ATR 42/72-600 Systems Guide — Chapter G: Electrical Systems

2026-06-23

Chapter G. ELECTRICAL SYSTEMS (FCOM DSC 24)¶

1. SOURCES OF POWER AND NETWORK ARCHITECTURE (Page 74)¶

The ATR 42/72-600 electrical network utilizes both Direct Current (DC) and Alternating Current Variable Wide Frequency (ACW) grids. The system isolates essential and non-essential loads to ensure structural survival timelines during multiple generator failures.

1.1. Generation Sources Matrix¶

Generation System	Source Components	Output Rating	Primary Distribution Targets
Direct Current (DC)	2 Starter-Generators

(Engine Driven) | 28V DC

400 Amperes each | DC Bus 1, DC Bus 2, Essential DC, Standby Networks, Battery Chargers. | | AC Variable Wide (ACW) | 2 ACW Generators

(Engine Driven) | 115V / 200V AC

2. DC-AC SCHEMATIC (Page 75)¶

Source: direct reading of PNG pages 77–82 (printed 75–80). Panel layout cross-referenced with Annex 1 cockpit panels (page 190).

ATA 24

The source PDF shows the DC Starter Generator driven by the HP spool through the Accessory Gear Box (AGB): - From 0 to 45% NH as a Starter - Above 61.5% NH as a Generator

The ACW Generator is driven by the Reduction Gear Box, and is available when NP > 66%.

In Hotel Mode or with the propeller feathered, there is no ACW. When unfeathered (CL in AUTO / 100% OVRD), the NP is maintained at a minimum of 70.8%, in order to have ACW (minimum 66%).

The MAIN ELEC PWR overhead panel (visible in source photos, pages 76–82) contains the following controls. Individual button positions are hard to resolve in the photos; labels below are confirmed from Annex 1 (page 190):

UNDV — DC STBY <19.5V. FWS.
BAT switch — with OVRD position (guarded). OVRD: transfer of STBY buses from HOT MAIN to HOT EMER BAT BUS.
INV — FAULT: inverter failure or supply loss. Auto-transfer of all AC buses to remaining inverter. FWS.
TRU — ON pb pressed IN (ON illuminated CYAN). TRU is connected to ACW BUS 2. ARROW illuminates green when supply of DC EMER BUS, DC STBY BUS, INV 1 (AC STBY BUS) and DC ESS BUS from TRU is effective. FAULT: battery charger failure detected by MFC.
SHED — One of the DC SVCE / UTLY 1 and 2 buses is shed (BPCU). FWS. OFF: all the DC SVCE BUS / UTLY 1 and 2 buses are shed.
DC GEN 1 / DC GEN 2 — Generator FAULT LT extinguishes above 61.5% NH if GEN is operating normally.
EXT PWR
ENG START — with START selector (CRANK / START positions). ON: starter engaged. At 45% NH, light out, starter disengages automatically. CRANK: dry motoring (no ignition). START: selects a start sequence. In flight regardless of start selection (A, B, A+B) both exciters are energized.
PROP BRAKE — not fully locked or not fully released. After 15 sec = FWS.

Note: The Gemini-generated ASCII panel in the original extraction showed a simplified layout with only BATTERY/BUS TIE/GEN 1/GEN 2 and an ACW section. The actual panel is significantly more complex. A precise ASCII recreation isn't attempted here because the source photos don't resolve all button positions clearly enough for confident placement.

3. DC-AC NETWORK SCHEMATIC: BATTERY-ONLY GROUND STATUS (Page 76)¶

This configuration outlines the active distribution channels when operating on the ground utilizing battery energy reserves prior to engine start or external power connection.

