They had flickering lights in a rural area. Fixed itself.

[11bgrunt]

Residential customer complaint of lights flickering a couple three times a day. They recorded the times they saw the lights flicker. This was checked and no cause found. A recorder was installed at the service entrance that did show the voltage drop from the AC but that was many times a day and didn't match their complaint. The recorder did show there was a voltage dip caused by a short 50 amp current rise during the times they saw the flicker. I checked the service by adding external loads to create phase unbalance and constant loading. No problem found.
Last week they called and said the flicker had stopped. In normal conversation they said they had just gotten their water well fixed.
Everyone, including me, is looking at the submersible water pump as the probable cause. After thinking about it for a while, I can't understand how a 240 volt pump motor can cause a current draw on one leg of the 240. If the motor or wiring was bad at the pump to create a fault, I would expect more current flow and tripping the OCPD. The well guys and the pump motor are gone now so I can't go backwards to find out.
If the pump motor is cap start and there is no fault, can the motor draw down only one leg of the 240 serving the well?

 

[GoldDigger]

The motor can draw more current from one leg of the 240 that the other only if there is a current path to EGC or ground (and therefore back to the supply transformer neutral) where the source of the fault current is closer to one leg of the 240 than to the other.
The fault current could be intermittent, influenced by the vibration of the motor, and thus explain the flickering.
An intermittent high current which never goes above the mag trip threshold of the breaker will not necessarily last long enough or be large enough to trip on thermal.
If the current path is actually through earth rather than through a wire EGC connected to the well casing, then it is perfectly reasonable that the breaker would not trip, especially if the pump only had power applied for a few minutes at a time under the control of the pressure switch.
Note also that some pressure switches will only interrupt one of the two hot leads to the pump. Or for larger pump motors the pressure switch may only control a contactor which could interrupt both legs.