Circuit breakers are used in a wide range of applications.  They are used in many environments and can handle currents and voltages of different ranges.

Circuit breakers are classified on the basis of different criteria.  Some of the classifications are below

Based on the interrupting medium
  1. Air circuit breaker
  2. Oil circuit breaker
  3. SF6 circuit breaker
Based on type of action
  1. Automatic circuit breakers
  2. Non-automatic circuit breakers
Based on the method of control
  1. Locally controlled circuit breakers
  2. Remotely controlled circuit breakers (remote control can be mechanical, pneumatic or electrical)
Based on the type of mounting
  1. Panel mounted circuit breakers
  2. Remote from panel mounted circuit breakers
  3. Rear of panel mounted circuit breakers
Based on location
  1. Outdoor circuit breakers
  2. Indooor circuit breakers
Based on voltage
  1. Low voltage circuit breakers
  2. Medium voltage circuit breakers
  3. High voltage circuit breakers

The relay in a protection system should be sensitive enough to operate when a fault occurs.  A sensitive relay improves the reliability of the system.

When the parameter exceeds the set value, the relay should start operating.

The sensitivity of a relay is mentioned as a ratio of the minimum value of short circuit current to the minimum value of the quantity for the operation.

The sensitivity is indicated by a sensitivity factor Ks

Sensitivity of a Relay

Is is the minimum short circuit current in the zone and
Io is the minimum operating current for the relay.

The sensitivity of a relay is also related to the VA of the input to the relay.  Lesser the VA of the input, greater will be the sensitivity and vice versa. For instance, a relay which has 1 VA as its measuring input will be more sensitive than a relay, which has 5 VA as its measuring input.

A reliable and effective protection system is a crucial part of any power system.  The protection system protects the equipment in the power system, such as generators, motors, transformers, etc from damage to faults.

The protection system also ensures reliability by localizing and isolating the fault and minimizing its impact on the rest of the power system.

The requirements of a power system are as follows

1. To isolate the equipment or component in which the fault has occurred.  The isolation should be quick enough to prevent damage to the component itself.  For instance, a short circuit inside can severely damage a transformer.  The differential relay, in such a situation, should immediately act and trip the transformer.

2. To isolate the smallest possible section of the power system to minimize the interruption.

3. To prevent disturbance or disruption to other parts of the power system.  The fault, if not isolated in time, can cause the upstream breakers to trip.

A well-designed protection system will greatly increase the reliability and performance of a power system.

The faults in a power system can be caused by a wide range of causes.  Below is a list of some of the most probable causes for electrical faults.

Overvoltage is caused due to surges such as lightning or due to switching loads on and off.  In generators, it can be caused due to the failure of the field controller.

Heavy winds which can cause lines to snap or trees to fall on them. This can cause open circuits, earth faults and short circuits.

Ageing which can result in weakening of insulation and failure. This can result in short circuits and earth faults.

Chemical pollution and deposition on the insulators that can result in flash overs

Faults due to wildlife, such as birds, snakes, mice, etc which can cause short circuits and earth faults.

Collision of vehicles on towers and transmission poles can cause the towers to fall.

The table below will give an idea of the percentage of the types of faults in the different equipment in a power system

Overhead lines                      50%
Transformers                         12%
Switch gear                            15%
Cables                                   10%
Miscellaneous                         8%
Instrument Transformers        2%
Control Equipment                 3%

Silicon Controlled Rectifiers
Silicon Controlled Rectifier
A silicon controlled rectifier is a three terminal electronic component.  It consists of an anode, a cathode and a gate.

The device is similar to a diode, except that it needs to be switched on with an external voltage applied to the gate during the positive half cycle.  Once, the SCR has been trigerred, it "fires" and conducts as long as it is positively biased.

In AC circuits, during the negative half cycle, the SCR switches off automatically.

SCRS find application in applications where the current needs to be controlled.  They are used in many electronic equipments, such as inverters, converters, speed controller, etc.

