Over Current Relay(Type-Application-Connection)


Types of protection:

  • Protection schemes can be divided into two major groupings:
  1. Unit schemes
  2. Non-unit schemes

1) Unit Type Protection

  • Unit type schemes protect a specific area of the system, i.e., a transformer, transmission line, generator or bus bar.
  • The unit protection schemes is based on Kerchief’s current law – the sum of the currents entering an area of the system must be zero. Any deviation from this must indicate an abnormal current path. In these schemes, the effects of any disturbance or operating condition outside the area of interest are totally ignored and the protection must be designed to be stable above the maximum possible fault current that could flow through the protected area.

2) Non unit type protection

  • The non-unit schemes, while also intended to protect specific areas, have no fixed boundaries. As well as protecting their own designated areas, the protective zones can overlap into other areas. While this can be very beneficial for backup purposes, there can be a tendency for too great an area to be isolated if a fault is detected by different non unit schemes.
  •  The most simple of these schemes measures current and incorporates an inverse time characteristic into the protection operation to allow protection nearer to the fault to operate first.
  • The non unit type protection system includes following schemes:
  • (A)  Time graded over current protection
  • (B)  Current graded over current protection
  • (C) Distance or Impedance Protection

(A) Over current protection

  • This is the simplest of the ways to protect a line and therefore widely used.
  • It owes its application from the fact that in the event of fault the current would increase to a value several times greater than maximum load current.
  • It has a limitation that it can be applied only to simple and non costly equipments.

(B) Earth fault protection

  • The general practice is to employ a set of two or three over current relays and a separate over current relay for single line to ground fault. Separate earth fault relay provided makes earth fault protection faster and more sensitive.
  • Earth fault current is always less than phase fault current in magnitude. Therefore, relay connected for earth fault protection is different from those for phase to phase fault protection.

 Various types of Line Faults:

NoType of FaultOperation of  Relay
1Phase to Ground fault (Earth Fault)Earth Fault Relay
2Phase to Phase fault Not with GroundRelated Phase Over current relays
3Double phase to Ground faultRelated Phase Over current relays and Earth Fault relays

 Over current Relay:

  • A relay that operates or picks up when it’s current exceeds a predetermined value (setting value) is called Over Current Relay.
  • Over current protection protects electrical power systems against excessive currents which are caused by short circuits, ground faults, etc. Over current relays can be used to protect practically any power system elements, i.e. transmission lines, transformers, generators, or motors.
  • For feeder protection, there would be more than one over current relay to protect different sections of the feeder. These over current relays need to coordinate with each other such that the relay nearest fault operates first. Use time, current and a combination of both time and current are three ways to discriminate adjacent over current relays.

 Over Current Relay gives Protection against:

  1. Over current includes short-circuit protection.
  2. Short circuits can be
  3. Phase faults
  4. Earth faults
  5. Winding faults
  • Short-circuit currents are generally several times (5 to 20) full load current. Hence fast fault clearance is always desirable on short circuits.

 Primary Requirement of Over Current Protection:

  • The protection should not operate for starting currents, permissible over current, current surges. To achieve this, the time delay is provided (in case of inverse relays).
  • The protection should be co-ordinate with neighboring over current protection.
  • Over current relay is a basic element of over current protection.

 Purpose of over current Protection

  • Detect abnormal conditions
  • Isolate faulty part of the system
  • Speed Fast operation to minimize damage and danger
  • Discrimination Isolate only the faulty section
  • Dependability / reliability
  • Security / stability
  • Cost of protection / against cost of potential hazards

Over Current Relay Ratings:

  • In order for an over current protective device to operate properly, over current protective device ratings must be properly selected. These ratings include voltage, ampere and interrupting rating.
  • If the interrupting rating is not properly. Selected, a serious hazard for equipment and personnel will exist. Current limiting can be considered as another over current protective device rating, although not all over current protective devices are required to have this characteristic
  • Voltage Rating: The voltage rating of the over current protective device must be at least equal to or greater than the circuit voltage. The over current protective device rating can be higher than the system voltage but never lower.
  • Ampere Rating: The ampere rating of a over current protecting device normally should not exceed the current carrying capacity of the conductors As a general rule, the ampere rating of a over current protecting device is  selected at 125% of the continuous load current

 Difference Between Over current Protection & Over Load Protection:

  • Over current protection protects against excessive currents or currents beyond the acceptable current ratings, which are resulting from short circuits, ground faults and overload conditions.
  • While, the overload protection protects against the situation where overload current causes overheating of the protected equipment.
  • The over current protection is a bigger concept So that the overload protection can be considered as a subset of over current protection.
  • The over current relay can be used as overload (thermal) protection when protects the resistive loads, etc., however, for motor loads, the over current relay cannot serve as overload protection Overload relays usually have a longer time setting than the over current relays.

  Type of Over Current Relay:

  • (A)  Instantaneous Over Current (Define Current) Relay
  • (B)  Define Time Over Current Relay
  • (C) Inverse Time Over Current Relay (IDMT Relay)
  • Moderately Inverse
  • Very Inverse Time
  • Extremely Inverse
  • (D) Directional over Current Relay.

