One is overcurrent protection:
I would recommend two on the firewall, an 80 or so for the alternator and a 40 for the battery feeder (providing you use a #8 jumper from the battery lug to the fuse block.) My reasoning on this is you do not want your "backup" feeder to accidentally pop EVER. So it is sized quite heavy for the sum of all loads plus a bit more.
Kevin also has a backup circuit for the Lightspeed EI, which made me slap my forehead over my leaving it off.
Here's the update with his additions. At some point I'll figure out how to make this diagram all nice and pretty. At the moment, it's written mostly in pencil. Click on the image to make it big.
Bob, looks like good progress. Backups are very subjective, but here's my 2 cents. Backups are necessary for critical avionics. Just like you have backup instruments in case your primary EFIS fails, you need backup power for citical avionics. The initial reaction many people have is to err on the side of too many backups. There is always a tradeoff. Backups do add weight, complexity, and failure points.
ReplyDeleteSo for example, backup flap power is usually not necessary because you can land with flaps up. Backup power to a single EI is usually not necessary becaue you can fly with a single remaining mag. In the latter example, you have to ask what is the chance the mag and the EI power source will fail simultaineously? If you believe this is high, then you should put in a backup. Conversely, what is the chance introducing more crimp connectors and a mechanical switch into the power path will fail the EI altogether?
Are you installing backup engine gauges for the D-180? If the D-180 fails you cannot go flying, and if flying will want to land soon. So why no backup gauges? The answer is because the chances of failure are low. Same goes for electrical system planning.
In the end, you should do what you are most comfortable with because it is your plane.
Just food for thought.
Marc @ VP