Overcurrent Relays

Overcurrent relays, connected to the secondary current of a current transformer, are crucial for protecting circuits.

There are two types: Instantaneous and time-delay overcurrent relays.

Instantaneous overcurrent relays react as soon as the input current exceeds a pre-set value, the pickup current. This action instantly energizes the circuit breaker trip coil.

Time-delay overcurrent relays also respond to their input current but include an intentional delay that depends on the input current's magnitude.

For example, consider a power substation. If a fault occurs, causing a surge in current, an instantaneous relay would immediately trip.

In contrast, a time-delay relay would allow minor surges without tripping but would still act quickly for larger faults.

These relays have two settings: current tap and time-dial.

The choice of relay and its settings depend on sources, lines, and loads. Different relays operate faster on higher fault currents.

An electromechanical time-delay overcurrent relay operates using a conducting aluminum disc, which rotates under a magnetic field induced by the input current. The larger the input current, the faster the contact closing.