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

Av8rRob

Well Known Member
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?
 
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.
 
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Yes, a few reasons mentioned in the book but I was wondering about the battery contractor failure specifically.
 
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.
 
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Contactors

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

30b311t.jpg
 
I think

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...
 
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.
 
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.
 
Real redundancy-

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?

The whole idea of an essential loads bus(ELB) is that no failure modes should separate it from available battery power, so it is crucal that there be no switching or contactors of any kind between it and the battery. The simplest ELB is the battery post itself, but all should be located as close as possible to the battery as they are always “hot”. Truely essential loads like electronic ignition and electric primary fuel pumps sould always connect directly to ELB’s and never to the main bus. Each essential device must be breaker-protected, switch-connected to the ELB, but unaffected by master switch position. Yes, you have to manually turn off each essential load at shutdown.

My RV has twin elecronic ignitions AND twin primary electric fuel pumps. I run a single B&C alternator so have identical-twin 16AH EarthX batteries. Each battery has a contactor to the main bus and its own ELB. It is completely symmetrical, so the master switch is an on-off-on switch allowing either but not both batteries to act as primary(*-see note). Each essential load is also powered through an on-of-on switch protected by two breakers that can route power from either but not both ELB’s. The batteries, the ELB’s, the essential load switches and breakers, and the contactors are all located in very close proximity in a console separating the pilot and passenger’ feet, with the switches in a vertical stack above the fuel selector valve. There is a fifth on-off-on switch there energizing a sub-bus of breakers feeding alternate power inputs on several Garmin devices with dual power inputs, allowing selected access to avionics and communications with the main bus shut down. This system is simple, symmetrical, and fool-proof. There is no “emergency mode”, just alternate, normal, everyday operation of essential loads. I can fly an entire tank of gas with one fully charged battery.- Otis

(*)- There is one exception- A guard-protected, spring-loaded to “OFF”, DPST switch will activate both main bus contactors when held “ON”. This allows switching between the two batterys in flight without interrupting main bus power, and it allows the option of starting the engine with both batterys working. Both batteries, the added busses, breakers, wiring and contactors all together weigh less than one PC680, but have four times the cold cranking Amps.
 
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The whole idea of an essential loads bus(ELB) is that no failure modes should separate it from available battery power, so it is crucal that there be no switching or contactors of any kind between it and the battery. The simplest ELB is the battery post itself, but all should be located as close as possible to the battery as they are always ?hot?. Truely essential loads like electronic ignition and electric primary fuel pumps sould always connect directly to ELB?s and never to the main bus. Each essential device must be breaker-protected, switch-connected to the ELB, but unaffected by master switch position. Yes, you have to manually turn off each essential load at shutdown.

My RV has twin elecronic ignitions AND twin primary electric fuel pumps. I run a single B&C alternator so have identical-twin 16AH EarthX batteries. Each battery has a contactor to the main bus and its own ELB. It is completely symmetrical, so the master switch is an on-off-on switch allowing either but not both batteries to act as primary(*-see note). Each essential load is also powered through an on-of-on switch protected by two breakers that can route power from either but not both ELB?s. The batteries, the ELB?s, the essential load switches and breakers, and the contactors are all located in very close proximity in a console separating the pilot and passenger? feet, with the switches in a vertical stack above the fuel selector valve. There is a fifth on-off-on switch there energizing a sub-bus of breakers feeding alternate power inputs on several Garmin devices with dual power inputs, allowing selected access to avionics and communications with the main bus shut down. This system is simple, symmetrical, and fool-proof. There is no ?emergency mode?, just alternate, normal, everyday operation of essential loads. I can fly an entire tank of gas with one fully charged battery.- Otis

(*)- There is one exception- A guard-protected, spring-loaded to ?OFF?, DPST switch will activate both main bus contactors when held ?ON?. This allows switching between the two batterys in flight without interrupting main bus power, and it allows the option of starting the engine with both batterys working.


Wow. I implemented Z14....bout as simple as a two buss architecture gets.
 
Wow. I implemented Z14....bout as simple as a two buss architecture gets.

