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Protection Relays

The relay is a well known and widely used component. Applications range from classic panel built control systems to modern interfaces between control microprocessors and their power circuits or any application where reliable galvanic separation is required between different circuits. Altough considered to be a relatively simple component, the electromechanical relay and its technology is complex and often misunderstood.

Relay control and protection guides
Relay control and protection guides

History of relay

The earliest electrical relays were developed in the 1830s, as people began to recognize that such switches could be extremely useful. Historically, electrical relays were often made with electromagnets, which continue to be used today, although for some applications solid state relays are preferred. They key difference between electromagnetic and solid state options is that electromagnetic relays have moving parts, and solid state relays do not.

Electromagnets also conserve more energy than their solid state counterparts do.

Usage of relay

One of the reasons an electrical relay is such a popular tool for electricians and engineers is that it can control electrical output which is higher than the electrical input it receives. In the example discussed above, if the ignition connected directly to the battery, heavy duty insulated wiring would be needed to connect the steering column to the battery, and the ignition switch would also need to be much more robust.

By using a relay, relatively lightweight wiring can be used, saving space and increasing vehicle safety.

A variety of circuits can be connected to an electrical relays. Relays can be used as amplifiers for electrical energy, as in the car example, and they can also connect to things like alarm switches, activating when a circuit is broken to trigger an alarm.

Many electrical failsafe systems utilize electrical relays which turn on or off in response to things like a current overload, irregular current, and other issues which may arise. These electrical relays trip to shut the system down until the problem can be addressed.

All documents, EE software and EE books are free to download.

No. Description Download
1 Relay Selection Guide .
2 Distance Relay Fundamentals .
3 Line Protection With Distance Relays .
4 Electrical Relay Diagram and P&ID Symbols New .
5 The philosophy of protective relaying New .
6 The Art and Science of Protective Relaying .
7 Protective Relaying – Principles and Applications .
8 Transmission Line Protection Principles .
9 Electrical Plan Review Booklet star .
10 The Basics Of Power System Protective Relaying .
11 Schematic Representation Of Power System Relaying New star .
12 Guide to protection of power systems with distributed generation New .
13 Operation, maintenance, and field test procedures for protective relays New .
14 Basic principles in modern substation automation protection systems New .
15 Guide through applications for SIPROTEC protection relays New .
16 Lecture notes on switchgear and protection devices for students New .
17 Calculation and prevention of short circuit currents in high voltage grids .
18 Line protection calculations and setting guidelines for relays installed at 765kV, 400kV, 220kV transmission systems .
19 Design Considerations For Protection System Of a Geothermal Power Plant .
20 Generating Power Plant and Transmission System Protection Coordination .
21 The Basics Of Overcurrent Protection .
22 Ground Fault Protection of Overhead Transmission Lines (Focus On Distance Relays) .
23 Principles and protection applications of low-impedance bus differential relay .
24 How to design Bay Control Unit (BCU) in a substation automation system .
25 Simulation of protection system with a source, circuit breaker, transformer and motor .
26 Circulating current scheme using IEC 61850 and relay logic for junior & graduate engineers .
27 Overcurrent and differential digital protective relays with monitoring capabilities .
28 132kV and 22kV busbar protection schemes of the new Hong Kong Electric’s stations .
28 Symmetrical components theoretical and real-world examples for relay engineers .
29 Course on switchgears and relay protection for students .
30 Arc-flash prediction and detection in MV/LV switchgear .
31 Secondary injection testing for transformer differential protection relay .
32 Unit protection of feeders (principles, schemes and applications) .
33 Restricted earth fault relay application within a 400kV shunt capacitor bank design .
34 Phase overcurrent protection on overhead MV feeders .
35 Analysis of power system faults (transformers, rotating machines, overhead and cable lines) .
36 Generator protection application and relay selection guide .
37 Lecture notes in relay protection for students (generator, motor and transformer) .
38 Software framework for automated testing of power system protection relays .
39 The breaker failure protection (BFP) schemes in utilities .
40 Protection of parallel (double) circuit transmission lines in modern power systems .
41 Practical protection coordination study for electrical substation 154kV / 34.5kV .
42 Protection of generators at main generation station that supply Sudan (500 MW) .
43 Analysis of protection coordination in MV distribution system of Sri Lanka .
44 Relay protection failures and their impact on the 380 kV substation reliability .
45 Analysis of causes and effects of faults in overhead transmission lines .
46 Power line carrier channel (PLCC) and application of transmission line relaying .
47 Design and implementation of fibre optical HV sensor in relay protection systems .
48 Design control and protection for medium voltage switchgear .
49 The basics of power system protection that every engineer should know about .
50 Practical handbook for relay protection engineers .
51 Centralized substation protection and control system for HV/MV substation .
52 Testing protection IEDs in IEC 61850 based substation automation systems .
53 Generator protection functions and test methods .
54 Overcurrent protection study for power network (solving relay setting miss-coordination) .

Relay book

No. Description Download
1 Introduction .
2 Relays – General .
3 Electromechanical Relays .
4 Contact System .
5 Magnetic System – Coil .
6 Mechanical Components .
7 Handling and Use in Production .
8 Quality Aspects .
9 Standards .
10 Electronics in and around Power Relays .
11 Relay definitions .

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  1. Amit
    May 28, 2020

    Electrical relay diagram and p nid symbool book not available pls upload again.. Thanks

  2. ZANE Ilyas
    Sep 17, 2019

    Very interested

  3. Md.Masum Billah
    Sep 15, 2019


  4. Richard Gayi
    Aug 20, 2019

    Thanks very much for this site. Thank your whole team for putting up this site.

