Buss Main Power Fuse?

[ty1295]

When powering the buss, are most people putting some type of protection beyond the ability to turn the master back off?

Been searching and no luck finding a direct answer.

Fuse-able link?
ANL Fuse?
Circuit Breaker?

[flyinga]

I used a 40a CB.

[Auburntsts]

I use ANLs on my B-leads between the alternators and the bus, but nothing from the battery contactor to the bus. FWIW, I use CB's on the alternator field wires and ATC type fuses for everything else.

[rzbill]

Assuming I followed Aeroelectric recommendations as intended, review of my schematic shows the lead from battery to master contactor is not current protected. Additionally, the lead from master contactor to main bus (in my case a distribution fuse block) is not current protected.

Your statement about reliance on the master contactor appears to be true.

On related notes:

The "B" lead from the alternator is protected with an appropriately sized ANL Fuse.

My battery bus line (direct from battery to battery bus fuseblock in the cabin) IS protected with a fusible link. May not have been in Aeroelectric. Did it on recommendation of A&P prior to phase 1 and I agreed.

[ty1295]

Originally Posted by rzbill

....Additionally, the lead from master contactor to main bus (in my case a distribution fuse block) is not current protected.

This is my exact question. 95% of my background is automotive, racing etc. Just trying to figure out the differences. Having a HOT lead passing thru the firewall with no protection other than to turn the master off just had me scratching my head.

Are others the same way?

I do understand the B Lead for alternator side. Makes sense.

[Auburntsts]

Originally Posted by ty1295

This is my exact question. 95% of my background is automotive, racing etc. Just trying to figure out the differences. Having a HOT lead passing thru the firewall with no protection other than to turn the master off just had me scratching my head.

Are others the same way?

I do understand the B Lead for alternator side. Makes sense.

I think the rationale is the battery lead to the bus tends to be quite large -- in my case 2AWG. If I've got something drawing enough amps to fry that wire then I've got bigger problems than not having circuit protection on said wire.

[johnbright]

Bob Nuckolls' comments on un-fused battery and bus feeder wires culled from Aeroelectric List:

Sec. 23.1357 Circuit protective devices.

(a) Protective devices, such as fuses or circuit breakers, must be installed in all electrical circuits other than--
(1) Main circuits of starter motors used during starting only; and
(2) Circuits in which no hazard is presented by their omission.

(b) A protective device for a circuit essential to flight safety may not be used to protect any other circuit.

(c) Each resettable circuit protective device ("trip free" device in which the tripping mechanism cannot be overridden by the operating control) must be designed so that--
(1) A manual operation is required to restore service after tripping; and
(2) If an overload or circuit fault exists, the device will open the circuit regardless of the position of the operating control.

(d) If the ability to reset a circuit breaker or replace a fuse is essential to safety in flight, that circuit breaker or fuse must be so located and identified that it can be readily reset or replaced in flight.

(e) For fuses identified as replaceable in flight--
(1) There must be one spare of each rating or 50 percent spare fuses of each rating, whichever is greater; and
(2) The spare fuse(s) must be readily accessible to any required pilot.

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"I'll direct your attention to paragraph (a)(2) which speaks to battery feeders and feeders to main power distribution busses that are typically the largest wires in the airplane and are not subject to being "smoked" by downstream faults. You can comfortably leave out the fuse you've cited.

When wiring with truly "fat" wires (6AWG or larger) they're not at high risk for burning due to shorts or overloads. Take a look at the wiring diagrams for any single engine TC aircraft and you'll find that few if any will incorporate fuses or current limiters in these pathways. This philosophy is echoed in the FARS.

Faulted robust wires generally arc to ground and burn their faults clear. Further, they're easily installed with attention to mechanical details such that faults to ground are as probable as loosing one's propeller due to bolt failure.

The Z-figures are crafted with this philosophy in mind supported by a confidence in nearly 100 years of field history. I'll suggest that none of your fat wires should be smaller than 6AWG and that protection beyond what is illustrated in the Z-figures is no-value-added weight, cost and complexity.

Bob..."