APSEEE – Page 3

What is Triac?

What is TRIAC?

Triac belongs to thyristor family. The triac is the most widely used device in an alternating current applications. Basically, it is a combination of two SCRs connected in anti-parallel manner. Triac can conduct in both directions positive as well as negative of an ac supply. A triac is widely used in industries for control purposes. The triac can conduct alternating current. So, it is used for ac current applications.

Construction of Triac

The triac is a three terminal, four layer bidirectional semiconductor device. The name of three terminals is MT1, MT2 and Gate(G). The same is used for the construction of triac that is used used for thyristor.


Basically, a triac consists of two SCRs connected in anti-parallel connection with a common gate.

antiparallel connected scr triac

Symbol of Triac

symbol of triac

I-V Characteristics of Triac

As we discuss above the triac has three terminals. MT1 MT2 and Gate (G). The triac can trigger by two methods.

Firstly when input voltage level reaches at breakover voltage or exceeds the breakover voltage.

Secondly, Gate signal is applied to the triac.

The first quadrant characteristics are just like an SCR but on the other hand, the third quadrant characteristics of a triac are identical to its first quadrant. Only polarities of the main terminals (such as MT1 and MT2) change.

vi characteristics of triac

The Triac can be operated in both the half-cycle of an ac supply. The output of the triac can be controlled by with the help of gate current.

Applications of Triac

  1. It is used to control power in steps.
  2. It is used as a fan regulator.
  3. It is used to control the speed of ac motors. Triac can control the speed by adjusting the gate current.

The triac are available in different sizes and ratings. They have the capacity to handle current from 0.5A to 100A and voltages upto 2000V. The size of the device depends upon its current carrying capacity. BT 151 is an example of triac.

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Conducting Materials

Conducting Materials

Conducting materials are those materials which offers easy path to the flow of electric current. It offers low resistance to the flow of current. Silver, Copper, Aluminium, iron are the example of conducting materials. Conducting materials are subdivided into low resistivity and high resistivity materials.

Low Resistivity Materials

There are the materials which offers low resistance to the flow of electric current through them. In order words, we can say that current can easily flow in these materials.

Low resistivity materials are used in house wiring, as conductors for power transmission and distribution of an electric power, in windings of electric machines like transformer, motors and generators. Basically, these materials are used where low power losses and voltage drops are required.

List of low resistivity materials are given below

  • Copper
  • Aluminium
  • Iron

Copper and Aluminium are used commercially because these materials have low resistivity and high conductivity. The conductivity of copper is higher than aluminium but due low cost of aluminium these are used in most of applications. Iron is used basically for manufacturing of earth wire or ground wire. The resistivity of iron is very high as comparatively to copper and aluminium.

Properties of Low Resistivity Materials

  1. It should have low temperature coefficient.
  2. The mechanical strength should be high.
  3. The material should be ductile which allow it to be drawn out into a wire.
  4. These materials should be such that it is not corroded when used in out-door services.

High Resistivity Materials

These materials are used for making resistance elements for heating devices filaments of lamps, rheostats etc. High resistivity materials are used in all such applications where a large value of resistance is required.

Tungsten, Nichrome etc are the examples of high resistivity materials.

Properties of High Resistivity Materials

The high resistivity materials should have following properties.

  1. These materials should have low temperature coefficient of resistance.
  2. These materials should have high meeting point. So it can withstand high temperature for a long time without melting.
  3. It should be able to withstand high temperature for a long time without oxidation.
  4. These materials should be ductile.
  5. High resistivity materials should have high tensile strength.

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Electrical Earthing

Electrical Earthing

Earthing means connecting of the non current carrying parts of electrical device or equipment or the neutral point of the power system to the earth through a wire or conducting materials having very low resistance. An electrical earthing is very necessary for equipment and power system. It protects the electrical equipment and power system against short circuit, fault currents and lightning. In this article, we will deals with objectives of earthing and type of earthing.

Objectives of Earthing

  1. It provides the safety of the personnel against electric shock. It insure the safety of human being during insulation failure.
  2. It protects the device or electrical equipment against damaged which may cause due to flow of overload current during short circuit.
  3. It limits the over voltages from neutral to ground or line to ground.
  4. Earthing suspress dangerous earth potential gradients.

Types of Electrical Earthing

Earthing can be divided into two types

  • Neutral Earthing
  • Equipment Earthing

Neutral Earthing

Neutral earthing deals with the earthing of the system neutral to ensure system security and protection. Neutral earthing is also known as system earthing. There are various methods for neutral earthing. The name of these methods is given below.

