Avionics bus relay NC or NO?

[bobmarkert]

My panel is going to paint this week so it won?t be long until Stein ships it to me (Stein, any chance of a sneak-peek once it?s back from paint?).

My question is: should the avionics bus relay be NC or NO?

At the Sport Air electrical workshop, the instructor said that if you use an avionics bus (that decision is made?. I will), you should use a NC relay. The logic is that if there is a power failure to the relay the NC relay would continue to power the bus. If you use a NO relay and there is a power failure to the relay you will lose your avionics bus. I asked if there would be a momentary power application to my avionics when I turned on main bus power (before the relay opened) and he said yes. He also said that some production airplanes (Piper?) use the same setup and you can actually hear a slight pop in the headset if you?re listening for it, but this will not harm modern avionics. All my buds at the airport that have an avionics bus have used a NO relay, not the suggested NC relay. Thoughts, input suggestions?
Thank You

[shuttle]

Bob,
I've used a high Amp-rated switch rather than a relay and so do not have to address your potential issue. Also reduces part-count. Any part removed cannot now fail.
Steve

[airguy]

Run through the "what-if" scenario a bit in your head, and I think you'll answer your own question.

You are flying along fat, dumb and happy when you hear a pop, smell electrical smoke, and some of your avionics go dark. How are you going to remove power from the avionics to stop the electrical fire? What are you going to do after that? There are a hundred different ways to wire it up - you just need to run through all the plausible emergency scenarios in your head to make sure you are OK with any of the failure modes.

I don't have a relay for mine, I'm powering the avionics bus directly through a switch-breaker, and each individual device has it's own breaker off that bus. Relays can and do fail, in my experience more often than switches. I can carry a spare switch in the airplane tool box and change it out a lot faster than I can change out a failed relay.

[N804RV]

Originally Posted by shuttle

Bob,
I've used a high Amp-rated switch rather than a relay and so do not have to address your potential issue. Also reduces part-count. Any part removed cannot now fail.
Steve

This!

The "spike" you're protecting your modern radios from is caused by the charging circuitry during engine start. Flipping the switch once the engine is running and electrical system voltage is stable is preferable.

Use a good switch, rated for 25%-30% higher D.C. current than your avionics suite's cummulatively rated current draw.

[deek]

Originally Posted by N804RV

This! Use a good switch, rated for 25%-30% higher D.C. current than your avionics suite's cummulatively rated current draw

Another vote for a quality switch and no relay. 4 homebuilts (several with extensive avionics) and NO avionics master relays.

[bkthomps]

Another vote for vertical power

[bobmarkert]

I have already purchased this SW to control the avionics bus relay:

Locking Toggle Switch SPST On/Off
Product ID: LT-001
MS24658-22D. Locked in both On & Off Positions. 20A Rating, screw terminals. Commonly used as Ignition Switches, Avionics Masters, etc.. or anywhere you need a switch that is cannot be accidentally actuated. Mounts in a 15/32 hole.

Based on the good advice here I will eliminate the relay. I now have to figure out if this SW will handle this load. If not, I'm back to a relay (I think...???).

Dual G3X screens
Dual ADAHARS
Dual magnatometeres
GTN 650 GPS/ RADIO
PAR 100EX RADIO/Audio panel
GMC 305 Mode Control Panel
GTX 23 remote transponder
GEA 24 engine sensor module
GAD 29 ARINC 429 interface
GDL 39 ADS-B receiver

Thanks to all for the sage advice

[WingsOnWheels]

On the fip side, I did use a relay. My peak current draw of the avi suite can be pretty high. However, it would be rare for all those items to be drawing their Max current at the same time. My relay is NO, the panel switch just grounds the coil (other side is connected to the relay incomming feed). I do have an alternate feed to my avionics buss from the battery buss, so that drove my decision.

-Lets say the AV relay fails, I turn on the alternate feed
-Main buss fail, alternate feed
-Short in the AV buss, trip AV beaker, SOL for the most part, can't use alternate feed, but EFIS has a battery backup.
-AV relay signal wire shorts to ground, relay stays on, no problem.
-AV relay signal shorts to Hot, same potential, no problem, use alternate feed.
-Short in the connection from the main bus to the AV breaker (short lead). Wire is toast, Likely blows FWF fuse on main buss feed. Turn on alternate feed.

As you can see from just those few senarios, if I didn't have the alternate feed I would have far more trouble. I used the relay since it kept my large wire runs short while still having my switch where I want them. Also, it reduced ammount of wire carrying power from one side of the panel and back. If one of those wires rubs through you can loose all of your avionics, less risk of that with a very short run.

[Bevan]

+1 I did the same as Colin for the same reasons.

Bevan

Originally Posted by WingsOnWheels

On the fip side, I did use a relay. My peak current draw of the avi suite can be pretty high. However, it would be rare for all those items to be drawing their Max current at the same time. My relay is NO, the panel switch just grounds the coil (other side is connected to the relay incomming feed). I do have an alternate feed to my avionics buss from the battery buss, so that drove my decision.

-Lets say the AV relay fails, I turn on the alternate feed
-Main buss fail, alternate feed
-Short in the AV buss, trip AV beaker, SOL for the most part, can't use alternate feed, but EFIS has a battery backup.
-AV relay signal wire shorts to ground, relay stays on, no problem.
-AV relay signal shorts to Hot, same potential, no problem, use alternate feed.
-Short in the connection from the main bus to the AV breaker (short lead). Wire is toast, Likely blows FWF fuse on main buss feed. Turn on alternate feed.

As you can see from just those few senarios, if I didn't have the alternate feed I would have far more trouble. I used the relay since it kept my large wire runs short while still having my switch where I want them. Also, it reduced ammount of wire carrying power from one side of the panel and back. If one of those wires rubs through you can loose all of your avionics, less risk of that with a very short run.

[WingsOnWheels]

On a side note: if you do decide to use a relay, don't forget to add a flyback diode on the coil, just like the FWF contactors. Some relays already have this built-in, most do not.