AN2551 3 pin plug

I understand the premise of the 3rd (smaller) pin (top of the image) is to delay switching power until the two main pins have made positive contact with the aircraft socket on plug insertion; and similarly, to disconnect power before the two main pins lose contact on plug removal.

This setup is desirable as it;

  • ensures correct polarity as the plug only fits into the socket one way
  • avoids arcing on insertion and removal which in turn;
    • removes a potential source of ignition (especially if refuelling)
    • prevents damage to the socket and pins

My understanding is that all pins on the plug are always live and that all control logic and switching is part of the aircraft's electrical system. ie: the switching occurs internal to the aircraft between the ground power socket and the main bus via a relay (powered by the external power source) that is ambiguously referred to as the external power relay (referring to its function, not its location).

I have several questions;

  • If the plug pins are always live, why does the third pin not present a potential arcing risk? (I assume the lower power has something to do with this but you can get a reasonable spark from a 9V battery).
  • How is the power on the 3rd pin limited?
  • Why not use the 3rd pin to switch the power at the source so that the positive pin is only ever live when connected? eg: short the 3rd pin to the ground pin on the aircraft socket to activate an external relay (part of the power supply - not to be confused with the relay in the aircraft referred to as the external power relay) to switch the power on/off before it even reaches the aircraft. As a bonus this would also (slightly) simplify the aircraft electrical system.
  • Is all the control logic always in the aircraft?
  • Is any aircraft power required? ie: will this work if the aircraft battery is completely dead?
  • Is the aircraft battery required to be physically connected (to complete the circuit for the relay)?

Sources that got me this far:


2 Answers 2


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The key to how it works is best explained with the 3rd link in OP question: the short pin provides power for a relay switch, powered = closed. When pulling the plug out the short pin disconnects first, opening the switch which removes power from the plus (middle) terminal of the plug before it is disconnected from the counter-plug.


Well, the power does not neccessarily be limited on the 3rd pin - and neither is there any substantial arcing, as it is plugged in and out without any substantial load (unless operated wrongly) - on plugging in, the external power switch should be still turned off, so nearly no current will flow (except to maybe a indicator light - but that has a low inductance, and thus little induced voltage).

Having the control inside the aircraft makes for a simple construction with very limited room for operational error - switching the power outside the aircraft might be a feasable option, but keep it simple...

Yes, this works without any power on the plane itself, though subsequent electrical systems might need the battery to be present (and with some residual charge) to operate.


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