In countries like UK and Australia, power is distributed to residential streets as a three-phase voltage, with 240V from each phase to neutral.

Small appliances are connected from phase to neutral, and operate from 240V AC.

However, if you have a high-powered motor, it is more efficient to power it from three-phase power. If you measure the voltage from phase-to-phase, it is 240V * √3 =415V AC*.

So your 415V transformer is a three-phase transformer.

*In more detail, you can represent 2 voltage phases as 2 sides of an isosceles triangle, with the phase-to-phase voltage being the third side. The angle between the sides is 120°. Applying the

cosine rule, the length of the third side is found to be 240√3 = 415.