E-Bus alternate feed: switch or relay?

[N941WR]

Originally Posted by KayS

Hi All,

didn't find an answer to my question in past threads... I want to install an E-Bus (name it essential, endurance, emergency or whatever) in my electrical system. The devices i intend to put on the E-Bus will draw well below 20 amps.

...

Cheers
Kay

I just checked some pictures i have taken of my SkyView in flight and it shows i was drawing only 6 amps.

With backup batteries on pretty much everything, do you really need an E-bus?

Just asking.

[tim2542]

I don?t know your typical mission, but for most of us Sunday flyers I also agree with reassessing the need for an E bus. I intalled one and have never used it once or known of anyone else ever using one. I also had an AUX bus for brown out. After flying several hundred hours I now think all that redundancy, for me, was silly and I always regretted adding the complexity and weight.
Keeping it simple and light is very good advice for most of us.


Tim Andres

[rocketman1988]

Something that no-one has mentioned is that using a relay will allow you to keep the big wire runs short and use a small wire for control.

For example:

If you are switching 20 amps, you will be using a 10 or 12 AWG wire, using a switch, those relatively large wires must run all the way to the switch.

Using the relay, you can place the relay near the load, and run a much smaller wire, like 22 AWG to the switch as a control wire.

Obviously, there are multiple ways to accomplish the switching task; the switch and relay are both tools...use the best tool for the job at hand.

[lr172]

Originally Posted by Walt

With todays modern glass panels that generally have dual power inputs for most critical items, and back-up batteries to support them, what advantage does a ESS buss have?

The way I see it the ESS buss adds both parts count and failure points, especially the diode which if it fails likely kills most of your avionics (perhaps at a critical time when power failure could cause high workload like IFR). Don't think diodes fail... think again.

I think the orginal premise for the ESS buss was easy load shed for a VFR aircraft, not so sure it's a great feature in today's typical glass panel design.

Walt,

Don't these modern systems just put the diodes inside the box, with theoretically the same risk?. I am not an electrical expert, but I thought there needs to be diodes whenever you have two battery sources to prevent cross-draining.

I am discussing an ESS buss fed from a second battery. I tend to agree that a second buss, just to have two, doesn't make that much sense.

Larry

[lr172]

Originally Posted by rocketman1988

Something that no-one has mentioned is that using a relay will allow you to keep the big wire runs short and use a small wire for control.

For example:

If you are switching 20 amps, you will be using a 10 or 12 AWG wire, using a switch, those relatively large wires must run all the way to the switch.

Using the relay, you can place the relay near the load, and run a much smaller wire, like 22 AWG to the switch as a control wire.

Obviously, there are multiple ways to accomplish the switching task; the switch and relay are both tools...use the best tool for the job at hand.

I am going through this right now. I prefer a switch, but in the case of the 10, that means running an unfused line, with no means for disconnection, the length of the fuselage and a relay back at the battery prevents that. I suppose I could add a fuse at the battery tap, but prefer a breaker on the panel in case it needs to be reset in flight.

Larry

[N941WR]

Originally Posted by tim2542

I don?t know your typical mission, but for most of us Sunday flyers I also agree with reassessing the need for an E bus. I intalled one and have never used it once or known of anyone else ever using one. I also had an AUX bus for brown out. After flying several hundred hours I now think all that redundancy, for me, was silly and I always regretted adding the complexity and weight.
Keeping it simple and light is very good advice for most of us.


Tim Andres

When I was designing my electrical system, I took that into account and understood where I was coming from. I had been flying antiques without any electrical system so I knew airplanes fly just fine without radios.

I also realized that with a battery backup in my EFIS (D100 at that time) and the handheld GPS that was always being charged, I knew I could keep the plane upright and navigate.

Now, three panels later, the 496 is still in there and will eventually be replaced by another handheld GPS, regardless of what other GPS nav system I might have on board. The SkyView has its own battery only now, in addition to that, I have a Dynon PocketPanel to help keep me upright.

My fear with all the complexity we are installing in our planes is that when things go bad, we will either take an incorrect action or forget about it.

Simple is always best.

So is practicing! I know of one RV builder/pilot who landed 15 miles short of his uncontrolled home airport because of an electrical problem that took out his radio. It was day VFR conditions and he told me how nervous he was making that landing and what a bad job he did of getting it on the ground.

While being on the ground is good, don't rush things to the point of making bad choices. In his case, he landed at an unfamiliar airport, one he had never landed at before, let his stress level rise, etc.

What I'm getting at is, if you install emergency systems, practice with them. Practice with them enough that they don't feel like emergency systems. (The same goes for crosswind, short, soft field, and engine out landings.)

[rv7charlie]

Ed,
That 6" spec was for *un*-fused (unprotected) feeders.

The 'essential bus' came about precisely to deal with alternator failure, and not for 'large' a/c; not very many homebuilts could be considered large.

