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The relays is the element that senses an abnormal condition in the circuit and commands the operation of the breaker. They can be classified in a number of ways:

  • According to the quantity sensed: current, voltage, active power, reactive power, impedance relays
  • According to the tripping: instantaneous trip, delayed trip, inverse timecurrent response
  • According to the operating principle: electromagnetic relays, induction relays, thermal relays, static or digital relays

More general classification distinguish two types of relays:

  • 1. Electromechanical relays
  • 2. Digital relays

Currently, electromechanical relay in all its forms is being replaced by static, digital and numerical relays, each change bringing with it reductions and size and improvements in functionality.

In most types (except digital relays) the relays contacts are closed by a moving part which senses a force proportional to the current (or other sensed quantity) in the circuit. A restraining force produced by a spring or other means sets the threshold above which the relay operates.

In a protection relay, the term "static" refers to the absence of moving parts to create the relay characteristic. Compared to static relays, digital relays introduced A/D conversion of all measured analogue quantities and use a microprocessor to implement protection algorithm. The distinction between digital and numerical relays rests on points of fine technical detail, and is rearly found in areas other than protection. They can be viewed as natural developments of digital relays as a result of advances in technology.

Relay timing mechanisms: a) Instantaneous, b) Inverse time, c) Timed

Overcurrent relays

IEC definition states that overcurrent relay is a measuring relay which operates when the value of the current exceeds the setting (operating value) of the relay. There are generally 3 types of these relays: instantaneous, inverse and induction.

Relays are adaptable to transmission lines, buses, feeder circuits, transformers and motors.

Overcurrent relays - instantaneous

Overcurrent relays are electromagnetic relays. They are based on electrodynamic force produced on a moving part by the current flowing through a coil. Instantaneous relays operate without intentional time delay. They are used for faults close to the source when the fault current is very high. The operating time is approximately 10 ms. The construction of instantaneous relays is usually moving armature, plunger, or induction disk.

An important characteristic of an instantaneous relay is a drop-out ratio.

drop-out ratio = (drop-out current) / (pick-up current)

Drop-out ratio is usually less than 1.

Instantaneous overcurrent relay
Instantaneous overcurrent protection
MCTI instantaneous overcurrent relay

Overcurrent relays – inverse time

Time overcurrent relays operate with a time delay, which is adjustable. For given setting, the actual time delay depends on the current through the relay coil. Higher current will cause a faster operation of the relay. The minimum relay operation current (pick-up current) is also adjustable.

The most commonly used overcurrent relay has both the instantaneous response for very large currents and the inverse time response for lower currents.

Time overcurrent relays come in five different versions that are defined by the steepness of the time-overcurrent characteristic:

  • definite time
  • moderately inverse
  • inverse
  • very inverse
  • extremely inverse
Inverse time overcurrent relay characteristics, different delay settings
Time overcurrent relay characteristics, all versions
Inverse time overcurrent relay

Overcurrent relays – induction disk

The inverse time response can be provided by an induction disk unit.

In the induction disk unit, a metal disk is mounted on a shaft that can freely rotate. The current coils are fixed and create magnetic field that induces eddy currents in the metal disk. The magnetic field of the eddy currents interacts with the magnetic field of the stationary coils and produce torque on the disk. The disk and its shaft rotate and bring the moving contact towards the fixed contact into a closed position.

The motion of the shaft is opposed by a spring that returns the disk and the moving contact into the open position when the current drops below a preset value. The time to close the contact depends on the contact travel distance which is set by a time dial. The pick-up current is adjustable by selecting current taps on the current coil. The relays are normally available with three ranges of current taps: 0.5 to 2.0 A, 1.5 to 6.0 A, and 4 to 16 A. The time dial has usually positions marked from 0 to 10, where for 0 setting the contact is permanently closed.

Induction disk overcurrent relay
Induction disk type relay

Recommended settings for overcurrent protection

More on recommended settings for overcurrent devices: Overcurrent Protection

Small air breakers for low voltage domestic circuits

Circuit breakers that are used at the distribution board in houses are called MCBs (minature circuit breakers). The distribution board is also called the consumer unit.


The load current is passed through a small heater, the temperature of which depends upon the current it carries. The heater will warm up a bimetal strip. When excessive current flows the bimetal strip will warp to trip the latch mechanism.

Some delay occurs owing to the transfer of heat produced by the load current to the bimetal strip. Thermal trips are suitable only for small overloads of long duration. Excessive heat caused by heavy overload can buckle and distort the bimetal strip.


The principle used in this type is the magnetic force of attraction set up by the magnetic field of a coil carrying the load current. At normal currents the magnetic field is not strong enough to attract the latch. Overload currents will increase the force of attraction and operate the latch to trip the main contacts.

Inside a 10 amp thermal-magnetic circuit breaker. These circuit breakers are standard in modern domestic and commercial electrical distribution boards.

