entire car shuts down. This could be unnerving, especially at night.
Click to expand...

That happened to me... once.

I'll go with Tom's answers. Let me add that mine is now set up with a 30a main (circuit breaker) and a 20a (circuit breaker) for the lighting circuit. I also have fuses on other circuits, including turn signal and fog/spot light circuits, among others.
 
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Your basic premise has a very large flaw.

Fuses are sized based on the gauge of the wire.

Follow this chart for a basic idea.

Wire to fuse chart

The real calculation is a bit more complicated. You need to take into account the type of load and the type of fuse. You also consider how tight the wire are together to allow for heat transfer and such. Breakers would use a different chart.

Just remember, a toaster does not pop a breaker and can cause a big fire. So when the 30 amp fuse does not blow cause it is not a full hard short and the wire is glowing red then you do not have a protected wire.
 
The Model A's electrical system really should be completely different. But the fact is that it works OK the way it is, I think adding the 30 amp fuse at the starter is a good idea, but the best bet is to make sure all the wiring and connectors are in good condition.
 
Put an ammeter in line with whichever circuit you are trying to protect. Then you know if the wire is adequately sized, and you can fuse it accordingly. Rule of thumb is fuse at twice the operating current. Some electrical loads have high inrush current, you can account for this two ways. Use a slow blow fuse or fuse at a higher rate.
 
That exact thing happened to me once- headlight shorted, blew the fuse, killed the engine ,while inside an unlit tunnel in Zion national Park in Utah. Dark car- dark tunnel- car dead. Damn near got killed. Pushed the car out of the tunnel, replaced the fuse and hoped their were no more tunnels until we fixed the lights.

It was while in the 1993 Great American Race.

That was the only breakdown, but I will never forget it....

Steve @ Bert's
 
To muse a fuse

A fuse isn't just a thingy which blows when it's rating is exceeded.

There are very fast acting fuses, fast acting, medium acting, dual element slow blow, and slow-blow fuses.

And there are shorts which are long lasting, shorts which are short lasting.

100A will not blow a 25A fuse if it lasts only 1 microsecond!
30A will not blow a 25A fuse ever!

Circuit analysis and component selection is what engineering is all about.
Lacking that, carry an extra box of fuses in the glove box.

Remember, shorts may sit idle for years waiting to catch you at your most inopportune moment -in the dark, in a hurry, no spare fuse.
 
I think the original question is ... will a smaller fuse blow first?

When I have had a headlight wiring upgrade on a couple of modernish cars, heavier wiring is used with relays and a couple of breakers. One on low, one on high.

The theory is if one beam (say high beam) shorts then flick the switch to the other (low beam).
 
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