There are variations on the meaning of 'simple'. One big bus and lots of switches is 'simple'. Two buses and flipping one or two switches when dealing with an alternator failure in a plane with both electrically dependent instruments and engine while in IMC...that might meet the definition of 'simple', too. If you're day VFR with magnetos, then multiple buses are pointless. Just turn off the master & fly home. But if you have an electrically dependent engine, or you're IFR, the math changes. Will your battery(s) carry you to the end of your IFR flight plan, including the alternate, without breaking a sweat? No? If simply flipping a switch or two could make that happen, would you consider it worth the trouble while wiring the electrical system?

On switches vs relays: Looks like Bob covered the best reason for a relay before I could hit 'send'.

[Walt]

Originally Posted by lr172

Walt,

Don't these modern systems just put the diodes inside the box, with theoretically the same risk?. I am not an electrical expert, but I thought there needs to be diodes whenever you have two battery sources to prevent cross-draining.

I am discussing an ESS buss fed from a second battery. I tend to agree that a second buss, just to have two, doesn't make that much sense.

Larry

Yes the boxes with dual power inputs do so with diode isolation.

An ESS buss with a second battery as the source is basically a back-up battery system, not an ESS buss as typically thoght of.

[rv7charlie]

So, who's defining 'essential bus' (or 'endurance bus')?

From the Aeroelectric Connection book:

A Short Discussion on the ?Endurance? Bus
For a number of years the ENDURANCE bus was called the
ESSENTIAL bus . . . bad choice. Words like ?emergency?,
?critical?, and ?essential? conjure up tense images of things
going badly in the airplane. I?ve had a lot of queries from
builders asking about running flaps, fuel pumps, and lighting
systems from the e-bus. The purpose of an e-bus with two
feeds was to provide reliable power for the minimum
equipment necessary for comfortable continuation for the en
route phase of flight using only the battery for power.
Unless you?re planning TWO alternators (Z-12, Z-13 or Z-
14) then the purpose of the e-bus is to provide a minimum
power consumption mode of operation in a battery-only
condition such that comfortable arrival is assured after you
have a clearance to land. Then, you can re-close the battery
master and run any accessories you like on whatever
remains of the battery?s energy. If the battery goes flat then,
it doesn?t matter.
What endurance do you want from the battery? If your
design goal is to permit only fuel aboard to dictate
endurance, then your battery capacity needs to be matched
to your e-bus loads such that a fully charged battery will
carry the e-bus for time equal to or exceeding fuel duration.
If your personal endurance value is less, then you can
increase the e-bus loads accordingly. Know further that a
battery?s useful capacity goes DOWN as load increases. An
18 AH battery may well have received that rating based on
a 20 hour discharge rate . . . or about 0.9 amps! If your
proposed e-bus loads are, say 3 amps, it is not reasonable to
expect 6 hours of performance from the 18 AH battery . . .
it WILL be less, probably more like 12-15 AH
Consider further that you?ll want to periodically test the
battery so as to KNOW its capacity or simply replace it
every year. The choice is yours. The goal is to KNOW how
long your battery will carry an e-bus load so that you can
depend upon it. Most single engine airplanes flying right
now have ?failed? batteries aboard . . . they cranked the
engine but do not carry enough energy for really useful
battery-only endurance. Worse yet, the pilots of those
airplanes don?t have a clue as to what the battery?s
capabilities are.
As you craft your dream project, keep in mind that the e-bus
and your battery maintenance philosophy can provide
system reliability that few single engine airplane drivers
enjoy. But you MUST understand how it works, what it?s
for and how to maintain it.

[bpattonsoa]

When I built my -10 I used a two bus system. The second bus was an endurance, not emergency in the AeroElectric terminology. It had the things critical to flying, Lightspeed ignition, Radio, Com (to keep passengers calm), transponder and trim. (flying a -10 without trim is a issue and a very small load.) The Dynon screens have their own 45 minute backup battery. Its only function is to be able to operate with a failed master or to be able to quickly reduced power demand by turning off the master and the alt feed on.


The E bus was initially fed from the main bus thru a 30 amp fuse to a blocking diode. (I use fuses rather than circuit breakers) The E-bus switch went directly to the battery with a 40 amp fuse. The Dynon voltage pickup was from the E-bus, what you want to see in an emergency.


In the 40 hour period I did not like that there was a .3 to .4 V drop across the blocking diode. I checked at the normal bus and voltage was the expected 14.2. I did not like to look at voltage and see lower than 14 V, it required a double look to assure it was still good.

As a result, I rewired it and got rid of the blocking diode. I used a single pole double throw switch (SPDT ON-ON) The two feeds were same as before, one from the normal bus with a fuse and the second from the battery with fuse. The center went to the E-bus. Down was "normal" feed so the switch is down, the normal position for most switches while not in use.


Now the E-bus voltage was the same as the alternator output . From a failure analysis standpoint I had removed the diode, a very low failure item. Switches are a extremely low probability for failure.


AS an added note, I don't use an avionics bus. I have a four pole single throw switch (from Digikey). To isolate a radio, I can pull its fuse. Saves a bunch of wires.