Protective Relay : Working, Types, Circuit & Its Applications

An electrically operated switch like a relay plays a key role in controlling an electrical circuit through an independent low-power signal, otherwise used where a number of circuits should be controlled through the single signal. First, relays were used as signal repeaters within long-distance telegraph circuits and after that, widely used in early computers & telephone exchanges to achieve logical operations. There are different types of relays available and each type is used based on the requirement. So this article discusses an overview of a protective relay or protection relay – working with applications.

What is a Protective Relay?

A protective relay definition is; a switchgear device used to detect faults & begin the circuit breaker operation to separate the faulty element of the system. These relays are self-contained & compact devices that detect abnormal conditions occurring within the electrical circuits by measuring the electrical quantities constantly which are dissimilar in fault & normal conditions. In fault conditions, the electrical quantities may change like current, voltage, phase angle & frequency. The protective relay diagram is shown below.

Protection Relay
Protection Relay

Protective Relay Working Principle

A protective relay is used to protect the device once the fault is detected within a system. Once the fault is detected, the fault location is found and then provides the tripping signal to the circuit breaker or CB. These relays work on the two principles like electromagnetic attraction & electromagnetic induction.

Electromagnetic attraction relay simply works on both supplies like AC & DC and it attracts the coil toward electromagnet poles. These types of relays instantaneously work & it doesn’t delay while the Electromagnetic induction relay simply works on only AC supply & it utilizes the induction motor to generate the torque. So these are regularly used like directional relays to protect the power system & also in high-speed-based switching operation applications.

Protective Relay Types

Protective relays are available in different types which are used based on requirements.

Overcurrent Relays

Overcurrent relays operate through the current. The overcurrent relays may get actuated through the current. This relay includes a pick-up value and this relay activates once the measurement and quantity of current exceed that pick-up value.

Overcurrent Relay
Overcurrent Relay

These relays are available in two types instantaneous and time-delay types where these two relays are often provided within a single container. These two are activated by a similar current; but, their separate pickup values can be separately adjusted by changing the tap settings within the input.

Overcurrent relays are not expensive, so used on low-voltage circuits and also in specific high-voltage system applications. The main disadvantage of this relay is, it may also select the fluctuations of current as well as the faults within the nearby zones.

Electromechanical Relays

Electromechanical relays are the earliest relays but they are using in many areas still today. This relay simply works using a magnetic field generated by an electromagnetic coil once a control signal is provided to it. This relay changes the voltages & currents to electric, magnetic forces & torques that push against spring strains within the relay. The spring strain & taps on the electromagnetic coils within the relay are the main processes through which a user sets a relay. Please refer to this link to know more about an Electromechanical Relay.

Electromechanical Relay
Electromechanical Relay

Directional Relays

These relays are activated by the flow of current in a certain direction. It may detect a variation between the actuating & reference current. This relay is used in combination with some other relays like over current relay so that the capacity & selectivity of the protective relay system will improve. This relay simply reacts to the variation of the phase angle between both the actuating & a reference current is known as the polarizing quantity.

Directional Type
Directional Type

Distance Relays

This distance relay is used to distinguish between normal operating conditions & a fault and also differentiates faults within a particular area & within a different element of the system. The distance relay operation is inadequate to a particular range of impedance pickup values. This relay picks up once the impedance measurement is low or equivalent to the preferred pickup impedance value.

Distance Type
Distance Type

In this relay, the parameters like voltage & current are balanced from each other & this relay reacts to the voltage & current ratio which is the transmission line’s impedance from the location of the relay toward the point of interest. This impedance is used to determine distance through a transmission line, thus it is known as a distance relay. These relays are available in different types like reactance, mho & impedance relays.

Please refer to this link to know more about Distance Relay.

