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What is a FUSE and How it Works?

A fuse protects an electrical circuit or device from excessive current when a metal element inside it melts to create an open circuit. With the excep­tion of resettable fuses, a fuse must be dis­carded and replaced after it has fulfilled its func­tion.

When high current melts a fuse, it is said to blow or trip the fuse. (In the case of a resettable fuse, only the word trip is used.) 

A fuse can work with either AC or DC voltage, and can be designed for almost any current. In residential and commercial buildings, circuit break­ers have become common, but a large cartridge fuse may still be used to protect the whole sys­tem from short-circuits or from overcurrent caused by lightning strikes on exposed power lines.

In electronic devices, the power supply is al­most always fused.

Schematic symbols for a fuse are shown in figure. Those at the right and second from right are most frequently used. The one in the center is approved by ANSI, IEC, and IEEE but is seldom seen. To the left of that is the fuse symbol understood by electrical contractors in architec­tural plans. The symbol at far left used to be com­mon but has fallen into disuse.

How a FUSE Works?

The element in a fuse is usually a wire or thin metal strip mounted between two terminals. In a cartridge fuse, it is enclosed in a glass or ceramic cylinder with a contact at each end, or in a small metallic can. (Old-style, large, high-amperage fuses may be packaged in a paper or cardboard tube.) The traditional glass cartridge allows vis­ual inspection to confirm that the fuse has blown.

A fuse responds only to current, not to voltage. When choosing a fuse that will be reliable in conditions of steady current consumption, a safe rule is to figure the maximum amperage when all components are functioning and add 50%. 

How­ever, if current surges or spikes are likely, their duration will be relevant. If I is the current surge in amps and t is its duration in seconds, the surge sensitivity of a fuse—which is often referred to verbally or in printed format as I2t—is given by the formula:

I2t = I² * t

Some semiconductors also have an I2t rating, and should be protected with a similarly rated fuse.

Any fuse will present some resistance to the cur­rent flowing through it. Otherwise, the current would not generate the heat that blows the fuse. Manufacturer datasheets list the voltage drop that the internal resistance of a fuse is likely to introduce into a circuit.

Types of Rechargeable Batteries

Rechargeable batteries are everywhere these days: cordless tools, laptop computers, cordless phones, and cell phones, just to name a few.

Rechargeable batteries for use with consumer electronic products are of four basic types: 

  1. Nickel cadmium (Ni-Cd), 
  2. Nickel metal hydride (Ni-MH) 
  3. Lithium ion (Li-Ion). 
  4. Lithium polymer (Li-Po)

Although these four types of batteries will not look much different from the outside, there are significant differences among them. We will explain a bit about each of them now.

Ni-Cd Batteries

Ni-Cd Batteries have been around the longest of these three types. There are many Ni-Cd batteries out there in use today. They have a good capacity and hold a very stable voltage between charges as they are being discharged.

The major Disadvantage of Ni-Cd batteries is the memory effect ( The decreasing useful time between charges for a rechargeable battery is due to what is called the memory effect )

Also, Ni-Cd batteries can only undergo a limited number of discharge-recharge cycles before they need to be replaced. They often last only one to two years.

Ni-MH (Nickel-metal hydride) Batteries

Ni-MH (Nickel-metal hydride) Batteries are a more recent development in rechargeable batteries.

They have many of the same advantages that the Ni-Cad batteries have. However they suffer much less from the memory effect than Ni-Cd batteries. There is some memory effect with Ni-MH batteries, but not nearly as much as with Ni-Cd. 

Also they can go through more discharge-recharge cycles than Ni-Cd batteries. Their typical useful life is more like 3 to 4 years. On the down side, Ni-MH batteries discharge more when not in use than Ni-Cds. 

After about a week of not being used, a fully charged Ni-MH battery will have lost about 20% of its charge. Also Ni-MH batteries cost more than Ni-Cd batteries, but their longer life tends to more than compensate for that.

Li-Ion (Lithium Ion) batteries

Li-Ion (Lithium Ion) batteries are the next newest type rechargeable battery to be commercially available.

They have all the advantages mentioned above for Ni-Cd and Ni-MH batteries and have a longer useful life than either of them. They do not suffer at all from the memory effect that is a problem for Ni-Cad and to a lesser extent for Ni-MH batteries. 

The main problem with Li-Ion batteries is that they lose about 10% of their useful capacity each year of use. This loss is due to chemical breakdown in the cells and currently there is no way to prevent or reverse this.

Li-Ion batteries typically last through about 300 to 500 discharge cycles, or about four to five years.

Li-Po (Lithium Polymer) Batteries

The next evolution of rechargeable battery technology is the Lithium Polymer Battery. 

Lithium Polymer or LiPo batteries are a great new way of storing energy for portable devices from cell phones, home electronics to RC hobby battery packs used in cars, boats and flight. 

They're great because they can store 350% (approximately) more energy than a typical NiCd/NiHm battery pack and weigh 10% - 20% less. They can also discharge much more current than a NiCd/NiHm battery and can be fully charged in about an hour. 

LiPo batteries also don't develop memory or voltage depression characteristics like NiCd/NiHm batteries, and do not need to be discharged before being charged.

But they're not without their downside. Mishandling of these batteries can lead to fire, explosions and toxic smoke inhalation.

Classification of Analog Instruments

An analog device is one in which the output or display is a continuous function of time and bears a constant relation to its input.

