Submersible tripping out on current unbalance.


Senior Member
3 phase submersible water well tripping out on current unbalance intermittently.
Pump is set 400' deep and served 480/277. Franklin submersible pump and motor with Franklin's SubMonitor control. Control is programmed to the limit of 10% current unbalance.
Checked today with a PQA at the starter and voltages were as good as anyone could expect. Within a volt of each other. Currents were 74.4, 74.9 and 66.1. THDv was under 3%. Sine wave was perfect. I use the rule of 1% voltage unbalance can cause 6-10% current unbalance. Rolled the conductors at the starter serving the motor and the high and low legs stayed on the line conductors.
This suggest a supply side problem to me. Thermal was used on the gear we could open with no hot joints identified. Measured currents and volts back at the POCO's metering set and saw the same good volts and the same unbalanced currents.
I can't explain the unbalanced currents that will exceed 10% and cause the SubMonitor to trip the motor out. I have not been at the well when it tripped out. We are installing a recorder tomorrow. How can the currents be so far out if voltage is balanced and the low current stays on the source phase not the motor lead?
This is a water supply company that has three submersibles. Only this pump is large enough that Franklin requires the SubMonitor be used to keep the warranty on the pump. Replacement is $8K, so it will not be run without the SubMonitor. There are no issues with the two smaller pumps that just have starters.

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Senior Member
NE (9.06 miles @5.9 Degrees from Winged Horses)
Electrical Contractor
Have you checked the connections at the well head? Megged the motor? Megged the wiring to the pitless or well head?

Our village had a similar problem but they kept restarting until the smoke was gone. Connections at the well head had failed. Probably could have saved it had they called us the first time vs waiting and fiddling around.


Staff member
San Francisco Bay Area, CA, USA
Electrical Engineer
I've run into this issue when the wells are located in areas with a lot of encroaching residential subdivisions. The utilities cannot keep the line voltage balanced when people come home from work/school etc., because they can't control which people come and go, yet every one of them is a single phase service tapping off of different phases of the 3 phase distribution. Your recorder will likely show you that the imbalance is temporary as the utilities change taps on transformers and/or move loads around, but it's long enough for the Phase Imbalance to trip. Complain enough and the utility will upgrade to a faster responding system, that's the only way I've seen it get fixed.