Blown Fuse

[RV12Pilot]

I bought a RV-12 in 2020 which was completed in 2013, with a Rotax 912 ULS, about 340 hours now.
Three weeks ago the fuse for my NAV lights blew. Replaced the fuse. The NAV light fuse blew again this afternoon.
I essentially have not had any issues since the purchase in June 2020.
I am not a mechanic.
Would anyone have any suggestions as to what might be the cause(s)?
TIA
Charlie

[Mike S]

Start tracing the wiring from the switch out to the light unit.

Methinks you should find a spot where the wire is grounding, thus blowing the fuse. As the fuse blowing was not as soon as you flew next, there is most likely not a constant short to ground, but something that is making intermittent contact.

If that doesn't work, look for high resistance like corrosion in the bulb socket, loose screws or push on tabs. Even loose fuse to holder contact.

[RV12Pilot]

Originally Posted by Mike S

Start tracing the wiring from the switch out to the light unit.

Methinks you should find a spot where the wire is grounding, thus blowing the fuse.

Thank you sir.

[Bob Y]

You might try pulling the wing and check the wing-root connector for misaligned pins that may be shorting together.

[RV12Pilot]

Originally Posted by Bob Y

You might try pulling the wing and check the wing-root connector for misaligned pins that may be shorting together.

Okay, thank you.
Charlie

[Desert Rat]

So since you said you're not a mechanic, perhaps a brief overview of what the heck a fuse actually does might help you in troubleshooting?

Basically, the fuse is there to protect the wiring. if you get a short, that short is literally a shortcut for a bunch of surplus angry little electrons to bypass the load (in this case a nav light) on their way back to the ground side of the battery. Like mike said, a bunch of extra resistance from a loose connection or corroded terminal can cause the system to try to suck more amps to overcome the increased resistance as well.

The wire is sized to handle the load designed in the circuit, so a bunch of extra amps cruising through that wire causes it to heat up and melt/burn/smoke/whatever.

The fuse in the circuit is sized so that it can handle the normal load, but will pop (burn open) at an amp rating slightly higher than the circuit pulls in normal operations, but less that what would allow the wire to heat up.

Therefore, if you're getting a popped fuse, it means that the safety feature is doing it's job. Bent pin, wire with damaged insulation, corroded terminal, whatever it is, it' something thats causing more amps to flow through there than the circuit can handle so the fuse is popping.

If it's intermittent, that just means that the short isn't happening all the time. Like maybe a damaged wire is flopping around and bumping up against a rib flange every once in a while. Heck, years ago I had an old motorcycle that would blow the headlight fuse seemingly randomly and it turned out to be a cracked connector housing that was allowing water intrusion in there when I washed the bike. Water is conductive, and if I turned on the headlight before it evaporated back out of there it popped the fuse.

Hopefully this helps as you trace this out.

[Norman CYYJ]

The color of the blown fuse where the fuse element broke can tell you a tale of the kind of problem you might have.

[RFSchaller]

If you pull the wings and the fuse still blows you will know the short is in the fuselage. Actually all you have to do is pull the wings out enough the break the wing connectors contacts. Then push them back one at a time if the fuse does not blow initially. The wing with the short will be obvious when you make contact again.

[Desert Rat]

his problem is an intermittent fault with two events 3 weeks apart. Those are always the hardest to find because the darn thing won't break and stay broken!

[John-G]

Charlie,

If your troubleshooting isolates the problem to fuselage wiring, one of the first places I would suggest looking is where the wiring is captured by a metal plate above the rudder pedals. That area is a really tight fit and it would be very easy for a wire to become pinched there.