Electromechanical Components

  • Know different types of switches.

  • Identify and list the different parts of a relay.

  • List applications of relay.

  • State the main function of a circuit breaker.

  • Understand the difference between a circuit breaker and a fuse.


In electrical and electronic system, a switch is a device, which can make or break an electrical circuit or we can say that switch is a controlling device, which interrupt the flow of current or direct the flow of current in another direction. Almost all the electrical and electronics systems contain at least one switch, which is used to make the device ON or OFF. In addition, a switch is used to control the circuit operation and user may able to activate or deactivate the whole or certain parts of the connected circuit. Generally, Switches can be categories as:

  1. Mechanical Switches

  2. Electrical/Electronic Switches

Mechanical Switches:

Mechanical switch is a switch in which two metal plates touch each other to make a physical contact for the current to flow and separate from each other to interrupt the flow of current. There are many types of Mechanical switches and they are also be categories on the basis of power handling capacity. The contact material is chosen by keeping in mind that the metal oxides, which produced due to corrosion, are mostly insulator and layers of such oxides on the switch plates will hinder the normal operation of the switch.

Depending upon the types there are four different types of Mechanical switches:

  1. SPST = Single Pole, Single Throw This is a simple ON/OFF switch. It is also called as One Way Switch (in the US, they called it Two-Way Switch). When a user press the button of the switch, then the plates of the switch connect with each other and the current starts to flow and vice versa.

  2. DPST = Double Pole, Single Throw This switch is basically two SPST switches in one package and can be operated by a single lever. This switch is mostly used, where we have to break both ground and lines at the same time.

  3. SPDT = Single Pole, Double Throw This button has three pins in which, one pin is used as common and called a Two-Way Switch (in US, they called it Three-Way Switch). We can send two different signals to same pin by using this switch. Because of this functionality, this switch is also called selector switch. Other switches related to SPDT are SPCO (Single Pole Changeover) and SPTT (Single Pole Center Off or Single Pole Triple Throw)

  4. DPDT = Double Pole, Double Throw: This switch is equivalent to two SPDT switches packaged in one pack. This switch has two common pins and four signal pins. Total four different combination of singles can be applied to the input pins of this switch. Another switch, related to DPDT is DPCO (Double Pole Changeover or Double Pole, Centre Off).


While a switch closes its contacts by the mechanical actuation of its lever, the relays do this by an electromagnetic coil pulling its contacts by the force of magnetism. A coil carries current and is wound over a core of soft magnet; the moving armature of the core causes the contact closure. In this way, by switching on the current through the coil of the relay, the relay contacts close. Relays can have several poles and several contacts. The contacts can be normally closed or normally open.

Terminology of a Relay:

  • Electromagnet - a coil of wire wrapped around a conductive core.

  • Armature -that can be attracted by the electromagnet

  • Spring- to keep the armature in a ’home’ or ’normal’ position

  • Control Circuit - connects to the coil of the electromagnet

  • Secondary circuit usually a higher voltage and amperage demand than the control circuit

  • Coil -the wire wrapped around the conductive core of the electromagnet assembly

Circuit Breakers:

All electric circuits needs a switching device and also a protective device. Switchgear is the general term covering a wide range of equipment connected with switching and protection. A circuit breaker is a switching and circuit interrupting device. A circuit breaker serves two purposes:

  • Switching on and off during normal operation for maintenance etc.

  • Switching during abnormal conditions- short circuits, earthing etc. to protect the associated equipment.

In short, a circuit breaker is a sort of automatic switch which can interrupt the fault currents.

Solved Examples

Solved Example:


The function of protective relay in a circuit breaker is:

Correct Answer: B

Solved Example:


A material best suited for manufacturing of fuse wire is:

Correct Answer: B

Solved Example:


A fuse is normally a:

Correct Answer: A

Solved Example:


Most of the fuses operate due to _______ effect of current: (SSC JE EE Paper 5- Jan 2018 Evening)

Correct Answer: A

Solved Example:


A fuse wire should have _______ specific resistance and ______ melting point.

Correct Answer: D

Solved Example:


A circuit breaker is: (Based on PSPCL JE EE 2015)

Correct Answer: D

Solved Example:


A circuit breaker will normally operate: (VIZAG MT Electrical 2017)

Correct Answer: D