For a long time, I had a simple rule in my mind: high current means fault.
If a transformer suddenly drew 5 times or 10 times its rated current, I would immediately think something was wrong. Maybe a short circuit, a protection issue, or some kind of system problem. Then I started learning about transformer energization and came across something interesting called Transformer Inrush Current.
What surprised me was that a transformer can draw a very large current when it is switched ON, even when there is no load connected and no fault in the system.
So why does this happen? The answer is inside the transformer core. When a transformer is switched OFF, a small amount of magnetism can still remain in the core. This is called remnant flux.
When the transformer is switched ON again, new magnetic flux is created. This new flux combines with the remnant flux already present in the core. If the total flux becomes too high, the transformer core becomes saturated.
