Definition – It is a device that detects fault & gives command to the breaker to isolate the faulty circuit.

WORKING PRINCIPLE – The relay circuit can be divided into three parts –

  • First part is the Primary Winding of CT (Current Transformer) which is connected in series with line to be protected,
  • Second part is Secondary Winding of CT&relay operating coil,
  • Third part is tripping circuit which may either a.c or d.c. It consists of a source of supply, trip coil of circuit breaker & relay stationary contacts.Most of the relays used in power system are actuated by current /voltage supplied by CT & PT (Potential Transformer) connected in various combinations. Having detected the fault, the relay operates the trip circuit which results in the opening of the circuit breaker and hence in the disconnection of the faulty circuits.


Being a fault sensing device, relay plays a very important role in protectionof equipment. While selecting a relay, we must remember following points –

  1. Selectivity – It means that relay settings/range should be selected correctly so that in case of fault, only faulty part should be disconnected without disturbing the rest of the system.
  2. Speed – The relay should disconnect the faulty section as fast as possible. If fault current remains in the system for a longer period, fault current may damage the equipment & its associated accessories.
  3. Sensitivity – Relay should get activated with low value of actuating quantity (volt –ampere).
  4. Reliability – The relay should operate under the predefined conditions.
  5. Simplicity – It should be simple so that it can be easily maintained.
  6. Economy – Also it should cost effective or economic.

Types of Relay

There are two types of relay used in power system – A) Electro-mechanical,   B) Electronic relays.

  1. Electro-mechanical relays These relays work on main two operating principles  – a)Electromagnetic attraction & b) Electromagnetic induction
  1. Electromagnetic attraction –It consist of an armature which is attracted towards poles of an electromagnet or a plunger being pulled into a solenoid. The shorting link attached at the end of the armature or plunger close the circuit (this is called tripping command) & breaker gets tripped.

Electromagnetic attraction relays are of 3 types –

  1. Attracted armature type,
  2. Solenoid type,
  • Balanced beam type,
  1. Electromagnetic Induction –Such relays operate on the principle of induction motor. These are widely used for protecting AC electrical circuits. This type of relay is not suitable for dc circuits because of principle of operations.

An induction relay consists of a pivoted aluminum disc place in two alternating magnetic fields of the same frequency but displaced in time & space. The torque is produced in the disc by interaction of these two magnetic fields.

Following types of designed are generally used which make the phase difference in fluxes which produces torque in induction relays –

  1. Shaded pole structure,
  2. Watt hour meter or double winding structure,
  3. Induction cup structure,


  • Relay Timing – The time of operation of a relay is very important. It means the length of time from the instant when the actuating element is energized to the instant when the relay contacts are closed. Sometimes is the requirement of the system to control the operating time of a relay & for this purpose, mechanical accessories are used with the relays.

There are three types of time delay in relays operations –

  1. Instantaneous relay,
  2. Inverse time relay,
  3. Definite time lag relay,
  4. Instantaneous relay – An instantaneous relay is one in which no time delay is provided. Hence, relay contacts are closed immediately when current is relay coil exceeds set value. These relays have operating time less than 0.1 sec.
  5. Inverse time relays –In this case, the operating time is inversely proportional to the magnitude of the current value. Inverse time delay can be achieved – i) by positioning a permanent magnet, ii) by dash pot, iii) a time limit fuse.
  6. Definite time lag relays – In this case, a definite time delay is provided between the closing of relay contacts & the fault/actuating current.
  • Pick up current –It is the minimum current in the relay coil at which relay starts to operate. If current is less than the pick -up value, the relay does not operate & breaker will remain closed & operational.
  • Current setting – It is normally desirable to adjust the pick-up current of the relay, this is known as current setting & is usually achieved by the use of tapings on the relay operating coil. The taps are brought out to a plug bridge. The plug bridge allows to change the number of turns on the relay coil which changes the torque on the disc & time of operation of the relay.

The value of each tapping is directly associated with the percentage of CT.

Pick up current = rated secondary current of CT X Current setting

Example – suppose an over current relay is having setting of – 110%, where CT ratio is 500/5. The rated secondary current is 5A. Therefore pick up value will be 10% more than 5A i.e. 5X1.1 = 5.5A. SO relay will operate when current in the relay coil is equal to or greater than 5.5A.

  • Plug setting multiplier (PSM) – It is the ratio of fault current in the coil to the pick- up current.
PSM = Fault current in the coil / pick up current
  • Time setting multiplier (TSM) – A relay is having a feature of adjusting the time of operation. Its in the form of dial which is calibrated from 0 to 1in steps of 0.05 sec.


Broadly following types of relays are used in HT & LT panels in a commercial building-

  • Over voltage relay,
  • Under voltage relay,
  • Earth fault relay,
  • Restricted earth fault relay,
  • Over current relay,
  • Automatic Power Factor Correction Relay,
  • Supervision relay,
  • Master Trip relay
  • Over/Under voltage relay –This relay is used to protect electrical system like generators, transformer, motors etc from Over/Undervoltage. These relays are installed separately in panels. These relays are connected with secondary of PT (potential transformer). These relays measure the phase to phase to voltage of the system. When voltage goes beyond the set value, relays gives tripping command to breaker, breaker trips & gives alarm, hence electrical equipment connected with this panel will be protected from over/under voltage.

These relays are available with wide range of voltages –

Over Voltage range – 105% to 180% in steps of 5%,

Under Voltage range – 95% to 20% in steps of 5%,

Operating time–These are available with definite time or inverse time stage.