  1. Isolated Earthing
  2. Resistance Earthing
  3. Reactance Earthing
  4. Solid Earthing

Equipment Earthing

Equipment earthing deals with earthing of non-current carrying parts of the equipment to ensure safety of personnel and protection against lightning. Equipment earthing is also known as safety earthing. The name of different types of earthing is given below.

  • Plate Earthing
  • Pipe Earthing

How earthing protects the personnel against electrical shocks?

In this example, we will explain about protection of personnel against electric shock.

In first case, a man touches an electrical motor. If short circuit occurs in a motor the short circuit current starts flowing through a human body.

without electrical earthing

In second case, an electrical machine is earthed. When a man touch with a motor. If short circuit occurs in motor the fault current flows through a earth wire instead of flowing through human body.

with electrical earthing

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Interconnected System

Interconnected System

The connection of many generating stations such as hydro power plant, Thermal power plant, Nuclear power plant etc running in parallel is called interconnected system. The interconnected system may be two types (i) Integrated and (ii) Uni-integrated. An integrated interconnection results in maximum overall economy. But most of the system interconnections are unintegrated. An unintegrated interconnection the identity of individual not lost. There is no central control office like integrated interconnection system.

Advantages of Interconnection or Interconnected System

The interconnected system has many advantages. There are given below.

  1. It increases the service reliability. Maintenance and replacement of any equipment is required during its operation so if the system is interconnected it will increase the reliability. An electrical energy is generated from different source, such as water, coal a nuclear etc. It the system is interconnected then we will choose that plant where will have to pay less cost. During peak load we can receive or transmit energy from large capacity plant.
  2. Load growth necessitates additional transmission facilities. If the system is interconnected them we have a provision to add and replace transmission. It makes the system more reliable.
  3. Reserve capacity required is reduced with interconnected system we do not need reverse capacity of plant.
  4. Reduction in total installed capacity. An interconnection decrease the installed capacity needed to meet the load requirement. Different areas have different demands of electrical energy. In areas required less energy and at the same other area required more electrical energy. If two such areas are interconnected, the diversity of load would cause the maximum combined demand to be less than the some of the individual maximum demands. In this way, diversity factor improves.
  5. The interconnected system improves the efficiency of plants.
  6. Economical operation of station is ensured. We can choose that plant which has lower running cost in this way the system become economical.
  7. With system interconnection capital and maintenance cost is reduced.

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Wattmeter is an instrument which is used to measure electric power in the circuit. It measure electrical power in watt. Electric power is the product of current and voltage but in case of ac circuits power factor (cos Φ) is multiplied by an electric power. In this article, we will discuss about basic construction and working of wattmeter.

Construction of Wattmeter

A wattmeter basically consists of two coils, one coil is called the current coil and the other one is called pressure coil. As from definition of wattmeter, it measures electric power. So current coil (cc) measures current and pressure coil (PC) measures voltage. The current coil (cc) has less number of turns, which carries the current in the load and has very low impedance. The pressure coil (PC) has more number of turns having high impedance which is connected across the load. The load voltage is immersed across the pressure coil.  Circuit diagram of wattmeter is given below.

Construction of wattmeter

From above figure, M and L represents current coil. Common and V represents pressure coil. The terminal M denotes the main side, L denotes the load side, common denotes the common point between current coil(cc) and pressure coil (pc) and V denotes the second terminal of pressure coil. A wattmeter is a four terminal device.

Working Principle of wattmter

When current flows through the current coil (cc) and pressure coil (pc), they set up magnetic fields in space. The electromagnetic torque is produced by the interaction of magnetic fields set up in current coil and pressure coil. In both coils, one coil is fixed coil and an other coil is moveable. Under the influence of the torque, the moveable coil moves on a scale. The produce torque in coil is directly proportional to the current flowing through the coil. Large current flows in a coil produce large torque and vice-versa. The torque which moves the pointer is called deflecting torque. This torque is controlled by controlling torque. Controlling torque is provided by spiral spring. The deflection is proportional to the average power (VI Cos Φ) delivered to the circuit.

Construction of wattmeter

Sometimes, meter gives backward deflection or downscale reading. This is due to improper connection of the current coil and pressure coil of wattmeter.