 (A) Instantaneous Over Current Relay (Define Current):

  • Definite current relay operate instantaneously when the current reaches a predetermined value.
  • Operates in a definite time when current exceeds its Pick-up value.
  • Its operation criterion is only current magnitude (without time delay).
  • Operating time is constant.
  • There is no intentional time delay.


  • Coordination of definite-current relays is based on the fact that the fault current varies with the position of the fault because of the difference in the impedance between the fault and the source
  • The relay located furthest from the source operate for a low current value
  • The operating currents are progressively increased for the other relays when moving towards the source.
  • It operates in 0.1s or less
  • Application: This type is applied to the outgoing feeders

(B) Definite Time Over current Relays:

  • In this type, two conditions must be satisfied for operation (tripping), current must exceed the setting value and the fault must be continuous at least a time equal to time setting of the relay. Modern relays may contain more than one stage of protection each stage includes each own current and time setting.


  • For Operation of Definite Time Over Current Relay operating time is constant
  • Its operation is independent of the magnitude of current above the pick-up value.
  • It has pick-up and time dial settings, desired time delay can be set with the help of an intentional time delay mechanism.
  • Easy to coordinate.
  • Constant tripping time independent of in feed variation and fault location.

Drawback of Relay:

  • The continuity in the supply cannot be maintained at the load end in the event of fault.
  • Time lag is provided which is not desirable in on short circuits.
  • It is difficult to co-ordinate and requires changes with the addition of load.
  • It is not suitable for long distance transmission lines where rapid fault clearance is necessary for stability.
  • Relay have difficulties in distinguishing between Fault currents at one point or another when fault impedances between these points are small, thus poor discrimination.

Application: Definite time over current relay is used as:

  • Back up protection of distance relay of transmission line with time delay.
  • Back up protection to differential relay of power transformer with time delay.
  • Main protection to outgoing feeders and bus couplers with adjustable time delay setting.

(C) Inverse Time Over current Relays (IDMT Relay):

  • In this type of relays, operating time is inversely changed with current. So, high current will operate over current relay faster than lower ones. There are standard inverse, very inverse and extremely inverse types.
  • Discrimination by both ‘Time’ and ‘Current’. The relay operation time is inversely proportional to the fault current.
  • Inverse Time relays are also referred to as Inverse Definite Minimum Time (IDMT) relay


  • The operating time of an over current relay can be moved up (made slower) by adjusting the ‘time dial setting’. The lowest time dial setting (fastest operating time) is generally 0.5 and the slowest is 10.
  • Operates when current exceeds its pick-up value.
  • Operating time depends on the magnitude of current.
  • It gives inverse time current characteristics at lower values of fault current and definite time characteristics at higher values
  • An inverse characteristic is obtained if the value of plug setting multiplier is below 10, for values between 10 and 20 characteristics tend towards definite time characteristics.
  • Widely used for the protection of distribution lines.
  • Based on the inverseness it has three different types.4

(1)  Normal Inverse Time Over current Relay:

  • The accuracy of the operating time may range from 5 to 7.5% of the nominal operating time as specified in the relevant norms.
  • The uncertainty of the operating time and the necessary operating time may require a grading margin of 0.4 to 0.5 seconds.
  • used when Fault Current is dependent on generation of Fault  not fault location
  • Relatively small change in time per unit of change of current.


  • Most frequently used in utility and industrial circuits. especially applicable where the fault magnitude is mainly dependent on the system generating capacity at the time of fault

(2)  Very Inverse Time Over current Relay:

  • Gives more inverse characteristics than that of IDMT.
  • Used where there is a reduction in fault current, as the distance from source increases.
  • Particularly effective with ground faults because of their steep characteristics.
  • Suitable if there is a substantial reduction of fault current as the fault distance from the power source increases.
  • Very inverse over current relays are particularly suitable if the short-circuit current drops rapidly with the distance from the substation.
  • The grading margin may be reduced to a value in the range from 0.3 to 0.4 seconds when over current relays with very inverse characteristics are used.
  • Used when Fault Current is dependent on fault location.
  • Used when Fault Current independent of normal changes in generating capacity.

(3)  Extremely Inverse Time Over current Relay:

  • It has more inverse characteristics than that of IDMT and very inverse over current relay.
  • Suitable for the protection of machines against overheating.
  • The operating time of a time over current relay with an extremely inverse time-current characteristic is approximately inversely proportional to the square of the current
  • The use of extremely inverse over current relays makes it possible to use a short time delay in spite of high switching-in currents.
  • Used when Fault current is dependent on fault location
  • Used when Fault current independent of normal changes in generating capacity.


  • Suitable for protection of distribution feeders with peak currents on switching in (refrigerators, pumps, water heaters and so on).
  • Particular suitable for grading and coordinates with fuses and re closes
  • For the protection of alternators, transformers. Expensive cables, etc.

 (4)  Long Time Inverse over current Relay:

  • The main application of long time over current relays is as backup earth fault protection.

