Fuses on the panel, or not

My big winter glider project is going to be re-doing my control panel. I have been looking at a lot of panel pictures recently and I notice that a lot of them have panel mounted fuse holders all over them. My experience with our panel mounted electronic gizmos is that if they blow a fuse, replacing it inflight is not going to do anything for you. Once the magic smoke gets out it's gone. So I'm looking at a terminal strip that incorporates the mini blade type fuses into the strip, and reducing the number of holes in the panel.

Has anyone ever replaced a glider fuse in flight and had something good happen?

SF

In a recent safety bulletin from the FAA, they noted that inflight resetting of circuit breakers should mostly be avoided and that in any case they should only be reset once and thereafter left triggered if it happens again; delaying further troubleshooting until the flight is completed. They suggest the tripped circuit breakers, found during a pre-flight, should not be reset without investigation by airframe or avionic technicians.

Fuses are not discussed, but the principle of protecting the main electrical system from a failed sub-system still applies. If it occurs in flight, do your best to do without it for the remainder of the flight.

Recently I was powering up a PIK-20 and after a few moments there was a smell of burning insulation and I powered the system off. In the following inspection we discovered that the VolkLogger had shorted and the complete harness from the VL to the power distribution had basically melted. The circuit was protected by a 2 Amp fuse that did blow but I have to believe there was much more than 2 Amps flowing to melt that wiring before the fuse melted.

I'm just delighted that it didn't happen after I was aloft. I might have not smelled the burnt wiring if there had been some airflow through the cockpit. Might have lite something else up before I know it.

We are planning on rebuilding the whole panel/electrical system this winter and I'll do some serious measuring before selecting the fuse current values. I'd rather the fuse blew early that late.

Mike

When I did mine I used a lot of solid state fuses to protect all the circuits. They'e a little device that if too much current flows they go high resistance. Take the fault away and they go back to low resistance. Most of mine are mounted on a circuit board behind the panel - so no panel space issues.

Paul

On Tuesday, October 15, 2013 5:15:26 AM UTC+1, SoaringXCellence wrote:
In a recent safety bulletin from the FAA, they noted that inflight resetting of circuit breakers should mostly be avoided and that in any case they should only be reset once and thereafter left triggered if it happens again; delaying further troubleshooting until the flight is completed. They suggest the tripped circuit breakers, found during a pre-flight, should not be reset without investigation by airframe or avionic technicians.



Fuses are not discussed, but the principle of protecting the main electrical system from a failed sub-system still applies. If it occurs in flight, do your best to do without it for the remainder of the flight.



Recently I was powering up a PIK-20 and after a few moments there was a smell of burning insulation and I powered the system off. In the following inspection we discovered that the VolkLogger had shorted and the complete harness from the VL to the power distribution had basically melted. The circuit was protected by a 2 Amp fuse that did blow but I have to believe there was much more than 2 Amps flowing to melt that wiring before the fuse melted.



I'm just delighted that it didn't happen after I was aloft. I might have not smelled the burnt wiring if there had been some airflow through the cockpit. Might have lite something else up before I know it.



We are planning on rebuilding the whole panel/electrical system this winter and I'll do some serious measuring before selecting the fuse current values. I'd rather the fuse blew early that late.



Mike

I agonised over this when I re did my own panel. My conclusions we

1) I would not reset or replace a fuse in flight so did not need them on
the panel.

2) Thermal trips need a large overcurrent for a long time before they go.
Low value trips introduce a large voltage drop.

So, I fitted in line blade fuses for all my equipment and use the panel
space for other things. I did buy a fuse block but decided it was too big
and made the wiring cumbersome.

Jim

Fuses rely on getting hot enough to melt the wire. Quick blow can easily go
on initial surge, and need considerably above the rating to blow on
continuous current. Slow blow take about the same length of time and
overcurrent as a thermal circuit breaker. Both thermal c/bs and fuses drop
voltage - that's how they get hot.

Magnetic trip breakers drop less voltage, and have a more precise trip, but
cost 2 - 3 times the price.

Fuses are to protect the wiring, if the wiring burns before the fuse blows
then the wire is much too low rated.