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A diac belongs to thyristor family. Once it is triggered, it starts conducting in both directions. It is similar to diode but there is a main difference between diode and diac.  The diode conducts in one direction, when it is forward biased. On the other hand, diac conducts in both directions or we can say that diac conducts both half cycle of the alternating current. That is why the diac is also known as a bidirectional avalanche diode. It is known as bidirectional avalanche diode because it is conducts only when applied voltage becomes equal or more the forward breakover voltage.

Basically, diac is a two terminal bidirectional semiconductor device that can conduct alternating current.

Di means (two terminal device)

Ac means conduct alternating current.

Construction of Diac

The material used for manufacturing of diac is silicon like thyristor. Silicon materials have many advantages.

A diac is a four layer (PNPN), two junctions (J1, J2) and two terminal (MT1 and MT2) semiconductor device.

MT1 is known as Main Terminal One.

MT2 is known as Main Terminal Two.

There is no control terminal in this device.


Symbol of Diac

symbol of diac

Working of Diac

AC supply is given to the input of circuit. Diac is connected in series with load. The current does not flow through the load because the voltage applied to the diac is less than breakover voltage and at this stage  diac remains in non-conducting state.

When input voltage exceeds the breakover voltage, the diac starts conduct and current start flowing through the load.

working of diac

I-V Characteristics of Diac

The device can be triggered by either positive or negative half of an ac cycle. When the supply voltage is less than the breakover voltage, a very small amount of current called the leakage current flows through the device as shown in figure.

i v characteristics of diac

Leakage current is not sufficient to conduct the device. The device remains practically in non-conducting mode.

When the voltage level reaches the breakover voltage, the device start conducting. The current increased exponentially at this stage.

For positive half cycle the characteristics obtained in the first quadrant.

Negative half cycle the characteristics obtained in the third quadrant.

Diac does not give controlled output like Triac.

Applications of Diac

Diac are basically used to triggering of triac. Triac and diac pairs are available in market.


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Underground Cables

Underground Cables

Underground Cables are used for transmission and electric poser in congested areas (Such as in cities, towns etc) at comparatively low at medium voltages. Usually, the underground cables used in congested cities or towns, substations, railway crossings and where safety is very important. However, recent improvements in the design and manufacture have led to the development of cables suitable for use at high voltage. In this article, we will discuss about definition of underground cable and construction of underground cables.

Define Cables

A conductor covered with a suitable insulation and protecting layer is called cable.

Requirement of Underground Cables

  • The conductor used in underground cable should be standard in order to provide flexibility. The conductor should have high conductivity so that current flows easily through them.
  • The area of cross section should be such that the cable carries the desired load current without overheating.
  • The voltage drop in conductor should be within permissible limits.
  • The cable must have proper thickness of insulation.
  • The cable must be provided with suitable mechanical protection.
  • The cable do not react with chemicals.

Construction of Cables

The cable have various parts. There are given below.


The cable consists of one or more than on core. It depends upon the typ0es of service for which it is intented. The conductors are made of tinned copper or aluminium . The stranded conductor are used in order to increase flexibility.

underground cables


Each conductor or core covered with proper thickness of insulation. Basically thickness of insulation depends upon the voltage level. The commonly used insulating materials are impregnated paper, varnished cambric etc.

Metallic Sheath

A metallic sheath is provided over the insulation in order to protect the cable from moisture, other liquids that are present in soil and atmosphere. The commonly used material for metallic sheath is lead or aluminium.


Over the metallic sheath bedding layer is provided which consists of a fibrous metarial like jute or hessian tape. Bedding protects the metallic sheath against corrosion and from mechanical injury due to armovring.


Armouring is provided over the bedding which consists of one or two layers of galvanized steel wire. It protect the cable from mechanical injury.


In order to protect armouring from atmospheric conditions, a layer of fibrous material provided over the armouring. This layer is called serving.



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Electrical Transducer

Electrical Transducer

There are number of physical or non-electrical quantities which can be measured with the help of electrical or electronic instruments. Transducer plays very important role in industries.  Basically, an instrumentation system consists of three main parts, namely input device, a signal conditioning device and an output device. The output device may be a simple indicating meter, display etc. An Electrical Transducer used in industries.

electrical transducer

The Input device is usually  known as transducer. In this article we will discuss about transducer, advantages of transducer classification of transducer.

What is Electrical Transducer?

An electrical transducer is a device that converts non-electrical or physical quantity into electrical signal or quantity.

