Friday, 10 April 2020

Miniature Circuit Breaker (MCB)- Construction, Working, Advantages, Applications.

● Miniature Circuit breaker. It also called as MCB. A circuit breaker is a switching (On/Off/Trip) device that is used for switching on and off the load and for inturreting the circuit under the fault condition.

● Circuit breaker make or break a circuit doing normal conditions. During the unhealthy conditions they break the circuit to prevent the spread of fault to other equipments.

Miniature Circuit Breaker
Miniature Circuit Breaker

● Circuit breaker is safety device used to protect your electrical appliances at home in case of short circuit or any other cause for excessive current. It operates manually as well as automatically.

● Circuit breaker have to be simple, because they are a minor safety measure in common haosehold and in the electrical grids.

Construction of MCB

There are any parts are arranged in circuit breaker.

Construction of MCB
Construction of MCB 

External casing
External casing is the insulation which hold all the components and protects them from the durt and dust.

Knob is the switch which used to turn on and off the MCB.

There are two contacts inside the circuit breaker one is fixed contact and other is movable contact.
To hold the contacts under the spring tension when MCB is ON position.

● Bimetallic strip
The bimetallic strip used to offer delayed overload protection by sensing the flow of current greater than its rated current.

Solenoid provides instantaneous protection against short circuit by releasing the mechanical latch.

Arc chutes
It is used for the splitting and quenching the arc.

Working of MCB

● There are two types of operation of an MCB. The first depends on the thermal effect of overcurrent.

● Thermal Effect
When the over current flows through the MCB the bimetallic strip present in MCB owing to the thermal expansion of metals is heated and deflects by bending.

● That deflection in the strip releases a mechanical latch and this operates the contacts of the MCB to connect the fault circuit.

● Electromagnetic Effect
In the short circuit conditions the fault of sudden rising of current causes electromechanical displacement of the plunger connected with the tripping coil or the solenoid of MCB.

● Then plunger strikes the trip level with the causing of immediate release of the latch mechanism consequently so the circuit breaker contacts gets open.

Advantages of MCB

● We can identify tripped MCB even in darkness.

● It gives trip free mechanism- cannot be switched back on while fault exists.

● It also acceptable as a isolator.

● There is no any wire to replace like fuse.

● Normally it do not requires replacement.

● They act and open the circuit in less than 5 milliseconds.

● Supply is restored by resetting it again.

● It requires less maintenance and it is more reliable.

Disadvantages of MCB 

● The MCB is costly than fuse.

● The cost of MCB distribution board is greater than desirable fuse board.

Applications of MCB

● New constructions uses MCB instead of the older type fuses.

● It is designed to protect the houses from circuit overload.

● Can be used for ground fault or arc fault mechanisms.

● Domestic appliances, power supplies, lighting circuits and motor circuits etc.

Sunday, 5 April 2020

What is SF6 Circuit Breaker? Working, Construction, Advantages, Applications

Circuit Breaker in which SF6 gas under pressure is used to extinguish the arc is so it is called SF6 circuit breaker.

SF6 means Sulphur Hexafluoride gas. It has excellent dielectric, arc quenching, and other physical properties. So this is used in circuit breaker.

SF6 circuit breaker are used in high voltage transmission and distribution systems. Because it has excellent arc breaking capacity.  In this there is no risk of fire or explosion. 

SF6 Circuit Breaker
SF6 Circuit Breaker

Construction of SF6 Circuit Breaker

Construction of SF6 circuit Breaker
      Construction of SF6 circuit Breaker

The construction of SF6 circuit breaker is simple in kind of other 
Circuit breakers. There are main two contact and fixed contact. 

This two contacts are placed in the interruption chamber. Then the SF6 gas is reservoir is connected to that interruption chamber. 

When the contacts of CB opens then it allows to high pressure SF6 gas from reservoir to flow towards the interruption chamber. 

Both the contacts and arcing horn are coated with the copper tungsten material which is arc resistant material. 

