Essential Bus and how often do battery contactor's fail?

Av8rRob · #1 03-01-2019, 09:57 PM

Looking over the Nuckolls Z diagrams part of the reason for the Ess bus is in case of a battery contractor failure. How often does this happen? Anecdotally, looking at the finished panels that Stein has been putting out I see lots of panels with a backup alternator, lots with an avionics switch but almost none with a Ess power switch? Is this no longer necessary in todays avionics? Did Stein convince you of a power distribution plan or did you tell them what you wanted?

N804RV · #2 03-01-2019, 10:03 PM

Actually, if I understand it correctly, the purpose of the E buss is to protect against a failed alternator that has to be taken off line from causing an emergency condition where you lose all electrical power. Having alternate feed switch allows you to de energize the battery contactor, powering down the main buss and reducing the load to just the E buss items. By selecting E bus and then turning off the master switch, you open the battery contactor, which by the way draws about 1 amp all on its own.

The combination of an E buss with alternate feed, and an "Crowbar" over voltage protection circuit will prevent either mode of alternator failure from becoming a total electrical failure.

Av8rRob · #3 03-01-2019, 10:18 PM

Yes, a few reasons mentioned in the book but I was wondering about the battery contractor failure specifically.

N804RV · #4 03-01-2019, 10:24 PM

I don't think they fail very often. I didn't get the impression that that was a primary concern. My impression the whole discussion on contactors was to explain the need for the diodes that are wired in parallel to the coil to prevent the flyback voltage spikes.

rocketman1988 · #5 03-02-2019, 03:24 AM

Reference the section in the book on contactors. I think the phrase that was used was contactor failure is ?exceedingly rare?...

rlmccarter · #6 03-02-2019, 06:19 AM

Pages 11-7 through 11-15 of AEC v12 list several failure modes of contactors from contact closure welding (addressed with a diode across the coil terminals) to what Nuckolls calls "environmental abuse" on p. 11-13. He goes as far as saying he would "happily use" a type I and carry spares.

Nuckolls also says "In-flight contactor failure is exceedingly rare" in http://forums.matronics.com/viewtopi...or+reliability.

Bob

maniago · #7 03-02-2019, 07:21 AM

I agree that failures are rare. But rare happens. In running my electrical gear I noticed that the contactors got pretty hot (~130F), tho with in design parameters. Still, hot electricals make me itchy, so I put a heat sink on all my contactors. That took them down about 5F in open air as I remember. So I put a small 1x1 computer fan on each heat sink and now they all run mid 90F. Will it really matter when flying and lots of air moving thru the firewall area? Probably not. But it cant hurt.....

rocketman1988 · #8 03-02-2019, 07:54 AM

Someone said that the likelihood of a contactor failure was about as much as a change in waterline on an aircraft carrier when a fly lands on it...

Captdf · #9 03-02-2019, 08:45 AM

In 44 years of general aviation and almost 30,000 hours I've had two master contactor failures. One in IMC conditions.

One of these was a failure of the master switch, but the result was the same as a contactor failure.

rv6ejguy · #10 03-02-2019, 08:52 AM

A friend had one fail during taxi. Rare but can and does happen. Best to have a backup method of getting power to essentials, especially if running an electrically dependent engine.