c|net's NASA troubleshoots mysterious ISS power issue links to the NASA Blog post Crew Waits for Dragon Mission While Teams Troubleshoot Power Issue which says:

Monday morning, teams identified an issue with the International Space Station’s electrical power system and are working to identify the root cause and restore full power to the system. There are no immediate concerns for the crew or the station. An issue is being worked with a Main Bus Switching Unit (MBSU) that distributes electrical power to two of the eight power channels on the station. Flight controllers have been working to route power through the remaining six power channels. Electrical power generated by the station’s solar arrays is fed to all station systems through these power channels. Discussions are underway to determine any impacts to SpaceX’s CRS-17 cargo resupply mission targeted for launch May 1.

Is this like house wiring, where a circuit breaker box will split the power and hard wire it to different locations, and if one breaker gets thrown, that area is blacked out until you route extension cords? Or can areas choose or select from at least more than one channel?


1 Answer 1


Eight solar array wings, eight power channels.

My answer to this question shows one power channel: How does electrical safety system work on ISS?

You can read about the gory details of each channel in section and subsequent of this document

This diagram shows how the eight channels can be interconnected through the four main bus switching units (MBSUs) (normally all the channels are isolated from each other). The small boxes inside the MBSUs are Remote Bus Isolators (RBIs) - essentially switches that can be commanded open or closed.

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The arrows are where the users of power connect. Depending on what the failure in the MBSU is, the ground may be able to command workarounds to restore the user power.

MBSUs have failed before and been replaced by EVA or robotics. The last time one failed, it simply lost the ability to be commanded so it was "stuck" in its last configuration but still provided power.

The Station’s four MBSUs all reside on the central S0 Truss and each accepts primary power from two of the station’s power channels which it then distributes downstream to direct-current converters that create the secondary power flow to the various users. MBSUs can be cross-tied to re-wire the Station’s power system architecture in the event of an upstream failure.

So depending on what failed, you could lose all the user power to one or two channels from an MBSU, or you could lose the ability to reconfigure the power supply to those channels. Or both.

That last linked article is pretty good and includes an explanation of how the crew can install "jumper cables" to regain some power if the MBSU goes down completely.

Update: This nasaspaceflight.com article reports that the failed MBSU was MBSU 3, and that it was a total failure, resulting in the loss of power channels 3A and 3B.

Major impacts included KU-band, Alpha Magnetic Spectrometer (AMS), CMG-3 and Space Station Remote Manipulator System (SSRMS) redundancy.

  • $\begingroup$ This helps a lot, thanks! Somehow the big loop in the middle bothers me. If it were ground, we might call it a ground loop and suggest that it can in some cases cause trouble $\endgroup$
    – uhoh
    Commented Apr 30, 2019 at 4:16
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    $\begingroup$ Agreed. I think normal ops is to have all 8 channels isolated from each other. The idea was that if an array or set of batteries went down, you could cross tie the user power on the dead channel to another channel input. But the cross-tying units seem to be what keep failing... $\endgroup$ Commented Apr 30, 2019 at 4:17
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    $\begingroup$ @uhoh The way I read that diagram, it would only be a loop if they configured it to be a loop. Every leg of the loop is switchable. They could connect everything together without creating a loop by just picking one segment to not close. $\endgroup$
    – Cort Ammon
    Commented May 1, 2019 at 0:38
  • $\begingroup$ @CortAmmon oh I see what you mean; those little squares are "switches". Yes that makes total sense, thanks! $\endgroup$
    – uhoh
    Commented May 1, 2019 at 0:43
  • $\begingroup$ Yes, they are "remote bus isolators" or RBIs. They can be commanded open or closed. At least, they can be if the MBSU is working. $\endgroup$ Commented May 1, 2019 at 1:13

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