Designing an electrical system

[Dust]

Well, it will be a while till i lay out the electrical system, but one thing i'll surly do is test all items under load for amp draw before i wire and install.

The reason is simple - problems i've seen or heard of.

One guy lost his entire system when the main 50 amp fuse blew - it was undersized.

another time - I was flying over livingston county airport and could not land cause there was a falco, gear up on the runway. Talked to an A&P at the field later that day and found out the the pilot and flight instructor on board had put the gear down and that the circuit had blown so the gear did not extend. the pilot had inherited the plane from the builder, his father, and his father had always cranked the manual handle to insure that the gear was down, the son did not do this as he figured that the gear would go down when the switch was pushed. My guess is that this was not the first time the breaker had misbehaved and the dad had got into the habit of makin sure the gear was down.

I got word that a current almost flyer is having similar problems, the circuits in the power board will not handle the nose gear and it keeps poppin and the lights had to be rewired to relays so the circuit would not trip. the lights now have in line fusses. Fortunately this is happening on the ground and knot in the air

I will test every item under full load and oversize by 25% to insure a bullet proof system. Don't believe the MFG claims of current draw - test it.

[dpaton]

Agreed, test, but make DAM sure your current shunt and meter are accurate. Also, your 25% derating is a little high...I prefer to go 10-20%, and make it a breaker, not a fuse. This comes from a few years of designing intrinsically safe, life safety, MSHA, FM, and ATEX electronic product. You can always turn things off and reset a breaker after a minute, but it's really hard to reset a fuse and keep going if you turn off the offending circuit.

Wait, you do plan on a way of isolating circuits for re-activation, yes? Plan for bad alternator, bad avionic piece, bad strobe, bad pump, etc. Breakers/disconnects all around. Build the E-busses like brick sh..houses so that they CANNOT be a short circuit point.

Oh yes, and a backup battery if you plan to have one o'dem glassy panels.

[Dust]

Just like my fuel system - simple and robust, course what i think is simple may not look simple

What is an "E-busses"

[Steve parkins]

i think you needs one of thems e-tickit to rides them there E-bussies

[dpaton]

That'd be an Elecktrickle bus, the big chunk of wire/copper/metal/screws/insulators that make up the + and _ distribution points, and where all of the wires join each other. No arcing, loose wires, or failures there will make the system a LOT more robust.

[Dust]

As i am not actually doing the electrical right now, i won't push you on specifics, but on the bussesses aren't they usually protected by a fuse?

[dpaton]

Designing for failure tolerance also requires the ability to recover from failures. The FAA says don't reset breakers and don't fix fuses in flight, as they may cause a larger problem. That's true, and good advice if things are designed for a profit motive. I'm more comfortable doing it my way though, having done some very hard science on failure modes of breakers, fuses, wires, connections, and batteries, not to mention electronics.

Protection by fuse is good in places, but in things like an aircraft, I strongly prefer breakers. If a fuse MUST be installed, it needs to be sized such that it only blows in the event of catastrophic failure, and there must be an adequate backup supply for critical systems. I HATE the idea of losing a complete bus with no recourse. Example:

Avionics get breakers, all of 'em.
Same for the retract pump/motor/whatever, trim motors, AP servo motors, strobes, ignition, etc.
All go to a common battery bus.
Critical systems are also fed (isolated of course) from an emergency bus.
If the main bus goes down, the e-bus can be kicked in, and things run for long enough to get home.
Only at the batteries would I place a fuse. Something huge, 80A or so, depending on max critical load and things like starter current.

Of course, these are only my opinions.

[Dust]

your thinking meshes with mine to the T

when i am in the midst of doing it - i'll bug you

[argoldman]

Originally Posted by dpaton

Designing for failure tolerance also requires the ability to recover from failures. The FAA says don't reset breakers and don't fix fuses in flight, as they may cause a larger problem. That's true, and good advice if things are designed for a profit motive.......


