[bhaikaju]

Can anybody tell me the purpose of the 3 pin ground power unit connector in aircraft, I want to ask why the third pin is short in length than the other two and is it positive or negative?

[KISS]

The power limited connection, I believe has a minimum of 4 pins and not 3 and limited to around 100W (don't quote me on the number).

There are two larger pins that carry DC power and two shorter pins.
On the device that is plugged in, those two pins are shorted together.

What this does is, it allows the device to be plugged in, the power connections made, and then the power turned on as the connecor is inserted. It also allows the power to be disconnected prior to the larger power pins clearing the connector.

This does a few things:
1. Control of the power can be logic level and some can be disconnected for power management.
2. It removes ignition sources such as sparks. Connection is made before power is supplied.
3. Increases the reliability of the connectors.
4. The connectors are not powered at all times.
5. A power limited switch (the two pins) can be anly low voltage. For instance 5V at 20 mA which cannot draw appriciable power.

The terminology is sometimes called "hot-swap".

In a different application such as tellecommunications:

A redundant power supply develops a fault and signals that it has one. The other one is now supplying power due to some diode oring. The technician removes the old one and places the new one in service without disconnecting power, because power is diconnected during the removal process. No sparks fly in the process and no connectors are damaged.

Here is a discussion:

http://www.ecnasiamag.com/article-65...hips-Asia.html

[bhaikaju]

Thanks a lot brother... Really helpful..

[Stratmando]

I believe many have at least 2, 3 and 4 prong GPU's, Here's 3 phase:
http://www.stat-con.com/GPU_Catalouge.pdf
http://www.teslaind.com/lspeed/suppo...00manual_L.pdf
Piper uses 2 prong. Many smaller aircraft use 3prong. Still looking for the purpose of the 3rd pin.
May be answered above.

[Spacewizard]

Typically 3 pin aircraft ground power connectors use two circuits to prevent arcing when the pins are plugged into the sockets.
Two larger pins provide power and ground for the main high voltage circuit. The shorter third pin powers a low voltage circuit that energises a solenoid to close the high voltage circuit after the longer pins have first been partially inserted into their sockets. The low voltage circuit makes use of the aircraft electrical systems common ground and therefore needs only one pin for power.

[Spacewizard]

Typically, a three pin aircraft ground power plug/receptacle are provided to prevent arcing. Two (usually larger pins) are ground and power connectors for the main high voltage power circuit. The third (shorter) pin powers a low voltage circuit that energizes a solenoid to close the high voltage circuit after the large prongs have first been partially inserted into their sockets. The low voltage circuit needs only one pin, because it makes use of the aircraft electrical systems common ground.

[Aircraftplugs]

Hi,

This is correct.

A three pin aircraft ground power plug/receptacle are provided to prevent arcing. Two (usually larger pins) are ground and power connectors for the main high voltage power circuit. But The third (shorter) pin has the same voltage, not po a low voltage circuit

It energizes a solenoid to close the high voltage circuit after the large prongs have first been partially inserted into their sockets. The low voltage circuit needs only one pin, because it makes use of the aircraft electrical systems common ground.

The 3 pins are also silver plated to avoid arcing.

You can find more information on the website of www.aircraftplugs.com

[Gary jp4]

Good answers. The concept is correct, but one correction.

In the typical 3 pin GPU plug commonly referred to as the "NATO connector" the third small pin is not a lower voltage. It is a lower amperage at the same voltage. The large positive pin and the small pin are connected together (shorted) within the plug, so they are the same voltage. The small pin is used for low amperage control, usually to close a relay in the aircraft used to make the high amperage circuit.

From the reference given (PRODUCER OF AIRCRAFTPLUGS - AN2551 ), under the literature button, read the PDF "Purpose of 3 PIN on AN2551 connector".

Here are a couple of excerpts:

"The purpose of the 3 pin ground power unit connector in aircraft, why is the third pin is shorter in length than the other two and is it positive or negative ? The two positive pins are shorted together. The smallest pin is limited to 100 W. There are two larger pins that carry DC power and one shorter pin. It allows the device to be plugged in, the power connections made, and then the power turned on as the connector is inserted. It also allows the power to be disconnected prior to the larger power pins clearing the connector."

And

"Typically 3 pin aircraft ground power connectors use two circuits to prevent arcing when the pins are plugged into the sockets. Two larger pins provide power and ground for the main high voltage circuit. The shorter third pin powers a low amperage circuit that energizes a solenoid to close the high amperage circuit after the longer pins have first been partially inserted into their sockets. The low amperage circuit makes use of the aircraft electrical systems common ground and therefore needs only one pin for power. All three pins are silver plated to avoid arcing as well. "