The separate blocking relay in series with the differential relay is used. The operation of this relay is based on harmonic component of inrush current. This relay consists of 100 Hz blocking filter in operating coil while 50 Hz filter in restraining coil. At the time of inrush current, second harmonic component is maximum and thus blocking relay is blocked with its contacts remain open. In short circuit case, the harmonic component is negligible and 50 Hz compensated is dominant. Hence the blocking relay operates to close its contact. This principle is called harmonic blocking.
