Electrical System EC145

What are the main power sources of the BK117 C-2 electrical system?

·         Two engine-driven starter-generators. These normally supply 28.5 VDC once the engines are running.

·         One onboard 24 VDC nickel-cadmium battery. The battery provides starting power when no EPU is used and emergency power during generator failures.

·         External Power Unit (EPU). The EPU can power the aircraft on the ground through the external power receptacle.

Checkride emphasis: know which source is supplying the buses in normal flight, on battery only, with EPU connected, and during generator failures.

What are the major parts of the electrical system?

The system is divided into power supply, power distribution, power consumers, and optional AC power.

·         Power supply: starter-generators, battery, and EPU.

·         Power distribution: bus bars, relays, and diodes.

·         Power consumers: essential, non-essential, and shed loads.

·         AC power: optional inverter-supplied AC voltages.

Checkride emphasis: explain power as source -> relay/bus control -> bus bars -> consumers.

What is the ESS BUS and why is it important?

ESS BUS means essential bus. Essential equipment is connected to ESS BUS 1 and ESS BUS 2. Examples from the manual include equipment such as the forward fuel pump and N2 trim.

Key points:

·         ESS buses remain the priority during degraded electrical operation.

·         They can be supplied by generator power in normal operation.

·         During emergency/battery operation, the ESS buses can still receive power through the BAT BUS and diode paths.

Oral answer: the ESS BUS powers critical equipment needed to continue safe flight and is protected by the system design so it remains powered when nonessential loads are removed.

What is the NON-ESS BUS?

NON-ESS BUS means non-essential bus. It supplies equipment that is not required for basic safe continuation of flight, such as utility receptacles and ventilation blowers.

Key relay logic:

·         NON-ESS BUS 1 connects to MAIN BUS 1 through its NON-ESS BUS relay.

·         NON-ESS BUS 2 connects to MAIN BUS 2 through its NON-ESS BUS relay.

·         The NON-ESS buses are normally available when both generators are operating properly or when EPU logic permits them.

·         If generator failure logic removes them, the aircraft preserves power for essential loads.

Oral answer: the NON-ESS BUS is a lower-priority load bus that can be shed to protect essential electrical power.

What is the SHED BUS?

The SHED BUS powers loads that can be removed automatically during emergency or degraded operation. Examples in the manual include aft fuel pump and copilot static port heating system.

Key relay logic:

·         SHED BUS 1 is connected to MAIN BUS 1 through SHED BUS relay 1.

·         SHED BUS 2 is connected to MAIN BUS 2 through SHED BUS relay 2.

·         In a double generator failure, both SHED BUS relays disconnect both SHED BUSes from the electrical system.

Oral answer: SHED BUS loads are important but not highest priority; they are removed during emergency electrical operation to conserve remaining power.

What is the purpose of the diode boxes?

Each diode box contains diodes used to ensure power supply to the ESS BUSes while preventing unwanted reverse current paths.

Plain-language explanation:

·         A diode is an electrical one-way valve.

·         It lets power flow toward the intended bus.

·         It prevents one bus or source from backfeeding another path in an undesired direction.

Checkride emphasis: the diode boxes are part of why the ESS BUSes can remain powered and isolated during abnormal power distribution.

What is the purpose of the bus tie relays?

The bus tie relays are high-power relays that connect the MAIN BUSes to the BAT BUS and allow system redundancy.

Specific functions:

·         K10003 connects MAIN BUS 1 to the BAT BUS.

·         K10004 connects MAIN BUS 2 to the BAT BUS.

·         When both bus tie relays are energized, their auxiliary contacts establish the equalizing line between GCU 1 and GCU 2 when both generators operate in parallel.

·         If a short circuit occurs on MAIN BUS 1 or MAIN BUS 2, the affected bus tie relay disconnects to isolate that bus from the rest of the system.

Oral answer: bus tie relays provide cross-power capability, support generator load sharing, and isolate a failed/shorted main bus.

What causes a bus tie relay to de-energize?

A bus tie relay can de-energize when:

·         The MAIN BUS short circuit protection unit detects undervoltage below 12.0 VDC on the associated MAIN BUS.

·         A bus tie control switch is placed OFF.

When the bus tie switch is OFF, only ESS BUS 1 and ESS BUS 2 are supplied by the onboard battery or EPU.

Checkride emphasis: bus tie relays are not just connection devices; they also help isolate faults and protect the remaining electrical system.

What happens when the BAT MSTR switch is momentarily placed to ENGAGE?

ENGAGE is spring-loaded to ON. Momentarily selecting ENGAGE energizes the bus tie control relays and closes a self-locking loop. This energizes and closes both bus tie relays, connecting MAIN BUS 1 and MAIN BUS 2 to the BAT BUS.

Why it matters:

·         It connects the main buses for battery/EPU power distribution.

·         It supports normal system configuration before/after start depending on the mode.

·         It allows bus tie relay closure unless protection logic prevents it.

Oral answer: ENGAGE closes the bus tie relays and ties the main bus system into the battery bus logic.