The function of the plug interlock relay(s) is to cause the output load contactor to open, via
the generator control module (GCM) in the event the cable plug connector becomes
accidentally disconnected from the aircraft during power delivery, or if an attempt is made to
deliver power when the output cable is not connected to the aircraft, or if the aircraft should
reject the GPU’s 400 Hz power for any reason. 28 VDC for operation of the relay is supplied
from the aircraft either through an on-board transformer-rectifier or from a 28 VDC electrical
system. Connection from the aircraft to the interlock relay is made through terminals E and F
on the output cable plug connector. GPU-4090 models have (1) output plug interlock relay as
standard and a 2nd is included with the optional 2nd 400 Hz output cable. All other GPU-
4000 models have (2) output plug interlock relays as standard.