Alt Circuit Breaker Popped Twice Yesterday

[ve6yeq]

@Cactus Charlie see link below. I’m working to download the actual data file and will post that up too.

SavvyAviation

apps.savvyaviation.com

From what I can make of the numbers, I would have to guess that something caused a loss of alternator voltage regulation and then an overvoltage shunt somewhere tripped the breaker. The one-second sample rate on the logs would not be fast enough to capture the actual trip event.

I am not familiar with the xcub electrical system, but it looks as if you have a lead acid main battery that is not in great condition from the amount of main bus voltage drop when the alternator dropped offline. Does the AMM that comes with the Xcub have the wiring schematics in the back?

 

[Neal]

@lrobichaux - the coil packs are part of the electronic (light speed) ignition system but I have no clue what's used in your plane, and quite frankly, stop at I have no clue.

Any talk with Pete on a warranty replacement of the push-pull breaker? That's the most expensive one in the plane so I'd see if CC can send you one. It's rare but breakers do go bad too. Also check wires on the breaker. My IBBS wire came off the breaker which caused a 6 month hunt for the "IBBS in use" message on my G3X until I noticed a pink wire dangling one day looking under the panel. I found a screw on the floor which I saved and then the puzzle came together. So check the security of the wires at the breaker as well.

 

[lrobichaux]

I am not familiar with the xcub electrical system, but it looks as if you have a lead acid main battery that is not in great condition from the amount of main bus voltage drop when the alternator dropped offline.

It has an EarthX ETX 900 vented LiFePO4. Seems to be in good shape, as I keep it connected to the proper battery tender.

Does the AMM that comes with the Xcub have the wiring schematics in the back?

CC has sent me all the wiring schematics.

I would have to guess that something caused a loss of alternator voltage regulation and then an overvoltage shunt somewhere tripped the breaker.

I'm not an electrical engineer, but your guess seems spot on to me. If this pattern repeats, I'll be filing a warranty claim for a new alternator.

 

[lrobichaux]

So check the security of the wires at the breaker as well.

Excellent suggestion!

 

[Cactus Charlie]

I looked at the data and it is consistent with an intermittent short to ground of the alternator output . This short must be down path of the CB and current sensing shunt.

Of general interest - the NX (at least this NX) instruments alternator current, not battery current, so not the same as EX-3 or FX-3.

 

[Cactus Charlie]

If this pattern repeats, I'll be filing a warranty claim for a new alternator.

Nothing wrong with the alternator but you may run up a few hours of labor if you pay someone to find the intermittent short circuit.

 

[lrobichaux]

I looked at the data and it is consistent with an intermittent short to ground of the alternator output . This short must be down path of the CB and current sensing shunt.

Of general interest - the NX (at least this NX) instruments alternator current, not battery current, so not the same as EX-3 or FX-3.

Since my EIS instruments alternator current, does this mean that I do not need a Hall effect sensor like you described above? If correct, why do I not get a CAS message when the Field CB is pulled? Is this just a G3X EIS set-up issue?

Also, if the intermittent short is downstream of the CB, does this mean the voltage regulator cannot be the problem?

 

[lrobichaux]

@Cactus Charlie what does the symbol next to #1 mean, and what does #2 do?

 

[ve6yeq]

Since my EIS instruments alternator current, does this mean that I do not need a Hall effect sensor like you described above? If correct, why do I not get a CAS message when the Field CB is pulled? Is this just a G3X EIS set-up issue?

Also, if the intermittent short is downstream of the CB, does this mean the voltage regulator cannot be the problem?

For the Carbon Cub SS/EX/FX the alternator circuit looks like:

You can see here the overvoltage "crowbar" that grounds the field if the field voltage is too high. This will blow the field breaker, which in turn then disconnects the alternator output via the "OV Relay". Remember that disconnecting the alternator from all loads without first disconnecting the field can cause really large voltage spikes that normally "let the smoke out" of something (such as the alternator's built-in regulator).

As you have an alternator "shunt" to measure the current somewhere in the XCub, there must be some changes in the alternator circuit from the SS/EX/FS to the XCub. Did you see the recent CubCrafters service notice about the XCub alternator?

https://cubcrafters.com/c/wp-content/uploads/2025/11/XK-SB004-Rev-NC-Alternator-B-Terminal.pdf

 

[Cactus Charlie]

The symbol next to 1 is an electrolytic capacitor. It is there to suppress alternator whine.

2 is the current shunt. It is a very low value resistor that passes the alternator current. There is a voltage drop across the shunt that is proportional to the current being passed. That voltage is sensed by the two lines below the shunt. The symbols in those two lines are fuses. After passing through the fuses the voltage is fed to GEA 24 and then displayed as current on the GDU.

You do not need to add a Hall effect sensor to read alternator current since it is measured by the shunt. If you wished you could add a Hall effect sensor to measure battery current. I like having alternator and battery current but it would only be useful to have both if you know how to interpret the data.

The electrical system of the NX is not the same as the EX/FX. Also, the X/NX schematics I have seen do not match the charging system of the aircraft under investigation. If it was my aircraft I'd be asking for the applicable revision of the DC power schematic.

All that said - I'm confident you have an intermittent short of the alternator output and it may not be easy to find. On my old PA-28 (now sold) I had an intermittent alternator short that was traced to the alternator wire fretting on the crankcase mounted support clamp. Over 40 years of vibration had worm through the rubber on the clamp and the insulation on the wire. Don't expect to find that sort of problem on a nearly new airplane.