(88) Location and function of the ground service switch:
MD-88
• APU PWR switch and EXT PWR switch, both located on overhead panel, remove and select their respective power to ground service bus.
(90) Location and function of ground service switch:
MD-90
Ground Service Power Switch, located on ovhd panel, performs following functions:
• OFF - Disconnects APU or external power from AC tie bus to ground service bus.
• ON - Connects APU or external power from AC tie bus to ground service bus, provided right generator bus is not powered.
- APU power has priority over external power for powering ground service bus.
(88, 90) Essential electrical equipment that may be powered during a complete loss of normal electrical power when positioning EMER PWR switch to ON or ARM(90):
- Captain’s PFD, ND, Airspeed, Altimeter, TA/VSI
- # 1 VHF COMM/NAV
- Interphone
- PA
- FGCP course/heading bugs (course/hdg displayed on ND)
- APU and Engine fire detection/protection
- Capt/FO/Cockpit white flood lights
- Cockpit thunderstorm lights
(88, 90) When does the battery charger operate?
- BATT switch ON
- EMER PWR switch OFF
- Ground service bus powered
(90) Time limitation on starting APU on initial electrical power up. Why?
On initial electrical power up (MD-90), crew must wait 90 sec before starting APU
- Allows EPCU built-in-test to finish
- Prevents ELEC SYS BIT FAIL msg on OAP from being triggered (requires maint. action to clear)
(88, 90) How to connect APU generator to electrical system:
- Check APU voltage and frequency: 115 ± 3 volts, 396 ± 8 Hz (ground), 400 ± 10 Hz (air)
- MD-88 - Placing L/R APU BUS Switch to ON selects APU power to respective buses. (Switches are left ON in-flight unless non-normal checklist calls for them to be off)
- MD-90 – APU GEN switch is normally left on to supply power to the respective generator bus provided all protection circuits are satisfied and generator is operating.
(88) Meaning of each blue light on MD-88 electrical panel
- APU PWR AVAIL: APU generator power available.
- EXT PWR AVAIL: external power available.
- POWER IN USE: selected power source connected to respective bus.
(90) Meaning of each blue light on MD-90 electrical panel
- EXT PWR AVAIL - external power switch ON and external power frequency and voltage are within required limits.
- APU POWER AVAIL - APU generator switch is ON, APU generator frequency and voltage are w/in required limits.
(88, 90) Where are voltage, frequency and amps read?
• AC VOLTS Meter, FREQUENCY Meter, and AC LOAD Meter are located on Electrical Power Panel of Forward Overhead Panel. DC Volts, Amps and Loads are adjacent to AC indicators.
(All) Components powered by the ground service bus:
Ground service bus provides power to only those circuits necessary for ground servicing.
(88, 90) How long do batteries last on emergency power?
- MD-88 - With complete loss of normal electrical power, ship’s batteries will provide both AC and DC emergency power when EMER PWR switch is ON for approx 30 min.
- MD-90 - Ship’s batteries will provide both AC and DC emergency power when EMER PWR switch is in ARM or ON for approximately 45 minutes.
(88) Function and positions of A/C BUS X TIE Switch:
- OPEN - Opens AC crosstie relay, isolates left and right generator buses. Prevents closing of relay.
- AUTO - AC crosstie relay will operate automatically.
AC Crosstie system has 3 functions:
• Prevents paralleling.
• Powers unpowered generator bus.
• Isolates generator bus faults.
NOTE: Under certain fault conditions, AC crosstie relay will lock open.
(90) Function & positions of BUS TIE switches on MD-90:
- OPEN – Isolates the L/R generator bus from the tie bus.
- AUTO – (Normal position) Electrical Power Control Unit (EPCU) automatically opens and closes bus tie relays as req’d to maintain elec. power to respective buses.
(88, 90) Positions and function of DC BUS X TIE Switch
- CLOSE - Closes DC cross-tie relay and connects left & right DC buses. Supplies DC power from one side to the other in case both transformer rectifiers fail on either left or right system.
- OPEN - (Normal position) Left and right DC buses are isolated.
(88, 90) Positions, function of EMER PWR switch (MD-88)
OFF - Removes batteries as source of emergency power.
ON
- Connects batteries as source of emer. AC and DC power.
- Power to DC emer bus now supplied from batt. direct bus.
- AC power supplied to AC emer. bus by emer. inverter
- Inverter powered from battery direct bus
- Standby lights illuminate
(88, 90) Positions, function of EMER PWR switch (MD-90)
- OFF - Disarms Emergency power and resets Automatic Emergency Power System.
- ARM - (Normal position)
- Places Emergency power relay in standby mode
- Automatic activation (requires at least 1 FUEL switch ON) when elec power control unit (EPCU) detects a power loss to emer AC, DC or DC transfer bus.
• ON - Emergency power is energized, prevents battery charger operation. Batt. supplies pwr for AC emer bus (through emer inverter), DC emer bus and DC transfer bus.
(88, 90) Positions and function of BATT switch:
- OFF - Removes batteries from battery bus, battery charger, and DC transfer bus.
- ON - Connects battery direct bus to battery bus. Selects batteries to battery charger and DC transfer bus. White line, positioned horizontally, indicates locked.
(88, 90) Normal AC power voltage and frequency for engine driven generators:
- 115 ± 3 Volts
* 400 ± 4 Hz