But maybe not so foolproof? Can you inadvertantly run down battery power after an unnoticed alternator failure? Did you verify function and charge level of the backup battery(if you have one)? Things get a bit more serious after you have eliminated the engine driven fuel pump!- Otis
 
One time

I have had a Cole Hersee battery contractor fail. But it failed closed. I had, at the time a separate avionics switch so it was a non-issue. Nevertheless, it was a frustrating experience to swap out because of the way it was mounted. Needless to say, I used a different method to mount the second one. I also added an EBUS feed contactor.
 
But maybe not so foolproof? Can you inadvertantly run down battery power after an unnoticed alternator failure? Did you verify function and charge level of the backup battery(if you have one)? Things get a bit more serious after you have eliminated the engine driven fuel pump!- Otis

I have my electrical backups; but I also count on myself as part of the backup system, not just a blob along for the ride.

In the end, as long as youre happy with the way you designed your system for the way you fly, then youre as safe as you can be. As it should be for all of us.
 
I have my electrical backups; but I also count on myself as part of the backup system, not just a blob along for the ride.

In the end, as long as youre happy with the way you designed your system for the way you fly, then youre as safe as you can be. As it should be for all of us.

Well, As one who has been flying for 39 years, I?ve had my share of humbling experiences. Pile on enough stress and every pilot will, at some point, be transformed into a real ?boob?. If you can design redundant systems that cannot be defeated and that do not require any sophisticated emergency interventions on the part of the pilot, you have gone a long way toward intrinsic safety.
 
If you look at when Bob designed the E-bus drawings, there weren't EFIS's back then and if there were, they were brand new technology in homebuilt aircraft. Thus there was a need for an E-bus.

I argue that with today's technology, backup batteries on everything, all-in-one units like the Dynon Pocket Panel, there is no longer a need for backup alternators, batteries, etc

In fact, the complexity of some of the panels people are putting in their planes are probably creating bigger risks of failure than had they keep things simple.
 
Well, As one who has been flying for 39 years, I?ve had my share of humbling experiences. Pile on enough stress and every pilot will, at some point, be transformed into a real ?boob?. If you can design redundant systems that cannot be defeated and that do not require any sophisticated emergency interventions on the part of the pilot, you have gone a long way toward intrinsic safety.

Well said Otis! +1 for sure.
 
Or consider a more exotic, expensive but extremely reliable switch such as a Gigavac MX11S**. As a bonus the hold current is a tiny fraction of the switching current, something that might be interesting if minimizing continuous current load is a goal.
 
I had a battery contactor fail in an 8A while IFR in VMC. Caused an entire electrical failure. Luckily the place I dropped into (Beckley, WV) still had a mech on duty. We diagnosed the problem, cannibalized a battery contactor from a 172 and I was on my way home about an hour later. After that I packed a spare contactor in the tool bag. Airplane had pretty standard Vans elec system. Single PP alt, single Odysee 680. My 14 I recently completed has two EarthX, each with their own batt switches and contactors. Two alternators as well. Probably overkill, but peace of mind and might allow me to get to the field of my choice if/when I experience another single failure.

Rick
 
Unfortunately

?...If you can design redundant systems that cannot be defeated...?

Unfortunately, you cannot. If it is a mechanical system, it can be defeated.

The best you can do is minimize the risk involved in a failure... and there are many, many opinions on how best to accomplish this...
 
Or consider a more exotic, expensive but extremely reliable switch such as a Gigavac MX11S**. As a bonus the hold current is a tiny fraction of the switching current, something that might be interesting if minimizing continuous current load is a goal.

Gigavacs are dabomb! If you can stand the price of admission. I looked at them, and eventually decided that for normal use, I have my engine driving 2 alternators capable of 100amps between them - more than a 1970s household. I can stand the 1.5a draw of the HerseeColes. And if SHTF, one HC pulling on my two EarthX 680Cs keeping my A-buss alive. And if SHTF and the balloon goes up, my MiniX with built-in backup, ought to get me on the ground.
 