  GROUND COLD-START TOPOLOGY (BATTERY ONLY)
  ┌────────────────────────────────────────────────────────┐
  │                  MAIN BATTERY (24V)                    │
  └───────────────────────────┬────────────────────────────┘
                              │
                              ▼
  ┌────────────────────────────────────────────────────────┐
  │                  HOT MAIN BAT BUS                      │
  │     (Supplies Refueling Loops & Fire Extinguishers)    │
  └───────────────────────────┬────────────────────────────┘
                              │ ◄── [BATTERY SW SELECTED ON]
                              ▼
  ┌────────────────────────────────────────────────────────┐
  │                     MAIN DC BUS                        │
  │        (Powers Basic Flight Deck Diagnostics)          │
  └───────────────────────────┬────────────────────────────┘
                              │
                              ▼
  ┌────────────────────────────────────────────────────────┐
  │                 STATIC INVERTER 1 (INV 1)              │
  │     (Converts 28V DC to Fixed 115V AC / 400 Hz)        │
  └───────────────────────────┬────────────────────────────┘
                              │
                              ▼
  ┌────────────────────────────────────────────────────────┐
  │                   AC ESSENTIAL BUS                     │
  │        (Powers Left PFD and Essential Sensors)         │
  └────────────────────────────────────────────────────────┘

3.1. Network Limitations¶

Operating in this configuration draws power exclusively from the batteries. To prevent battery depletion, a hardware protection timer automatically isolates the main DC bus contactors after 10 minutes unless an external power source or engine generator comes online.

4. DC-AC NETWORK SCHEMATIC: GROUND EXTERNAL POWER ACTIONS (Page 77)¶

Connecting a certified 28V DC Ground Power Unit (GPU) to the fuselage external receptacle updates the distribution routing across the main buses.

  EXTERNAL DC POWER DISTRIBUTION MATRIX
  ┌────────────────────────────────────────────────────────┐
  │                  EXTERNAL GPU (28V DC)                 │
  └───────────────────────────┬────────────────────────────┘
                              │
                              ▼ [EXT PWR SWITCH PRESSED ON]
  ┌───────────────────────────┴────────────────────────────┐
  │                EXTERNAL POWER CONTACTOR                │
  └─────────────┬────────────────────────────┬─────────────┘
                │                            │
                ▼                            ▼
  ┌────────────────────────┐    ┌────────────────────────┐
  │        DC BUS 1        │    │        DC BUS 2        │
  └─────────────┬──────────┘    └────────────┬───────────┘
                │                            │
                ▼                            ▼
  ┌────────────────────────┐    ┌────────────────────────┐
  │  BATTERY CHARGER BCU 1 │    │  BATTERY CHARGER BCU 2 │
  │ (Charges Main Battery) │    │ (Charges Emer Battery) │
  └────────────────────────┘    └────────────────────────┘

4.1. Network Balance¶

The Bus Tie Contactor (BTC) closes automatically to link the networks.
Static Inverters 1 and 2 operate simultaneously, energizing the entire AC 115V/26V system architecture.
All ACW networks remain unpowered because the engine-driven ACW generators are stationary.

5. DC-AC NETWORK SCHEMATIC: HOTEL MODE POWER PROCESSING (Page 78)¶

Hotel Mode describes the operation of the Right Engine (Engine 2) running on the ground with the propeller brake engaged. This allows the engine core to turn and drive its onboard generators without turning the propeller blades.

5.1. System Capabilities and Generation Output¶

Propeller State: The mechanical prop brake locks the low-pressure turbine and propeller assembly safely at 0 RPM.
Core Output: The high-pressure engine spool spins normally, turning DC Generator 2 and ACW Generator 2.
DC System Balance: DC GEN 2 supplies DC BUS 2 directly. The Bus Tie Contactor (BTC) closes automatically to route power from DC BUS 2 over to DC BUS 1. This energizes the entire DC network.
ACW System Balance: ACW GEN 2 energizes ACW BUS 2 directly. The ACW bus tie tie contactor closes to cross-feed ACW BUS 1. This provides full aircraft environmental conditioning and galley power on the ground without generating thrust or prop wash hazards.

6. DC-AC NETWORK SCHEMATIC: NORMAL FLIGHT DISPATCH (TWO GENERATORS) (Page 79)¶

With both engines running normally and prop brakes disengaged, the electrical network operates in its optimal split-isolated configuration.

