120Mission Specialist Paolo Nespoli practices using a tool on the bus switching unit that is part of the payload.

In the Orbiter Processing Facility bay 3, STS-120Mission Specialist Paolo Nespoli practices using a tool on the main bus switching unit that is part of the payload on the mission. 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