There are different methods of trigerring the gate of a Silicon Controlled Rectifiers (SCRs).  They are

Using DC Voltage
By applying a positive voltage to the gate with respect to the cathode, the junction J2 can be forward biased.  This will switch on the SCR.  This process requires a constant dc voltage to be applied between the Gate and the Cathode, which is a disadvantage.  Besides, there is no isolation between this triggering dc voltage and the main dc supply

Using AC voltage
In AC applications, the trigger voltage can be obtained from the AC voltage suitable reduced.  The phase of the AC voltage is modified and applied to trigger the SCR at the desired instant.  The SCR will continue to conduct till the negative half cycle.

A separate transformer is required for the trigger circuit which increases the cost

Pulse triggering
This is the most widely used form of triggering.  In this method, a pulse of a small duration is applied to the gate to switch it ON.  Sometimes, a series of pulses are applied.  The pulse need not be continuous.  This reduces the losses in the gate.

Commutation in dc machines refers to the changing of current flow from one circuit to another.  In SCRs ( Silicon controlled Rectifiers) and thyristors, it refers switching off a conducting electronic component.

In AC circuits, SCRs are commutated by the negative half cycle which reverse biases the anode and cathode terminals.  This is known as natural commutation.

However, in DC circuits, special circuits should be designed to switch off the SCRs once, they have been switched on.  The current is reduced to zero by means of external circuits.  This is known as Forced Commutation

The SCR cannot switch on on its own once its anode and cathode are connected to the positive and negative terminals respectively.

The following are some of the methods.

Gate Triggering

This is the most popular method.  A single pulse or a train or pulses are applied to the gate terminal of the SCR.  This creates a forward bias across junction J2 and switches on the SCR.

Thermal Triggering
When the SCR is heated above a certain value, more hole-electron pairs are produced this increases the charge carriers and can cause the SCR to switch on.

Light Triggering

When light is made to fall on the junction in reverse bias, hole-electron pairs are created due to the energy of the incident light.  This can cause the SCR to fire.  Special components such as LASCR (Light activated Silicon controlled Rectifiers) and LASCS ( Light activated Silicon controlled Rectifiers) work on this principle.  This method of triggering is cheaper when designing components of higher ratings.  The light is conducted to the junction by means of optical cables.

dv/dt triggering
In this method of triggering, a rapid change in the voltage current to flow through the junction J2 which acts as a dielectric between two conductive junctions (J1 and J3).  The SCR will switch on even if the voltage is low provided the rate of change of the voltage is high.

The advantages of grounding (earthing) the neutral are as follows

  1. Sensitive current protection schemes can be used to quickly identify an earth fault.
  2. The external surges and overvoltages caused by lightning or switching are discharged to the ground.  If the neutral is not grounded, these waves will get reflected and cause overvoltages in the system.
  3. The phase voltages are within limit and the value is the voltage between the phase to ground.
  4. Arcing grounds, which occur at the location of an earth fault are avoided.

The disadvantages of grounding the neutral are

  1. The system will trip even for a minor earth fault.  This affects the reliability of the power supply.
  2. The zero sequence currents which flow through the neutral can cause interference to telecommunication lines.

The Generator Neutral Breaker is used in systems, which are grounded through low resistances or solidly grounded (without a resistance).  In such systems, the fault current in the line due to an earth fault will be high.

The current flowing through the equipment due to an earth fault can be limited if a breaker is connected in series with the neutral.  This breaker is opened simultaneously with the armature and the field breaker.  This will bring the fault current to zero quickly.

Circuit breakers used in switching of long transmission lines have a resistors which is pre-inserted between the contacts before the contacts are closed. This resistor is called the Pre-insertion resistor. The function of this resistor is to limit the initial charging current of the line. The resistance of the line is around 500 ohms.

Once the closing command is given to the breaker, the resistor is first connected across the contacts. This resistance in series limits the line current. A few milliseconds later, the contacts are closed. 

While opening the breaker, the pre-insertion resistor is first disconnected before the contacts are opened by the circuit breaker. Pre-insertion resistors are also used in lines which have transformers to limit the high inrush current.

