Main Bus Switching Units (MBSU)

Description:

• The Main Bus Switching Units (MBSUs) is a component of the ISS United States Orbital Segment (USOS) Electrical Power System (EPS).

Locations, Installation and changing date (on 11 Nov 2017):

11.04.2002   installation of S0 Truss with four MBSU′s
03.08.2005   installation of ESP-2 with one spare MBSU on FRAM-4
30.10.2007   installation of one spare MBSU, delivered on STS-120, on ESP-2 FRAM-6
25.08.2013   installation of one spare MBSU, delivered on HTV-4, on ESP-2 FRAM-4 

30.08.2012   changing the MBSU, delivered on ESP-2 with one MBSU failed on S0
28.01.2013   relocation of the failed MBSU to ELC-2
08.02.2016   deinstallation/reinstallation of the MBSU on ELC-2 for test purposes
04./05.05.2017   changing the MBSU, delivered on HTV-4 with one MBSU failed on S0
08.08.2017   relocation of the failed MBSU from ESP-2 to JEM
20.10.2017   deinstallation of the MLI from one MBSU on ESP-2
04.11.2017   relocation of the repaired MBSU from JEM to ESP-2

• 26 Jan 2018: In preparation for the upcoming Main Bus Switching Unit (MBSU) maintenance, today the crew extended the A/L slide table into the JEM pressurized module (JPM), removed the passive capture mechanism, and installed JEM ORU Transfer Interface (JOTI) on the slide table. Next week, robotics ground controllers will remove an MBSU from its location outside the station and place it on the JEM A/L slide table for transfer inside. Once inside, station crew will perform maintenance on the MBSU to recover its functionality and then pass it through the JEM A/L again for reinstallation outside the station.
  It is possible, that this will be delayed due to the problems with the LEE. (see ext. links)
• On May 1-2, 2019 the Canadarm-2 and Dextre arms were used to replace a failed Main Bus Switching Unit on the ISS S0 truss. The failed MBSU supported the channel 3 solar arrays. It was exchanged with an MBSU on External Stowage Platform 2. This replacement unit was originally the channel 2 MBSU, which failed in 2017, was brought inside ISS, repaired, and replaced on ESP-2.
• There are 7 MBSUs on ISS (on May 2019):

     Launch order   Launch year   Current role
     MBSU 1         2002          Failed, on ELC-2 since 2013
     MBSU 2         2002          Active as MBSU-3 since 2019
     MBSU 3         2002          Failed, on ESP-2 since 2019
     MBSU 4         2002          Active as MBSU-4 since 2002
     MBSU 5         2005          Active as MBSU-2 since 2017
     MBSU 6         2007          Active as MBSU-1 since 2012
     MBSU 7         2013          Spare on ESP-2 since 2013
  

External links:

• NasaSpaceflight: 01 Sep 2012: ISS program approve new EVA following MBSU-1 installation failure
• NasaSpaceflight: 05 Sep 2012: second EVA installs troublesome MBSU-1