Pilot Relays

The pilot relay is used to determine whether a fault is inside or outside the protected line. If the fault is internal toward the protected line, then all the circuit breakers (CBs) at the line terminals are tripped at maximum speed. Similarly, if the fault is external toward the protected line, then the circuit breaker tripping is blocked or prevented. There are three types of pilot relays available wire, power line carrier & microwave pilot which are used for protective relaying.

Pilot Relay
Pilot Relay

Differential Relays

A differential protective relay simply works by contrasting the main difference among the entering & the leaving current magnitude as well as values. If the difference is above the pickup value then the system may be separated & the breaker circuit (CB) is triggered.

Differential Type
Differential Type

Protective Relay Circuit

The protective relay is used to detect abnormal conditions within the electrical circuits by measuring the different electrical quantities constantly under normal as well as fault conditions. The electrical quantities which may vary in fault conditions are; current, voltage, phase angle & frequency.

A typical protective relay circuit is shown which can be separated into three parts which are discussed below.

Protective Relay Circuit
Protective Relay Circuit
  • The first part of the circuit is the primary winding of a CT which is also called a current transformer. This CT is connected with the transmission line in series to be protected.
  • The second part includes the secondary winding of the current transformer, CB & the operating coil of the relay.
  • The final part of the circuit is the tripping circuit which may be either AC/DC. So it mainly includes a source of power supply, the circuit breakers trip coil & the stationary contacts of the relay.


Once a short circuit at the ‘F’ point on the transmission line occurs, then the flow of current within the transmission line will increase to an enormous value. So this causes to flow heavy current throughout the relay coil and makes the protective relay function by simply closing its contacts.

Consequently, it closes the trip circuit of the CB and makes the CB open & separating the faulty segment from the system. So in this manner, this protective relay ensures the security of the equipment of the circuit from breaking & typical working of the system.

Protection Relay Codes

In electrical power system design, the ANSI codes indicate what features a protective device supports like a relay/circuit breaker. These devices simply protect electrical systems as well as components from injury once an electrical fault takes place. ANSI codes are very useful in identifying medium voltage-based microprocessor device functions. The protection relay ANSI codes are listed below.

Protection of Current Functions

The protection of current functions with codes is listed below.

ANSI 50/51 indicates phase over current.
ANSI 50N/51N (or) 50G/51G indicates an earth fault.
ANSI 50BF indicates breaker failure.
ANSI 46 indicates an unbalanced or negative sequence.
ANSI 49 RMS indicates thermal overload.

Directional Current Protection

The protection of directional current with codes is listed below.

ANSI 67 indicates the directional phase over-current.
ANSI 67N/67NC indicates a directional earth fault.

Directional Power Protection Functions

The protection of directional power with codes is listed below.

ANSI 32P indicates directional active over power.
ANSI 320/40 indicates directional reactive over power.

Machine Protection Functions

The machine protection function with codes is listed below.

ANSI 37 indicates phase undercurrent.
ANSI 48/51LR/14 indicates a locked rotor or extreme starting time.
ANSI 66 indicates starts per hour.
ANSI 50V/51V indicates voltage/ restrained over current.
ANSI 26/63 indicates Buchholz/thermostat.
ANSI 38/49T indicates monitoring of temperature.

Voltage Protection Functions

The voltage protection function with codes is listed below.

ANSI 27D indicates a positive sequence under voltage.
ANSI 27R indicates remanent under voltage.
ANSI 27 indicates under voltage.
ANSI 59 indicates overvoltage.
ANSI 59N indicates displacement of neutral voltage.
ANSI 47 indicates a negative sequence overvoltage.

Protection Functions of Frequency

The protection functions of the frequency with codes are listed below.

ANSI 81H indicates over-frequency.
ANSI 81L indicates under frequency.
ANSI 81R indicates frequency rate change.
ANSI 81R indicates frequency rate change.

Protection Relay Testing

In current power systems, protection relays play a key role so their reliable operation has to check at all times. So, these relays should be tested during their life cycle. Additionally, relay testing on a normal basis is required to make sure the right operation is maintained. If the testing of the protection relay is not performed well on a regular basis, electrical faults may occur and cause equipment damage & harm to workers.