  NORMAL IN-FLIGHT BALANCED NETWORK
  ┌────────────────────────┐    ┌────────────────────────┐
  │  ENGINE 1 DC GEN 1     │    │   ENGINE 2 DC GEN 2    │
  │     (Powers DC BUS 1)  │    │      (Powers DC BUS 2) │
  └─────────────┬──────────┘    └────────────┬───────────┘
                │                            │
                ▼                            ▼
  ┌────────────────────────┐    ┌────────────────────────┐
  │   BATTERY CHARGER 1    │    │   BATTERY CHARGER 2    │
  │ (Main Battery Isolated)│    │ (Emer Battery Isolated)│
  └────────────────────────┘    └────────────────────────┘
                       \            /
                        \          /
                    [ BUS TIE CONTACTOR ]
                    [   (OPEN STATE)    ]

6.1. Operating Configurations¶

Isolation Law: The Bus Tie Contactor (BTC) is commanded OPEN. This isolates the left and right electrical grids. A short circuit or component failure on one side cannot propagate across to take down the opposing side's instruments.
ACW Independence: ACW Generator 1 powers ACW BUS 1, and ACW Generator 2 powers ACW BUS 2. The ACW tie contactor remains open.

7. EMERGENCY SCHEMATIC: DUAL DC GENERATOR LOSS OPERATION (Page 80)¶

If both engine-driven DC generators experience a simultaneous failure in flight, the network drops into a survival configuration driven by automatic load shedding.

7.1. Automatic Load-Shedding Sequence¶

Bus Isolation: The main DC BUS 1 and DC BUS 2 contactors open automatically, stripping away non-essential utilities (such as main cabin lighting, utility galley blocks, and secondary comfort systems).
Battery Power Path: The Main Battery and Emergency Battery change from a charging state to an emergency discharging state.
Essential Bus Tracking: The batteries feed power directly onto the EMER DC BUS and ESSENTIAL DC BUS.
AC Survival Path: The EMER DC BUS supplies power to Static Inverter 1 (INV 1). This keeps the AC ESSENTIAL BUS energized, preserving power to the Captain's primary displays, the Flight Warning System (FWS), and primary communication channels.
Placing the cockpit BATTERY toggle switch to the OVRD position manually ensures these critical connections remain locked online, guaranteeing a minimum of 30 minutes of battery life for emergency instrument operation.

Chapter G. ELECTRICAL SYSTEMS (CONTINUED)¶

7.2. EMERGENCY SUPPLY: BATTERY EXTENSION & VOLTAGE RECOVERY (Page 81)¶

When operating in a total dual DC generator loss environment, the duration of the battery reserve depends heavily on proactive power management and bus protection relays.

In-Flight Shedding Sequences: The automatic opening of the main DC line relays sheds approximately $70\%$ of the aircraft's non-essential electrical footprint.
Thermal Protection Interlocks: If battery internal temperatures spike during high-rate emergency discharge states, internal sensing nodes communicate warnings to the Flight Warning System (FWS).
Voltage Decay Boundaries: Standard 24V Nickel-Cadmium cells decay linearly under load. If system distribution buses drop below $21\text{V}$ DC, automated instrumentation displays blank sequentially to preserve computing capacity for the primary Integrated Modular Avionics (IMA) cabinets.

7.3. EMERGENCY SUPPLY: TRU (TRANSFORMER RECTIFIER UNIT) LOGIC (Page 82)¶

The Transformer Rectifier Unit (TRU) serves as a critical bridge between the Alternating Current Wide-Frequency (ACW) grid and the Direct Current (DC) essential distribution lines during compounding failure events.

  TRU POWER CROSS-FEED TOPOLOGY
  ┌────────────────────────────────────────────────────────┐
  │              ACTIVE ACW GENERATOR (1 OR 2)             │
  └───────────────────────────┬────────────────────────────┘
                              │
                              ▼
  ┌────────────────────────────────────────────────────────┐
  │                      ACW BUS 1                         │
  └───────────────────────────┬────────────────────────────┘
                              │
                              ▼
  ┌────────────────────────────────────────────────────────┐
  │           TRANSFORMER RECTIFIER UNIT (TRU)             │
  │     (Steps down 115V ACW & Rectifies to 28V DC)        │
  └───────────────────────────┬────────────────────────────┘
                              │
                              ▼ [AUTOMATIC TRANSFER RELAY]
  ┌───────────────────────────┴────────────────────────────┐
  │                 DC ESSENTIAL BUS NETWORK               │
  │     (Powers Critical Displays & Restores Bat Charge)   │
  └────────────────────────────────────────────────────────┘

