The biased beam relay also called percentage differential relay is designed to respond to the differential current in terms of its fractional relation to the current flowing through the protected section. It’s called percentage differential relay because the ratio of differential operating current to average restraining current is a fixed percentage. It’s called bias relay because restraining known as biased coil produces the bias force. Fig 17 a, shows the schematic arrangements of biased beam relay. It is essentially an over current balanced beam type relay with an additional restraining coil. The restraining coil produces a bias force in the opposite direction to the operating force.

Under normal and through load conditions, the bias force due to restraining coil is greater than operating force. Therefore, the relay remains inoperative. When an internal fault occurs, the operating force exceeds the bias force. Consequently the trip contacts are closed to open the circuit breaker. The bias force can be adjusted by varying the number of turns on the restraining coil.

The equivalent circuit of a biased beam relay is shown in fig 17 b. The differential current in operating coil is proportional to i₂ – i₁ and the equivalent current in the restraining coil is proportional to (i₂ – i₁)/2 since the operating coil is connected to the mid-point of the restraining coil. It is clear that greater the current flowing through the restraining coil, the higher the value of current required in the operating winding to trip the relay. Thus under heavy load, a greater differential current through the relay operating coil is required for operation, than under light load conditions

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