What Is a Protective Relay?

Article Details
  • Written By: Allan Robinson
  • Edited By: A. Joseph
  • Last Modified Date: 11 November 2014
  • Copyright Protected:
    Conjecture Corporation
  • Print this Article
Free Widgets for your Site/Blog
A red blood cell can pass through a person's entire body in 20 seconds.  more...

December 19 ,  1998 :  President Bill Clinton was formally impeached by the House of Representatives.  more...

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.


More from Wisegeek

You might also Like

Discuss this Article

Post your comments

Post Anonymously


forgot password?