Personally I use breakers, and appropriate gauge wiring. It means I don't
have to have a bunch of spare fuses handy. Murphy guarantees that a fuse
will blow just before take off on the best day of the year, and that
someone wil have 'borrowed' the little bag of spare fuses.


At 08:23 15 October 2013, Jim White wrote:
I agonised over this when I re did my own panel. My conclusions we

1) I would not reset or replace a fuse in flight so did not need them on
the panel.

2) Thermal trips need a large overcurrent for a long time before they go.
Low value trips introduce a large voltage drop.

So, I fitted in line blade fuses for all my equipment and use the panel
space for other things. I did buy a fuse block but decided it was too big
and made the wiring cumbersome.

Jim

Ū1jd6

2) Thermal trips need a large overcurrent for a long time before they go.
Low value trips introduce a large voltage drop.

I would add to this that thermal trips will reset after a period of time. If the original short persists they will cycle on and off quite quickly (in my experience a few times a second) and will NOT protect equipment from overheating. I had them on my batteries and got lucky to that the output to the 5V converter shorted on the ground - the 12 to 5v converter overheated, caused a permanent short and ended up well above too hot to touch. I now have a normal fuse and redundancy provided by a second battery.

I would be concerned that any self-resetting circuit would behave the same, perhaps with a longer cycle. If you can't physically isolate then you are stuffed when you are flying... Bet you can't land quickly enough

On Tuesday, October 15, 2013 11:39:59 AM UTC-4, 7C wrote:
If you can't physically isolate then you are stuffed when you are flying.... Bet you can't land quickly enough

Someone told me a story about bad smoke developing in the cockpit of a glider. The pilot ejected the canopy. At that point the increased airflow stoked the smoldering fire and something burst into flames. Story goes, he landed and the fire went back to smoldering.

Does this sound plausible or is it yet another BS story?

On Tuesday, October 15, 2013 8:39:59 AM UTC-7, 7C wrote:
2) Thermal trips need a large overcurrent for a long time before they go.

Low value trips introduce a large voltage drop.



I would add to this that thermal trips will reset after a period of time. If the original short persists they will cycle on and off quite quickly (in my experience a few times a second) and will NOT protect equipment from overheating. I had them on my batteries and got lucky to that the output to the 5V converter shorted on the ground - the 12 to 5v converter overheated, caused a permanent short and ended up well above too hot to touch. I now have a normal fuse and redundancy provided by a second battery.



I would be concerned that any self-resetting circuit would behave the same, perhaps with a longer cycle. If you can't physically isolate then you are stuffed when you are flying... Bet you can't land quickly enough

Uh? Why are you talking about "thermal trips" and automatic resetting? These thingies are called circuit breakers. Yes they they internally thermally trip, but so what. An aviation circuit breaker like the popular Klixon "TC" product lines will not automatically reset. You've got to push 'em back in.. For good reason.

Darryl

On Tuesday, October 15, 2013 1:23:20 AM UTC-7, Jim White wrote:
I agonised over this when I re did my own panel. My conclusions we

1) I would not reset or replace a fuse in flight so did not need them on
the panel.

2) Thermal trips need a large overcurrent for a long time before they go.
Low value trips introduce a large voltage drop.

So, I fitted in line blade fuses for all my equipment and use the panel

space for other things. I did buy a fuse block but decided it was too big

and made the wiring cumbersome.

Jim

I bought and tested Klixon CBs and the standard German panel mounted fuses when I re-did my panel last winter. The CBs had a significant voltage drop - corresponding to about 20 minutes of battery life at the end of a flight if I recall correctly. I was unwilling to give up the extra duration off my battery, so I stayed with fuses.

My panel is curved and my instruments are basically square behind the panel so there were several good triangular-shaped spots that could accommodate up to 5 panel-mounted fuse holders. I have never replaced a fuse in flight, but I could imagine a circumstance where a current spike (maybe from keying the radio) could blow a fuse that I might want to replace. Plus it's easier not to have to take off the canopy to get behind the panel. I've seen fuses mounted in the leg tunnel under the panel, which is an interesting alternative.

I'm a little nervous about putting automotive parts in my glider - for instance, I replaced all my wiring with Tefzel after experimenting with high current through the vinyl shielded Radio Shack variety.

9B