This contacts are fixed like hollow cylindrical shape. Moving contacts contains rectangular which has holes in sides to allow the SF6 gas go through these holes flowing and across the arc.

This gas is recycled and reused by the suitable auxiliary system after every operation or use of breaker. This is because of the SF6 gas is costly.

Working Of SF6 Circuit Breaker

  The contacts of circuit breaker are closed at the normal operating condition of working.

When some fault occurs in the system contacts are open and arc is struck between them.

The movement of moving contacts cause to occur operate valves at the same time which enters the high pressure SF6 gas in the arc interruption chamber at some pressure.

This SF6 gas absorbs free electrons in the arc path and forms ions that ions does not carries the charge. This ions increases the dielectric strength of the gas and hence the arc get extinguished. 

This gas is stored in low pressure reservoir and this is pulled back to high pressure reservoir for reuse it.

Advantages of SF6 Circuit Breaker

 The atmospheric conditions does not affects on the performance of the SF6 circuit breaker.

The cost of it is less due to compressed air system is required.

Maintanence of this circuit breaker is less.

  It has high dielectric strength so the electric clearance gets reduced.

Arc gets extinguished at natural current zero so there is no voltage problem. There is no risk of fire.

The gas used is non-inflammable and it has chemically stable.

  Same gas is re- circulated into the circuit there by reducing the requirement of SF6 gas.

The SF6 CB can  perform various duties like clearing short line faults, opening unloaded transmission line, capacitor switching transformer reactor switching etc. Without any problem.

Disadvantages of SF6 Circuit Breaker

The parts in CB needs more cleaning during periodic maintenance under clean and dry environment.

It causes failure when the moisture enters in CB.

The SF6 gas is too costly and it needs more facility for transformation and maintanence of quality of gas.

 Arced SF6 gas is poisonous and should not be inhaled.

Applications of SF6 Circuit Breaker

  Used for high voltage Applications

For switching related to high voltage and extra high voltage applications.

Friday, 3 April 2020

What is Vaccum Circuit Breaker? Working, Construction, Advantages, Application

Vaccum Circuit Breaker (VCB) arc quenching takes place in vaccum medium. 

All operations of switching that is on and closing of current carrying contacts are take place in vaccum chamber present in the circuit breaker so it is called vaccum circuit breaker.

Vaccum offers the highest insulation strength it has for superior arc quenching properties than the other medium. 

Vaccum Circuit Breaker
Vaccum Circuit Breaker

Construction of Vaccum Circuit Breaker

Construction of the vaccum circuit breaker is simple than the other circuit breakers. 

There are main three components are arranged in VCB i.e. fixed contact, moving contacts, and arc shield. This all components are fixed inside the arc interrupting chamber. 

The fixed and moving contacts of the breaker are placed inside the arc shield. The moving contacts of circuit breaker are move through a distance of 5 to 10 mm but it depends upon the operating voltage. 

Cross arm section of Vaccum Circuit interruption
Cross arm section of Vaccum Circuit interruption

From the stainless steel the metallic bellows are formed and this metallic bellows used to move the moving contacts. 

The ability of VCB is depends on the life of the all components are made and their arrangement it requires the repeated operations satisfactorily. 

Working Of Vaccum Circuit Breaker

When the contacts of the breaker  are opened in vaccum, an arc is produced between the contacts by the ionisation of metal vapours. 

However, the arc is quickly extinguished because of the metallic vapours, electrons and ions produced during arc rapidly condense on the surface of the CB constants resulting in quick recovery of dielectric strength. 

Cross arm section of Vaccum Circuit interruption Electrons and ions produced during arc are diffused in a short time and seized by the surface of moving and fixed members and shields. 

Arc get quickly extinguished due to the fast rate of recovery of dielectric strength in vaccum. 

Advantages Of Vaccum Circuit Breaker

Vaccum Circuit breaker has excellent recovery. 

For only half cycle less arcing occurs after the proper contact separation. 

