In the Orbiter Processing Facility bay 3 a worker checks the cover on a main bus switching unit.

In the Orbiter Processing Facility bay 3, a worker checks the cover on a main bus switching unit, part of the payload on mission STS-120. A main bus switching unit is used for power distribution, circuit protection and fault isolation on the space station's power system. The units route power to proper locations in the space station, such as from solar arrays through umbilicals into the U.S. Lab. The unit will be installed on the external stowage platform 2 attached to the Quest airlock for temporary storage. Discovery is targeted to launch mission STS-120 no earlier than Oct. 20. Photo credit: NASA/Jim Grossmann