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Home / Technical Articles / Comparison of Protection Relay Types
Comparison of Protection Relay Types
Comparison of Protection Relay Types (photo by

Comparison Table

This comparison summarize characteristics of all protection relay types described in previously published technical articles:

  1. Using Protective Relay For Fighting Against Faults
  2. The Good Old Electromechanical Protective Relay
  3. The Solid State Relay (Static Relay) Overview
  4. Few Words About Digital Protection Relay
  5. Flexibility and Reliability of Numerical Protection Relay
CharacteristicEl. Mech. RelayStatic RelayDigital RelayNumerical Relay
Technology Standard1st generation relays.2nd generation relays.Present generation relays.Present generation relays.
Operating PrincipleThey use principle of electromagnetic principle.In this relays transistors and IC’s been usedThey use microprocessor. Within built software with predefined valuesThey use microprocessor. Within built software with predefined values
Measuring elements/ HardwareInduction disc, electromagnets, induction cup, balance beamR, L, C, transistors, analogue ICs comparatorsMicroprocessors, digital ICs, digital dignal processorsMicroprocessors, digital ICs, digital signal processors
Measuring methodElectrical qtys converted intomechanical force, torqueLevel detects,comparison withreference value in analogue comparatorA/D conversion, numerical algorithm techniquesA/D conversion, numerical algorithm techniques
Surrounding EnvironmentDepend upon gravitation and the value changes to the surrounding magnetic fields also.There value may vary with respect to temperature also.
CharacteristicEl. Mech. RelayStatic RelayDigital RelayNumerical Relay
Relay SizeBulkySmallSmallCompact
Speed of ResponseSlowFastFastVery fast
Timing functionMechanical clock works, dashpotStatic timersCounterCounter
Time of AccuracyTemp. dependantTemp. dependantStableStable
Vibration ProofNoYesYesYes
Requirement of Draw OutRequiredRequiredNot requiredNot required
CT BurdenHighLowLowLow
CT Burden8 to 10 VA1 VA< 0.5 VA< 0.5 VA
CharacteristicEl. Mech. RelayStatic RelayDigital RelayNumerical Relay
Reset TimeVery HighLessLessLess
Auxiliary supplyRequiredRequiredRequiredRequired
Range of settingsLimitedWideWideWide
Isolation VoltageLowHighHighHigh
FunctionSingle functionSingle functionMulti functionSingle function
MaintenanceFrequentFrequentLowVery Low
Resistance100 mille ohms10 Ohms10 Ohms10 Ohms
Output Capacitance< 1 Pico Farad> 20 Pico Farads> 20 Pico Farads> 20 Pico Farads
CharacteristicEl. Mech. RelayStatic RelayDigital RelayNumerical Relay
Deterioration due to OperationYesNoNoNo
Relay ProgrammingNoPartiallyProgrammableProgrammable
SCADA CompatibilityNoNoPossibleYes
Operational value indicationNot PossiblePossiblePossiblePossible
Visual indicationFlags, targetsLEDsLEDs, LCDLEDs, LCD
Self monitoringNoYesYesYes
Parameter settingPlug setting, dial settingThumb wheel, dual in line switchesKeypad for numeric values, through computerKeypad for numeric values, through computer
Fault Disturbance RecordingNot possibleNot possiblePossiblePossible