System Performance & Auto-Schedules:¶

Activation Law: If both main DC generators fail but at least one engine is spinning fast enough to turn its engine-driven ACW generator, the TRU receives 115V ACW input directly.
Conversion Output: Converts AC power down to a stable 28V DC output stream limited to a maximum continuous load of $200\text{ A}$.
System Isolation Relief: The TRU automatically feeds the DC ESSENTIAL BUS and EMER DC BUS. This completely halts emergency battery discharge loops and provides infinite emergency timeline coverage, bounded only by the remaining usable fuel inside the wing tanks.

3. DC-AC PANEL (Page 83)¶

Source: direct reading of PNG page 85 (printed 83) — annotated panel diagram

ATA 24

The source page shows the MAIN ELEC PWR overhead panel with full callout annotations. The panel has a central DC voltmeter/ammeter and buttons arranged around it. Annotations from source:

Left side (DC section): * UNDV: DC STBY < 19.5V. ELEC STBY UNDV on FWS. * OVRD: Transfer STBY buses from HOT MAIN to HOT EMER BAT BUS. * Arrows: Emergency supply indicators (respective battery discharging). * TRU: ON PB in: TRU is connected to ACW BUS 2. Arrow illuminates green when DC EMER, DC STBY, INV 1, AC STBY and DC ESS BUS are supplied by TRU. * EMER BAT CHG: FAULT: overload detected by MFC or failure of contactor. ELEC EMR CHG on FWS. Contactor auto opens if: thermal runaway / bus voltage <25V / start sequence / bat sw on OVRD. * DC BUS OFF: Respective bus not supplied. ELEC DC 1/2 on FWS. * DC SVCE/UTLY BUS: DC SVCE supplies power in flight and on ground during airplane servicing operations. UTLY buses supply non-essential loads. In automatic shedding mode, one of the DC SVCE/UTLY 1 and 2 buses is shed by the BPCU — SHED is illuminated (amber). OFF: all DC SVCE/UTLY 1 and 2 buses are shed. * DC GEN 1/2: Generator FAULT Lt. Extinguishes above 61.5% NH if GEN is operating normally. * X START FAIL: Opposite generator has not come on line to assist start at 10% NH. ELEC X START on FWS.

Right side (AC / switching section): * BUS OFF: Associated bus de-energized. ELEC AC 1/2 on FWS. * INV FAULT: Under/over V or INV output (Hz failure or supply loss). Auto-transfer of all AC buses to remaining inverter. ELEC INV 1/2 on FWS. * OVRD: Insures basic mode operation by overriding all other protections. * ON: Basic mode: STBY buses supplied by HOT MAIN BAT BUS. Ext or gen pwr: DC STBY bus transferred to HOT EMER BAT BUS, AC STBY BUS to DC BUS 1. * OFF: ESS BUS, DC STBY BUS + INV 1 are supplied from HOT MAIN BAT BUS. DC EMER BUS is isolated from HOT EMER BAT BUS. * BTC: The BPCU controls BTC operation. Ext power, hotel mode or single gen operation — BTC is closed (green flow line). * ISOL: BTC and BTR opened (released). * DC GEN 1/2 FAULT: Protection trip by GCU. Auto reset if underspeed. BTC auto closes. ELEC DC 1/2 on FWS. * AVAIL: GPU has been checked by BPCU for over/under voltage, over current and polarity. * EXT PWR: AVAIL → ON. GPU has priority on both generators.

9. ELEC DC SYSTEM DISPLAY (SD) PAGE TOPOLOGY (Page 84)¶

The Multi-Function Display (MFD) utilizes a dedicated systems synoptic architecture to track live generation values and bus configuration metrics.

