No fuse for battery contactor

[Veetail88]

Folks,

I'm using Knuckolls' figure z-13 for my base electrical system architecture. This figure is for single battery-dual alternators.

I've noticed that there is no fuse protection in the line between the main power switch and the battery contactor. There IS one for the standby alternator contactor, so this doesn't make much sense to me.

I've looked at some of his other architecture drawings and they're all the same in this regard.

Anyone know why this is done?

[PaulR]

I believe the master is activated by a ground and it picks up the postive 14v internally.

[Auburntsts]

Jesse,
Which version of the z-diagrams are you looking at? The current rev (12A) doesn't have a z-13 that uses an ebus contactor. I do have a copy of an older version that does have the 2 contactors, but in that one both the lines from the master switch to the battery contactor and from the e-bus swtich to the ebus contactor are the same (un-fused). Just trying to make sure I'm looking at the same thing you are.

[f1rocket]

I asked Bob about that once when I saw him at AirVenture and his response was that due to the large size of the wire (2AWG or 4AWG) that there was minimal risk exposure because of the current capacity of those wires. There are other wires that are unprotected as well such as the feed wire to a battery bus. If you want, you can create a fuseable link on this wire with a piece of 8AWG wire or other size that is also capable of carrying your maximum load. A fuseable link is just a short piece of smaller wire covered by some non-flamable sleeve such that when there is a short, the smaller wire gets hot and melts, thus protecting the larger wire. Kind of like a real slow blowing fuse.

[rzbill]

Originally Posted by Veetail88

Folks,

Anyone know why this is done?

PaulR is right but let me expand a little more. You are speaking of the wire that goes from the contactor to the master switch and then to ground. In operation (master switch on) the wire is dead shorted to ground so it is seeing the most current and voltage at the hot end it is ever going to see. The current through this grounded wire is controlled by the resistance of the solenoid in the master contactor. What happens if this wire rubs though and shorts to ground somewhere else? Nothing. You just can't turn off the master solenoid when you land, so next flight you have a dead battery .

The wire is already pretty small (22ga on some prints). That means a fuseable link would have to be a 26ga. Too small for my liking. That link might fail because of my fat fingered construction before the 22 gets fried from some wierd solenoid resistance drop.

[Veetail88]

for the response.

Bill, the ground issue makes sense. I'll run with that and not worry.

Thanks again.