• A typical relay circuit is as shown in figure 1.1. This diagram shows one phase of the 3Φ system for simplicity. The relay circuit connections can be divided into three parts viz.

1. First part is the primary winding of a current transformer (C.T.) which is connected in series with the line to be protected.

2. Second part consists of secondary winding of C.T. and the relay operating coil.

3. Third part is the tripping circuit which may be either AC or DC. It consists of a source of supply, the trip coil of the circuit breaker and the relay stationary contacts.

Fig. 1.1 Basic Trip Circuit

• When a short circuit occurs at point ‘F’ on the transmission line, the current flowing in the line increases to an enormous value. This results in a heavy current flow through the relay coil, causing the relay to operate by closing its contacts. This in turn closes the trip circuit of the breaker, making the circuit breaker open and isolating the faulty section from the rest of the system. In this way, the relay ensures the safety of the circuit equipment from damage and normal working of the healthy portion of the system.