Asking for advice on : circuit breaker. ⁉️

[rv7charlie]

We're each going to do what we're going to do. But I've found it instructive that there have been airliners lost while (read that: *because*) a 3 person highly trained crew tried to troubleshoot a non-critical electrical problem in flight.

[Canadian_JOY]

This topic quickly digresses into a religious discussion. We each have our opinions as to how best to achieve circuit protection. To each his own.

I'll try to add a little value here by suggesting that, while I'm very strongly a supporter of "pullable circuit breakers" such as the Klixon 7277 or similar, I have used fuses in other applications. In fact I used LED indicating fuses in one application. I find the concept of troubleshooting fuses to be a challenge. Not because I don't understand how they work but because either a continuity tester is required or a significant amount of light is required to determine whether a fuse truly is "blown". For these reasons I would not choose to install fuses in an aircraft unless they were of the LED indicating style. One wants to be able to troubleshoot quickly and easily in austere conditions (including waaaay down at the end of the unlit visitor aircraft parking area). The LED indicating fuses support this methodology.

There's one aspect of this discussion that gets very little air play. Often times we will see electrical devices produce higher-than-normal loads in a one-off fashion. One of these that comes to mind is the flap motor on a cold morning where there might have been a drip of moisture that froze on a flap track. The flap motor suddenly encounters a brief load spike, thus producing a brief current spike. Some circuit protection devices are not sized with adequate "overhead" to allow a load to function under such slight over-load conditions. If a fuse was protecting the flap motor in this example, the fuse would blow and one would be stuck scrambling for a replacement. This is an example where a resetable circuit breaker has an advantage. Reset it once - if it pops again, you have more serious troubleshooting to perform. The nugget in this paragraph is that one never wants to have their circuit protection device sized to just barely cover the operating load of whatever is hanging off the far end of the wire. Size the wire for the max current of the load, then size the circuit protection device to protect the wire. I'm guilty of not having strictly followed this recommendation and I rue the day that I deviated from it because the results have always been frustrating.

Now for a question to those who have extensive experience with fuse-equipped aircraft. Where do you keep your spare fuses, and how do you keep them organized? I ask because I was sure I had kept a link to a compact fuse holder for the standard automotive fuses - it looked like a compact fuse block. For the life of me I can't find that reference, hence my ask about how you keep your spare fuses organized and easy to access.

[9GT]

Originally Posted by Canadian_JOY

Now for a question to those who have extensive experience with fuse-equipped aircraft. Where do you keep your spare fuses, and how do you keep them organized? I ask because I was sure I had kept a link to a compact fuse holder for the standard automotive fuses - it looked like a compact fuse block. For the life of me I can't find that reference, hence my ask about how you keep your spare fuses organized and easy to access.

In the RV-10 I kept an assortment of blade fuses in a small snack size zip-lock and that was left in the map box. I'll do the same for the -9A. I numbered all the blade fuse slots and made up a legend that I laminated and kept in the map box also: slot number, item protected, and fuse amperage.

[Waiex-guy]

1. If there are extra slots in your fuse box, which is likely, store extra fuses there. You probably will never need one, certainly not many.

2. ?No reset of c/b > 5 amps in flight?. This is the limitation on the Part 25 Jet I fly. As someone pointed out, aircraft have been lost while flight crew attempted to diagnose problems. Do that on the ground. The one reset idea is out of date and, literally, playing with Fire.

3. Fuses are a good idea and very cost effective compared to c/b?s. A few c/b?s are needed where an item might need to be DE-powered in flight.

Roman, your plane doesn?t have to look like an airliner from the 60?s with c/b?s covering every more or less flat surface in the cockpit. Design it howbit makes sense to you.

[jliltd]

Beware. I used to use LED lit fuses as they seemed like a good idea. A couple of years ago I delivered a newly-built experimental Backcountry Super Cub to the owner flying from FL to New Mexico. The aicraft had an automotive fuse panel on the kick panel filled with blade fuses that lit up on failure. The onboard USB charge port seemed to work fine for.about two hours and then stopped charging. When a portable device was plugged into the port it would recognize the power and show it was charging. But it would not charge and would actually discharge. Ultimately the culprit was a failed fuse. The small current used to feed the failure light was enough to keep the USB port lit up and cell phones and iPads tricked into thinking they were charging but not enough to have it act properly. Now when something still powers up, there doesn't seem an immediate need to check the fuse. After hours of trouble shooting replacing the fuse was the answer. Adding to the frustration the failure light was too dim to make out in daylight and a crude continuity check showed things good.

Once I stopped to think about it I realized that upon failure there has to be enough current to light up the failure light. Not a problem for most legacy circuits but definitely a problem for modern low-power digital devices. And the continuity checker could pass the LED (probably in only one polarity so I guess I hit the 50/50 chance).

I switched thenwhole.bus to non-indicator blade fuses and the failure gremlins went away.

So fuses are great. But beware the "illuminating upon failure" type. ��

Jim

[GeoffP]

As a kind of half & half solution, I bought some resettable c/b's that will fit in a blade fuse holder, though mostly so I could safely bench test electrical systems as I build. Possibly I may fit these in the finished plane (in several years time), but mainly for low power and non-critical things. These have a distinct pop-up tell-tale when they trip, and won't reset while o/l current is still there. The only downside is they are 15mm or so taller than a fuse so they need a little more space, but still relatively cheap compared to a "proper" c/b.

[Waiex-guy]

Good thread. Thanks Jim for that information, never considered how the led comes on when the fuse burns out, but now I can see the problem. Sounds like a good electrical diagram with clearly labeled fuses is the way to go for troubleshooting.

[Canadian_JOY]

Originally Posted by GeoffP

As a kind of half & half solution, I bought some resettable c/b's that will fit in a blade fuse holder, though mostly so I could safely bench test electrical systems as I build. Possibly I may fit these in the finished plane (in several years time), but mainly for low power and non-critical things. These have a distinct pop-up tell-tale when they trip, and won't reset while o/l current is still there. The only downside is they are 15mm or so taller than a fuse so they need a little more space, but still relatively cheap compared to a "proper" c/b.

Where did you source these wee beasties? They sound like an interesting compromise perhaps...

[romanov]

Originally Posted by romanov

👍👍👍


Thanks Guys,

The thread is super educational about real
life experience. My conclussion is like that:
if I am fine with the blade fuses form of
manual disconnecting I can go on and arrange
the board. Still good to be aware about light
voltage that can be enough to trick USB devices
into charging mode.

Thanks a lot to all of you.
Roman

[Canadian_JOY]

I should have added to my post above... the fewer different amperage ratings of fuses or CB's one has in the airplane, the fewer spare parts one has to stock. A Grumman Cheetah I used to fly had its ash tray filled with glass fuses!