Guardian 4721, Repair Manual

Page: 66 / 115

PART 4
INTRODUCTION
The schematic diagram on this and the following pages shows the DC control system in three (3) major areas,
i.e., the standby generator area, the engine, and the prepackaged transfer switch.
CIRCUIT CONDITION - UTILITY SOURCE VOLTAGE AVAILABLE
• Utility source voltage is available to transfer mechanism Terminal Lugs N1, N2, via installer attached utility
source lines.
• Transfer mechanism main contacts are at their “Utility” source side, i.e., “Utility” voltage is across the closed
main contacts and to transfer mechanism Terminal Lugs T1 and T2. Loads connected to lugs T1 and T2 are
powered by the “Utility” supply.
• Utility source voltage is available to transfer mechanism terminals A1/A2, via Wires N1 and N2, normally-
closed transfer relay (TR) contacts, and Wire 194.
• Limit switches XA1 and XB1 in the transfer mechanism are closed to their terminals B1/B2 side. Thus, the
A1/A2 circuit to the main contacts actuating coil (C) is open and the actuating coil (c) is de-energized.
• Transformer (T1) action induces a reduced voltage (about 14 volts AC) into the transformer (T1) secondary
coil. This reduced sensing voltage is delivered to the control board. As long as the sensing voltage remains
above 60 percent of nominal rated line-to-line voltage, the control board takes no action.
• The transformer reduced sensing voltage is used to operate a “trickle charge” circuit to the unit battery. This trickle
charge output is delivered to the battery from the control board via Wire 15, a 15 amp fuse (F1), and Wire 13.
• Battery power is available to the AUTO-OFF-MANUAL switch (SW1) via Wire 13, terminal 13 of terminal board
TB1, Wire 13, 15 amp fuse (F1), and Wire 15. With the AUTO-OFF-MANUAL switch set to AUTO, battery
power is available to the control board via Wire 15A.
• Line-to-line load voltage from transfer mechanism Terminal Lugs T1/T2 is delivered to the control board via
Wires T1 fT2, transfer switch terminals T1/T2, installer connected wires, and terminals T1/T2 of the control
board’s terminal board. This power is used by the circuit board to operate a 7-day exerciser circuit.Circuit
Condition- Initial Utility Voltage Dropout
Page 62
DC CONTROL
LIQUID COOLED
ENGINE UNITS
SECTION 4.2
OPERATIONAL ANALYSIS