main relay (battery contactor)

[EricBrown]

Question for any electrical gurus: All modern light planes have a starter contactor and/or solenoid to handle the starter current. They also have a switch to turn off the regulator/alternator, which could be putting out a large number of amps.

So the question is:
If the purpose of the main relay is to avoid switching high current with the master switch, but you turn off all your lights/pumps/avionics/etc before you turn off the master switch, then why would you need a main relay? Likewise, if you don't turn on your lights/pumps/avionics until after you turn on the master switch and get the engine running, then the main relay is pointless, no?

[mburch]

Originally Posted by EricBrown

why would you need a main relay?

Among other reasons, in case of smoke.

mcb

[gasman]

The largest load on the main relay is the start load. After that, you need a way to remove all of the load from the battery if needed. Like a stuck starter relay or elect. fire.

[EricBrown]

Stuck starter relay not good. Thanks.

[ken westlund]

Eric, The purpose of the main relay, in my oppinion, is to completely isolate the cabin/cockpit of any electrical power. The main relay is closed with a ground signal, this means that the only power on the interior side of the firewall is the signal wire coming from the master relay, which when grounded closes the relay. Therefore, all power will be elliminated to all sources if the master relay is opened. K

[EricBrown]

Aeroelectric shows one schematic without a main relay, and I've seen two others. A master switch (30-amp breaker type) could be used to manually switch the main buss, but a stuck starter solenoid would be problem. So you use two starter solenoids, both controlled from the starter button/switch?