Protection by fuse is good in places, but in things like an aircraft, I strongly prefer breakers. If a fuse MUST be installed, it needs to be sized such that it only blows in the event of catastrophic failure, ....

Same for the retract pump/motor/whatever, trim motors, AP servo motors, strobes, ignition, etc.
All go to a common battery bus.
Critical systems are also fed (isolated of course) from an emergency bus.
If the main bus goes down, the e-bus can be kicked in, and things run for long enough to get home.
Only at the batteries would I place a fuse. Something huge, 80A or so, depending on max critical load and things like starter current.

Of course, these are only my opinions.

I gotta chime in here so I don't blow my fuse (or break my breaker)

The fuse/breaker value is chosen to protect the wire supplying the device, not to protect the device. Therefore if the fuse/breaker blows/brakes this is to prevent a catastrophic event, not in case of one.


I am curious what the profit motive of the FAA is to say "Don't reset the things?"

I do agree with you in certain circumstances.

When electric motors are used, as the force they have to provide increases, so does their current draw. This property is used as protection in electric nose lifts to prevent damage if the nose gear is cocked on retraction and contacts the bottom of the fuselage instead of tucking itself into the well. Since the motor is activated (as opposed to being shut off by microswitches in the well, as it stalls, on the fuselage in it's retraction cycle, the draw increases and at the selected amperage (I believe Jack suggests 10A) the breaker blows.

Realizing this, one can reverse the switch and reset the breaker and watch the gear go down. Breakers here or similar setups where they are designed to protect a mechanism and not the wire are greatly to be desired.

However if you have a lead going to your com radio, or your electronic ignition, or your pacemaker, and an event happens which draws enough current to blow the breaker/fuse short of causing a fire, why would you want to tempt fate and walk on that knife edge again. The best thing that could happen is that the breaker would blow again.

You may be too young to remember when, when we used screw-in fuses, to prevent having to go to the hardware store to get a fuse, many people put pennies in the socket. The circuits magically worked, until a fire followed.

Now as to the backup, I totally agree with you. With electronic ignitions/injectors, duplicated, separate fed systems are vital.

The system should not be designed so that a failure of one fuse will blow out a whole buss

Why not use a circuit breaker at the batttery???? instead of a fuse????
If your wire from the battery will only handle 60A (not including starter system), why would you want to use a fuse of a large level higher than that (assuming you have calculated a safety factor for temperature rise) Better yet to select the wires with extra capacity and fuse to protect the wire.
When those wires get hot, the first thing they do is burn their own insulation, they then burn anything near them.

Protect the wire not the device, use what ever protection you want and unless you have designed resetting the breaker into the system, don't reset, or change the fuse.

My opinion, however subject to change if appropriate facts (or lack of panel space) come my way.

[Dust]

Originally Posted by argoldman

I gotta chime in here so I don't blow my fuse (or break my breaker)

The fuse/breaker value is chosen to protect the wire supplying the device, not to protect the device. Therefore if the fuse/breaker blows/brakes this is to prevent a catastrophic event, not in case of one.

....
When electric motors are used, as the force they have to provide increases, so does their current draw. This property is used as protection in electric nose lifts to prevent damage if the nose gear is cocked on retraction and contacts the bottom of the fuselage instead of tucking itself into the well. Since the motor is activated (as opposed to being shut off by microswitches in the well, as it stalls, on the fuselage in it's retraction cycle, the draw increases and at the selected amperage (I believe Jack suggests 10A) the breaker blows.

Realizing this, one can reverse the switch and reset the breaker and watch the gear go down. Breakers here or similar setups where they are designed to protect a mechanism and not the wire are greatly to be desired.

....
The system should not be designed so that a failure of one fuse will blow out a whole buss


My opinion, however subject to change if appropriate facts (or lack of panel space) come my way.

Points well made - love the panel space comment - may be all too real a limitation.