Electrical Bus Switching

Question 1

GRD PWR switch

Answer 1

• controls external power to the ac transfer buses through the external power contactor (EPC) and the bus tie breakers (BTBs)
• both AC transfer buses receive power when you put the ground power switch to the ON

Question 2

BUS TRANSFER switch
AUTO- (3)
OFF-

Answer 2

AUTO-

• provides 3 things
• the BTBs, the DC bus tie relay, and the TR3 transfer relay work automatically as necessary
  1) BTBs work automatically to maintain power to the transfer buses from any operating generator (engine or APU) or external power
  2) the DC bus tie relay automatically provides normal or isolated operation as required
  3) if transfer bus 2 looses power, TR3 transfer relay automatically energizes so that TR3 gets power from transfer bus 1

Question 3

TRANSFER BUS OFF light

Answer 3

• the TRANSFER BUS OFF lights give indication of power status on each transfer bus
• illuminated means that transfer bus is not powered

Question 4

SOURCE OFF light

3

Answer 4

• no source has been manually selected to power the related transfer bus, or
• the manually selected source has been disconnected, or
• the related transfer bus is not powered by the last selected source

Question 5

GEN OFF BUS light

Blue

Answer 5

• illuminated means the IDG is not supplying power to its related transfer bus
• the engine generator control breaker (GCB) is open

Question 6

APU GEN OFF BUS light

Blue

Answer 6

• illuminated means APU is up to speed (95%), frequency is above 380 Hz and ready for load, however, the APU is not powering a bus

Question 7

GENERATOR switch

Answer 7

• each IDG has a generator switch
• each switch supplies electrical on/off signals to let the generator control unit (GCU) close its generator breaker (GB)

Question 8

APU GENERATOR switch (1 and 2)

Answer 8

• the APU GEN switches can supply power to the AC transfer buses
• there are two switches - one for each bus tie breaker (BTB)

Question 9

GRD POWER AVAILABLE light

Blue

Answer 9

• light is on when external AC power is connected and the power quality is in limits
  panel
• the bus power control unit (BPCU) does a check of power quality and supplies the ground for the light to come on