Introduction

• As already pointed out, the most obvious effect of a fault is a sudden build up of current, therefore the over-current protection is the most widely used form of protection. “A protective relay which operates when the load current exceeds a preset value is called an over-current relay.” “The value of the preset current above which the relay operates is known as its pick-up value.” An over-current relay is used for the protection of distribution lines, large motors, power equipment etc. “A scheme which incorporates over-current relays for the protection of an element of a power system is known as an over-current scheme.” An over-current scheme may include one or more over-current relays. At present Electromagnetic relays of induction disc type construction are widely used for the over-current protection. 

• The type of protection which depends on only the magnitude of the current, without taking any cognizance of its phase angle, is known as the non directional overcurrent protection. However, many times it is required to discriminate between faults in front of the breaker and faults behind the breaker. This is possible if we take into account not only the magnitude of the current but also its phase with respect to the voltage at the relay location. In such cases the protection is known as the directional overcurrent protection. However, protective relays are classified depending on the technologies they use for their construction and operation: 

1. Electromagnetic and electro-thermal

2. Static using devices like IC’s, transistor, diodes, logic gates etc

3. Microprocessor based

4. Numerical based

• There are various types of protective relays in each category, depending on the operating principle and application. The entire gamut (scale, compares, extent) of protection can be captured in the table.