It does not requires the aditional filling of Air or oil in it.

Breaker unit is compact and self contained.

Requires less maintanence.

Disadvantages of Vaccum Circuit Breaker

Loss of vaccum occurs because of transit damage 

It requires surge suppressors for the interruption of low magnetizing current in a certain range.

Applications of Vaccum Circuit Breaker

Industrial application where the high speed making switches requires because of VCB has excellent recovery.

It is also usefull for system which requires voltage from 11 to 33 KV.

Thursday, 2 April 2020

Air Circuit Breaker- Working Principle, Construction, Types, Advantages, Applications

It is an automatically operated electrical switch which protects the system or device from any fault overload or short circuit. It protects to the Electrical equipments.

If fault is detected by the circuit breaker it interrupts the flow of current and protects to the system and for the interruption it uses air that's why it is called Air Circuit Breaker.

Air Circuit breaker used compressed air to blow out the arc. It operates in air at atmospheric pressure. 

Air Circuit Breaker
Air Circuit Breaker

Working Principle of Air Circuit Breaker

Working principle of this circuit breaker is different from the other circuit brekers in some manner. For interrupting arc it creates an arc voltage in excess of the supply voltage.

When fault occurs in any part of the system the trip coils of the circuit breaker get energised and the moving contacts are pulled apart by some mechanism thus opening the circuit.

  When the contacts of circuit breaker are seperated under fault conditions, an arc is struck between them. The current is thus able to continue until the discharge ceases.

  The production of arc not only delays the current interruption process but it also generates enormous heat which may cause damage to the system or circuit breaker itself.

The circuit breaker increases the arc voltage by three ways
1. It may increases the arc voltage by cooling the arc plasma. If temperature of arc plasma decreases then the mobility of particle in arc plasma reduces. So that's why more voltage gradient is required to maintain the arc.

2. It may increases the arc voltage by lenghning the arc path.

3. Another one way is splitting up the arc into many series arcs also increases the arc voltage.

Construction of Air Circuit Breaker

Construction of Air- Blast circuit Breaker
Construction of Air- Blast circuit Breaker 

  The air circuit breaker consists of three types of contacts in a compressed air chamber.

1. The main contact-
This main contact carries the load current. They are made up of good conducting material, such as silver or copper.

It avoids over heating of the contacts while in service.

2. Arcing Contacts Of ACB-
It is made up of harder material which has high resistance such as tungsten.

When main contacts opens the arching current flows through the arching current with high resistance path.

3. Arching horns and chutes -
Arcing horns carries the arching current so they are made up of the high resistance and from harder material.

Chutes are made up of fire proof insulating materials because there are the last contact arc.

Cooling metal plates are connected to the chutes to convert the hot gasses and gives the way and cool the contacts arc the extinction of arc.

These all components are filled in circuit breaker chamber with compressed air which acts as dielectric material.

Types of Air Circuit Breaker

There are main four types of ACB.

1. Plain Break type air circuit breaker or cross blast air circuit breaker.

2. Magnetic blowout type air circuit breaker.

3. Air chute air break circuit breaker..

4. Air blast circuit breaker.(ABCB)


• This is cheap and free available of the interruption medium.

• ABCB are chemically stable.

• Gives high speed operation.

•  It requires less maintenance.


• Current chopping problem occurs.

• Produces high noise.

• Problem of restraining voltage.

• Compressor plant requires some maintanence.


• Used for protection of plants.

• Protection of electrical machines.

• For the protection of transformer, capacitor, generator.

• For low and high voltage and current applications.

Wednesday, 1 April 2020

Oil Circuit Breaker- Working Principle, Construction, Types, Advantages

• Circuit Breaker is device used to break circuit during load in on and due to load current flow in the circuit.
• Oil Circuit breaker is breaker used to break the circuit when abnormal voltage appears and it will protects electrical equipments from abnormal voltage.
• Oil Circuit breaker is used for two purposes one is arc quenching medium and other is for cooling purpose.

