Phase Rotation with 300KVA XFMR

[kwired]

Then how did he know the rotation was wrong?

OP said the source had been changed as well which could be a reason why rotation was changed if he was careful with marking everything but another party was responsible for source. Breaker in question did hold the first time, but was discovered rotation was wrong and then it would not hold after reversing rotation even with no load. Not necessarily his exact words but kind of how I interpret them.

 

[LarryFine]

Breaker in question did hold the first time, but was discovered rotation was wrong and then it would not hold after reversing rotation even with no load.

To which I suggested returning the primary to the direction that would hold, and reverse the secondary instead.

 

[kwired]

To which I suggested returning the primary to the direction that would hold, and reverse the secondary instead.

I sort of have no problem with that either. could change rotation at every piece of equipment if you wanted and it is still effective.

A B and C phase mean little until you identify one as A B or C until you do that all you have is clockwise or counter clockwise in the current order of things.

If you swap A to B location then swap C to A location you have all three conductors in different spots than the original connection, but rotation is still same and that generally is all that matters in most applications. High leg system with neutral loads would be one exception.

 

[Tulsa Electrician]

I don't believe he mention system types and voltages. Unless I missed it. That may or may not shed some light.

 

[Tulsa Electrician]

For all we know it could be a reverse-fed transformer just thinking out loud.

 

[Jraef]

If you swap A to B location then swap C to A location you have all three conductors in different spots than the original connection, but rotation is still same and that generally is all that matters in most applications. High leg system with neutral loads would be one exception.

I agree. I have had to "roll" the rotation on connecting transformers before to prevent the breaker from tripping on magnetizing inrush. It's a known phenomenon, taught to me in the field by an old EE decades ago. By rolling the rotation (A to B, B to C, C to A), you are finding out if there is a natural impedance difference in the transformer that can help out (if the situation is marginal enough). I've done the same when there is a little too much voltage imbalance. You can only try it twice though...