- Oct 18, 2003
- Saline Michigan
Let's start by knowing that we have been putting electrical systems in airplanes for about a century now. Power generation for lighting, avionics, starters, fans, and so on.My two cents; first, use a CB or fuse sized to protect the device, not the wire. Many times I've heard that the CB should be sized to protect the wire under maximum load, while the device (or load) on that wire is drawing only a fraction of the current that the wire can manage. That makes no sense to me... if you protect the device, you've by definition protected the wire...
A lot of really smart people have worked on airplanes in that century. Lots and lots of innovation in every area of flying machines.
Circuit protection has been thought about a bunch over that century by these smart people.
Do any of us really think that in all that effort over all that time by all those smart people, that protecting the device with a fuse instead of protecting the wiring with a fuse might have been overlooked completely? I suspect not. With the choices on circuit protection boiled down to: Protect the device from burning; Protect the wiring from burning; or a Combination, I suspect that it has been thought through.
I suspect that a more likely event (orders of magnitude more likely) is the scheme of sizing wire for adequately low line loss and low temperature build up, then sizing the circuit protection to keep us from burning those wires came out of a lot of painfully acquired experience. And there is evidence. The regs have bits and pieces scattered about that drives us to the standard approach. Yeah, the regs do not apply to us, but they are still a pretty good idea, as they have largely been written in the blood of victims. A fire in the cockpit is a nightmare I would rather leave out of my flying experience.
Building avionics to live despite the slings and arrows of life in an airplane has been mandated for a while now. Document for certified airplane avionics is DO-160 and any successors, then there are Military specs as well. Our experimental systems do not rigorously have to meet these specs, but look through Dynon and GRT's web sites, and you will find that they do. These documents specify minimum spikes, surges, and noise that each type of device must with stand without failures and allow immediate return to functionality when the event ceases. It also addresses faults and failure trapping inside the device. The levels spec'd are pretty impressive. For 14 volt systems, spikes of 300 volts for 100 microseconds, surges of 40 volts for 100 milliseconds, 20 volts for 1 second, it goes on in the areas of noise and faults too. The spec is met by internal design of devices through a combination of sturdiness, filters, and other protective schemes. If the hardware is good per the specs, you are unlikely to size your circuit protection device adequately to protect the device. And if the device fails in a way that would cause it to draw excess current for the ship, you are not going to keep it from being broken with a fuse or circuit breaker, but you can keep it from lighting your airplane on fire.
Do I see harm in the approach posited above? Well, one might succeed in using lower amp rated fuses or circuit breakers and have a workable airplane. I do not think that one will have any increased risk of fires, but you might need to run a lot more wires and circuit protection to protect each device at its level, with all of its attendant complications to design, build, install, use, and service. The increase in wire footage and connections does carry some risk of increased failure exposure with decreased dispatch rates and increased repair work. I suspect that in developing your scheme to protect the devices, you wlll get nuisance interruptions and have to up size your circuit protection. You may even find that you approach the nominal scheme as you do so.
In short, this proposed scheme is unlikely to protect the devices it is intended to protect and is likely to make for a poorer airplane.
Please read the AeroElectric Connection. At least then you are going onto whatever path you choose with conscious decisions.