Gigavacs are dabomb! If you can stand the price of admission. I looked at them, and eventually decided that for normal use, I have my engine driving 2 alternators capable of 100amps between them - more than a 1970s household. I can stand the 1.5a draw of the HerseeColes. And if SHTF, one HC pulling on my two EarthX 680Cs keeping my A-buss alive. And if SHTF and the balloon goes up, my MiniX with built-in backup, ought to get me on the ground.

Heh... if SHTF and the balloon goes up, haven't heard that in a long time.

I've got dual alternators and a single ETX900 battery, with Dynon Skyview having it's own battery backup for the screen as well. In the off chance that I happen to loose both alternators in a single flight, or have a major electrical snafu that requires simply hitting the master, then the Skyview will give me a minimum of 45 minutes and I've got a dedicated emergency hot wire to power my fuel pumps (electric only, no engine drive) to keep the prop turning for close to 2 hours.

If my situation gets worse than that, well then I just hope Jesus ain't mad at me today.
 
If my situation gets worse than that, well then I just hope Jesus ain't mad at me today.

Haha! I forgot to mention that I also have the min set of steam gauges and magnetos, but this is a thread about electrical stuff.......
 
Contactor failure

While contactor failure itself is rare there is a little more to think about. Any issue with the master switch, related wires, terminals, crimped connections ect can cause the master contactor to open and un-power everything downstream. The primary alternator will probably loose field power and go off line as well. Back when I was a shop rat I saw most of these problems first hand. For my plane a E Buss with its own switch and alternate wire path will allow access to whatever stored amps are in the main battery. I?m planning 2 screen ifr panel, 1 B&C primary alternator, a back up battery and a G5 with its integral back up battery.

Don Broussard
RV9 Rebuild in Progress
57 Pacer
 
I read most of the alternator reliability poll a couple of times. My build has a single B&C belt driven alternator, and two, switchable B&C voltage regulators. According to the poll, and my own experience, the vast majority of alternator failures are actually VR failures.

So I chose to carry 2 VR's, instead of 2 alternators. I haven't decided on one or two batteries yet. I might start with 1 ETX900, and one 1 lbs scooter start battery, also from Earthx. The second battery would just be a "load" for the alternator to help stabilize voltage in case the primary battery (or BMS) goes offline.

My build is VFR, but has SDS EFI. So I do need to juice to keep flowing.
 
Big flashing light

But maybe not so foolproof? Can you inadvertantly run down battery power after an unnoticed alternator failure? Did you verify function and charge level of the backup battery(if you have one)? Things get a bit more serious after you have eliminated the engine driven fuel pump!- Otis

The B&C voltage regulator has an output to flash a light when the buss voltage drops below 13. That gets your attention as soon as the alternator goes offline so you can start conserving electrons by turning stuff off. One could also set up a low voltage alarm on an EFIS.

Ed Holyoke
 
The B&C voltage regulator has an output to flash a light when the buss voltage drops below 13. That gets your attention as soon as the alternator goes offline so you can start conserving electrons by turning stuff off. One could also set up a low voltage alarm on an EFIS.

Ed Holyoke

And there is a valuable point for discussion and/or dissension. I have glass screens that monitor voltage and amperage draw from the battery with alerts for off-nominal condition, and with dual alternators of which only one is on at any time. The idea is that I would get an alert from my EFIS if/when my primary alternator goes TU, and then I manually activate my backup alternator. I don't WANT it to be automatic without pilot action, because I want to absolutely ensure the pilot is in the loop and KNOWS the primary alternator is on vacation, and take positive action to bring the backup online. Then, should the backup fail as well, the EFIS will again do its job to alert the meat servo that he's on battery power only and the clock is ticking to find a place to put down.

Some things should be automatic backup without pilot action, with an alert after the fact - but I want my airplane to require positive pilot intervention to make certain the pilot is aware of the problem. YMMV.
 
