On Nov 1, 7:37 am, "Peter Dohm" wrote:

At the moment, I am not entirely sure why a "typical" homebuilt would use a
master contactor and I suggest that you take a look at what the professional
designers may have done. For example, to the best of my recollection, the
Cessna 150 and 152 and also the Piper Tomahawk had starter contactors (a/k/a
solenoids) and a had master breakers that also functioned as switches; but
did not have master contactors--and I really have difficulty understanding
why a well designed aircraft in that size and weight range would need one.

I suggest that you determine whether your battery is located in an unusual
way and then ask a mechanic what was used in reasonably similar factory
built aircraft. For example: I would certainly expect a master contactor
in a Piper Cheyene; but I would not extpect to find one in a Cherokee 140.

It's there so the battery can be totally isolated from the
electrical system in case of electrical fire or forced landing. The
Cessna 150, I can tell you, did have a master contactor, as did all
the 172s including the nice new 172S G1000 one in our fleet. Every
airplane I ever worked on has had a master contactor except for my
1946 Auster, which had a huge toggle switch on the panel, and my
Jodel, whose tiny battery powers a handheld radio. You DO want to be
able to cut off all electrical power in flight if it becomes
necessary, because getting out and running is hardly an option when
the smoke and flames start up, or if a forced landing is going to tear
up metal and start shorting stuff and making sparks around spilled
fuel.

At the same time, I don't understand why we don't have a solid-state
device for this application by now.

Dan