Capacitance is the phenomenon of holding electrostatic charge. In electrical systems, long transmission lines, power cables and capacitor banks can have large amounts of capacitance. In a circuit containing capacitance, the current will lead the voltage by 90 degrees. This means that at the instant of the current zero crossing, the voltage across the breaker contacts will be the maximum. If a circuit is isolated at this instant, the high system voltage will be retained by the line capacitances. If the breaker is opened when the current is zero and the voltage is maximum, half a cycle later when the supply voltage reaches maximum in the opposite direction, the voltage across the breaker contacts will be 2V. This can result in a restriking voltage being developed and a flashover occurring across the circuit breaker. Once the flashover due to the restrike occurs, oscillations are set up in the line between the system inductance and the capacitance. These oscillations and the restrikes they cause can result in the line voltage reaching up to 4 times the voltage (4V). Hence, in lines with high capacitances, air blast circuit breakers or multi break circuit breakers are used for isolation.

A Silicon controlled Rectifier (SCR) is a semiconductor device which conducts in only one direction.  It has three terminals.  An anode, a cathode and a gate.  Unlike a diode, however, it needs to be switched on by a pulse applied to the gate.  

The circuit below shows the method of switching on an SCR using a resistor.  The power source is connected across the SCR.  The gate voltage is provided by the voltage divider circuit.  The variable resistor, R4 is used to control the firing angle.  

The Diode D1 prevents negative voltage from reaching the gate during the negative half cycle.  The SCR will be switched off during the negative half cycle by the supply voltage

Resistor switching of an SCR
Circuit Diagram - Resistor switching of an SCR

The Transient Stability Analysis is done to determine the behaviour of a system during transients or sudden changes in a power system.

Transients occur when there are power flows from one source to another.  They also occur during faults when a load or a generating point is cut off.  This can cause oscillations in the voltage or power.  Many of these oscillations will quickly get resolved and the system will return to its steady state operation.

However, in certain situations, the oscillations may can increase in severity and can cause fluctuations in the voltage or power which can affect the system and can cause trippings.  Hence, it is necessary to ensure that the system will be stable in the event of a transient.

The stability of a system is classified into

Steady State Stability and
Transient Stability

Steady state stability is the ability of the system to respond to small oscillations in the voltage or slow changes in the load.

Transient stability is the ability of the system to respond to sudden, unexpected changes such as the tripping of a power source or a fault in a tranmission line.

Transient stability analysis is used in relay setting and in determining the clearing time of breakers.  They are also used to determine the voltage level of a power system and the power transfer capability between different systems.

A Fault Analysis is a study which describes the fault currents and the behaviour of a power system during an electric fault.  Faults may be line to line faults or line to ground faults. The fault analysis provides information about the  The Fault Analysis is used to determine the ratings of fuses and circuit breakers.

Using the fault analysis, we can determine the maximum current which will be developed during a fault.  The bus bars, breakers and other transmission equipment should be equipped to withstand the heavy current which flow during a fault.

The protective relays are set based on the current calculated during the fault analysis.

The Load Flow Analysis is done to determine the voltage, current, real and reactive power in a particular point in a power system as well as the flow from one point to another. 

The Load Flow Analysis helps understand the operation and behaviour of the system when a generator trips or when a big load is suddenly cut off.  Load Flow Analysis also helps identify routes to transfer power when a transmission line has to be isolated due to a fault.  This ensures reliable power supply and ensures quick restoration in the event of blackouts. 

Load Flow Analysis is done during the design of the power system.  It should also be done before any modification of the power system such as the addition of loads or generating units.

A Power system has three main components
They are

  • The Generating System
  • The Transmission System 
  • The Distribution System

Generating System
The Generating System is the source of the power.   The generation can be from generators, solar panels, etc.  Power can be generated from different sources such as hydropower, wind turbines, nuclear plants,etc.

Components: Synchronous Generators, induction generators, solar panels,

Transmission System
The transmission system transmits the generated power over large distances to the distribution centres such as industries and cities.  The distribution areas can be thousands of kilometres away from the generating stations.  The voltage is stepped up to high values to minimize the losses using transformers.  The power is then transmitted through the power lines to the distribution areas.

Transmission systems can be categorized into

Primary Transmission Systems, which transfer power at voltage of 110 kV and above.  These lines are hundreds of miles long.  They are connected to secondary receiving substations

Secondary Tranmission Systems, which receive the power from the primary transmission system send it to the distribution systems.  The voltage levels in the secondary transmission systems are about 33kv to 66kV

Components: Transformers, Circuit Breakers, Overhead Transmission Lines, Underground Cables.