  5. Padmanaban A
    Jan 14, 2019

    Thank you!, sir, your relay protection updating details have very useful for me for relay testing.

  6. Pedro Valverde
    Nov 19, 2018

    Today (11/19/2018) you published an article on protective relays polarity. I would like to download a copy. Thankyou.

  7. dinesh
    Oct 22, 2018

    Dear sir, i would have the guideline document related to scheme preparation of protective lines , transformer , generator etc.

  8. chatha thai
    Aug 15, 2018

    I want to download this file

  9. Syed Zohaib Hassan
    May 10, 2018

    sir pls tell me how differential relay works ?

  10. Satish kumar
    Feb 09, 2018

    I have a crompton greaves drive in a equipment, i am unable to vary its speed from HMI. Also unable to find drive manual.
    Can you help with probable solution

  11. Pedro
    Aug 25, 2017

    Thanks for information

  12. Hassan M. Fallatah
    Aug 05, 2017

    Tahnk you very much for this website and for it’s team

  13. Jawaid Akhound
    Mar 05, 2017

    In India What % of T&D Losses are standardized.

    • Madhav
      Jan 11, 2018

      Hello Jawaid,
      Here is a short reply to your query..
      Here in India 33kV, 22kV and 11kV level is considered as sub-transmission or HV distribution voltage level. Being a vast country distribution network and parameters differ widely from State to State and so does the T&D loss level. The loss level (which includes losses in HV lines and LT lines) in some pockets is around 10% and even less than that. However, it is as high as 30 to 40% in some rural and remote pockets.

  14. yasaamaan
    Sep 15, 2015

    We have two Thyro-A (AEG) in our substaion and both of them flashing on pulse inhibit.
    we checked LED MESSAGES and have been detected that one of Frequency error or SYNC error can be occured, and SYNC error has more probability. If SYNC error be right fault, how can i remove it? Can you help me please?

  15. Dinu
    Aug 25, 2015

    Good work. thanks for the information

  16. Abhishek shukla
    Aug 09, 2015

    10+2 + iti electrician one year exp appertince plz sir my carreyear job requirements ho to sir plz bta na ya my number 9811092589

  17. Kousha CS
    Jul 18, 2015

    I am looking for Protection engineer who know completely about MIcom P 44 series distance Relays any body interested please contact me immediately
    Salary package will be good
    Name: koushal CS
    Mobile: 9986101267

  18. Bipin Shah
    Jun 03, 2015

    I would like to know the reason for equi- potential wires getting disconnected at CT body end from main transformer incomer feeder. Feeder tripped on transformer differential & LV side restricted earth fault protection. Pls Explain…

    • madhusudhna rao
      Aug 22, 2015

      yes it is possible, when ever equipotential ring is disconnected from ct there is a chance of occuring error on measuring of ct, actual current flow on ct and measured current on ct will differ in that situation u get the trip from both differential and ref protection.

  19. Asghar
    May 18, 2015

    please explain can we use measurement CT’s for protection relay ? if not so why ?

    • sunilkumar
      Jul 24, 2015

      because of the ct winding always short suppose it is used for protection it will be open that time huge current flow through the ckt and it will damage the property or human body.

  20. jegatheshwaran
    May 13, 2015

    i had no idea about numerical relay, please tell full detail about that how to make setting,progarmming,configuration and progarmming logic. please sir,

  21. Daniel Zebene
    Apr 11, 2015

    thanks u very much, I am interested for particpate and to became member of this website.

  22. syed aijaz mohiuddin
    Sep 13, 2014

    please, can u give me the details of dry type transformer for the maintenance point of view.and also the over -under voltage relay…feeder protection relays..
    thanking you..

  23. iman ramang
    Aug 19, 2014

    As i know for Voltage Drop maximum allowable on VAC is 5%

  24. Moloi
    Jun 12, 2014

    What is the recommended voltage drop in Alternating Current{AC} and Direct current {DC}

  25. hadi218
    Jul 12, 2013

    Thanks for information

  26. Avinash
    Feb 26, 2013

    can any one share “pilot protective relaying by walter Elmore” or “Numerical Differential Protection by Gerhard Zeigler”

  27. kalkish
    Aug 08, 2012

    Good site..Happy to see that the data are given thro industrial products(GE,Siemens,ABB like)

  28. marg
    Jul 16, 2012

    i want all information about relays.and specially information on ‘numerical relay protection for motor’

  29. bahaa91
    Apr 26, 2012

    Congratulations for this website
    This website is very helpful
    B R I

  30. Bishoy
    Mar 29, 2012

    Congratulations for this website
    This website is very helpful

  31. Vinay Kumar
    Feb 07, 2012

    Good Job boss, really inspired especially application enginners like us….

    Jan 11, 2012

    You have done a excellent job. This website is very useful for begineers and even for expert as well.

  33. seanmoo
    Jan 07, 2012

    Just joined after seeing reference on Linkedin and really impressed by the sites contents. Great work.


  34. harjit
    Dec 13, 2011

    dear u really did great job ., i just want to know that ,can you provide me some elecrical formules for calculation for relay , like for didtance protection relay , or diff. protection relay
    thnx a lot

    • Edvard
      Dec 13, 2011

      Thank you for kind words Harjit! I wrote down your request, and I’ll try to find something.


  35. Gilmar
    Oct 28, 2011

    Congratulations for this website .


    São Paulo -Brazil

    • Edvard
      Oct 28, 2011

      Thank you Gilmar! Me and my team did a great efforts to make EEP to be like it is now.


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