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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.

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,… Read more

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… Read more

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… Read more

Mar 15, 2014

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.);… Read more

Nov 05, 2011

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… Read more

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… Read more

Oct 13, 2010

Relay Selection Guide


    Jul 07, 2022

    Hello we are a supplying company based in kenya we would like to inquire about some protection relays

  2. Amit
    May 28, 2020

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

  3. ZANE Ilyas
    Sep 17, 2019

    Very interested

  4. Md.Masum Billah
    Sep 15, 2019


  5. Richard Gayi
    Aug 20, 2019

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

  6. Padmanaban A
    Jan 14, 2019

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

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