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Relays Part 4: The Protective Relay Basic Theory

by: Dec 12,2023 1390 Views 0 Comments Posted in PCB Basic Information

Summary:       Several types of relays for different purposes exist in the area of power electronics and in this article, we are going to introduce engineers to the protective relays working principle, their existing types, circuit diagrams, and where they find application.


Power electronic relays are important components when it comes to controlling voltages and currents in an electrical circuit by involving a low-power signal. There exist so many different types of electric relays that serve us according to the requirements of the circuits. From protection to power flow control and so on, relays have proved to be compulsory devices in many power systems. In this article, the discussion will focus on the protective power relays with a complete overview of all the relevant technical and theoretical information that an electrical engineer should have before making use of them. Let us begin!

Defining Protective Relays

A protective relay can be defined as an electrical switchgear gadget that is deployed in an electrical circuit to detect any electrical faults and trigger the circuit breaker functioning to separate the element that is faulting in the electrical circuit. During a fault condition, there is a change in electrical quantities such as the voltage, current, frequency, and phase angle and it is the work of the self-contained relay to constantly and consistently detect these faults.

The Working of Protective Relay

They serve the purpose of protecting devices once an electrical fault is detected in the electrical circuit. It finds the fault and then triggers the electrical circuit breaker that is connected to the trip. They operate under two principles to achieve their function. These are electromagnetic induction and electromagnetic attraction principles.

The electromagnetic attraction protective relays are applicable in both AC and DC power and attract their poles towards the electromagnetic poles. This type of power protective relay works instantaneously without any delay.

The electromagnetic protective relays work on AC power only and it makes use of the connected induction motor the produce the necessary torque. Therefore, they are used in most cases as directional relays for high-velocity switching operations and as directional relays for protecting power systems.

Existing Types of Protective Relays that Engineers Should Know

Different types of protective relays that find different applications in electrical circuits exist. They include overcurrent, electromechanical, directional, distance, pilot, and differential relays. Let us discuss all of them in detail below.

Overcurrent Relays

This type of protective relay makes use of the current to operate. Their actuation is based on the available current. The device has a set pick-up value and it will only activate once the measured current exceeds the set pick-up value.

They exist in two types namely time delay and instantaneous where the two types are available in a single enclosure. They are activated by the same current but might have different set pick-up values through settings adjustment.

Overcurrent relays are cheap and they are used in circuits of low-voltage and some specified high-voltage power system applications.

 They have the disadvantage of one point towards the targeted area of interest. They are of mho, reactance, and impedance types distance relays.

Pilot Relay

This is a type of protective relay that is used to determine if the fault has occurred internally or externally of the protected electric line. If the detected fault is internally and towards the protected electric line, then the tripping of all the connected circuit breakers occurs at a maximum speed. On the other hand, if the detected fault happens externally, the circuit breaker trips do not happen. This relay exists in power line carrier, wire, and microwave relay types.

Differential Protective Relay

This is used to compute the difference between the signals that enter the system and the one that leaves the system. It compares their magnitude and values and if it finds a difference that is above the pick-up value, the system breakers are triggered.

The Circuit Diagram of the Protective Relay

Figure 2: Protective Relay Circuit

Protective relays play a role in detecting unexpected conditions that occur in the electric system circuits. The relay circuit above can be divided into three important parts that are discussed below.

  • Current transformer primary windings: these windings are connected in series to the electrical transmission line that is to be protected.
  • Current transformer secondary windings, relay’s operating coil, and circuit breaker.
  • The tripping circuit: This can be either AC or DC and is made up of the trip coil of the circuit breaker, power source, and the relay’s stationary contacts.

When a short circuit occurs at point F located on the transmission line, it creates current flow on the transmission line to an unexpected value. The effect is that more current flows through the connected protective relay causing its contractors to trip. The CB trips to separate the segment that is faulty from the whole system, therefore, protecting the system.

Codes of Protection Relay

ANSI codes are used to indicate the features supported by any protective device such as the circuit breakers and the relays. These devices play the role of protecting the components and the power systems. Below is the table of the ANSI codes of the protective relays.

Current Protection Codes


Table 1: Current Protection Codes


Voltage Protection Codes



Table 2: Voltage Protection Codes


Testing of Protection Relay

Three types of protection relay testing exist, that is, bench testing, maintenance testing, and commissioning testing.

Bench Testing

Done to check if the relay is constructed as per design specifications and it helps to avoid more costly problems that might occur at an advanced stage.

Commissioning Testing

This helps to check that the system that has been installed is functioning as expected. For example, a relay that is connected to the electrical switchgear should work as expected.

Maintenance Testing

This involves routine checking of a working relay to ensure it serves its purpose. It involves the reprogramming and repair of existing running relays.

Protection Relay Application

  • Electrical protection
  • Problem detection in electrical circuits
  • Tripping of CB immediately after a problem is detected


From the article:

  •  A protective relay has been defined as a switchgear deployed in an electrical circuit to help detect any electrical fault.
  • The protective relays operate under two principles electromagnetic induction and electromagnetic attraction.
  • The types of protective relays that exist are overcurrent, electromechanical, directional, distance, pilot, and differential relays.
  • The circuit diagram of the protective relay is made up of current transformer primary windings, current transformer secondary windings, relay operating coils, circuit breakers, and the tripping circuit.
  • The protective relays communicate through codes that have different meanings such as the current protection codes and the voltage protection codes.
  • Protective relays are tested through three methods bench, commissioning, and maintenance testing.
  • Protective relays find application in fault detection in a circuit, electrical protection, and triggering tripping of the circuit breakers.





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