  MFD DC SYNOPTIC PAGE ARCHITECTURE
  ┌────────────────────────────────────────────────────────┐
  │  [GEN 1] 28.2V  095A                  28.2V  092A [GEN 2]│
  │     │                                       │          │
  │  [DC BUS 1] 28.1V                       28.1V [DC BUS 2]│
  │     │                                       │          │
  │     ├─────────────► [ BTC LINE ] ◄──────────┤          │
  │     │               (Open/Closed)           │          │
  │     ▼                                       ▼          │
  │ [MAIN BAT] 27.5V +012A                27.5V +010A [EMER]│
  └────────────────────────────────────────────────────────┘

Visual Element Conventions:¶

Green Lines/Boxes: Confirm a system component is fully online, operating inside safety margins, and maintaining normal connection paths.
Amber Lines/Boxes: Identify a tripped breaker, a failed generator source, or an unpowered bus line.
White Symbols: Represent an unselected standby element or a component intentionally isolated by manual pilot configuration inputs.

10. SD PAGE BATTERY TELEMETRY AND ALERTS (Page 85)¶

The battery status symbols dynamically adjust text colors and directional vectors to communicate charge degradation values.

Arrow Parameters: - An arrow pointing inward toward the battery symbol box confirms a positive charge profile ($+$ Amps vector), indicating the battery chargers are functioning normally.
An arrow pointing outward away from the battery symbol box warns the crew that the battery is actively discharging onto the system buses ($-$ Amps vector).
Thermal Runaway Fault Protection: - If internal battery temperatures breach $50^\circ\text{C}$, the numerical text value flashes amber, and the ELEC BAT OVHT master caution triggers.
The associated Battery Charge Unit (BCU) opens its line contactor automatically. This isolates the overheating battery cell from all charging paths to prevent a catastrophic thermal cabin smoke event.

11. ACW (ALTERNATING CURRENT VARIABLE WIDE FREQUENCY) ARCHITECTURE (Page 86)¶

The Alternating Current Variable Wide Frequency (ACW) system manages high-voltage alternating current requirements without the mechanical weight penalties of constant-speed drive gearboxes.

11.1. Physical Characteristics¶

Power Generators: Two engine-driven brushless AC generators directly coupled to the auxiliary component gearboxes of Engine 1 and Engine 2.
Voltage Properties: 115V AC / 200V AC across a standard 3-Phase framework.
Wide-Frequency Spectrum: Output frequency scales dynamically based on propeller rotational speed ($N_P$):
Minimum Frequency: $400\text{ Hz}$ at minimum operational prop settings.
Maximum Frequency: $600\text{ Hz}$ during maximum takeoff RPM configurations.

12. ACW SYSTEM DISTRIBUTION SCHEMATIC (Page 87)¶

Under normal operational flight conditions, the ACW system executes an isolated dual-channel delivery map to power high-draw structural sub-systems.

  ACW SPLIT-BUS OPERATION MAP
  ┌────────────────────────┐    ┌────────────────────────┐
  │   ACW GENERATOR 1      │    │    ACW GENERATOR 2     │
  └─────────────┬──────────┘    └────────────┬───────────┘
                │                            │
                ▼                            ▼
  ┌────────────────────────┐    ┌────────────────────────┐
  │      ACW BUS 1         │    │       ACW BUS 2        │
  └───────┬──────────┬─────┘    └─────┬──────────┬───────┘
          │          │                │          │
          ▼          ▼                ▼          ▼
     [Wing De-Ice] [INV 2]       [Aux Pump] [Galley Power]
          │                           │
          └────────────► [ ACW BTC ] ◄┘
                         (OPEN STATE)

12.1. System Cross-Feed Automation:¶

If one ACW generator fails, the central ACW BTC (Bus Tie Contactor) closes automatically. This cross-feeds the unpowered side bus from the surviving generator.
Automated Load-Shedding: To prevent overload tripping on the remaining operational generator, non-essential comfort utilities—such as main galley ovens and secondary cabin recirculation blowers—are shed immediately via automatic contactor open loops.

13. ELEC ACW OVERHEAD CONTROL PANEL LAYOUT (Page 88)¶

Located on the lower portion of the forward overhead electrical panel assembly.

