Fuses on the panel, or not

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

On a German forum earlier this year a Glider Instrument supplier started an enquiry to see if there would be any demand for a Centralised Power distribution unit. Forum is in German but the first message shows a picture and some description what they had in mind. I understand that the project was never realised as to many people had to many different demands.

http://forum.segelflug.de/showthread...zentraleinheit

At 14:02 31 October 2016, Muttley wrote:


On a German forum earlier this year a Glider Instrument supplier
started
an=
enquiry to see if there would be any demand for a Centralised Power
distri=
bution unit. Forum is in German but the first message shows a
picture and
s=
ome description what they had in mind. I understand that the project
was
ne=
ver realised as to many people had to many different demands.

http://forum.segelflug.de/showthread.php?
t=3D8160&highlight=3Dzentraleinhei=
t

please use this url

http://tinyurl.com/z7678rz

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?

More air flow can fan or extinguish a fire.

In his shoes, I'd have stepped over the side. I'd really hate to have a
major part of the glider burn away while on final approach!


"son_of_flubber" wrote in message
...
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?