Oil Circuit Breaker
Oil Circuit Breaker 

Working Principle Of Oil Circuit Breaker

• Oil Circuit breaker is used as arc quenching medium and for cooling purpose.
•  As the time of arc oil absorbs heat and split into ions and molecules and releases gases hydrogen, ethane etc.
• Which releases high amount of hydrogen gas which leads to turbulence (force) of the oil the length of the arc increases and resistance of the arc decreases but as we know resistance is inversely proportional to the current finally arc will be extinction.
• The arc extinction is occured mainly by two purposes
•  Firstly the hydrogen gas has high heat conductivity and cools the arc thus aiding the de-ionization of the medium between the contact.

• Second the gas sets up turbulence in the oil and forces it into the space between contacts, thus eliminating the arcing products from the arc path.
Thus arc extinguish and circuit current interrupts.

Construction of Oil Circuit Breaker

Construction of Oil Circuit Breaker

• It consists of current carrying contacts enclosed in a strong weather tight earth tank.
• That tank is filled with oil that is transformer oil. The oil acts as arc extinguishing medium and as an insulator between the live part and earth.
•  At the top of oil, air is filled in the tank which acts as cushion to control the displaced oil on the formation of gas around the arc.
• Oil Circuit Breaker consists gas outlet which is fitted in the tank cover for the removal of the gases.

Types of Oil Circuit Breaker

There are main two types of the oil Circuit breaker are following

1. Bulk oil Circuit breaker
Bulk Oil Circuit Breaker is again classified into two types
a. Plain break oil Circuit breaker
b. Arc Control Circuit Breaker

Bulk oil Circuit breaker
Bulk oil Circuit breaker

Minimum Oil Circuit Breaker
Minimum Oil Circuit Breaker

Advantages Of Oil Circuit Breaker

• Oil has good insulating properties which keeps arc formation to a minimum when the breaker is open or closed.
• Oil has high dielectric strength.
• Oil absorbs arc energy while decomposing.
• Good cooling property of the gas formed due to the decomposition.
• It acts as an insulator between the live parts and earth.

Disadvantages of Oil Circuit Breaker

• It takes long arcing time.
• It do not permit high speed of interrupt.
• Arc interruption control can be obtained only by increasing length of arc.

Tuesday, 31 March 2020

Circuit Breaker- Working Principle, Construction, Types.

Circuit Breaker
Circuit Breaker
  • The electrical circuit is an switching device that interrupts the fault current or the abnormal current. Circuit breaker is a safety device used protection of electrical equipments. 
  • It can be operated manually and automatically. It protects an electrical circuit from damage caused by abnormal or excess current from an overload or short circuit.
  • In indoor applications of Circuit breaker we also use it as an isolator.

Construction of Circuit Breaker

There are many components in circuit breakers.

1. Frame
To all the circuit breaker components frame provides the insulation. Frame is constructed by thermal set plastic such as glass polymer.

2. Contacts
Contacts are the way through which the current flows in circuit breaker. When fault current flows and circuit breaker turns off then C.B. interrupts the flow of current by seperating it's contacts.

Contacts are of two types
1. Straight-Through Contacts
2. Blow-Apart Contacts

3.Operating Handle
Operating handle is connected to the movable contacts arm through an operating mechanism. Operating handle is moved from the OFF to the ON position.

4. Trip Unit
To providing the open and close its contacts manually circuit breaker must automatically open its contacts when fault current is sensed. It determines when the contacts will open automatically.

Construction of Circuit Breaker

Construction of Circuit Breaker

Working Principle of Circuit Breaker

  • A circuit breaker sense the electrical current flowing through it and breaks that current if current exceeds the limit that is abnormal current.
  • There are two common ways that circuit breaker sense the current Thermal and Magnetic
  • Small breakers are usually thermal and work by passing the current through a heating element and break the current if that element gets too hot.
  • In simple terms, it's a switch which turns itself off. Its just two metal contacts which can be moved apart, like any other switch and a solenoid which is an electromagnetic device which moves one of the contacts. 
  • The solenoid is calibrated so that at a certain electrical current it activates and turns the switch off. 
  • When a circuit has a breaker in it and the circuit tries to draw more current than it can handle it turns off and deactivates the whole circuit once the problem with the current is fixed, we can turn the switch back on, and the circuit is fine again. 

