To Shunt or not to Shunt

As I was wiring my GEA24 today I realized there was a decision to be made about where and if to install a shunt for measuring current. I already actually installed a shunt on the firewall to measure the current coming in from the alternator B-Leads to the Main Bus. (again, I will go over my bus decisions later, but this is a Z-14 inspired design where there are two alternators and two batteries). Thinking more about that though, I am not sure I want/need a shunt up there.

The decision impacts what information I will on my EFIS display. I am already pretty accustomed to monitoring Bus1 and Bus2 Amps and Volts from flying a G1000 and G500 system with dual busses. I like having that information available to me as a pilot.

The thing about my design is that the VPX-Pro already speaks to the G3X over RS-232 to relay the main bus voltage and current draw to the EFIS. So the EFIS can be configured to use that VPX information about the Main bus. The downfall of my setup is that the VPX current draw is not the full story for my entire electrical system. Some of the current is being sourced from the AUX bus which the VPX does not have visibility of.

So my original thinking was that I wanted to be able to see how hard the alternator was working. Only one of my alternators will be active at a time because they both feed the same bus. In thinking more about it though, I think it would make more sense to know how much current my AUX bus is using.

Sources of data:

• Main Bus Volts: VPX
• Main Bus Current: VPX
• AUX Bus Volts: GEA24 J244-Pin28 (attached to the aux fuse block)
• AUX Bus current: Rear mounted shunt (or maybe I am going to co-located this up front with the aux fuse block)

The thing that I find a little confusing about this is that the "Main Bus Current" is not the current that is going over the 2GA wire going to the battery box, it is what the VPX is outputting. So if I was flying and wanted to know how stressed my electrical system is in total, there has to be some math involved. That said, there was no way around that. Ideally I would talk to Garmin and they would provide a touch screen option to the EFIS page that when you tapped current you could flip flop between battery current and alternator current with both busses being depicted. I think for now though I am happy. (although I have to go remount my shunt to a new location)

Here are the relevant sections from both the G3X manual and the Vertical Power manual regarding sensing current.

G3X installation Manual

Current
The GEA 24 and GSU 73 have provisions to monitor bus current from two different sources. Current can be measured either using a shunt resistor such as the UMA 1C4 (50mV/100A type) or a Hall effect sensor such as the Amploc KEY100 series.

Shunt sensors: Shunt sensors are connected to the SHUNT 1 and SHUNT 2 inputs on the GEA 24 and GSU 73 (see Figure 30-2.2 and Figure 30-3.1). The SHUNT 1 input can be configured to display either "Bus 1 Amps" or "Main Bus Amps". The SHUNT 2 input can be configured to display either "Bus 2 Amps" or "Essential Bus Amps". 

Hall effect sensors: Hall effect current sensors are connected to the GEA 24 or GSU 73 general purpose (GP) inputs (see Figure 30-2.2 and Figure 30-3.1). Hall effect sensors can optionally be calibrated to adjust for installation differences (see Section 34.4.19.4). The supported configurations for Hall effect current sensors on GP inputs are similar to those supported for shunt current sensors:

• Bus 1 Amps (Hall)
• Bus 2 Amps (Hall)
• Main Bus Amps (Hall)
• Essential Bus Amps (Hall)

Vertical Power: When using a Vertical Power unit, configure SHUNT 1 to "Vertical Power Main Bus
Amps" or "Vertical Power Bus 1 Amps" to use primary bus current data from the Vertical Power unit.
Configure Shunt 2 to "Vertical Power Bus 2 Amps" to use secondary bus current data from the Vertical Power unit.

Vertical Power Installation Manual

3.7 Alternator Current Sensing (Shunt) When planning your aircraft electrical wiring you must consider whether to wire an ammeter (usually a shunt or hall effect sensor provided with the engine monitor) on the wire connecting the alternator(s) to the main bus. The ammeters indicate the amount of current the alternator is providing. 

A shunt is not required to tell if the alternator is working. It is very easy to tell if the alternator is working correctly by simply looking at voltage. If you see 14 (or so) volts with the engine running then it is working. If you see 12 (or so) volts it is not working or not turned on or the devices are drawing more current than the alternator can provide (note, engine must be running). If you set your low voltage alarm on the EFIS at 13 volts, then you will get a low voltage alarm if the alternator fails. 

Since the VP-X provides basically the same information as a shunt installed on the alternator b-lead, our position is that adding a shunt gives you no additional meaningful information. 

A shunt on the alternator b-lead shows the amount of current the alternator is providing to power the devices and charge the battery. The VP-X total current reading shows the total amount of current the devices attached to the VP-X are using. The delta between the two is the battery charging current, which goes to (basically) zero after re-charging any loss from starting the engine or charging a run-down battery. 

If the battery charging current is important to you, then you should install a shunt. If not, then simplify your wiring and don’t install it. Your call. And of course each builder’s needs are different so there is no absolutely right answer. 

The EFIS displays a VP-X page which shows individual device current as well as total current through the VP-X. The EFIS also has an ‘Amps’ gauge that is used to show the readings from the shunt. In some cases the EFIS ‘Amps’ gauge can be used to display total system current from the VP-X. Please check with your EFIS manufacturer for details. 

If you don’t install the shunt then the shunt wires on the engine monitor/ EFIS are not used