Keep in mind that the battery is not powering the electrical system, the alternator is. And many alternators will continue to function if the battery is disconnected. Mine did. My PC680 failed internally due to a broken weld. The affect was identical to a battery contactor failure. Everything electrical kept working, but the voltage fluctuated causing alarms to go off on the Dynon D-180. Closing the throttle for landing caused very low voltage, not desirable if IFR or at night.
 
Keep in mind that the battery is not powering the electrical system, the alternator is. And many alternators will continue to function if the battery is disconnected. Mine did. My PC680 failed internally due to a broken weld. The affect was identical to a battery contactor failure. Everything electrical kept working, but the voltage fluctuated causing alarms to go off on the Dynon D-180. Closing the throttle for landing caused very low voltage, not desirable if IFR or at night.

Yeah, I have heard that as well. I just don't know how well. My system will be drawing about 20 amps, most of that for the EFI. Maybe that is enough of a load I don't need the second battery. I will try it both ways on the ground.
 
And there is a valuable point for discussion and/or dissension. I have glass screens that monitor voltage and amperage draw from the battery with alerts for off-nominal condition, and with dual alternators of which only one is on at any time. The idea is that I would get an alert from my EFIS if/when my primary alternator goes TU, and then I manually activate my backup alternator. I don't WANT it to be automatic without pilot action, because I want to absolutely ensure the pilot is in the loop and KNOWS the primary alternator is on vacation, and take positive action to bring the backup online. Then, should the backup fail as well, the EFIS will again do its job to alert the meat servo that he's on battery power only and the clock is ticking to find a place to put down.

Some things should be automatic backup without pilot action, with an alert after the fact - but I want my airplane to require positive pilot intervention to make certain the pilot is aware of the problem. YMMV.

Yup. Gotta immediately and for sure know when battery is all you have left. And you gotta know for sure how much battery. Capacity check at annual is the only way you can be sure of your battery's condition. Need to do an alternator out load calculation so you can run your cap check at a similar load and get a real life time to all gone number in minutes. Set a stopwatch when the battery only ops begins and land with some reserve.

Ed Holyoke

Ed Holyoke
 
“...If you can design redundant systems that cannot be defeated...”

Unfortunately, you cannot. If it is a mechanical system, it can be defeated.

The best you can do is minimize the risk involved in a failure... and there are many, many opinions on how best to accomplish this...

Depends upon what you mean. Obviously, you can always be really stupid and shut your airplane off in flight. What I mean by cannot be defeated is that there is no combintion of pilot inputs like switch positions that can render the redundant feature unavailable. That would describe my electrical system. It is also pretty much impossible to inadvertantly discharge both batteries after an alternator failure.- Otis
 
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The B&C voltage regulator has an output to flash a light when the buss voltage drops below 13. That gets your attention as soon as the alternator goes offline so you can start conserving electrons by turning stuff off. One could also set up a low voltage alarm on an EFIS.

Ed Holyoke

That is true, but pilots have on occasion shown an incredible talent for missing glaring warning devices- ususlly under conditions of elevated stress.- Otis
 
...The idea is that I would get an alert from my EFIS if/when my primary alternator goes TU, and then I manually activate my backup alternator....Some things should be automatic backup without pilot action, with an alert after the fact - but I want my airplane to require positive pilot intervention to make certain the pilot is aware of the problem. YMMV.

That's entirely sensible. However, note there's nothing very urgent about an alternator failure. The battery supplies electrons without pilot intervention. The pilot action merely re-configures for extended duration.

The urgent things should always be automatic, or if not fully automatic, the necessary action should be a trained response common to the pilot community. Urgent things are mostly in the category of "Keep the fan turning". The test is the classic departure scenario; engine goes silent 300 feet above the trees.
 
?Protect Essential? ... haven?t heard that one in a few years.

Fastest way there was to bust an FE checkride. Fun times!
 
The whole idea of an essential loads bus(ELB) is that no failure modes should separate it from available battery power, so it is crucal that there be no switching or contactors of any kind between it and the battery. The simplest ELB is the battery post itself, but all should be located as close as possible to the battery as they are always ?hot?. Truely essential loads like electronic ignition and electric primary fuel pumps sould always connect directly to ELB?s and never to the main bus. Each essential device must be breaker-protected, switch-connected to the ELB, but unaffected by master switch position. Yes, you have to manually turn off each essential load at shutdown.