 (D) Directional Over current Relays

  • When the power system is not radial (source on one side of the line), an over current relay may not be able to provide adequate protection. This type of relay operates in on direction of current flow and blocks in the opposite direction.
  • Three conditions must be satisfied for its operation: current magnitude, time delay and directionality. The directionality of current flow can be identified using voltage as a reference of direction.

 Application of Over Current Relay:

  • Motor Protection:
  • Used against overloads and short-circuits in stator windings of motor.
  • Inverse time and instantaneous over current phase and ground
  • Over current relays used for motors above 1000kW.
  • Transformer Protection:
  • used only when the cost of over current relays are not justified
  • Extensively also at power-transformer locations for external-fault back-up protection.
  • Line Protection:
  • On some sub transmission lines where the cost of distance relaying cannot be justified.
  • primary ground-fault protection on most transmission lines where distance relays are used for phase faults
  • For ground back-up protection on most lines having pilot relaying for primary protection.
  • Distribution Protection:
  • Over Current relaying is very well suited to distribution system protection for the following reasons:
  • It is basically simple and inexpensive
  • Very often the relays do not need to be directional and hence no PT supply is required.
  • It is possible to use a set of two O/C relays for protection against inter-phase faults and a separate Over Current relay for ground faults.

 Connection of over current and Earth Fault Relay:

(1)  3 Nos O/C Relay for Over Current and Earth Fault Protection:

  • For 3-phase faults the over current relays in all the 3-phases act.
  • For phase to phase faults the relays in only the affected phases operate.
  • For single line to ground faults only the relay in the faulty phase gets the fault current and operates.
  • Even then with 3 Over current Relay, the sensitivity desired and obtainable with earth leakage over current relays cannot be obtained in as much as the high current setting will have to be necessarily adopted for the Over current Relay to avoid operation under maximum load condition.


  • Over current relays generally have 50% to 200% setting while earth leakages over current relays have either 10% to 40% or 20% to 80% current settings.
  • One important thing to be noted here is that the connection of the star points of both the C.T. secondary’s and relay windings by a neutral conductor should be made.
  •  A scheme without the neutral conductor will be unable to ensure reliable relay operation in the event of single phase to earth faults because the secondary current in this case (without star-point interconnection) completes its circuit through relay and C.T. windings which present large impedance. This may lead to failure of protection and sharp decrease in reduction of secondary currents by CTs.
  •  It is not sufficient if the neutral of the CTs and neutral of the relays are separately earthed. A conductor should be run as stated earlier.

 (2)  3 No O/C Relay+ 1 No E/F Relay for Over Current and Earth Fault Protection:

  • The scheme of connection for 3 Nos Over current Relay 1 No Earth Fault Relay is shown in figure.


  • Under normal operating conditions and three phase fault conditions the current in the 3-phase are equal and symmetrically displaced by 12 Deg. Hence the sum of these three currents is zero. No current flow through the earth fault relay.
  • In case of phase to phase faults (say a short between R and Y phases) the current flows from R-phase up to the point of fault and return back through ‘Y’ phase. Thus only O/L relays in R and Y phases get the fault and operate.
  • Only earth faults cause currents to flow through E/L relay. A note of caution is necessary here. Only either C.T secondary star point of relay winding star point should be earthed.
  • Earthing of both will short circuit the E/L relay and make it inoperative for faults.

 (3)  2 No O/C Relay + 1 No E/F Relay for Over Current and Earth Fault Protection:

  • The two over current relays in R&B phases will respond to phase faults. At least one relay will operate for fault involving two phase.


  •  For fault involving ground reliance is placed on earth fault relay.
  • This is an economical version of 3-O/L and 1-E/L type of protection as one overcurrent relay is saved. With the protection scheme as shown in Figure complete protection against phase and ground fault is afforded

 Current Transformer Secondary Connections:

  • For protection of various equipment of Extra High Voltage class, the Star point on secondary’s of CT should be made as follows for ensuring correct directional sensitivity of the protection scheme
  • Transmission Line , Bus Bar & Transformer:
  • For Transmission Lines – Line side
  • For Transformers – Transformer side
  • For Bus bar – Bus side8
  • Generator Protection:
  • Generator Protection – Generator Side
  • 9The above method has to be followed irrespective of polarity of CT’s on primary side.
  • For example, in line protection, if ‘P1’ is towards bus then ‘S2’s are to be shorted and if ‘ P2’ is towards bus then ‘S1’s are to be shorted.

 Standard over Current & Earth Fault Protection:


Name of the Equipment



11 KV Feeders

(A) 2 No Over Current and one no Earth Fault IDMT relays
(B) 2 No Instantaneous Over current (highest) and one no Instantaneous Earth fault relay


8 MVA Capacity  OR Two Transformer in a Sub Station ( Irrespective of Capacity)


HV side : 33 KV Breaker ( Individual or Group Control with 3 Over Current and One Earth Fault IDMT relaysLV Side: Individual 11 KV Breakers with 3 Over Current and One Earth Fault IDMT relays


8 MVA Power Transformer

Differential relays OR REF relays on LV side


Only one PTR in a Sub Station (Less than 8 MVA)

HV Side : HG fuseLV Side : 11 KV Breaker with 3 Over Current and one  E/F IDMT relay