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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.

Relay standards

Compliance with National and International standards ensures the safety of electrical appliances and minimises the risk to life, health and the possibility of fire. In some countries component approval to… Read more

Oct 07, 2010

Quality Aspects

Product quality is judged according to criteria such as reliability, design. life expectancy etc. Certain quality parameters such as AQL, failure rate, and failure probability are applied to obtain an… Read more

Oct 06, 2010

Handling and Use in Production

Relays should be stored in a clean area within the specified temperature limits. Extreme humidity and condensation can cause corrosion of the metal parts on the inside as well as… Read more

Oct 06, 2010

Mechanical Components

Some power relays have a mechanical indicator fitted. The indicator is connected to the armature. actuator or contact springs to show the actual switching position of the contacts. Mechanical indicators,… Read more

Oct 06, 2010

Relay Magnetic System – The Coil

The magnetic, coil system or motor unit is also called the energizing or primary side of a relay. lt consists of all the parts used to transform the electrical energy… Read more

Oct 06, 2010

Contact System

The word “contact” not only describes the conductive connection of two mechanically separate electrical conductors, but also the conductive parts (contacts) even if they are not touching. Contacts comprise: Non… Read more

Oct 06, 2010

Electromechanical Relays

The basic design of electromechanical power relays is the same regardless of whether it is a miniature PCB relay or an industrial power relay. The most important components are: fixed… Read more

Oct 06, 2010

Relays – General

The relay was invented more than 160 years ago. During the last 50 years it has undergone considerable change, the most obvious of which is its reduction in size. Using… Read more

Oct 05, 2010

Introduction to relay

The relay is a well known and widely used component Applications range from classic panel built control systems to modem interfaces between control microprocessors and their power circuits or any… Read more

Oct 05, 2010

40 Comments


  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

    I LIKE THE BOOK


  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.

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