Types Of Circuit Breaker 

  • Circuit Breakers are mainly classified on the basis of rated voltage.
  • Circuit breaker below the rated voltage of 1000V are the Low Voltage Circuit Breakers and above the voltage 1000V are the High Voltage Circuit Breakers.
  • They are mainly classified on basis of the medium of arc extinction are following.

1. Oil Circuit Breaker 
2. Bulk Oil Circuit Breaker
3. Minimum Oil Circuit Breaker
4. Minimum Circuit Breaker
5. Air Blast circuit Breaker
6. Sulphur Hexafluoride Circuit Breaker
7. Vaccum Circuit Breaker
8. Air break Circuit Breaker

Friday, 6 March 2020

Types of Fuses and Classification

  • In Electrical or Electronic Engineering the fuse is used for safety of the devices. The fuse is usually an electronic device, which is used to protect circuit from over current, overload and make sure the protection of the circuit.
  • Is composed of an alloy which has a low melting point. The strip of this fuse is placed in series with circuit.
  • The fuse is mainly classified into two types. And depending on the input supply voltage they are classified in AC fuses and DC fuses. AC and DC fuses are again classified are following. 
Classification of Fuse
Classification of Fuse

AC Fuses

  • In AC system, there is easy to extinguish the arc produced due to the over current because in AC system the voltage is with 50 to 60 Hz frequency and it changes its amplitude from 0 to 60 in one second. 
  • Because of this AC fuses are little small than DC fuses. AC fuses are again classified in Other types.

DC Fuses

  • In DC system, because of over heating when the metallic wire melts and arc produces and it is very difficult to extinguish this arc due to the DC constant value.
  • In order to minimize the fuse arcing the DC fuses are bigger than the AC fuses.  

Rewirable Fuse

Rewirable Fuse

Rewirable Fuse

  • This kind of fuse is most commonly used in the case of domestic wiring and small scale usage. Another name of this type of fuses is kit-kat type fuse. 
  • The main composition is of a porcelain base which holds the wires. The fuse element is located inside a carrier that is also made out of porcelain. It is possible to remove the fuse carrier without any risk of electric shock. 
  • The main advantage of this type of fuse is that it is easy to install and also replace without risking any electrical injury. 

Cartridge Fuse

Cartridge Fuse
Cartridge Fuse
  • Cartridge fuses to protect many of the electrical appliances such as motors air-conditions, Pumps, Refrigerators, etc. It was developed to overcome the disadvantages of the former. 
  • The fuse wire is enclosed in a glass tube, with two metal caps at both ends. So the fuse element does not deteriorate. Replacement is expensive. This fuses are available 600A and 600 V AC and has used for Industries, Commercial Area.
  • This fuses are of two types 
                                         1.D- type cartridge fuse and 
                                         2. Link type cartridge fuse

1. D- Type cartridge Fuse
  • This can not be interchanged and comes with the fuse base and cap, adapter ring, and the cartridge. The main feature of this fuse is its reliability. 
2. Link type cartridge fuse or HRC High Rupturing Capacity Fuse

HRC Fuse
HRC Fuse

  • The link type cartridge fuse is also called the HRC fuse means High Rupturing Capacity fuse. The fuse wire or element can carry short circuit heavy current for a known time period. 
  • During this time if the fault is removed, then it does not blown off otherwise it blow off and melt. The enclosure of fuse is either of glass or some other chemical compound. 
  • The Operation of HRC fuse is that when the over rated current flow through the fuse element of HRC fuse the element is melted and vaporized. 
  • The filling powder is of such a quantity that the chemical reaction between the silver vapor and filling powder forms a high resistance substance which very much helps in quenching the arc.

