If I walk into one of our locations with a scope, I can easily determine phase rotation. That's really all you know unless you have a wild leg. Consider an aux transformer with four unmarked conductors coming out.(safely landed for my metering pleasure.) I would know after a few seconds of metering, which conductor to mark as the neutral. I can scope a phase I choose to call "A" to neutral, and positive trigger on it. If the second phase I observe has its positive peak 5+ mS (120 degrees) later, I would call it "B". If 11+ mS (240 degrees), I would call it "C". But what I think is ABC, might teally be BCA, or CAB. Its usually just the rotation that matters, unless we have bus ties. Even if the phases are landed correctly, opposite sides of a bus tie might not be in sync. If I have a Wye source that I trust the phasing on, I can meter known Phase "A" to unknown phase "A","B","C". If I guessed right, "A" should give me the lowest AC voltage.
In DC traction power, we deliberately use out of phase sources. A 2 rectifier house will feed one rectifier Delta : Delta, and the other Delta:Wye. This helps smooth out the ripple since there is only rail inductance and no, building sized, filter capacitors to smooth out the 600 VDC.
Joe
[This message has been edited by JoeTestingEngr because a smile ate my
elta
[This message has been edited by JoeTestingEngr (edited 04-07-2006).]