It's informative that the Electrician noted transformer core steel is chosen to reduce the presence of remnant flux to zero in an unenergised transformer.

Not quite zero.

For some reason, the early researchers in magnetism used the spelling "remanence" and "remanent magnetism", rather than the usual English "remnant" for the residual flux in a core left after an external magnetizing force is removed. See:

http://en.wikipedia.org/wiki/Remanence

Presumably a remnant flux under the "right" conditions would lead to even higher inrush current.

Quite true. Transformer steel exhibits a rather low, but non-zero, remanence. I can tell the difference in the surge at turn on for different turn-off states. The difference is small, but detectable.

 

Thanks for the correction on distinction regarding remanent. English usage isn't my strongest suit.

Have had similar previous faux pas pointed out.

One of my classics was referring to the Rhyme of the Ancient Mariner .....

 

For steady state operation, when the applied voltage crosses zero on the way up, the core has been magnetized to maximum flux in the negative direction (so to speak) by the previous negative half cycle (rather than starting at zero flux as when the voltage has been disconnected for a while), so application of the volt-seconds in the positive half cycle doesn't take the core beyond saturation in the positive direction.

But, when applying a full positive half cycle when the core has become unmagnetized by having been turned off for a while, those volt-seconds are sufficient to drive the core well past saturation, leading to a large current surge.

Of course, so simple! Great explanation!