AC & DC Metering panel - Classic

737 ELC NG

AC & DC Metering panel - NG

This panel is slightly non-standard because it contains the optional APU BAT position on the DC side. Most classics don't have this second battery.

The Residual Volts button (not installed on the NG) may be used to test a generator that has dropped off a bus. When pressed, if a voltage is seen then the generator is still turning, therefore a generator showing zero residual volts has failed and will not reconnect. Residual volts is the only selection to use to the 30V scale on the AC voltmeter, for this reason residual volts should never be pressed when a generator is connected to a bus (will get 115V).

Notice the new CAB/UTIL & IFE/PASS SEAT switches that replace the GALLEY switch. These control the following services:



Recirculation fan(s)

115V AC audio IFE

Door area heaters

115V AC video IFE

Drain mast heaters

28V DC video IFE

Lavatory water heaters

Airphone equipment

All galley buses

Pax seat elec outlets

Shaver outlets

Logo lights

Potable water comp

The purists may like to know that the DC voltages are measured at the following points:

DC Selector switch Voltage measurement point Typical Voltage Typical Current
STBY PWR DC Standby bus 24-30 N/A
BAT BUS Battery bus 24-30 N/A
BATT Hot battery bus 22-30* 0
TR1 DC bus 1 24-30 20-25
TR2 DC bus 2 24-30 20-25
TR3 TR 3 24-30 10-15
TEST Power system test module See table See table
  • May be up to 33V during pulse mode charging.

Do not leave the DC meter selector at BAT on a dead aircraft, because indicating draws some current and will eventually drain the battery.


The TR's convert AC into DC. The check for TR serviceability is current, not voltage, because the TR voltage indicates that of the associated DC busses (for TR's 1 & 2). TR's should always be checked before commencing an autoland because the TR3 disconnect relay / cross bus tie relay opens at glideslope capture and this will leave DC Bus 1 unpowered if TR1 had previously failed. NG's have a TR UNIT light which illuminates if either TR1 or TR2 and TR3 fail in flight or if any TR's fail on the ground. The TR's are unregulated and output rated to 50 Amps.

TR voltage range: 24-30V
Battery voltage range: 22-30V (May be up to 33V during pulse mode charging)

The TEST positions are used in conjunction with the Power System Test panel (1-500 see below). This test information is all contained within the metering panel on the NG.

Gen Drive & Standby Power panel


Gen Drive & Standby Power panel - Classic


Gen Drive & Standby Power panel - NG

LOW OIL PRESSURE and HIGH OIL TEMP cautions are replaced by a single DRIVE caption on the NG. This will only illuminate for an IDG low oil pressure, since the IDG's will auto disconnect for a high oil temperature. They will also illuminate for an under-frequency.

A higher than normal rise (ie above 20C) indicates excessive generator load or poor condition of drive. These temperature gauges were deemed to be redundant and have been removed from the NG.

Max gen drive rise: 20°C
Max gen drive oil temp: 157°C

If aircraft is fitted with a VSCF, must operate within 45mins of a suitable aerodrome.

Generator Bus panel


Gen Bus panel - NG


Gen Bus panel - Classic

The amber TRANSFER BUS OFF light comes on when the respective AC transfer bus does not have power.

The amber BUS OFF light (classics) indicates that the respective AC generator bus is not energized.

The amber SOURCE OFF light (NG's) indicates that the respective AC transfer bus is not energized by the source you last selected.

The engine and APU generator OFF BUS lights illuminate when the respective generator is running and of the correct quality.

The blue GND POWER AVAILABLE light on classics only means that the GPU is physically plugged in to the aircraft and gives no indication about the quality of the power. You may not be able to connect the ground power to the busses even if the light is illuminated. ON NG's the quality is checked and the light will only illuminate when external AC power is connected and the quality is good.

There are three golden rules of 737 electrics:

1. There is no paralleling of AC power.

2. The source of AC power being connected to a generator bus takes priority and automatically disconnects the existing source.

3. A source of AC power does not enter the system automatically (when it reaches proper voltage & freq). It must be manually switched on.


AC Busses - Classics

Gen busses – Point of connection for the power sources (engines/APU/GPU). Used for heavy, important loads eg hydraulic pumps. Effectively now renamed transfer busses on the NG

Main busses – Fed from the respective gen bus. Used for heavy non-essential loads eg fuel boost pumps.

Transfer busses – Normally powered by respective gen bus. If these fail, will feed from other gen bus if BUS TXFR switch is in AUTO. Used for essential loads eg trim.

AC standby bus – Powered by transfer bus 1 or the battery via an inverter. Used for essential loads eg ATC 1

AC Busses - NG's

Transfer Busses - Point of connection for the power sources (engines/APU/GPU). Used for heavy, essential loads eg hydraulic pumps.

Main Busses - Fed from respective transfer bus. Used for non-essential loads eg recirc fans. The main busses are next to be load shed after the galley busses

Galley Busses - First in line to be load shed.

AC standby bus – Powered by transfer bus 1 or the battery via an inverter. Used for essential loads eg ATC 1

DC Busses

DC busses – Powered by the respective transfer busses via a TRU.

DC standby bus – Powered by DC bus 1 (Classics) / TRs (NGs) or battery bus (Classics) / battery (NGs).

Battery bus – Normally powered by TR3, alt power is battery. Powered when the battery switch is ON or the standby power switch is BAT.

Hot battery bus – Always live, used for fire extinguishing & Captains clock.

Switched hot battery bus - Only powered when the battery switch is on.

Standby Busses

Are for essential AC & DC loads and are guaranteed for 30mins from the battery.

SBY AC bus – Is powered from AC transfer bus 1 or the battery via an inverter.

SBY DC bus – Is powered from DC bus 1 or the battery via the battery bus.

Bus transfer switch - when off will completely isolate left & right sides of the electrics.


Battery - Is a 36 ampere-hour, 24 volt, 20 cell, Nickel-cadmium battery and should provide 30 minutes (20 mins 1/200's) of standby power if all other generators fail.

APU Battery - This is a customer option that I have only seen on Series 500 aircraft. It is primarily used for starting the APU but also works in parallel with the main battery to provide 45mins of standby power. One of its best applications is that power is retained on the Captains EFIS with the loss of all generators, similar to latest build classics.

Aux Battery - This is a reserve battery on the NG which is normally isolated unless the main battery is powering the standby system when it operates in parallel with the main battery. The aux battery combined with the main battery will provide 60 minutes of standby power

The NG also has 2 extra dedicated batteries for the engine and APU fuel shut off valves and the ISFD (150mins capacity).

BAT OVHT & APU BAT OVHT lights are a customer option on classics. They are located on the aft overhead panel and no crew action is required if they should illuminate.

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