There are three types of protection relay tests that are performed bench testing, commissioning testing, and maintenance testing which are discussed below.

Bench Testing

This test is performed to test the relay on its own & that it equals the design. This avoids more costly as well as time-consuming troubles from occurring at later stages within a project.

Commissioning Testing

When the electrical system has been designed, commissioning the protective relay involves checking the larger system works as expected. So, for instance, once the protective relay is connected to the switchgear, then it should work as expected, and respond to interlocks & other replicated conditions. In the future, the function of the relay will have been verified.

Maintenance Testing

Once maintenance testing is performed then the whole design purpose is assumed, however, the behavior of the protective relay should be verified for below operation. Apart from particular failures, this relay cannot notice changes within the characteristics of a system like network loads being modified over time. So these long-term changes may need the protection relay to be reprogrammed to make sure the estimated operation is maintained.

While doing protection relay testing there are many parameters that need to be tested frequently based on the type of test like the relay’s visual inspection, connection parts, opening & closing of circuit breaker (CB), protection functions, logic functions, protective relay binary & analog input and outputs, Primary injection, Insulation resistance testing & secondary injection testing.

Advantages & Disadvantages

The advantages of a protection relay include the following.

  • This relay monitors different parameters continuously like current, voltage, power & frequency.
  • It Improves system stability through the isolation of defective section
  • This relay clears the error in no time, so it reduces the damage.
  • This relay detects failures & faulty sections in the system.
  • It reduces the fire risk.
  • It provides electrical security & protects a person while working on the system.
  • It improves the performance, stability & reliability of the system.
  • The operation of these relays is very fast & also very fast to reset.
  • These can be utilized in both the power supplies like AC & DC.
  • These relays simply work in milliseconds & the outcome is instant.
  • These are the most reliable, robust, compact & very simple.
  • It is applicable in different fields.

The disadvantages of a protection relay include the following.

  • A protective relay cannot avoid faults within a power system, so, this relay spends more time in the power system monitoring.
  • It needs periodic maintenance as well as testing not static relays.
  • The operation of this relay can be simply affected because of the component’s aging, pollution & dust which results in false trips.
  • These relays provide security and consistency which is required to operate with confidence.


The applications of a protection relay include the following.

  • A protection relay is used in serve electrical protection.
  • The protection relay detects a problem during its early stage & significantly reduces or eliminates damage to equipment.
  • This relay device is mainly designed to trip a CB (circuit breaker) once a fault is noticed.
  • This relay works like a detecting device, so it detects the faults, knows its position & lastly it provides the tripping signal to the circuit breaker
  • This is a switchgear device used to detect the faults & begins the circuit breaker operation to separate the faulty element from the system.
  • These are very helpful in high-voltage & medium-voltage protection and overcurrent to complex distance protection.

What are the Key functions of Protective Relays?

The main functions of protective relays are;

  • It detects the presence of a fault.
  • It detects the fault location.
  • It detects the presence of fault type.
  • It closes the trip circuit & operates the CB (circuit breaker) to separate the faulty system.

What type of protective relay is used in an induction motor?

The MPR or motor protection relay is used for protecting the high-voltage induction motor.

What are the essential elements of a protective relay?

The essential elements of a protective relay mainly include a sensing element, comparison element, and control element.

What are protective relays used for?

A protective relay is used to detect faulty equipment and monitors the current & voltage with CTs & PTs.

What are the types of relays used for 3-phase protection?

A 3-phase voltage control relay is used in three-phase protection.

Thus, this is an overview of a protective relay – working with applications. In order to operate the protective relay satisfactorily, it must have these qualities like speed, selectivity, reliability, simplicity, sensitivity, economy, etc. Here is a question for you, what is a circuit breaker?