  ELEC ACW PANEL DETAIL
  ┌────────────────────────────────────────────────────────┐
  │                       ELEC ACW                         │
  │                                                        │
  │     ACW EXT PWR             ACW GEN 1     ACW GEN 2    │
  │      [ AVAIL ]              [ FAULT ]     [ FAULT ]    │
  │      [  ON   ]              [  OFF  ]     [  OFF  ]    │
  │                                                        │
  └────────────────────────────────────────────────────────┘

ACW EXT PWR Pushbutton: - AVAIL (Green text): Confirms an external AC ground cart is plugged into the fuselage receptacle and matches correct voltage, frequency, and phase parameters.
ON (White text): Closes the external power contactor to energize the entire ACW ground network when the engines are shut down.
ACW GEN 1 / ACW GEN 2 Pushbuttons: - FAULT (Amber text): Illuminates if the ACW Generator Control Unit tracks an over-voltage, zero-output, or under-frequency condition, pulling the generator offline.
OFF (White text): Illuminates when the pilot manually deselects the pushbutton to isolate that specific ACW generation circuit.

14. ACW SYSTEM DISPLAY (SD) PAGE VISUAL REPRESENTATION (Page 89)¶

The ACW synoptic presentation displays live operational parameters for high-voltage alternation networks.

Generator Parameter Lines: Display real-time 3-phase line voltage output values, system current delivery metrics, and active operating frequency counters (e.g., 115V 482Hz).
Dynamic Loading Connections: The display automatically redraws power lines to map active delivery paths when high-amperage sub-systems—such as the AUX PUMP or the structural WINDSHIELD HEATING elements—are switched on by the flight crew.

15. CIRCUIT BREAKER PANELS ARCHITECTURE (Page 90)¶

Thermal-trip circuit breakers provide localized physical circuit defense boundaries across all electrical networks.

15.1. Structural Breakdown Locations¶

Overhead Breaker Panels (120VU / 121VU): Located directly above the cockpit crew seats. These safeguard low-amperage command paths, engine validation sensors, logic line computers, fuel valve actuators, and basic panel illumination networks.
Main Cabin Electrical Rack (90VU): Located directly behind the First Officer's flight deck seat structure. This zone houses primary high-amperage bus bars, large system relays, transformer units, and main distribution contactor assemblies.

15.2. Safety Guard Identification Conventions¶

Primary flight-critical safety breakers—such as the flight data recorder loops or fire detection lines—are retrofitted with distinct physical plastic collar rings. These high-visibility indicators prevent flight crews from accidentally pulling or resetting critical circuits during standard operating regimes.

Chapter G. ELECTRICAL SYSTEMS (CONTINUED)¶

15.3. SD ACW / DC GRAPHIC LOGIC SUMMARY (Page 91)¶

The active configurations shown on the MFD System Display (SD) pages use color-coded logic vectors to dynamically communicate the status of line contactors and isolation relays.

ACW Synoptic Paths: Displays the status of the main ACW generators, the state of the ACW Bus Tie Contactor (ACW BTC), and live frequency/amperage telemetry for active consumers like the hydraulic auxiliary pump and structural leading-edge de-icing matrices.
DC Synoptic Paths: Outlines battery charge current directions ($+/-$ indications), line potential values across DC Bus 1 and DC Bus 2, and the physical alignment of the main cross-feed BTC line bar.

15.4. OVERHEAD CIRCUIT BREAKER PANELS (120VU / 121VU) (Page 92)¶

Located directly on the flight deck ceiling framework above the Captain and First Officer operating stations. These panels safeguard low-amperage systems, primary digital processors, and cockpit monitoring lines.

