aviation fuse dimension variation?

[GeorgeC]

Airplane owner continuing education, year 5...

I ASSumed that all my fuses were all 0.25" x 1.25" and they all seemed to be bussmann AGC in various amp ratings. I learned the hard way that they are not interchangeable in my fuse holders (for the same amp rating, of course). If I use my finest #1 alibabazon calipers, for example, I measure...

0.248 x 1.244 <- looks about right
0.24 x 1.185 <- visibly short
0.28 x 1.22 <- visibly fat

The fuse holders themselves seem similar, but different- some have springs that put tension on the fuses, some are quarter turn, some are screw in... what gives?

Somehow, I was also expecting them to be individually connected to a buss bar, but instead they are soldered to a wire that is stripped in multiple locations.

The 20A avionics breaker switch is also downstream of a single fuse, which seems weird to me, but what do I know...

 

[flyingron]

Do you have a separate avionics master? Are you sure that breaker isn't the emergency bypass. I have a 25A breaker in my plane that remains pulled in normal operation. You push it in in case the avionics master relay fails and it engages the avionics bus for you.

 

[Bell206]

I ASSumed that all my fuses were all 0.25" x 1.25" and they all seemed to be bussmann AGC in various amp ratings.

Aviation fuses fall under AGS and are 9/32" OD. AGC are automotive fuses. There are also a number of other alternates to AGS like 4AG or ABS and others. All 9/32" OD. And usually when you see different fuse holders is due to newer upgrades, etc. However, without an aircraft year/model or schematic can't offer much more on the other points..

 

[GeorgeC]

Do you have a separate avionics master? Are you sure that breaker isn't the emergency bypass. I have a 25A breaker in my plane that remains pulled in normal operation. You push it in in case the avionics master relay fails and it engages the avionics bus for you.

There's an avionics master (20A breaker switch) in series with the fuse.

 

[GeorgeC]

without an aircraft year/model or schematic can't offer much more on the other points..

1950 Cessna 140A.

 

[GeorgeC]

Aviation fuses fall under AGS and are 9/32" OD.

Perhaps it has accumulated incorrect fuses over time, some of which just happen to work?

 

[Gary Ward]

There’s 2 of them in my 172 that I know about. I found AGS fuses at McFarlane and probably others. Not cheap.Spruce sells them also from McFarlane.
AGC fuses were in the plane when I bought it. They worked 95% of time.

I tested with AGC until I was sure and then put in the right fuses.

Otherwise just wrap foil around them….
Kidding.

 

[Bell206]

1950 Cessna 140A.

Not much on the internet but did find a "simplified" 140 wire diagram. Do you know for an original 140 were the fuse holders panel mounted or inline?
The diagram didn't really show but if we could figure that out it would help to determine what you have now. But 10 to 1 there were additions and they used whatever fuse holder they had available.
The circuit breaker in line with a fuse was probably an "upgrade" at some point and the fuse was the original protection for that wire. Sometimes you wonder what people are thinking.
You may want to look into cleaning this up at your next annual. Best route would be to get an OEM diagram and put it back stock with any newer additions properly added. I wouldn't think it would be a big project on a 140. One option is to build a new electrical harness on the table then install and terminate. There were other diagrams that showed how the harness ran but I do not know how accurate they are.

Perhaps it has accumulated incorrect fuses over time, some of which just happen to work?

It can be somewhat common for this to happen. The AGS vs AGC is one of things that is not talked about much.

 

[GeorgeC]

You may want to look into cleaning this up at your next annual.

I was already leaning in that direction, or seeing if the local shop would supervise me. The avionics bus already has six push-pull breakers, two of which seem to do nothing. There are six fuse holders below the piano switches that should be converted to push-pulls. There are also a couple of inline fuses for the ELT and 12V outlet.

I wouldn't think it would be a big project on a 140.

I've heard that before

 

[GeorgeC]

I found AGS fuses at McFarlane and probably others. Not cheap.Spruce sells them also from McFarlane.

ebay aviation looks promising.

 

[Bell206]

There are six fuse holders below the piano switches that should be converted to push-pulls.

FYI: based on the wire diagram I saw, my guess is these 6 fuses are the original requirement and all the other fuses are related to upgrades. Now whether those original 6 fuses are still wired the same no clue.