Relay’s Nomenclature as per ANSI

NoType of Relay
2Time delay relay
33 Checking or Interlocking relay
2121 Distance relay
25Check synchronizing relay
27Under voltage relay
30Annunciation relay
32Directional power (Reverse power) relay
37Low forward power relay
40Field failure (loss of excitation)
46Negative phase sequence relay
49Machine or Transformer Thermal relay
50Instantaneous Over current relay
51A.C. IDMT Over current relay
52Circuit breaker
52aCircuit breaker Auxiliary switch “Normally open” (‘a’ contact)
52bCircuit breaker Auxiliary switch “Normally closed” (‘b’ contact)
55Power Factor relay
56Field Application relay
59Overvoltage relay
64Earth fault relay
67Directional relay
68Locking relay
74Alarm relay
76D.C Over current relay
78Phase angle measuring or out of step relay
79AC Auto reclose relay
80Monitoring loss of DC supply
81Frequency relay
81 UUnder frequency relay
81 OOver frequency relay
83Automatic selective control or transfer relay
85Carrier or pilot wire receive relay
86Tripping Relay
87Differential relay
87GGenerator differential relay
87GT Overall differential relay
87UUAT differential relay
87NTRestricted earth fault relay
95Trip circuit supervision relay
99Over flux relay
186AAuto reclose lockout relay
186BAuto reclose lockout relay

Relays for Transmission and Distribution Lines protection

1400 KV Transmission LineMain-I: Non switched or Numerical Distance Scheme
Main-II: Non switched or Numerical Distance Scheme
2220 KV Transmission LineMain-I : Non switched distance scheme (Fed from Bus PTs)
Main-II: Switched distance scheme (Fed from line CVTs) With a changeover facility from bus PT to line CVT and vice-versa
3132 KV Transmission LineMain Protection: Switched distance scheme (fed from bus PT).
Backup Protection: 3 Nos. directional IDMT O/L Relays and
1 No. Directional IDMT E/L relay.
433 KV LinesNon-directional IDMT 3 Over Current and 1 Earth Fault relays
511KV LineNon-directional IDMT 2 Over Current and 1 Earth Fault relays

– Handbook of Switchgear – Bhel – Digital/Numerical Relays – T.S.M. Rao

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More Information

Jignesh Parmar

Jignesh Parmar has completed M.Tech (Power System Control), B.E (Electrical). He is member of Institution of Engineers (MIE), India. He has more than 20 years experience in transmission & distribution-energy theft detection and maintenance electrical projects.


  1. Prasad Kesarkar
    Dec 25, 2020

    Best Information sir,
    We have China made Microprocessor based relay in hydro power projets, now we are finding alternative to same in Indian manufacturing relays. can you advice the indian componies?

  2. Mumtaz Ur Rehman
    Jul 23, 2019

    Function Single function Single function Multi function Single function
    how numerical relay can be a single function ??
    please explain

  3. Arun k
    Mar 02, 2017

    Its very informative artical.thank you sir

  4. Rafiq Ahmad
    Jul 30, 2014

    Dear Jignesh
    A very well compiled work. Hats off.
    A very important things that needs to be noted is that all the three types of these Relays(Considering Digital & Numerical to be one) are SCADA Compatible. However you will need different types of Interfaces at the RTUs. A different interface port for the Hot signals (with varying voltages) for the E&M and Static Relays while as a different Communication port for the Soft signals from the Numerical Relays.

  5. Dan Thompson
    Jun 11, 2014

    I found the article very informative thank you

  6. Suvra Pattanayak
    May 31, 2014

    The articale is very nice but i have few question.
    1.what is difference between numerical relay & digital relay?(many literature say both are same-Digital/Numerical Relays – T.S.M. Rao)
    2.why In comparison table relay resistance are different unit? (for EM relay 100 mile ohm,other relay are 10 ohm)

  7. maghboul
    Jan 12, 2014


  8. shahid jamil
    Dec 05, 2013

    This portal is very useful for Electrical Engineers.

  9. bitoha
    Apr 29, 2013

    Why the numerical relay is single function type? I think it is multi-function type?

  10. Jay
    Mar 09, 2013

    Good work Jignesh, But for El Mechanical Relay like CDD21 (Alstom) Aux. Supply is not required and all the Numerical Relay are Multi function relays.

  11. sharathrsidenur
    Feb 28, 2013

    very good information

  12. sagar77777
    Feb 17, 2013

    this portal is very helpful to all the electrical engineering student as well as professor and technical engineer.It’s very nice and helpful for all in electrical engineering to get practical knowledge & idea very easily.

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