High Voltage Fuse
  • All fuses used in power system from 1.5kV up to 138 kV are categorized as high voltage fuse. High voltage fuse are classified to protect instrument transformer used for electricity material, or for small power transformer where the expense of a circuit breaker is not warranted. 
  • When the fuse blow, heat from the arc causes the boric acid to evolve large volume of gases. The associated high pressure and cooling gases rapidly quench the arc. High voltage fuses rated for more than 1500V and up to 13kV.

Monday, 2 March 2020

Electric Fuse and Why are Fuses used? Working, Types, Applications

Electric Fuse 

  • In Electrical or Electronic Engineering the fuse is used for safety of the devices. The fuse is usually an electronic device, which is used to protect circuit from over current, overload and make sure the protection of the circuit. 
  • An electrical fuse operates on the principle of heating effect of electric current. There are many types of fuse elements available but function of most of these fuses is the same. 
  • Ordinarily, a fuse is a short metal wire or strip, connected to a conductor on each end, and usually enclosed in a glass or other non conductive container.  The fuse is mostly used for protection purposes in residential.

Electric Fuse
Electric Fuse

Working Of Fuse

  • During normal operating condition, the current flowing through the fusing element is such that the rate of heat production in the fuse element is nearly equal to rate of heat dissipation from the fuse element and thereby they are will be no considerable increases of temperature of that fuse element.
  • It usually consists of a low resistance metallic wire enclosed in a non combustible material. 
  • Whenever a short circuit, over current or mismatched load connection occurs, then the thin wire melts because of the heat generated by the heavy current flow. 
  • This disconnects the power supply from the connected system. In normal operation of circuit, fuse wire is just a low resistance component and does not affect the normal operation of system connected to the power supply. 
  • In order to minimize the fuse arcing, DC fuses tends to be bigger than an AC fuses which increases the electrodes  to reduce the arc in the Fuse.

Why they are used?

  • Whenever there is a short circuit, overload or earth fault in the system, huge current starts flowing through the system conductor which leads to hide temperature rise of the conductor itself. 
  • If this huge current is not prevented, after certain time the heat generated from the conductor part is so high, it can melt the conductor itself along with damage the insulation parts of the equipment.
  • Even there may be a chance of fire hazard due to extremely high temperature in the system. As a whole  there will be permanent damage in the equipment.
  • To prevent this, the huge short circuit current should be allowed to flow through the system for very short time so that there would be any serious temperature rise. 
  • Actually during fault nobody can prevent the flowing of  huge over current through the system but the during of flowing of this huge over current can be limited by using electrical fuse.

Types Of Fuses

Fuses can be divided into two main categories according to the types of input supply voltage.
  1. AC Fuses
  2. DC Fuses

1. Ac Fuses
  • In the AC system voltage frequency changes its amplitude from zero to 60 times every second, so arc can be extinct easily. 
  • AC fuses are little bit small in size as compared to DC fuses. 

2. DC Fuses
  • In DC system when the metallic wire melts because of the heat generated by the over current, then then arc is produced and it is very difficult to extinct this arc because of DC constant value.
  • So in order to minimize the fuse arcing, DC fuse are bigger than the AC fuses which increases the distance between the electrodes to reduce the arc in the fuse.

Applications of Fuses

Fuses are used in many industrial electrical and electronic devices for protection 
  • Hard Disk Drives
  • Laptops
  • LCD Monitors
  • Automotive Systems
  • Battery Packs
  • Gaming Systems & Portable Electronics
  • General Appliances and Devices
  • Cell Phones
  • Printers/ Scanners

Saturday, 29 February 2020

How works Electromechanical Relay? Construction, Types, Applications

 Electromechanical Relays

  • These relays are constructed with electrical, magnetic & mechanical components & have an operating coil & various contacts.
  • If a relay has moving parts, it is called electromechanical relay.  
  • These are very robust & reliable. Based on the construction, characteristics, these are classified in three groups.
Electromechanical Relay