Distribution Systems
The distribution system receives power from the transmission system and distributes the power to the individual customers at the required voltage.  The industrial supply voltage can be 33kV or 11kV.  The domestic supply voltage is 440 or 220V

Components: Transformers, underground and overhead transmission lines.

A Power System Analysis is a very important exercise in the design and operation of a power system.  The Power System Analysis is used to evaluate the performance of a power system. 

Power System Analysis deals, chiefly, with three important parts

They are

  1. Load Flow Analysis
  2. Short Circuit Analysis and 
  3. Stability Analysis

A Power System Analysis helps the following aspects.

  1. Study the ability of the system to respond to small disturbances caused by the application/removal of small or large loads.
  2. Design of the breakers and isolating equipments.
  3. Plan for future expansion of the power system
  4. Study the response of the system to different fault conditions.
  5. Observe and monitor the voltage, real and reactive power betwee different buses.
  6. Calculate the setting of the relays and the design of the protection system.

The power balance equation describes the relation between Power Demand and Power Generation in a power system.

The equation is


PD is the Total Power Demand
PG is the output of individual generating stations

The sum of the power generated should equal the demand for power.

Arduino is an open source platform used in embedded systems. Arduino has its own hardware and software. Since it is an open source project, it is used for numerous projects by many hundreds of people around the world.  The Layout and the production files are available in the public domain.

The Arduino board is powered by the Atmel 8-bit AVR microcontroller.  The Flash memory and other features may vary among different boards.

Programming the Arduino
The program for the Arduino can be written in any high level programming language with a compiler which can generate machine level code for Arduino.  However, Arduino has its own IDE (Integrated Development Environment).  The Arduino can be used using the IDE.  A program for the Arduino is called the sketch.

Programs can be written using the C and C++ language

Arduino has a well developed ecosystem consisting of numerous manufacturers and developers.  Many professional projects can be built with Arduino.  There are many peripherals such as sensors and actuators which can be linked to the Arduino to create a range of products from robots to security systems.

Many manufacturers and hobbyists create projects based on Arduino.

Useful Links

Software refers to the non physical parts of a computing system.  Examples are the programs which contain the instructions.  The software is written in the programming language such as VB, Java and C

Firmware is the program written on an embedded device such as a microprocessor or a microcontroller.  It controls the functioning of the microprocessor IC

It is written in the assembly level language. It is called firmware as it interfaces between the software and the hardware.

Hardware refers to the physical components of a computing system such as the processor, memory and the peripherals.

The key difference is that in a microcontroller, the memory (ROM and RAM) and the peripherals are fabricated on a single IC. A microprocessor, on the other hand, does not contain the memory and the peripherals in itself.  They are separately mounted and connected.

Microcontrollers are used for specific operations, such as to control and operate a washing machine or a traffic signal.  A microprocessor can be installed for a specific function in a larger system.  It is not designed for a single operation.

The speed of a microprocessor is above 1 GHz while the speed of the microcontroller is around 50 MHz.

Microprocessors can handle greater complexity as compared to microcontrollers.   They also use more power than microcontrollers.

Embedded Electronics, as the name suggests, refers to electronic hardware and software that is embedded or attached to the equipment being controlled.  

The component may be a robotic arm in an assembly line or a life support device in an ICU.  Today, Embedded Electronics can be found in all areas of life.  The washing machine and the refrigerator at home are also controlled by embedded electronics.

The advantages of embedded systems are their small size, low cost and power consumption and their rugged construction.  The program and the logic of machine operation can be easily modified.  The cost of embedded systems are lower as they are mass produced which reduces cost.  

Embedded systems can be built using both microprocessors and micro controllers.  Embedded systems can be used as standalone units or as part of a larger network controlling a bigger system.  

Programming Embedded Systems

Embedded systems can be programming using assembly level languages.  The assembly level languages are compiled into machine level using compilers.  The program is stored in the nonvolatile memory of the system.  Microprocessors and microcomputers will have their own programming languages specified by the manufacturers.  A good understanding of the C programming language will be useful in programming embedded systems.

