# preventing blown fuses in simple circuit

I am trying to prevent blowing fuses in a simple circuit: that consists of a battery(12V), fuse, and 2 lights(50W).

There are no switches; the lights are connected and disconnected from the live wire each time... (its just a simple circuit for outdoor use, so I kept it simple).

However, at least 1-2 month, on connecting, there is a small spark between connectors (random, rain, etc.), and the fuse blows.

The fuse is only really on there for recharging; whats the simplest solution for daily use.

thanks

--edit lamps are 40W combined parallel i dont recall the wire/fuse rating(guessing 21G and 10amps).

• did switch to a slow fuse(no luck), whats the upper range I can safely go up to with the fuse?

• ok, interesting about the garden lamps, cold-resistance spiking the amperage... Does that mean grounding the wire/ shoddy soldering (had been a long time) are not possible solutions/culprits? When a spark flies, does that spike the amps?

thanks again!

• What rating of fuse...? Is it two 50W lights, or two lights totalling 50W? Commented Jul 10, 2014 at 17:17
• use a slow fuse Commented Jul 10, 2014 at 17:22

Fuses don't protect light bulbs or lamps - they can do this themselves by burning out and then you replace them. In the main, a fuse is there to prevent a fire and this means it protects the wiring from over-heating and melting the insulation and thus causing a fire.

So, rate your fuse accordingly to protect the wire. If your wire is rated continuously for 10 amps then use a 10 A fuse and it doesn't need to be a quick blow either - anything a little bit over 10 amps might take several minutes to blow but the wire won't have got that hot in that time. 20 amps would probably take the fuse out in about 1 second. Here's an example of a 30 amp fuse: -

And here's another guide that shows the process of selecting the right size cable and providing an adequate fuse (taken from this very good page): -

The practical solution is a switch. Like radioshack or autozone for 4 bucks.

Other solutions include using dialectic grease to prevent moisture from getting into the connecter, shrouded connectors, resetable fuses, or a single ended connector (with ground wired to a common tie point on the frame).

Make sure your fuse isn't undersized. It should be round 10~20% greater than the average load, and the cable should be able to handle it too (12V/50W means ~4.2A, off the top of my head I think that mean 17AWG or greater)

With a 12V source, a little rain or some sparks made when connecting/disconnecting the lamps won't cause a fuse to blow.

However...

The cold resistance of an incandescent lamp is about 10% of its hot resistance, so a 12 volt 50 watt lamp will initially draw about 40 amperes from the battery.

If you have two lamps wired in parallel and you connect them both to the battery simultaneously, that'll be about 80 amperes for a little while, so it's likely that the fuse is giving up the ghost on one of those times.

The simplest solution for daily use, if you're using the fuse to protect something while the battery is charging, is to disconnect the fuse while you're running the lamps and re-connect it while you're charging the battery.