Different Types of Relays used in Protection System and their Applications

What is Relay :

RelayA relay is an electro-mechanical switch which is used to open or close a circuit . It’s operated by a relatively small electric current, which is capable of sensing abnormal conditions in electrical circuits. Relays are used where it is necessary to control a circuit by a separate low-power signal, or where several circuits must be controlled by one signal.

How many types of Relay in protection system:-

1.) Electromagnetic Relays : 

electromagnetic relays

These types relays are those relays which are operated by electromagnetic action. Its are constructed with electrical, mechanical and magnetic components, and have operating coil and mechanical contacts. It will be used in Magnitude measurement, Comparison, Ratio measurement , etc. It will take much longer time to replace all electromagnetic relays by micro processor based static relays. This will be operated in A.C supply or D.C supply.

2.) Solid State Relays :

A solid-state relay (SSR) uses solid state components to perform the switching operation without moving any parts. The main advantages of this relay are its long life span and faster switching operation compared to the other relays. This types relay have fast switching speeds compared with electro-mechanical relays, and have no physical contacts to wear out.

Used of Solid state relay:-

  •  Thyristor, TRIAC or other solid-state switching device.
  • Power semiconductor devices such as Thyristors and Transistors, to switch currents up to around a hundred amperes (100Amp).
  • For faster switching operation.

3. Hybrid Relay :

A “Hybrid” solid state relay is one that combines the benefits of both SSR and EMR, a solid-state relay (SSR) technology for switching the outlets. While solid-state relays are more reliable than electro-mechanical relays, the electro-mechanical relays are much cheaper and consume less power than solid-state relays. As mentioned above, the primary advantage EMRs have over a typical solid-state relay is the low impedance of its contacts. Quite commonly this can be as low as one or two milliohms. Therefore, a SSR switching a 50 amp load will dissipate 50 Watts of power, but a mechanical contactor in the same application may dissipate less than 5 Watts (P = I2R). Such an application would definitely require the use of a heat sink with the solid state relay and adequate panel ventilation, ultimately increasing the cost of the solution.

4.Thermal Relay :

A thermal relay is a protector that uses the thermal effect of current to cut off the circuit. Thermal overload relays are economic electro-mechanical protection devices for the main circuit. The thermal energy of the heater coils heated the bi-metallic strips. It often work with ac contactor to form an electromagnetic starter, and then to be used in the control and protection circuit of the three-phase asynchronous motor.

Applications Of Thermal Relay :-

  • The working principle of the thermal relay is that the current flowing into the heating element generates heat to make the bi-metal of different expansion coefficients to deform.
  • The “thermal relay” is used for overload protection in the motor
  • Adjustable current setting ranges
  • Overload protection with phase loss sensitivity
  • Temperature compensation up to +60°C
  • Automatic or manual reset, sealable
  • Stop and test function.

5.) Reed Relay :

A reed relay is a type of relay that uses an electromagnet to control one or more reed switches. This types of relays are small and fast acting; they require a much lower level of power to actuate them than other traditional types of relay and as a result they find many uses in various forms of electronic circuitsPickering Interfaces only uses reed relays assembled by our sister company, Pickering Electronics, because these offer the very best performance.

6. Safety relay :

Safety relays are devices that generally implement safety functions. In the event of a hazard, a safety relay must work to reduce risk to an acceptable position. The safety relay must initiate a safe and reliable response as well as monitor the integrity of its function.


Used of Relay in Protection System:-

  • Relays are used in the traffic signal controllers, temperature controllers.
  • Used in variety of applications like Eectric power systems, Home appliances, Automobiles, Digital computers, Industrial equipments, etc.
  • The main operation of this device is to make or break contact with the help of a signal without any human involvement in order to switch it ON or OFF.

Inventor of Relays:- 

One of the most brilliant scientist Carl Friedrich Gauss and Wilhelm Weber developed an electromagnetic relay in 1833.

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