Conductivity is an important parameter of industrial liquids.  Conductivity is measured for liquids almost all liquids.  The conductivity of the liquid gives an idea of the ions in the liquid.

The conductivity of a liquid is measured using special conductivity sensors.  The unit of conductivity is siemen/cm.  A siemen is 1/ohm.  The unit of conductance is sometimes referred to mho (ohm written in reverse).

The conductance is usually a very low value for conducting liquids such as water.  It will be of the order of a millionth of a siemen, in microsiemens.  Highly pure water, for instance, will have a conductivity of 1microsiemen/cm.

Measurement of conductivity
Conductivity is measured by measuring the conductivity of a liquid between two electrodes whose area and distance between each other is fixed.  This is known as a cell constant.

A cell constant of 1 implies that the electrodes will have a surface area of 1 cm2 and will be spaced 1 cm apart.

Magnetic flow meters are used to measure flow of liquids that are conductive.  Magnetic flow meters do not have to physically be in contact with the medium.

Magnetic Flow meters, or Magmeters as they are otherwise called work on the basis of Faraday's law which states that the voltage produced by a moving conductor in a magnetic field is proportional to the velocity of the conductor.

In a Magnetic flowmeter, the conductive liquid such as water is passed through a constant magnetic field.

As the conductive liquid flows between a magnetic field, a voltage is induced in direction perpendicular to the magnetic field.  This voltage is measured by a pair of probes.

The flowrate can be calculated from the voltage induced in these probes.

The magnetic field is produced by a pair of electromagnets whose polarity is constantly reversed. The reversal of polarity is essential to prevent interference due to electrochemical potentials induced where the probes come in contact with the liquid.

The voltage is proportional to the velocity of the liquid, the width of the pipe (diameter), and the magnetic field strength.

Latching current is the minimum current which is required to flow from the anode to the cathode to switch "ON" the SCR.

Holding current is the minimum current which needs to keep flowing to keep the SCR in the 'ON' state.

The Latching current will be greater than the Holding current for an SCR.

The bearings can be one of the two types, a plain bearing (sliding contact) or an anti-friction bearing (rolling bearing), depending on the design parameters of the machine element, each of the two types of bearings, plain and anti-friction, is available for design with linear motion, radial loads and axial loads.

Bearings may be classified into three general classes
Guide or flat bearings, which support linear motion in machine tables and slides.

Thrust bearings, which support rotational motion in machine elements that have axial loads i.e., the load is applied along the central axis of the rotating shaft

Radial bearings, which support rotational motion in shafts with radial loads i.e., the load is applied along the radius of the rotating shaft.

Anti Friction or Roller Element bearings

Anti – friction bearings or roller – element bearings, as they are often called, use a rolling element (ball or roller) between the loaded surfaces.
Anti-friction bearings are divided into two categories,
a) ball bearings
b) Roller bearings.

Ball bearings have five general types:
Guide, Radial, Thrust, Self – aligning and Angular contact.

Roller bearings have four general types: Cylindrical, Thrust, Spherical and Taper.

Roller and Ball bearing types
Guide bearing: The ball guide bearing is used for linear motion where very low co-efficient of friction and extreme smoothness in operation are desired.
Radial bearing: The first radial bearing is the single – row, deep – groove ball bearing, most widely used anti – friction bearing. Second radial bearing is the cylindrical roller bearing is capable of carrying larger radial loads at moderate speeds than those carries by radial ball bearings using the same size bearing.
Thrust bearing: First the ball thrust bearing is designed for axial (thrust) loads only – no radial loads. Second spherical roller thrust bearing is capable of very heavy axial loads as well as moderate radial loads.
Angular contact ball bearing: The shoulders in this provides for thrust (in one direction only) that is larger than the single row, deep radial ball bearing can handle.
Taper roller bearing: A pair of taper roller bearing is capable of handling both very large axial and radial loads.

A compound motor is a combination of shunt and series motor i.e., a series field winding, wound with heavy copper conductor on top of the shunt field winding. The series field winding is connected in series with the armature. So that its mmf will be proportional to the armature current and in the same direction as the shunt field mmf

Typical compound motors designed for industrial application obtain approximately 50% of their mmf from the series field wen operating at rated load.

