12.5.7 AC System
The AC electrical schematic is shown in Figure 12.5-18.
Electrical power sources for the AC portion of the EPGDS include:
• Two alternating current variable frequency generators
• AC External
The power sources supply power to the following buses in order of priority:
• Left or Right AC bus
• Left or Right Galley bus
During normal mode of operation, one AC source supplies its dedicated bus. AC power is
required for the following systems:
• De-icing/Anti-icing heaters
• Standby hydraulic pump
• Galley loads
• Transformer Rectifier Units (supplements DC power)
• Auxiliary fuel pumps
AC Electrical Outlet
A 115 V AC, 15A, 400 Hz electrical outlet for vacuum cleaning requirements is installed on the
inboard face of the G1 galley.
12.5.7.1 Variable-Frequency AC Power
Two 115 V, 45 KVA AC generators (mounted on the propeller reduction gearbox) supply variable
frequency (340 to 560 Hz) AC power. The AC power is supplied to the left and right AC buses.
AC power sources are prevented from being operated in parallel.
AC power is available once the condition levers are out of START & FEATHER in the MIN/850 to
MAX/1020 range, and the GEN 1 and GEN 2 switches on the AC CONTROL panel are on.
If one AC generator fails, the #1 AC GEN or #2 AC GEN caution light comes and the remaining
generator is capable of carrying the aeroplane’s AC electrical load except galley power. An auto-
matic cross tie function, controlled by the AC GCU logic circuits, ensures that all variable-fre-
quency buses are powered when only one AC generator is on line. Whenever a fault condition
exists, the GCU of the inoperative generator issues a transfer request signal to the operational
side AC GCU. The operational side AC GCU will issue a CLOSE command to the failed side line
contactor. In this configuration, the remaining generator will power both AC buses. In this situa-
tion the load shedding relays will not allow power to the galley buses.
The AC generators are protected from bus faults by the GCUs that detect any excessive load
that might result from a short circuit on a bus. Once a heavy load is detected, the GCU isolates
the bus, and turns on the appropriate L AC BUS or R AC BUS caution lights.
The # 1 AC GEN HOT or # 2 AC GEN HOT caution lights come on whenever an AC generator
overheats. The AC generator must be switched off.
NOTE: All AC and DC aeroplane services can be operated from the AC generators or the
AC external power alone.
Dash8 - Q400 - Electrical
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