Fuses for backup power

[VictorV]

I am planning to install a VPX for primary power distribution. For backup power I plan to use the TCW IBBS. The output of the IBBS goes to
a backup power bus which is then distributed to the "Power In 2" pins on the Garmin G3x suite. Do I need to install fuses between the IBBS and
the individual Garmin components? I could put a fuse on the output of the IBBS but if that blows, everything goes out rather than the one
faulty component.

Victor

[Eric Page]

The installation manuals for the various 12V IBBS models are a little different in how they word it, but all of them say more or less similar things. Two examples:

The output wires of the IBBS system are internally protected with a single 8 amp, 5 x 20 mm fuse. No additional external fusing is required; however, use at least 20 awg wire for the output wires.

The output wires of the IBBS system are protected with a single 10 amp, MINI fuse. This fuse is accessible on the side of the enclosure.

[Geek]

I am planning to install a VPX for primary power distribution. For backup power I plan to use the TCW IBBS. The output of the IBBS goes to
a backup power bus which is then distributed to the "Power In 2" pins on the Garmin G3x suite. Do I need to install fuses between the IBBS and
the individual Garmin components? I could put a fuse on the output of the IBBS but if that blows, everything goes out rather than the one
faulty component.

Victor

Which IBBS did you use? 3AH or 6AH. Max current out of the 3AH is 5 amp continuous and 10 amps "instantaneous for radio transmission". I am using the VPX and the 3AH IBBS as well and am not using fuses. My reasoning to do it that way ---All my power wires are 22 AWG which is rated at 7A so there wouldn't be any real protection to the wires that a fuse would provide. The IBBS is really only there to help supplement the battery if I were to lose an alternator and give you a little more time (maybe 40 min looking at my electrical load) to deal with finding a place to land the way I understand it. If it was only one component drawing too much current, the VPX would shut it down and the rest would still be good to go.

Gary

[VictorV]

I'm using the 6ah IBBS. The Garmin components automatically select the power input based on which one has the higher
voltage so it can correct for lower voltage during engine start. If the VPX detects an over-current and trips the fuse
the IBBS would then be providing the power and, based on Eric's response, the IBBS's fuse would then blow. Unfortunately
once the IBBS fuse blows nothing will have power including the engine. So, theoretically, if the generators are out on
the engine, a single Garmin component can cause a power loss to the engine. Extremely unlikely that you'd have both
generators fail and an avionics component causing a short so I don't think it's a big issue.

Victor

[Eric Page]

Wait... you're using the IBBS as a backup for the engine? What about the ship's main battery? I don't think TCW intend for the IBBS to be used as a backup for engine electronics, but I could be wrong. In any case, even the 6Ah version is far too small for that task (see below).

I'm using a 912iS. The electrical systems of our engines are virtually identical. As I'm sure you're aware, if both of the engine's alternator/regulator systems fail, the engine will quit. You then close the Battery Backup switch and restart the engine. From that point on, the engine is running on battery power only. You might want to think about a minimum equipment list for battery-only operation, and pare it to the bone.

My plan is this:
1. If the "A" alt/reg quits, the Fuse Box switches the engine to alt/reg "B" and the airframe operates on battery only. Load shedding to be done manually at pilot's discretion.
2. If the "B" alt/reg quits, the engine continues to run on alt/reg "A" and the airframe operates on battery only. Again, load shedding to be done manually at pilot's discretion.
3. If both "A" and "B" alt/regs quit, the engine dies and the airframe remains powered by battery. Close the Battery Backup switch and re-start the engine.

In my design, under scenario 3 a significant portion of the airframe will be automatically load-shed to maximize time of powered flight. All that will remain powered is the comm radio, cabin lights, landing lights (off until needed), pitch trim and a multi-function standby instrument (MGL Blaze ASV-2) -- plenty to get the aircraft safely on the ground, day or night. Notice that I'm purposely sacrificing my entire EFIS network (Dynon, in my case). This is an extremely rare and unlikely emergency; I don't need gee-whiz avionics to land the plane and I'll have an iPad to give me GPS directions if needed. And yes, I'm going to lose engine instruments. The chances of some other engine anomaly happening on the same flight as a dual alt/reg failure is virtually nil, and some digital gauges aren't going to prevent it anyway.

Be sure to read section 24 (electrical) of the Rotax installation manual carefully. Kitfox recommends a battery (EarthX ETX-680) that doesn't meet the minimum capacity requirements listed in the Rotax iS engine manuals. I'm using an ETX-900 for this reason. It's the same case size as the -680 so it will fit in the Kitfox battery box. The -900 is still a hair short of the Rotax capacity specification, but it's very close and my system load-sheds aggressively.

I'm using a DC-DC boost converter that weighs a few ounces to maintain power to the Dynon suite during engine start, so no avionics backup battery is needed for that purpose. I won't have to carry its weight and I won't have to test/maintain it. Since I'm not building an IFR machine, I see no value in keeping an attitude instrument running (except perhaps in the edge case of flying on a moonless or overcast night over featureless, unlit terrain -- something that's below my personal minimums in this kind of airplane).

Anyway, just some food for thought...

[VictorV]

Wait... you're using the IBBS as a backup for the engine?

Sorry. My own personal brain fart. Yes, the main battery is the backup for the generators and THEN the IBBS kicks in if
the battery is dead. Definitely never going to happen

V.

[VictorV]

Great points Eric. I, too, am planning on using the ETX-900 based on Rotax requirements. I believe the engine needs 8 amps to run so the ETX-900
should keep the engine going for 2 hours assuming it's fully charged. In my case the IBBS output is not even attached to the main bus.

V.