For most of the instrumentation systems, the input quantities are non-electrical, but these quantities are required to convert into electrical. Because electrical signal or quantity can be easily measured and processed.


  • Electrical signals can be easily processed as per need.
  • The electrical signals can be converted any desired form.
  • The power required for electrical or electronic system is very small.
  • These signals can easily transmitted and processed for the purpose of measurement.

Classification of Transducers

The transducer are classified on the basis of power utilized them for operations, there are two basic types of transducers (i) Active transducer (ii) Passive transducer.

Active Transducer

Active transducers are those transducers that does not require external power for their operation. Active transducers, also called self generating transducer. In these transducers, a voltage or current produced proportional to the quantity under measurement. The example of active transducer is piezoelectric crystal. When external force is applied to piezoelectric crystal it converts mechanical energy into desired output.

Passive Transducer

Electrical transducers are those transducers that require external power for their operation. In these transducers, some parameters is varied by the measurand which in turn varies the electrical quantity such as voltage or current.

The example of such type of transducers are resistive, inductive and capacitive. These are the basic types of transducer and further classified into many types.

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Buchholz Relay

Buchholz Relay

Buchholz relay is a protective and gas actuated relay . It is used in that transformer whose winding is  immersed in oil and rating having more than 500KVA. Buchholz relay is not used in small transformer because it increase the cost. It is used to protect transformer against internal or incipient faults only. Basically, it is used for detection of winding faults.

Buchholz relay is invented by Buchholz in 1921.

Buchholz Relay Construction

Buchholz relay is installed in the pipe connecting the conservator to the main tank. There are two mercury switches used in the relay. One of the mercury switches is attached to the upper float. This switch send signal to the alarm circuit when relay operates. Whereas the other switch is mounted on lower hinged type flap. This switch send signal to the trip circuit. A release cock is mounted top of the chamber.

Buchholz Relay


Operation or Working of Buchholz Relay

When a minor fault occurs in a transformer, heat is produced, oil gets heat up. Hydrogen gas is produce in transformer. The gases being light, try to go into conservator tank but gas is trapped in Buchholz-Relay which is connected between main tank and conservator tank. It is collected in top of the chamber while passing to the conservator tank. When the predetermined amount of gas is accumulate in the upper part of the relay, oil level falls due to gas pressure. This tilt the upper float downward and closes the alarm circuit contacts through mercury switch attached to the float arm. The alarm rings bell and gives the warning signal.

In case of series fault within the transformer, large amount of gas in evolved in the main tank. In this time, lower mercury switch of the buchholz relay operates and sends the signal to the trip circuit. This completes the trip coil circuit of the circuit breaker. Thereafter, the transformer is disconnected automatically.

A release cock is provided at the top of the relay chamber so that after operation, gas pressure is released through this cock.

‘Buchholz Relay’ indicates internal faults or incipient faults in the transformer, such as inter turns fault, insulation failure. It does not respond to external faults.




Transistor is a three terminals (namely emitter, collector, base) three layer and two junction semiconductor device. It is used to amplify or process an electrical signal. A transistor consists of two two pn junctions formed either P type or N type semiconductor between a pair of opposite types. There are two types transistor.

  • PNP Type Transistor
  • NPN Type Transistor

PNP Type Transistor

In this type, the N type is sandwiched between two P-type layers. A PNP transistor composed of two p-type semiconductors separated by thin section of n-type semiconductor. PNP transistor shown in figure

pnp transistor

NPN Type Transistor

In this type, the P-type is sandwiched between two N-type layers. A NPN transistor composed of two n-type semiconductors separated by thin section of p-type semiconductor.

NPN transistor

Symbol of Transistor

symbol of transistor

Transistor Terminals

There are three terminals in transistor, called emitter, collector and base. We have discussed above the transistor has three different layers or sections. The all layers or sections are different in size and having different doping level. According to this, different terminals are explained below:-


The emitter is heavily doped and moderate in size. Emitter supplies a large number of majority carries. The emitter is always forward biased w.r.t to base. So that it can supply a large number of majority carries to its junction with the base.


The other outer layer of the transistor that collects the majority carries supplied by the emitter is called collector. The collector-base junction is always reverse biased.

It is moderately doped and larger in size, so that it can collect most of the majority carries supplied by the emitter.


The middle of the transistor of the transistor. The base is lightly doped and small in size so that it can pass most of majority carries supplied by the emitter to the collector. The base forms two circuits.

Connections of Transistors

connections of transistor