two breakers trip but only one circuit is faulted

[7bomber7]

In a 277 lighting panel, two 20 amp breakers tripped. Each is on a different phase from the other. One of the circuits (circuit #2) could be reset but only if the other circuit (circuit #1) stayed off. Circuit #1 would not reset. The load on both are outdoors and are 100 feet, say, from the panel. I am asking why circuit #2 trips but then can be reset?
From my investigation, I believe the two circuits share a single neutral conductor. I believe circuit #1 is faulted to the neutral (not to ground). When both circuits are simultaneously energized, circuit #2 is also faulted through the neutral and the fault of circuit #1 to the line voltage of circuit #1 (we now have phase to phase voltage on the load of circuit #2). Therefore, it trips. But, so long as #1 circuit is tripped, the line conductor of circuit #1 is open and, therefore, circuit #2 does not trip.
Whaddayuh think? Has anyone experienced this before? where a shared neutral resulted in the trip of an unfaulted circuit because of a fault in the other circuit on the neutral? I am supposing that the long neutral is a factor because the voltage drop in the neutral would cause circuit #2 to be reactive to the condition of circuit #1.
Any experience or thoughts is greatly appreciated. Bomber

 

[Dennis Alwon]

Sounds like you have a double feed on the circuit. When both are on you have a phase to phase short. However

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