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So, the PoCo connected their Xformers in a Closed 3 Wire Delta configuration, instead of a (commonly used) 480Y/277 VAC 3Ø 4Wire config.?!?!?!
Very odd (to me), especially since (in my areas) the 480 VAC 3Ø 3 Wire Delta is a special order / situation only setup!

As to the Meter's reading of ± 277 VAC to Ground, this must have been measured with a High Input Impedance Volt Meter - like a DVM (Digital Voltage Meter) or a DMM (Digital Multi Meter).

Was the measurement done with a DVM/DMM, or a typical "Wiggy" Solenoid + Neon type Voltage Meter?

If DVM/DMM, these will read the "Capacitive Coupling Effects" between an Ungrounded AC System's Line Conductor(s), and a Conductor / Metallic Enclosure which has been Bonded to an Earth Reference.
"Shunting" the Meter's leads with a "Wiggy" will remove nearly all the Cap. effect, and give a much closer to accurate response.
(Shunting the leads = connect the "Red" leads of both meters to one test conductor, and the "Black" leads of both meters to the Grounded reference).

If the tester was a "Wiggy", and the readings were a "Solid" + 240 VAC (between 240 VAC and 480 VAC), and each Line was equal to Ground, then this is a new one on me!
The Wiggy will nearly always act like it is discharging a Capacitor (quickly "flash" a reading in a high range), then either drop off completely, or slightly hum a few seconds until responding with a dead indication.

If the Wiggy showed a "Solid" reading, which was "Somewhat Equal" on 2 Lines to Ground, but "Noticeably Higher" on the 3rd Line to Ground test, then this would indicate the Identified Conductor (the intended to be Grounded "Neutral" Conductor) was connected to the Center Tap of one Transformer, and the configuration is a Delta.

Let me know what tester(s) used, and any other questions.


Scott " 35 " Thompson
Just Say NO To Green Eggs And Ham!