Electromechanical Relay

Construction And Work Of Electromagnetic Relay

  • Relay are typically used when it is necessary to switch a small amount of power. Relay contains several electronic parts to make them work.
  • These includes an electromagnet, which controls opening and closing of the relay.
  • Next is the armature, or the moving part, which is the electronic part that opens and closes.
  • A spring is also used in relay. This is the part that forces the relay back to its original position after each revolution.
  • In addition, a set of electric contacts is needed in order to transfer the power.
  • The power source is given to the electromagnet through a control switch and through contacts to the load. When current starts flowing through the control coil, the electromagnet starts energizing and thus intensifies the magnetic field. 
  • Thus the upper contact arm starts to be attracted to the lower fixed arm and thus closes the contacts causing a short circuit for the power to the load. On the other hand, if the relay was already de-energized when the contacts were closed, then the contact move oppositely and make an open circuit.
  • As soon as the coil current is off, the movable armature will be returned by a force back to its initial position. This force will be almost equal to half the strength of the magnetic force. This force is mainly provided by two factors. They are the spring and also gravity.
Construction, Working of Relay

Construction, Working of Relay

Types Of Electromagnetic relay

There are three types of elctromagnetic relay
  1. Attraction relays
  2. Relays with movable coils
  3. Induction relays

1.Attraction Relays

  • Attraction relays can be AC & DC and operate by the movement of a piece of iron when it is attracted by the magnetic field produced by a coil. 
  • There are two main types of relays:

                        1. The attracted armature type
                        2. Solenoid type relay

1. Attracted armature relays
  • Consists of a bar or plate (made of iron) that pivots when it is attracted towards the coil.
  • The armature carries the moving part of the contact ,which is closed or opened, according to the design, when the armature is attracted to the coil.

2. Solenoid type relays
  • In this a plunger or a piston is attracted axially within the field of the solenoid. In this case, the piston carries the moving contacts.
  • In order to control the value of current at which relay operates, the parameters resultant force and restraining force  may adjusted. Attraction relays effectively have no time delay and are widely used when instantaneous operation is required.

Relays with movable coils
  • This type of relay consists of a rotating movement with a small coil suspended or pivoted with the freedom to rotate between the poles of a permanent magnet. 
  • The coil is restrained by two special springs which also serve as connections to carry the current to the coil.
  • The relay has inverse type characteristic.

Induction relays
  • An induction relay works only with AC.
  • It consists of an electromagnetic system Which operates on a moving conductor, generally in the form of a DISC or CUP.
  • Classification of induction relays
  1. Shaded pole relay
  2. Watthour- meter type relay
  3. Cup type relay

1.Shaded pole relay

  • The air gap flux produced by the current flowing in a single coil is split into two out of phase components by a so called „Shading Ring‟ generally of copper, that encircles part of the pole face of each pole at the air gap.
  • The shading ring may be replaced by coils if control of operation of the shaded pole relay is desired.
  • The inertia of the disc provides the time delay characteristics

2. Watt hour –meter structure

  • This structure gets its name from the fact that it is used in watt hour meters.
  • As shown in the top figure below, it contains two separate coils on two different magnetic circuit, each of which produces one of two necessary fluxes for driving the rotor, which is also a disc.


  • This type of relay has a cylinder similar to a cup which can rotate in the annular air gap between the poles & the fixed central core. 
  • The operation of this relay is similar to that of an induction motor with salient poles for the windings of the stator. 
  • The movement of the cup is limited to a small amount by the contact & the stops. A special spring provides restraining torque.
  • The cup type of relay has a small inertia & is therefore principally used when high speed operation is required, for example in instantaneous units.

Use of Electromechanical Relay
  • Relay typically are used in modern household appliances such as hair dryers, kitchen appliances, and lights that need to be switched on and off. 
  • They are also used in cars where things need to be turned off and on.
  • In fuel pumps. 
  • Industrial Applications- Where the control of high voltage and current is intended.
  • Controlling large Power loads.

