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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 all documents in this section are free to download. Navigate through sub-pages to discover all documents.

Schematic Representation Of Power System Relaying

A schematic is a diagram that represents the elements of a power system using abstract, graphic symbols rather than realistic pictures. The reason for not emphasizing reality is to communicate function, instead. A schematic usually… Read more

Released on May 28, 2016

Schematic Representation Of Power System Relaying

The basics of power system protective relaying

Protective Relaying The IEEE defines protective relays as: “Relays whose function is to detect defective lines or apparatus or other power system conditions of an abnormal or dangerous nature and to initiate appropriate control circuit… Read more

Released on Dec 19, 2015

The basics of power system protective relaying

Electrical Plan Review Booklet

Content of Electrical Plan Review Booklet: Overcurrent Protection and Devices, Short-Circuit Calculations, Component Protection, Selective Coordination and Other Considerations By reviewing this brochure, the Electrical Inspector, Electrical Contractor, Plan Examiner, Consulting Engineer and others will… Read more

Released on May 16, 2015

Electrical Plan Review Booklet

Transmission Line Protection Principles

The protection system selected should provide redundancy to limit the impact of device failure, and backup protection to ensure dependability. The high level factors influencing line protection include: The criticality of the line (in terms… Read more

Released on May 31, 2014

Transmission Line Protection Principles - General Electric

Protective Relaying – Principles and Applications

Protective relays using electrical quantities are connected to the power system through current transformer (CT) or voltage transformer (VT). These input devices or instrument transformers provide insulation from the high-power system voltages and reduce the… Read more

Released on Mar 15, 2014

The Philosophy Of Protective Relaying

The function of protective relaying is to cause the prompt removal from service of any element of a power system when it suffers a short circuit, or when it starts to operate in any abnormal… Read more

Released on Nov 06, 2011

Electrical Relay Diagram and PID Symbols

Disconnect; Circuit Interrupter; Circuit Breaker; Limits (normally open, normally closed, neutral position, maintained position, proximity switch); Liquid Level; Vacuum & Pressure; Temperature; Flow (Air, Water); Foot; Toggle; Cable Operated (Emerg.); Plugging; Nonplug; Plugging w/Lockout Coil;… Read more

Released on Nov 05, 2011

Fundamentals of line protection with distance relays

Distance relaying should be considered when overcurrent relaying is too slow or is not selective. Distance relays are generally used for phase-fault primary and back-up protection on subtransmission lines, and on transmission lines where high-speed… Read more

Released on Dec 24, 2010

Fundamentals of line protection with distance relays

Distance Relay Fundamentals

Distance functions have been in use for many years and have progressed from the original electromechanical types through analog types and now up to digital types of functions. The purpose of this paper is to… Read more

Released on Dec 24, 2010

Distance Relay Fundamentals

Relay Selection Guide

Power distribution systems serving industrial or commercial facilities represent a variety of costs to the owner. Obviously, the capital cost of the equipment is a concern, as is the cost of the energy which that… Read more

Released on Oct 13, 2010

Relay Selection Guide

Relay definitions

Due to the self induction of the coil and the inertia of the parts to be moved, on activating a relay the operations do not take place suddenly. The function diagram below shows the different… Read more

Released on Oct 13, 2010

Electronics in and around Power Relays

In addition to standard transistor control circuits. (see chapter 5) other electronic circuits are widely used to extend a relays function such as in timer relays or to reduce the power consumption of the relay’s… Read more

Released on Oct 12, 2010


  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
    Email: [email protected]

  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….

  32. [email protected]
    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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