A protective relay is a device that trips, or opens, a circuit breaker when it detects a fault. It monitors an electrical circuit for a set of parameters, such as voltage and time, and trips the circuit breaker when it detects overload conditions. A protective relay’s parameters are selectable, unlike a switching relay, in which the parameters are fixed.
A circuit breaker in a common household circuit uses a simple bimetallic strip that bends as its temperature increases. This causes the circuit to trip when it reaches the threshold temperature. Industrial circuit breakers, on the other hand, require a protective relay to signal to the circuit breaker when to open. These types of circuit breakers have an electromagnetic coil called a trip coil that opens the circuit when activated. The protective relay activates the trip coil if the circuit meets the specified overload conditions.
The design of a protective relay can be elaborate. Common components of a protective relay include induction disks, operating coils, shaded-pole magnets and solenoid operators. The first protective relays were purely electromechanical devices, but modern protective relays use microprocessors. Microprocessor-based protective relays are more precise than electromechanical versions, and they typically perform multiple functions.
Protective relays can respond to a variety of conditions. An over-current condition occurs when the voltage in a circuit is above the desired voltage. Similarly, an over-voltage condition occurs when the circuit’s voltage is above its desired level. Over-frequency and under-frequency conditions are also common types of circuit faults. Some protective relays can also estimate the distance to the fault.
An over-current relay is a common type of protective relay. It’s typically connected to a transformer and calibrated to the desired maximum current. The contacts in the relay operate when the current in the circuit exceeds this level, thus breaking the circuit.
A distance relay is a general type of protective relay that detects the distance to the fault. This is the most common type of protective relay on a high-voltage transmission line. A distance relay detects the current and voltage on the circuit. The impedance per unit distance on the circuit is a known quantity, which allows the distance relay to calculate the distance to the fault.