Why incorporate a master relay ?
 

I have been corresponding an RV builder regarding system plan for dual EI. I have some schematics from Dan H suggesting direct battery connections for EI #1 and EI#2, independent of master relay. Recognizing every component eliminated is one less failure point, what is the master relay for ?

If avionics, com, strobes, landing lights etc. are all on a separate switch, why have a master relay to energize a "main" buss? Yes, the alternator field would require a switch, not a breaker, and the battery buss conductor should be a safe "fat wire" per Nuckols.
It appears to me 3 switches would control power and ignition:
IGN #1
IGN #2
Alt field
So, what am I missing ?

Better Question
 

Why not eliminate the avionics master, and just use the master to turn on the avionics?

With an all glass panel, I cant figure out why I need an avionics master and a master relay. aren't they doing almost the same thing with an all glass panel?

A lot of options. I use a master relay on each battery, and I use an avionics master on each battery as well - the avionics masters go directly to one or the other battery, not via either master.

Why:
- Master relays are needed to isolate fat wires close to the battery(s) in case of a fault.
- All non-panel loads are on the master buss. This provides a rapid way to isolate a fault or to dump loads. Master relays also tend to be one of the larger loads if only on battery power.
- Separate avionics master relays (100ma draw, 30 amp contact relays) each provide power to half the panel (e.g. EFIS 1, Comm 1 on battery #1, EFIS2, Comm 2 on battery #2). This way it would be hard to lose both sides of the panel on a single fault.
- To take it one level further, I can power both sides of the panel from one battery as one backup mode.

I use two identical batteries (PC-625s).

So assume something will go wrong. Determine the result and then decide if you can live with it. If not, then design in the needed mitigation.

Carl

Master switch/relay: Just a convenient way to cut all non-essential power drain.

By essential I mean stuff that needs power to keep engine running.

Of course there is a gray area. For me I choose to also put the following items on the always hot engine bus:
Engine monitor
Oxygen sensor
Backup EFI pump
Left-to-right tank transfer pump

Engine controller A, B, monitor and O2 sensor each have their own fuse and are turned on by the same 4-pole switch.

Leading coils and trailing ignition coils each have their own fuse and switch.
Primary and secondary fuel injectors each have their own fuse and switch.

Why all these switches? During engine run-up I can check these items separately. If an injector hangs up open or closed and cause rough engine running I can turn its opposite off.

Finn

Larry,
In case of smoke or fire in the cockpit, the master can be shut off.
In case of an imminent forced landing, shutting off the master will minimize the chances of fire after a collision.
A master switch makes it easy to shut off all electrical loads after flying to prevent the battery from discharging.

John S
You are right. The avionics master switch is an unnecessary failure point.
Read what Bob Nuckolls has to say about it.
http://www.aeroelectric.com/articles/avmaster.pdfhttp://www.aeroelectric.com/articles/avmaster.pdf

Bob clearly hasn't paid Garmin's flat repair fee to fix a broken on-off switch on a piece of avionics!

The problem with that article is that many of todays avionics don't have power switches.

I feel like this is the main point. Here, the master acts as a device to reliably kill all power to the electrical subsystems from a point physically close to the power source. If the big cable carrying electricty from the battery to the main bus gets shorted somewhere along its run, there's only one way to de-energize it, that's the master. Generally it seems in small aircraft there isn't a main (automatic) circuit breaker close to the power source.

Otherwise, it's a convenient way to shut everything down and make sure there aren't any drains on the battery when the aircraft is not in use.

Ever since the advent of modern avionics that are tested to RTCA DO-160D or later for voltage spikes AND the use of diodes across relay coils, the need to turn avionics off for starting has become an old habit that is slow to die. As was pointed out, all of the flat screen equipment has no way to turn it off, simply because there is no reason, and you need the EMS diesplay on for engine start anyhow. Modern equipment is designed to handle the relay switching spikes and more.

I design electrical systems for certified aircraft and we no longer consider avionics masters a good thing to do.