E-Bus or Main?

[Mich48041]

I agree with Jesse. Sometimes fuses fail for no reason. After all they are designed to be the weak link in the circuit. A poor connection in a fuse holder can generate heat which will blow a fuse. Likewise, a loose screw on a circuit breaker will generate heat that can cause the breaker to trip. I think that having two fuses or breakers in series is bad practice. Even if one of them is much larger than the other, a hard short circuit can blow both simultaneously
An E-Bus relay located near the battery will give the pilot the ability to shut off the circuit at the source in case of electrical smoke. Good workmanship and double insulation will minimize the chances of the E-Bus feeder shorting to ground, eliminating the need for feeder short-circuit protection. The same reasoning is used for the main power bus feeder.

[RV7A Flyer]

Originally Posted by Mich48041

I agree with Jesse. Sometimes fuses fail for no reason. After all they are designed to be the weak link in the circuit.

They don't fail "for no reason". There is always a reason. If you don't have one, then you haven't looked hard enough to find it (and fix it).

A poor connection in a fuse holder can generate heat which will blow a fuse.

That's a reason. So make sure the connections are good and solid, use high-quality fuse blocks, etc.

Likewise, a loose screw on a circuit breaker will generate heat that can cause the breaker to trip. I think that having two fuses or breakers in series is bad practice. Even if one of them is much larger than the other, a hard short circuit can blow both simultaneously

Again, I'm no EE, but *is this true*? It may be bad practice, but is there an actual analysis that shows this? Or some dataset of empirical evidence? Has anyone here seen this happen?

Right now, all we have is someone (or some people) saying "that's bad...that bigger fuse could blow also", without no actual reason WHY that could happen.

An E-Bus relay located near the battery will give the pilot the ability to shut off the circuit at the source in case of electrical smoke. Good workmanship and double insulation will minimize the chances of the E-Bus feeder shorting to ground, eliminating the need for feeder short-circuit protection. The same reasoning is used for the main power bus feeder.

I think I could concur on this approach. As a matter of fact, this whole discussion *has* caused me wonder why Z-11 has a fuse on the feeder from the hot buss, but not from the battery (main buss tap). Seems one or the other should be done, but not both...?

Where's Bob Nuckolls to explain this?

[Bill Dicus]

If you normally land with flaps they could be on E buss. They would only draw current for 20 seconds or so at the end of the flight and you don't have to use them if stdby battery is critically low. Having them available might be good, especially if the runway is short. Also the com and nav parts of the SL-40 can be fused separately on the main buss. Good of you to gather ideas. Good luck.

[az_gila]

A quick look at the specifications shows that fuses do "pop" quicker than a breaker, but it should not be "unexplained" or at random.

An ATC with 200% overload has an average "melt time" of 0.7 second, a Tyco W23 breaker has a pop time of 6 to 23 seconds at 200% of it's rating.

If you are relying on the extra time for the breaker to not pop during operation then something is drastically wrong with either your equipment or with the selection of the fuse rating.

[rv7charlie]

Originally Posted by RV7A Flyer

snipped...
I think I could concur on this approach. As a matter of fact, this whole discussion *has* caused me wonder why Z-11 has a fuse on the feeder from the hot buss, but not from the battery (main buss tap). Seems one or the other should be done, but not both...?

Where's Bob Nuckolls to explain this?

He's on the Matronics Aeroelectric list, where the doc was created largely with community input. That's the best place to get answers to questions about the doc.

Specific to the question in the paragraph: The unfused always hot bus (fuse block) is intended to be within a few inches of either the battery, or the supply side of the master contactor. By being there, it's within the design criteria for unprotected loads recommended by the FAA for certified a/c. The always hot bus feeds not only the endurance bus, but other items as well, and each downstream load gets protection.

Guys, almost everything being discussed in this thread has been hashed out over on the Aeroelectric list. One of those things is that the Z diagrams in the book are *guidelines* for safe, functional, reliable electrical systems. They are not intended to be locked-down circuit diagrams that cannot be changed, as in certified systems. Any diagram can be modified, and Bob has said repeatedly that they should be, to fit the user's individual needs. You just need to analyse any changes for unintended 'gotchas'.

Charlie

[Tbone]

Well I built a fusible link to supply power to the Ebus. It is now a 14 gauge with a short piece of 18 gauge as the sacrificial link. My battery bus supply wiring is slightly longer than the suggested but is very, very protected. I'm not concerned with it in the least. I did have to leave a few items on the Ebus because of space constraints but they are not used during level flight. Moved xpndr to main, flaps to Ebus......Yada..yada. Thanks for the discussion. I for one have learned something from it and absolutely feel that we all love this hobby!