Abstract: A control unit accommodates various equipment, such as a circuit breaker, an electromagnetic switch, a zero-phase current transformer, and an operating transformer. The zero-phase current transformer is generally triangular-shaped to fit more compactly within the control unit. More specifically, the zero-phase current transformer comprises an annular core having a shape which is substantially triangular, wherein the annular core encloses an aperture region having a shape which is also substantially triangular. A primary conductor is disposed within the aperture region, and a secondary conductor is wound on the annular core.