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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
Characteristic El. Mech. Relay Static Relay Digital Relay Numerical Relay
Technology Standard 1st generation relays. 2nd generation relays. Present generation relays. Present generation relays.
Operating Principle They use principle of electromagnetic principle. In this relays transistors and IC’s been used They use microprocessor. Within built software with predefined values They use microprocessor. Within built software with predefined values
Measuring elements/ Hardware Induction disc, electromagnets, induction cup, balance beam R, L, C, transistors, analogue ICs comparators Microprocessors, digital ICs, digital dignal processors Microprocessors, digital ICs, digital signal processors
Measuring method Electrical qtys converted intomechanical force, torque Level detects,comparison withreference value in analogue comparator A/D conversion, numerical algorithm techniques A/D conversion, numerical algorithm techniques
Surrounding Environment Depend upon gravitation and the value changes to the surrounding magnetic fields also. There value may vary with respect to temperature also.
Characteristic El. Mech. Relay Static Relay Digital Relay Numerical Relay
Relay Size Bulky Small Small Compact
Speed of Response Slow Fast Fast Very fast
Timing function Mechanical clock works, dashpot Static timers Counter Counter
Time of Accuracy Temp. dependant Temp. dependant Stable Stable
Reliability High Low High High
Vibration Proof No Yes Yes Yes
Characteristics Limited Wide Wide Wide
Requirement of Draw Out Required Required Not required Not required
CT Burden High Low Low Low
CT Burden 8 to 10 VA 1 VA < 0.5 VA < 0.5 VA
Characteristic El. Mech. Relay Static Relay Digital Relay Numerical Relay
Reset Time Very High Less Less Less
Auxiliary supply Required Required Required Required
Range of settings Limited Wide Wide Wide
Isolation Voltage Low High High High
Function Single function Single function Multi function Single function
Maintenance Frequent Frequent Low Very Low
Resistance 100 mille ohms 10 Ohms 10 Ohms 10 Ohms
Output Capacitance < 1 Pico Farad > 20 Pico Farads > 20 Pico Farads > 20 Pico Farads
Characteristic El. Mech. Relay Static Relay Digital Relay Numerical Relay
Deterioration due to Operation Yes No No No
Relay Programming No Partially Programmable Programmable
SCADA Compatibility No No Possible Yes
Operational value indication Not Possible Possible Possible Possible
Visual indication Flags, targets LEDs LEDs, LCD LEDs, LCD
Self monitoring No Yes Yes Yes
Parameter setting Plug setting, dial setting Thumb wheel, dual in line switches Keypad for numeric values, through computer Keypad for numeric values, through computer
Fault Disturbance Recording Not possible Not possible Possible Possible

Relay’s Nomenclature as per ANSI

No Type of Relay
2 Time delay relay
3 3 Checking or Interlocking relay
21 21 Distance relay
25 Check synchronizing relay
27 Under voltage relay
30 Annunciation relay
32 Directional power (Reverse power) relay
37 Low forward power relay
40 Field failure (loss of excitation)
46 Negative phase sequence relay
49 Machine or Transformer Thermal relay
50 Instantaneous Over current relay
51 A.C. IDMT Over current relay
52 Circuit breaker
52a Circuit breaker Auxiliary switch “Normally open” (‘a’ contact)
52b Circuit breaker Auxiliary switch “Normally closed” (‘b’ contact)
55 Power Factor relay
56 Field Application relay
59 Overvoltage relay
64 Earth fault relay
67 Directional relay
68 Locking relay
74 Alarm relay
76 D.C Over current relay
78 Phase angle measuring or out of step relay
79 AC Auto reclose relay
80 Monitoring loss of DC supply
81 Frequency relay
81 U Under frequency relay
81 O Over frequency relay
83 Automatic selective control or transfer relay
85 Carrier or pilot wire receive relay
86 Tripping Relay
87 Differential relay
87G Generator differential relay
87G T Overall differential relay
87U UAT differential relay
87NT Restricted earth fault relay
95 Trip circuit supervision relay
99 Over flux relay
186A Auto reclose lockout relay
186B Auto reclose lockout relay

Relays for Transmission and Distribution Lines protection

No Line Protection
1 400 KV Transmission Line Main-I: Non switched or Numerical Distance Scheme
Main-II: Non switched or Numerical Distance Scheme
2 220 KV Transmission Line Main-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
3 132 KV Transmission Line Main 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.
4 33 KV Lines Non-directional IDMT 3 Over Current and 1 Earth Fault relays
5 11KV Line Non-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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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 13 years experience in transmission & distribution-energy theft detection and maintenance electrical projects.


  1. 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

  2. Arun k
    Mar 02, 2017

    Its very informative artical.thank you sir

  3. 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.

  4. Dan Thompson
    Jun 11, 2014

    I found the article very informative thank you

  5. 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)

  6. maghboul
    Jan 12, 2014


  7. shahid jamil
    Dec 05, 2013

    This portal is very useful for Electrical Engineers.

  8. bitoha
    Apr 29, 2013

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

  9. 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.

  10. sharathrsidenur
    Feb 28, 2013

    very good information

  11. 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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