Request for PA-30B external power plug wiring insight

[Gomer Pound]

I am trying to understand something about the external power plug wiring. Mine is hooked up so after you plug in the external power with power to it, you must turn on the Master Switch for the panel to come on and to start the engines. Some airplanes, like some Cessnas, I hear, have the external power plug wired so when you plug in the external plug and apply power, the Master Switch is bypassed and the panel comes alive, able to use the starter with the Master Switch off.
In my case, I believe the battery remains in the circuit after the power is applied via the external power plug.
What I am having difficulty understanding when my external power plug is connected with power (I use a StartPac) and the Master Switch is turned on, I will see a 10 or 12 amp discharge on my EI VA-1A Volt/Ammeter, yet almost no amps will be going through the StartPac DC wires to the power plug as measured by my Fluke clamp ammeter, at least in the beginning. The VA-1A is wired to only come on with the Avionics Master Switch on (which I am having changed to come on with the Master Switch). The voltage will only be up to 13 volts while the avionic are on. I would expect the Voltage to be 13.9 V like I see with when the alternators are charging and the avionics are on. I would expect the StartPac to immediately try to bring the voltage in the circuit up to 14.2.
The StartPac specs state the unit outputs up to 50 amps of 14.2V DC current. That is a lot of current, equal to one of my alternators at 100% rated output. I guess I am confused as to why the Volt/ammeter reads a higher voltage with the alternators running (which the ammeter says are only putting out 3-4 amps) when the StartPac connected via the external power plug only gets a voltage reading of 13 V. Does it have something to do with where in the circuit the Voltage is being measured?
I know this is a bit lengthy and obtuse, but if anyone has some insight, I would appreciate your input so I might understand what is going on.I saw the same thing before I had the external power plug components changed. The gear was swung with a Red Baron power supply connected, but I did not see any amps being delivered by the Red Baron. I wonder if the external power supplies deliver power differently in airplane where the circuit bypasses the battery and goes directly to the main bus?

[Larry Martin]

Sorry, I don't have the answer. A good question, I would like to know why too.

larry

[AlanBreen]

The external power wiring is different depending on your serial number however there is only one PA30B (S/N 1717) that was different from the rest.The wiring was changed from 1745 thru to 2000, these were all PA30C's I think.

The external power is effectively connected via the external power solenoid to the same point where the battery positive lead is connected to the Master solenoid. The Master switch needs to be on for the external power solenoid to be operated to deliver power to the aircraft. So really the external power connects to the aircraft system at exactly the same place as the battery.

You are correct in your diagnosis of how the external power plug connects.

Now to explain the discrepancies you are seeing with the current flows.

There are two electrical busses in the PA30. They are connected by the shunt that is used to measure the current flow you see on your EI or original Piper ammeter.

The primary bus is connected directly to the battery Master solenoid and there are four components that draw their power from the primary bus. The two starter motors, the landing gear and the flap motor. The operation of any of these devices will not show up as a current flow change on the EI ammeter.

The only current flow showing on the EI ammeter is power being used by all other items other than the four items I listed above.

I wonder that your problem with your external power supply is that it's not actually connected to the aircraft properly. You didn't state where you measured the current flow from the Red Baron, i.e. using a clamp meter or the aircraft EI ammeter. If it was using a clamp meter in the lead from the Red Baron then I would definitely expect t see some current flow. If not then I question your external plug connection.

Also the fact you say you see a 10 to 12 amp discharge on the EI ammeter but nothing on your clamp meter on the StarPac tends to confirm to me the Start Pac isn't actually connected to the aircraft electrical system and there's an issue with your external power plug/socket or the wiring for that circuit. What components did you change?

I know our external plug is extremely flakey, the problem is the socket it self not always making a good connection with the plug that is inserted into it. Do you know for sure your StartPac (or Red Baron) are actually supplying power. Have you checked the voltage output from these, perhaps the StartPac is only giving 13 volts.

Measure the StartPac voltage and your battery voltage and then check the battery voltage with the StartPac connected. I would take the nose cone off on measure the voltage at the battery before and after you switch on the Master with the StartPac connected.

Can you feel or hear the External Power solenoid operate? One way to check this is to power everything off including the aircraft Master Switch. Unplug the StartPac, turn on the aircraft Master and then plug in the StartPac while listening/feeling for the solenoid to operate

Also remember the StartPac will deliver 50 amps only when there is enough load to draw 50 amps.

I've attached the External Power plug wiring diagram

I hope this helps