  OVERHEAD BREAKER ARCHITECTURE (120VU / 121VU)
  ┌────────────────────────────────────────────────────────┐
  │  [MFC 1A]  [FGC 1]  [EEC 1]   │   [EEC 2]  [FGC 2]  [MFC 2A] │
  │   ( 5A )    ( 5A )   ( 7.5A)  │  ( 7.5A)   ( 5A )    ( 5A )  │
  │                               │                               │
  │  [FUEL TR] [CCAS 1][ADC 1]    │   [ADC 2] [CCAS 2][FUEL TR]  │
  │   ( 10A)    ( 5A )   ( 5A )   │   ( 5A )   ( 5A )    ( 10A)  │
  └───────────────────────────────┴───────────────────────────────┘

System Configuration Safety Rules:¶

Collar Identifiers: High-criticality breakers, such as those for the Flight Warning System (CCAS) or engine control computers (EEC), are fitted with raised plastic mechanical safety collars to prevent accidental manual isolation in flight.
Reset Protocols: Tripped breakers for high-amperage motors or fuel pump circuits must not be reset in flight unless explicitly directed by a QRH emergency checklist procedural action line.

15.5. MAIN CABIN ELECTRICAL RACK (90VU) (Page 93)¶

Located inside the forward cabin zone directly behind the First Officer's seat structure. This rack acts as the primary power distribution hub for high-amperage components and system relays.

Content Elements:¶

Primary Bus Bars: Houses the heavy high-voltage structural copper bus bars for DC Bus 1, DC Bus 2, ACW Bus 1, and ACW Bus 2.
Contactor Modules: Contains the main electrical line relays, Transformer Rectifier Units (TRU), and automatic bus-tie contactor (BTC) mechanical switches.
Thermal Protection Blocks: Equipped with integrated cooling ventilation pathways to prevent thermal localized tracking faults from escalating into open smoke hazards.

16. SYSTEM INTERCONNECTION SUMMARY MATRIX (Page 94)¶

The operational behavior of the electrical system changes dynamically based on the available power sources on the ground and in flight:

Power Configuration	DC Bus 1 / 2 Status	ACW Bus 1 / 2 Status	BTC Auto-Logic
Battery Only (Ground)	Powered (10 Min Limiter)	Completely Isolated	BTC Closed
External DC GPU Only	Fully Powered	Isolated	BTC Closed
Hotel Mode (Right Engine)	Fully Powered via Gen 2	Bus 2 Active / Cross-Fed	DC BTC Closed / ACW BTC Closed
Normal Flight (Dual Gen)	Fully Powered (Isolated)	Fully Powered (Isolated)	Both BTC Lines Open
Dual DC Generator Loss	Emergency Shedding Active	Fully Powered (If Engines Spin)	DC BTC Closed (Batteries / TRU)

17. NON-NORMAL ELECTRICAL FAULT CONDITIONS (Page 95)¶

The Generator Control Units (GCUs) continuously monitor the system grid to protect upstream avionics from voltage or current anomalies.

Bus Fault Isolation: If a short-circuit condition is localized on DC Bus 1, the associated GCU opens its line contactor immediately, and the automatic logic locks the BTC open to protect the healthy side bus (DC Bus 2).
Battery Charger Failures: If a Battery Charge Unit (BCU) detects an internal thermal tracking fault, it isolates its charging relay to prevent battery cell cooking, generating an ELEC BCU FAULT caution on the EWD.

18. EMERGENCY ELECTRICAL RECONFIGURATION (Page 96)¶

This section outlines the specific manual workflows required when the automated load-shedding architecture fails to isolate a system failure cleanly.

Manual Bus Tie Management: If a contactor welds shut during a short circuit, pilots must manually isolate the faulty side using the overhead BUS TIE pushbutton to force a split-bus configuration.
Emergency Battery Deployment: If all primary and secondary generation fields fail, pulling the BATTERY switch to OVRD bypasses the automatic 10-minute ground timers to maintain power to the emergency distribution line until touchdown.

19. ELECTRICAL SYSTEM SYNOPTIC DATA READOUTS (Page 97)¶

A summary of the normal, acceptable telemetry limitations displayed on the MFD system monitoring pages:

DC Voltage Tolerances: Normal operating range spans $27.0\text{V}$ to $29.0\text{V}$ DC.
DC Amperage Limits: Maximum nominal continuous load must remain below $400\text{A}$ per engine-driven starter-generator.
ACW Output Spectrum: Nominal output must hold steady at $115\text{V}$ AC / $3\text{-Phase}$, with frequency scaling smoothly between $400\text{Hz}$ and $600\text{Hz}$ based on propeller RPM ($N_P$).