Alarm/trip options– These relays are available with alarm & trip options. Alarm & Trip voltage value can be set in the relay.

  • Earth fault relay – An earth fault relay is one that operates in case of current leakage in electrical system. Earth fault relays are used for 3 phase & 1-phase electrical system.
  1. Earth fault protection in 3ph system – Normally in a 3 phase electrical system, current flows in all three phases & the vector sum of this current is 0. This current is called residual current. In other words the value of residual current is 0.


IR + IY + IB   = 0

In 3 phase system, earth fault relays are connected with CTs. CTs are mounted on each phase & secondary of CTs are connected with the relay. EF relays some times are separately or sometimes along with Overcurrent relay.

Under normal operating condition, equal current flows through the different phases of electrical system & hence the vector sum/algebraic sum are zero. SO zero current flows through the coil of relay.

However when an earth fault occurs (it means phase wire touches with body of the equipment), the fault disturbs the balancing of currents& hence the vector sum will no longer be zero & this unbalanced current starts flowing through the coil of the relay or in other words, now residual current has some value. When the value of residual current is more than the pickup value, the earth fault relay operates.

Note: Earth fault relay may operate under unbalanced currents in each phase also. Unbalanced current flows due to unbalanced loading on each phase. So, due to unbalanced current, vector sum no longer remains zero & this unbalanced current flows through the coil of the relay & relay operated.

Earth fault (EF) relay setting – EF relays are available with settings – 10%, 20%, 30% … of rated load current.

  • Restricted Earth fault relay – Restricted earth fault detects faults from a restricted zone which is created by CTs & it doesn’t sense fault from outside zone. It is mostly used for the protection of winding of transformers, alternators etc.


  • Over current relay (OC Relay) – It operates when current in the circuit exceeds its set value (or predetermined value). This condition may arise due to overloading or short circuit in the system. Over current relays work on three principles – IDMT, DMT & Instantaneous.

IDMT (Inverse Definite Mean Time) Relay – It works on the principle that the more current is put through the relay, the faster it operates& vise-versa.

DMT (Definite Mean Time) Relay – It operates when current in the circuit exceeds a set value & exceeded value of current remains in the circuit for a period/duration set in the relay.

Instantaneous rely – It operates instantaneously/immediately when current in the circuit exceeds the set value/pick up of the relay.

  • Power Factor Correction relay-This relay is used in electrical system to improve the value of power factor. The value of power factor lies between 0 – 1. Ideally the value of power factor should be 1. But due to inductive load (like induction motors, transformers, induction furnace, fluorescent light etc) in the system, value of p.f. decreases. Low pf means more reactive power in the system& reactive power is the power which is the wastage power &is not used in the system. In other words we can say that load draws more power than rated power when pf is low.Following example will help you to understand about value of pf –

Power P = V I Cos Ø

Where V = Supply voltage, I = Current, Cos Ø  = power factor

Now, I = P/( V Cos Ø)  or I is inversely proportional to pf. Now we can say that value of current will be less on unity power factor and when value of pf will decrease the value of current will increase for the same load.

So in order to increase the value of pf, power factor correction relay is used.

Working of Power Factor Correction relay – It is installed in the Main LT panel. Main LT panel is equipped with capacitors whose value depends on the value of the load connected with main panel. The capacitors are installed in the form of banks like 25KVAR, 50KVAR, 100KVAR etc. These capacitor banks are connected with the Power Factor Correction relay. There is a set pointfor pf in the relay which is to be maintained. PF correction relay checks on the actual power factor & compare it with the set value of pf & switch ON capacitor banks if pf is low. The no. of capacitor banks depends on the value of actual pf.

  1. Supervision relay – The function of supervision relay is to show the status of the trip circuit of the breaker that the trip circuit in the breaker is healthy & breaker will trip in case of fault. It is connected with an indicating light named “Trip Circuit Healthy” & a push button. If we press the push button, the Trip Circuit Healthy light will glow if trip circuit is ok otherwise it will bot glow. In this way we come to know that there is some problem with circuit (like loss of voltage, circuit breakage etc) & rectify the problem.
  2. Master Trip relay – It is also called lock-out relay. After giving tripping command to breaker it remains in lock out position until it is hand reset. A mechanically operated hand reset type flag is provided as an indicator.

Master trip relay is the main trip relay. Other protection relays are connected with this relay only. When other relays detect fault, they give command to Master Trip Relay & Master trip relay will give tripping command to circuit breaker.

  1. ElectronicrelaysElectronic relays are Solid State electronic devices that use power semiconductor devices such asthyristors&transistors. Solid State devices are switching devices that are on or off when a small external voltage is applied across its terminals. These relays can be used in place of electromechanical relays in electrical system due to following advantages –
  • It has no moving parts which increases accuracy in operations,
  • It is smaller in size,
  • It has higher operating speed,
  • It provides wide range of settings on account of time & current/voltage,
  • It is more accurate in operations,
  • It has long life,


  • All these relays are either self-reset type or manual reset type. A mechanically operated hand reset type flag is provided as an indicator in the relays. In self reset condition – flag will be down but relay get resets automatically for the next operation but in manual reset type, relay needs manual/hand reset for the next operation. In manual reset relay, breaker will not close if relay doesn’t get reset manually.
  • Some relays are like over current & earth fault relays come separately and/or in combination such as combination of 2-over current + 1- earth fault. It means over current relay is connected with R & B phases while earth fault relay is connected with Y-phase only.