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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. 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.

  2. dinesh
    Oct 22, 2018

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

  3. chatha thai
    Aug 15, 2018

    I want to download this file

  4. Syed Zohaib Hassan
    May 10, 2018

    sir pls tell me how differential relay works ?

  5. 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

  6. Pedro
    Aug 25, 2017

    Thanks for information

  7. Hassan M. Fallatah
    Aug 05, 2017

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

  8. 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.

  9. 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?

  10. Dinu
    Aug 25, 2015

    Good work. thanks for the information

  11. 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

  12. 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
    Email: [email protected]

  13. 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.

  14. 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.

  15. 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,

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