Electrical power distribution and control

The EPDC subsystem distributes 28-volt dc electrical power and generates and distributes 115-volt, three-phase, 400-hertz ac electrical power to all of the space shuttle systems' electrical equipment throughout all mission phases. The EPDC subsystem consists of a three-bus system that distributes electrical power to the forward, mid-, and aft sections of the orbiter for equipment used in those areas. The three main dc buses are main A (MNA), main B (MNB) and main C (MNC). Three ac buses, AC1, AC2 and AC3, supply ac power to the ac loads. Three essential buses, ESS1BC, ESS2CA and ESS3AB, supply control power to selected flight crew controls and operational power to electrical loads that are deemed essential. Nine control buses, CNTL AB1, 2, 3; CNTL BC1, 2, 3; and CNTL CA1, 2, 3, are used only to supply control power to flight crew controls. Two preflight buses, PREFLT 1 and PREFLT 2, are used only during ground operations.

Electrical power is controlled and distributed by assemblies. Each assembly-main distribution assembly, power controller assembly, load controller assembly and motor controller assembly-is an electrical equipment container or box.

The dc power generated by each of the fuel cell power plants is supplied to a corresponding DA. Fuel cell power plant 1 is supplied to DA 1, FCP 2 to DA 2 and FCP 3 to DA 3. Each DA contains remotely controlled motor-driven switches called power contactors used for loads larger than 125 amps. The power contactors are rated at 500 amps and control and distribute dc power to a corresponding mid power controller assembly, forward power controller assembly and aft power controller assembly. Power contactors are also located in the APCAs to control and distribute GSE 28-volt dc power to the orbiter through the T-0 umbilical before the fuel cell power plants take over the supply of orbiter dc power.

Each of the mid, forward, and aft PCAs supplies and distributes dc power to a corresponding mid motor controller assembly, forward motor controller assembly, forward load controller assembly, aft load controller assembly and aft motor controller assembly and dc power to activate the corresponding ac power system.

Each PCA contains remote power controllers and relays. The RPCs are solid-state switching devices used for loads requiring current in a range of 3 to 20 amps. The RPCs are current protected by internal fuses and also have the capability to limit the output current to a maximum of 150 percent of rated value for two to three seconds. Within three seconds the RPC will trip out, which removes the output current. To restore power to the load, the RPC must be reset, which is accomplished by cycling a control switch. If multiple control inputs are required before an RPC is turned on, hybrid drivers are usually used as a logic switch, which then drives the control input of the RPC.
Each LCA contains hybrid drivers, which are solid-state switching devices (no mechanical parts) used as logic switches and for low-power electrical loads of less than 5 amps. When the drivers are used as a logic switch, several control inputs are required to turn on a load. Hybrid drivers are also used in the MPCAs. The hybrid drivers are current protected by internal fuses. Hybrid relays requiring multiple control inputs are used to switch three-phase ac power to motors.