Simple MCB
Cross-section of the thermal-magnetic circuit breaker
Typical magnetic circuit breaker
Thermal-magnetic circuit breaker (1Ph + N) 2A-250VAC

Number description:

1. Actuator lever - used to manually trip and reset the circuit breaker. Also indicates the status of the circuit breaker (On/Off/tripped).
2. Actuator mechanism - forces the contacts together or apart.
3. Contacts - Allow current to flow when touching and break the flow of current when moved apart.
4. Terminals
5. Bimetallic strip
6. Calibration screw - allows to adjust the trip current of the device after assembly.
7. Solenoid
8. Arc divider / extinguisher

Differential (unit) protection

Differential protection is a very reliable method of protecting generators, transformers, buses, and transmission lines from the effects of internal faults. In normal operating conditions, the current through the CTs is the same so the relay sees no differential current. This is also the case for external faults. Differential protection can be used for protecting generators from faults to ground. Differential protection of busbars in substations uses one CT for each incoming line. All incoming currents are added up and compared to the sum of all outgoing currents.

General schematics of the differential relay
Microprocessor-based SEL-387L line current relay

Differential (unit) protection for transformers

In applying the principles of differential protection to transformers, a variety of considerations have to be taken into account. These include:

a) correction for possible phase shift across the transformer windings (phase correction)
b) the effects of the variety of earthing and winding arrangements (filtering of zero sequence currents)
c) correction for possible unbalance of signals from current transformers on either side of the windings (ratio correction)
d) the effect of magnetising inrush during initial energisation
e) the possible occurrence of overfluxing

The primary and secondary currents have different magnitudes and phases, so the magnitude and the phase shift must be balanced by appropriate ratio of the CTs. If the transformer is Y-Δ, the currents also have 30° phase shift. The phase shift is corrected by connecting the CTs on the Δ side in Y, and on the Y side in Δ.

Differential protection in transformers
Cooper Power Systems iXP-420 differential relay, suitable for transformers

When used on the overhead lines, the differential protection has the practical length limitation of 40 km for problems with the pilot wire (wire used to transfer state data of the protected line, i.e. to send information if the fault has occured).

Distance relays

The impedance of a transmission line is proportional to its length, for distance measurement it is appropriate to use a relay capable of measuring the impedance of a line up to a predetermined point (the reach point). Such a relay is described as a distance relay and is designed to operate only for faults occurring between the relay location and the selected reach point, thus giving discrimination for faults that may occur in different line sections.

The basic principle of distance protection involves the division of the voltage at the relaying point by the measured current. The apparent impedance so calculated is compared with the reach point impedance. If the measured impedance is less than the reach point impedance, it is assumed that a fault exists on the line between the relay and the reach point. The reach point of a relay is the point along the line impedance locus (set of points) that is intersected by the boundary characteristic of the relay. Since this is dependent on the ratio of voltage and current and the phase angle between them, it may be plotted on an R/X diagram. The loci of power system impedances as seen by the relay during faults, power swings and load variations may be plotted on the same diagram and in this manner the performance of the relay in the presence of system faults and disturbances may be studied.

General setting of the distance protection
Three-zone step distance relaying to protect 100% of a line and backup the neighbour line
TecQuipment PS250 distance relay

As the power systems become more complex and the fault current varies with changes in generation and system configuration, directional overcurrent relays become difficult to apply and to set for all contingencies, whereas the distance relay setting is constant for a wide variety of changes external to the protected line.

Following topics give the main overview over the different distance relay types:

Impedance relays

Impedance relays are used whenever overcurrent relays do not provide adequate protection. They function even if the short circuit current is relatively low. The speed of operation is independent of current magnitude.

The relay consists of a balanced beam. At each end of the balanced beam is a coil that exerts a force on the beam at that end. One coil is connected to a current transformer, the other coil to a potential transformer. Under normal conditions, the contact of the relay is kept open. During a fault, the voltage drops, and the current rises. The torque due to the current coil overpowers the torque due to the voltage coil, and the relay closes its contact.

The impedance relay has a circular characteristic centered at the origin of the R-X diagram. It is nondirectional and is used primarily as a fault detector.

Admittance relays

The admittance relay is the most commonly used distance relay. It is the tripping relay in pilot schemes and as the back-up relay in step distance schemes. Its characteristic passes through the origin of the R-X diagram and is therefore directional. In the electromechanical design it is circular, and in the solid state design, it can be shaped to correspond to the transmission line impedance.

Reactance relays

The reactance relay is a straight-line characteristic that responds only to the reactance (XL) of the protected line. It is nondirectional and is used to supplement the admittance relay as a tripping relay to make the overall protection independent of resistance. It is particularly useful on short lines where the fault arc resistance is the same order of magnitude as the line length.


MCB short overview: