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Avionics Master fused separtely?

polishpilot

Well Known Member
Just wondering if Avionics Master should be fused (or circuit breaker) separately from the individual systems it controls. Say I have 20 amps total draw from the individual systems; do I also install a 20 or 25 amp fuse between the power switch and the avionics buss?

Thanks for any help.
Jim
 
I say NO!

A serial avionics fuse just adds a single point of failure for your whole avionics suite. Though it's a low failure (except overload of course) mode device, it still could take everything off line with ONE overload (grounded power wire for example :mad:).

YMMV. Have Fun
 
It's my understanding, if you plan to have an "avionics switch" you should have a fuse upstream of the switch rated at or below the rating of the switch. A high current relay solves this but I'm not a fan of a switch PLUS a relay.

If I'm wrong, I likely over complicated my panel design.
 
Last edited:
OOPS!

Spoke too soon :eek:. Just looked at my diagram and Bob Nuckolls' tome and I do indeed have such a breaker in my system. Trick is, I have TWO additional ways to feed the endurance bus (basically my avionics); directly off either the left OR right battery bus (always hot). This allows me to protect the long wire with the breaker we are talking about. This wire goes all the way from the right side of my panel (main breaker bus)to the left side (avionics master switch) and back to the right side (endurance buss). This wire is way too long to have unprotected with either a fuse or a breaker...

Hope this makes sense.
 
Remember breakers protect the wiring, not the radios. If there is a large copper buss bar (only) the CB can be very large. But no smaller than the load plus 20%.
 
I say yes

I'll illustrate with a couple of pictures.
Avionics switch (10 amp SCB) in my RV-10 serves avionics bus. 16 G wire
protected with 10 amp SCB. You need a switch anyway if you want an avionics switch, might as well use a SCB. Total draw from the TX Com 1 and 2 plus the audio panel not anywhere near 10 amps. If it ever trips I can go to 15 amps on that 16G wire and still have the supply wire well protected.
Same is true for the instruments except here I have a pullable CB only.
I need the EFIS and Engine monitor to come on when I turn the master on
and I can still pull it if I needed to unload the system in an electrical emergency.

IMG_2258.JPG


IMG_1054.JPG


IMG_2257.JPG
 
Thanks for the information guys. I like the thought of a CB on the main feed to the avionics switch. I DON'T like the the idea of that CB tripping and losing all the Avionics. I would be concerned enough if that happened to get on the ground quick. The back-up power source is something to think about.

I appreciate the input.
Jim
 
Keep in mind that if the avionics CB trips you most likely have some kind of feed short and will need to cut power to all the avionics on that bus anyway. If it is a problem with an individual component only that CB should trip leaving the remaining avionic powered.
 
Jim
Yes, I thought about that. If that Avionics main CB blew, it would probably be a good time to find a good landing strip. All of the components will be individually fused; and the com, nav, autopilot, and EFIS are switched.
 
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