My RV has twin elecronic ignitions AND twin primary electric fuel pumps. I run a single B&C alternator so have identical-twin 16AH EarthX batteries. Each battery has a contactor to the main bus and its own ELB. It is completely symmetrical, so the master switch is an on-off-on switch allowing either but not both batteries to act as primary(*-see note). Each essential load is also powered through an on-of-on switch protected by two breakers that can route power from either but not both ELB?s. The batteries, the ELB?s, the essential load switches and breakers, and the contactors are all located in very close proximity in a console separating the pilot and passenger? feet, with the switches in a vertical stack above the fuel selector valve. There is a fifth on-off-on switch there energizing a sub-bus of breakers feeding alternate power inputs on several Garmin devices with dual power inputs, allowing selected access to avionics and communications with the main bus shut down. This system is simple, symmetrical, and fool-proof. There is no ?emergency mode?, just alternate, normal, everyday operation of essential loads. I can fly an entire tank of gas with one fully charged battery.- Otis

(*)- There is one exception- A guard-protected, spring-loaded to ?OFF?, DPST switch will activate both main bus contactors when held ?ON?. This allows switching between the two batterys in flight without interrupting main bus power, and it allows the option of starting the engine with both batterys working. Both batteries, the added busses, breakers, wiring and contactors all together weigh less than one PC680, but have four times the cold cranking Amps.

I?m quoting myself here for a couple of reasons. First is to stress that this setup, using twin and symmetrical main batteries, each capable of full aircraft operation, is a departure from convention that is made possible by powerful, lightweight lithium batteries. It guarantees an ample, uninterrupted supply of power for electron-dependent aircraft with little or no pilot intervention. I?ll be covering this in due course with a dedicated thread including photos and a wiring diagram.

Second is to describe a sublety not previously mentioned for simplicity sake. The On-Off-On master switch is actually a DPDT, whose function makes it impossible to discharge both batteries inadvertantly in the event of an alternator failure, but also also to guarantee that the battery acting as ?backup? at any given moment remains fully charged even if some of the essential loads are using it in normal flight.

One pole of the master switch energizes the contactor for either but not both batteries to the main bus. It absolutely does not matter which battery is serving as ?primary? for a given flight. The second pole energizes one but not both of two lightweight solenoid switches that feed main bus voltage through a 25Amp Schottkey diode, protected by a 20Amp breaker, to the OPPOSITE battery serving as backup.

This will allow the ?backup? battery to be recharged if it was used in tandem with the other during engine start, and to support any essential load that happens to be using the ?backup? battery during flight. SOP will be to have one of the two electronic ignitions connected to it, but that is a mere 1.5Amp draw in my case.
 
While contactor failure itself is rare there is a little more to think about. Any issue with the master switch, related wires, terminals, crimped connections ect can cause the master contactor to open and un-power everything downstream. The primary alternator will probably loose field power and go off line as well. Back when I was a shop rat I saw most of these problems first hand. For my plane a E Buss with its own switch and alternate wire path will allow access to whatever stored amps are in the main battery. I’m planning 2 screen ifr panel, 1 B&C primary alternator, a back up battery and a G5 with its integral back up battery.

Don Broussard
RV9 Rebuild in Progress
57 Pacer

My only electrical failure in my 40+ years of flying was my $4.79 battery master switch on my -8.
 
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There I was

My only electrical failure in my 40+ years of flying was my $4.79 battery master switch on my -8.

I was flying a Seneca at night when everything went dark due a master switch failure. The alternators went off line as well. It was IFR, but at the time I was in the clear above a undercast. I had recently flown over a airport that I saw thru breaks in the clouds so was relatively low stress to do a 180 fly back to there, free fall the gear and land. It was a controlled field and the controllers figured out I was having troubles when I turned around and they couldn?t contact me.

Don Broussard
RV9 Rebuild in Progress
57 Pacer
 
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