3 phase a/c over/under voltages

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Service call today at a theater. 3 rooftop ac units (3 phase 460v): 2 units working, 1 unit not. Unit 1 not working: measured voltages at line side of disconnect. Phase A to ground 460v. Phase B to ground 13v. Phase C to ground 453v. Phase A to B 460V. Phase A to C 460v. ETC. ETC. Units 2 and 3 operational. Voltage check at units 2 and 3 line side are identical to unit 1. We shut down all units and measured the voltages at all points indicated above. They were as follows : A TO GROUND 263V. B TO GROUND 260V. C TO GROUND 273V. ANY LINE TO LINE COMBINATION 460V. We restored power to unit 1 (non functioning unit). Voltages remained correct across all phases. We restored the power to unit 2 (functioning unit). Voltages spiked and remained on A AND C AS NOTED ABOVE AND DROPPED ON B AS NOTED. We restored the power to unit 3 (functioning unit). Same result as unit 2. We checked the voltage on the primary and secondary of the transformer under no load. The voltages were correct. As soon as the load from the functioning units (2 and 3) were applied the voltage spiked and dropped as indicated. Suggestions for the cause of this would be greatly appreciated.


Staff member
You have an ungrounded delta and you have ground faults on the load side of the disconnects for units 2 and 3. This is most likely not related to the reason why unit one is not working.
bulldog thanks for the reply. I belive the problem may be in both units. We ran the units in all senarios and the voltage anomalies occured in all instances.
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