There are two types of compound motors connection,

If the connection to the series and shunt winding is in such a way that their respective mmfs are additive is called cumulative compound motor.

If the series field is reversed with respect to the shunt field, its mmf will subtract from the shunt field mmf, causing the net flux to decrease with increasing load, resulting in excessive speed, which is differential compound motor.

Stabilized - shunt motor
Compound motors, whose series field are designed to provide just enough mmf to nullify the equivalent demagnetizing mmf of armature reaction and provide a very slight speed droop, are called stabilized – shunt motors. The series field winding of such machines generally have one – half to one and half turns / pole and depending on the application, provide approximately 3 to 10 percent of the total field mmf at rated load. The speed of stabilized – shunt motors is fairly constant, with only a slight droop in speed with increasing load. Stabilized – shunt motors are used in applications that require a fairly constant speed and a moderate starting torque.
Reversing the direction of rotation of compound or stabilized – shunt motors is accomplished by reversing the armature branch or reversing both the series field and the shunt field.

Torque in DC
The direction of the developed torque may be determined from an end view of the conductors and magnet poles. The direction of flux due to the known direction of current was determined by the right – hand rule and the direction of the mechanical force on each conductor, due to the interaction of the magnetic fields, was determined by the flux bunching effect.

BP = Flux density in air gap produced by shunt field poles (tesla)
IA = Armature current (ampere)
KM = Constant
Constant KM depends on the design of the motor and include the number of turns, effective length of armature conductors, number of poles, type of internal circuitry and units used.
The torque developed by a DC motor is proportional to the flux density in the air gap and the current in the armature.

Torque in AC
The torque developed by AC motors has two components: A Reluctance – torque component and a magnet – torque component. The reluctance – torque component is due to the normal characteristic of magnetic materials in a magnetic field to align themselves so that the reluctance of the magnetic circuit is minimum

The magnet – torque component is due to the magnetic attraction between the field poles (magnets) on the rotor and the corresponding opposite poles of the rotating stator flux.
It is also justified for salient – pole motors operating from 50 percent rated load to above 100 percent rated load, with power factors ranging from unity to leading, the reluctance torque for such loads is significantly smaller than the magnet torque.

Direct current (DC) generators are machines that convert mechanical energy into electrical energy. This conversion of energy is based on the principle of the production of dynamically induced electromotive force.

Types of DC Machines
Homo polar machines: These types of machines are used where low voltage and high currents are required e.g., Faraday’s disc dynamo
Hetero polar machines: The DC machines that are commonly used fall under this category.

The type of generator used in welding is homo polar machine. Differential compound generator also belongs to homo polar machines. The generator is so designed that it delivers a voltage high enough to start the arc and reduce the voltage as required to maintain the arc during the welding.
Unlike AC, DC flows continuously in one direction from a negative charge to positive charge. Although DC flows only one way, you can manipulate it to flow in the appropriate direction, which is its polarity.

DC welding is preferred when using high welding speeds and when welding is out – of – position.

Fusing Current refers at which the fuse is designed to melt and disconnect the circuit.  Wiring rules specify the fusing current for different circuits.  It is also known as minimum fusing current.  

Fusing Factor
The Fusing Factor is the ratio of the rated current and the fusing current

Fusing Current = Fusing Factor x Rated Current

The fusing current will always be more than the rated current.  Thus, the fusing factor will always be greater than one.  

‘Polarization’, denoted by ‘P’ is defined as the difference between the induced electric field ‘D’ and imposed electric field ‘E’ within a dielectric medium due to immovable and free charge carriers respectively.

This phenomena occurs if an electric field distortion takes place between negatively charged electrons around positively charged atomic nuclei giving rise to slight difference of charges.

Mathematically, it is stated as-
P = (D-E)/4π

Polarization can also be expressed in terms of electric susceptibility Хe, such as,
P = є0 Хe E

Where, є0 = permittivity of free space, (= 8.85 x 10-12 Farad/metre).

Quantitatively, it is denoted by-

P = p/V
Where, p =amount of dipole moment

  V = volume of polarized material