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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.

Browse guides and papers

Note that not all documents in this section are free to download. Navigate through sub-pages to discover all documents.

Lecture notes in power system protection

Protection is the art or science of continuously monitoring the power system, detecting the presence of a fault and initiating the correct tripping of the circuit breaker. The objectives of… Read more

Jun 22, 2020

Lecture notes in power system protection


  1. Daniel Zebene
    Apr 11, 2015

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

  2. 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..

  3. iman ramang
    Aug 19, 2014

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

  4. Moloi
    Jun 12, 2014

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

  5. hadi218
    Jul 12, 2013

    Thanks for information

  6. Avinash
    Feb 26, 2013

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

  7. kalkish
    Aug 08, 2012

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

  8. marg
    Jul 16, 2012

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

  9. bahaa91
    Apr 26, 2012

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

  10. Bishoy
    Mar 29, 2012

    Congratulations for this website
    This website is very helpful

  11. Vinay Kumar
    Feb 07, 2012

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

  12. [email protected]
    Jan 11, 2012

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

  13. seanmoo
    Jan 07, 2012

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


  14. 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.


  15. 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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