Whenever a power transformer is isolated from the power system, residual magnetism remains in its core due to a phase shift. Residual magnetism also remains after a DC voltage has been applied to the transformer core, for example during routine winding resistance tests in the field or factory.

Due to residual magnetism in the core, high inrush currents, up to the maximum short-circuit current, can occur. This puts undesired stress on the transformer when it is switched back into service. In addition, many diagnostic measurements can be affected by residual magnetism, making a reliable assessment very difficult.