DPDT Toggle switch for BAT and 2 Alternators?

To keep things simple I was thinking about using the Honeywell DPDT on/on/on toggle switch for switching on BAT + Alternator 1 & Alternator 2 (aux alt) at the same time. The design would include one main battery and the IBBS standby bat.

Switch position logic:

1. BAT + ALT 1 & ALT 2
2. BAT
3. OFF

To manually switch of either ALT 1 or ALT 2 I would have to pull either ALT1 or ALT2 field CBs positioned after the main switch. I understand 2 possible design problems:

1. The wire going to the main switch feeding the ALT fields has no protection
2. The switch would be a single point of failure

-any other problems I am missing with this setup?

thanks!
https://www.steinair.com/product/locking-toggle-switch-dpdt-ononon/

I used 2 switches:

Off-Batt-Alt1
Off-Ebus-Alt2

A nice locking toggle switch for Master(s) is always nice. I?ve used various brands, right now I?m using this one: https://www.alliedelec.com/nkk-switches-s6al/70192232/

I use the same type of switch for ignitions as well as control of battery mounted 30 amp relays to feed the separate left and right Avionics busses.

I recommend you consider a different approach for the two alternators. As you need a breaker anyway, skip the switch and just use a switch breaker: http://www.aircraftspruce.com/catalog/elpages/pbcircuitbrkr2.php

You will want a separate switch breaker for each alternator - that way you can test the standby alternator from time to time. Normal ops just leave the alternator breakers on. There is no need to switch it on and off like you did with a 1960 Cessna split rocker switch.

Carl

That Honeywell locking toggle switch is the exact same one I use for a master switch, with the circuits wired Off/Batt only/Batt+Alternator.

It's very solid and robust and fits the purpose of the master being the one switch I didn't want inadvertently knocked on, or off. That wiring also prevents the alternator being run without the battery. It's a good choice for a master switch.

However I'm not certain about having both a main and aux/emergency alternator on the same switch. The issue isn't reliability or single-point-of-failure (it's a very high quality switch), but more to do with the general principle of physically separating the operation of main systems, and backup/emergency systems. I like crabandy's philosophy above.

Mike, you got it. The 2 switches keep the systems separate and very easy for checklist flow -flying- emergency ops. Both switches on for normal ops, if (already had and flew halfway across the country on my backup alt) my volts/amps alarm and turn Batt/Alt1 off to load shed and keep flying on the Ebus/alt2.

crabandy said:

Mike, you got it. The 2 switches keep the systems separate and very easy for checklist flow -flying- emergency ops. Both switches on for normal ops, if (already had and flew halfway across the country on my backup alt) my volts/amps alarm and turn Batt/Alt1 off to load shed and keep flying on the Ebus/alt2.

Click to expand...

Crabandy, does it mean that Alt2 feeds the Ebus only?

Not sure how I would implement this with the IBBS backup bat..

Can you share your schematics?

thanks

Thank you guys,

after some head scratching, I decided to drop the backup battery and go with a single bat and 2 alternators architecture.

I like crabandy's idea..

Off-Batt-Alt1
Off-Ebus-Alt2

I've wired quite a few systems and recently switched my own setup to:

Master=off/bat/bat-alt (this is pretty standard and hasn't changed)

Backup alt if installed: no backup alt switch, always on with the bat bus. This keeps things really simple from an operational standpoint, if the main alt goes off line the backup automatically takes over but at a lower voltage setting than the primary. The voltage alarm will let you know that the backup is running as the voltage is now 13.5 vs the normal 14.5.

This also has the advantage of the backup supplying current to the system if the load increases beyond the capacity of the main alt (I use a B&C 40A main and B&C 20A backup).

The only down side is the backup field is powered on when the bat switch is on, so when doing ground mntc/testing I pull the backup alt field CB to reduce the battery load.