Creation: 01.31 - 2014
1-1-12
GPU - 406
B. Relay Panel Components
(1) Current Transformers – T1, T2, T3
The relay panel assembly contains 3 current transformers which send current signals from the
three phases to the Generator Control Module (GCM) for load sensing and line drop
compensation, for instrument panel indication, and for over-current protection. The transformer
converts current readings at a ratio of 400:5; that is, for every 80A of output current, 1A of
sensing current flows through the GCM. For 180 KVA units, this ratio is 600:5.
(2) Contactor(s) – K16, K17
The function of the contactors is to allow, or not allow, 400 Hz output power from the generator to
the aircraft. The contactor states, open or closed, is controlled by the 24 VDC contactor coil.
GPU-406 and GPU-409 have (1) contactor as standard and a 2nd is included with the optional
2nd 400 Hz output.
(3) Generator Neutral Distribution Block – TB2
The purpose of the neutral distribution block is to provide a means to connect the generator
neutral to the aircraft neutral through a cable. TB2 is for connecting the 400 Hz output cable(s)
Neutral wire “N” to the Generator “N” cable.
(4) 400Hz Control Signal Terminal Blocks – TB8, TB10
TB8 and TB10 connect the E and F pins of the 400Hz AC output cable to the generator
controller module (GCM). This provides the GCM with a 28 Volt Signal once the aircraft has
accepted power from the GPU. TB10 provides the signal for AC Output #1, while TB8 (optional)
provides the signal for AC Output #2.
(5) Battery Ground Terminal Block – TB5
TB5 provides the ground connection for all components inside the electrical box. It is connected
directly to the battery negative terminal.
(6) Switched +24V Terminal Block – TB3
TB3 provides 24V power when the GPU is turned on for all components inside the electrical
box. It is connected to a relay that is active after the GPU is turned on with the green power
button, or after it wakes up when the battery switch is engaged.
(7) Battery Power Terminal Block – TB7
TB7 provides battery power to components inside the electrical box. It is directly connected to
the battery positive terminal.
(8) Engine Running Relay – K3
The purpose of a relay is to provide a controlled connection. A relay is also used to connect a
high-power circuit by using a low-power circuit to control the connection. K3 is the engine
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