Related Terms:

Friday, 28 February 2020

Different Types Of Relay

Types Of  Relays

  • Relay is an electrical switch which is used to open or close a circuit with electromagnetic relay or electronically. It operates manually as well as automatically.
  • Relay is used in circuit for protection of electrical circuit, system and electrical equipment. In any electrical system protection of the system is given utmost priority.
  • The purpose of an Electric Power System is to generate and supply electrical energy to consumers. The power system should be designed and managed to deliver this energy to the utilization points with both reliability and economically.
  • Any abnormal operating state of a power system is known is FAULT. And over come this faults the relay gives the protection to the device or system. This faults are of two types 
  1. Short Circuit Fault - Current 
  2. Open Circuit Fault - Voltage

  Types Of Relays

Types Of Relays

Types Of Relay

Classification Of Relay

1. According to the Construction Principle

Depending upon the principle of construction, relay is classified in the following four categories.

  • Electromechanical 
  • Solid State
  • Microprocessor 
  • Numerical

2. According to the Actuating signals

  • Current
  • Voltage
  • Power
  • Frequency
  • Temperature
  • Pressure

Thursday, 27 February 2020

Directional Over Current Relay- Use of directional and non directional relay in over current relay

Directional Over Current Relay

  • Directional Over Current Relay is used to transmission line where the power flow is associated in a special direction.
  • From the word over current, it means that the relay will be operated for only one case if the current value rises from the set value.
  • And directional word is for the current direction that is if relay is set for only forward direction current, it will sense forward direction current, it will not sense reverse direction over current and vice versa.
  • These are suitable in ring main system and radial system.

How does the directional Relay Works?

Directional Over Current Relay
Directional Over Current Relay 

  • Directional over current relay respond to excessive current flow in a particular direction in the power system. The relay typically consists of two elements. One is directional element, which determines the direction of current flow with the respect to the voltage reference.
  • When this current flow is in the predetermined trip direction, this directional element enables (turn on) the other element, which is the standard over current relay, complete with taps and time dial, as found on a normal non- directional over current relay. 
  • Because of these relay is designed to operated on fault current, the directional unit is made so that it operates best on a highly lagging current, which is typical of faults in power systems. So the zone of fault is identified with the help of relay characteristics and maximum angle setting.
  • Directional over current relay are normally used on over coming line circuit breakers on buses which have two or more sources. 
  • They are connected to trip an incoming line breaker for fault current flow back into the source, so that a fault on one source is not fed by the other sources. 
  • In complex distribution or sub-transmission network, these relay may be used to improve coordination of the system.

What is Non Directional Relay?
  • When there is fault in power  system, power flows through fault. Non directional relay operates irrespective of direction of flow of current. 
  • Ex. Breaker at generator end. If there is fault on generator secondary relay has to operate to open circuit breaker.  
  • If there is fault in a winding generator and its drawing power from grid then also circuit breaker has to operate. So we use a non directional relay. It has to operate in fault conditions irrespective of direction of power flow. 

Use of Directional and Non- Directional Relay in over Current Relay

  • In an over current relay we use both directional as well as non directional over current relay.
  • Directional relay is connected towards load side of the line which will trip if over current flows from load towards the source and 
  • The non directional relay is connected at the source side which will trip if fault occurs irrespective of the direction of current. 

  • Now a days we use multiple conductors or transmission line forming a ring structure so as to avoid interruption of power supply to the costumers in case of fault in any line.  
  • Therefore when any of line undergoes a fault. The non Directional relay at the source will automatically trip and separate the source from fault point but load side is still connected to the fault point. 
  • Due to the other transmission lines present in the system the power will still reach to the costumers. As the fault is still present therefore other transmission line will also feed this fault point and thus system might get damage. 
  • Therefore the directional relay we connected at the load side will detect the reverse flow of current and thus trip and avoids other transmission to feed the fault location.

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