Emergency panel back feed with a single 120V source with SPD

[Bobakman]

In an emergency if power a  panel board  so that both 120V busses are powered by a single 120V generator source (Generator is an inverter type with only 120V, 240V is not available) will it damage any SPD's in the panel (All 240V breakers will be open in such a case so no 240V equipment will be powered)? Specifically I am concerned about a model 

QO2175SB Surge Q model since it does not have a two pole breaker for isolation being hard wired to the panel it will be connected to both busses powered only by one 120V source.
Please see generator wiring diagram attached.

[S-W]

Hello @Bobakman, thank you for your post, we'll come back to you!

[Bobakman]

Thank you I look forward to a reply.
Bob

[Manon_Chaboud · Accepted solution]

Hello @Bobakman, 

Thank you for your question regarding the use of a single 120V inverter-type generator to backfeed both 120V busses in a panel that includes a QO2175SB Surge Protective Device (SPD).

In this setup, where both panel busses are energized from the same 120V source and all 240V breakers are open, there is a potential concern with the SPD. The QO2175SB is designed to operate across a standard 120/240V split-phase system, where it expects a 240V differential between the two hot legs. When both busses are fed from the same 120V leg, there is no voltage difference between them. This can lead to abnormal operating conditions for the SPD, which may result in reduced protection performance or even damage to the device over time.

Additionally, since the SPD is hardwired and not connected through a two-pole breaker, it remains continuously connected across both busses. This means it will still be exposed to whatever voltage conditions exist between those busses, even if they are not ideal for its intended operation.

While your approach of opening all 240V breakers is a good safety measure to protect connected equipment, it does not address the SPD’s exposure. In some cases, it may be more appropriate to use an SPD designed specifically for 120V single-phase systems when operating under these conditions.

Have a good day!