The analog instruments find extensive use in present day applications although digital instruments are increasing in number and applications. The areas of application which are common to both analog and digital instruments are fairly limited at present. 

Hence, it can safely be predicted that the analog instruments will remain in extensive use for a number of years and are not likely to be completely replaced by digital instruments for certain applications.

Classification of Analog Instruments

Broadly, the analog instruments (and for that matter digital instruments) may be classified according to the quantity they measure.  For example an instrument meant for measurement of current is classified as an Ammeter while an instrument that measures voltage is classified as a Voltmeter. Thus we have wattmeters, power factor meters, frequency meters etc...

Electrical instruments may also he classified according to the kind of current that can be measured by them. Electrical instruments may be classified as instruments for:

  1. Direct Current (DC)
  2. Alternating Current (AC)
  3. Direct and Alternating Curent (DC/AC)

Instruments depend for their operation on one of the many effects produced by current and voltage and thus can be classified according to which of the effects is used for their working.
The analog instruments are also classified as : 
  • (a) Indicating 
  • (b) Recording, 
  • (c) Integrating.

Indicating Instruments

Indicating Instruments are those instruments which indicate the magnitude of a quantity being measured. They generally make use of a dial and a pointer for this purpose. Ordinary voltmeters, ammeters and wattmeters belong to this category.

The analog indicating instruments may be divided into two groups :
  • (i) electromechanical instruments, 
  • (ii) electronic instruments. ‘
Electronic instruments are constructed by addition of electronic circuits to electromagnetic indicators in order to increase the sensitivity and input impedance.

Recording Instruments

Recording Instruments give a continuous record of the quantity being measured over a specified period. 

The variations of the quantity being measured are recorded by a pen (attached to the moving system of the instrument ; the moving system is operated by the quantity being measured) on a sheet of paper carried by it rotating drum. 

For example, we may have a recording voltmeter in a sub-station which keeps record of the variations of supply voltage during the day.

Integrating Instruments

Integrating Instruments totalize events over a specified period of time. The summation, which they give is the product of time and an electrical quantity. 

Ampere hour and watt hour (energy) meters are examples of this category. 

The integration (summation value) is generally given by a register consisting of a set of at pointers and dials.

The analog instruments may also be classified on the basis of method used for comparing the unknown quantity (measurand) with the unit of measurement. The two categories of instruments based upon this classification are:
  1. Direct Measuring Instruments
  2. Comparison Instruments

Direct Measuring Instruments

These instruments convert the energy of the measurand directly into energy that actuates the instrument and the value of the unknown quantity is measured or displayed or recorded directly. 

The examples of this class of instruments are ammeters, voltmeters, wattmeters and energy metres.

Comparison Instruments

These instruments measure the unknown quantity by comparison with a standard. 

Direct measuring instruments are the most commonly used in engineering practice because they are the most simple and inexpensive. Also their use makes the measurement possible in the shortest time

The examples of comparison type instruments are DC and AC bridges.
Comparison instruments are used in cases where a higher accuracy of measurement is required.

What is Ladder Diagram and How to Draw a Ladder Diagram?

This is a continuation of the previous article (Introduction to PLC Ladder Programming) in PLC Programming series. Please go through introduction to Ladder Programming before reading this post. In this post we will about the what is ladder diagram and how to draw a ladder diagram.

Ladder Diagram

A very commonly used method of programming PLCs is based on the use of ladder diagrams. 

Writing a program is then equivalent to drawing a switching circuit. The ladder diagram consists of two vertical lines representing the power rails. Circuits are connected as horizontal lines, i.e. the rungs of the ladder, between these two verticals.

How to Draw a Ladder Diagram?

In drawing a ladder diagram, certain conventions are adopted:
  • The vertical lines of the diagram represent the power rails between which circuits are connected. The power flow is taken to be from the left-hand vertical across a rung.
  • Each rung on the ladder defines one operation in the control process.
  • A ladder diagram is read from left to right and from top to bottom. Figure shows the scanning motion employed by the PLC. The top rung is read from left to right. Then the second rung down is read from left to right and so on. When the PLC is in its run mode, it goes through the entire ladder program to the end, the end rung of the program being clearly denoted, and then promptly resumes at the start. This procedure of going through all the rungs of the program is termed a cycle. The end rung might be indicated by a block with the word END or RET for return, since the program promptly returns to its beginning.

  • Each rung must start with an input or inputs and must end with at least one output. The term input is used for a control action, such as closing the contacts of a switch, used as an input to the PLC. The term output is used for a device connected to the output of a PLC, e.g. a motor.
  • Electrical devices are shown in their normal condition. Thus a switch which is normally open until some object closes it, is shown as open on the ladder diagram. A switch that is normally closed is shown closed.
  • A particular device can appear in more than one rung of a ladder. For example, we might have a relay which switches on one or more devices. The same letters and/or numbers are used to label the device in each situation.
  • The inputs and outputs are all identified by their addresses, the notation used depending on the PLC manufacturer. This is the address of the input or output in the memory of the PL.

Introduction to PLC Ladder Programming

Programs for microprocessor-based systems have to be loaded into them in Machine code, that is in a sequence of binary code numbers to represent the program instructions. We can also use assembly language based on the mnemonics, e.g. LD is used to indicate the operation required to load the data that follows the LD, and a computer program called an assembler is used to translate the mnemonics into machine code.

Programming can be made even easier by the use of the so-called high level languages, e.g. C, BASIC, PASCAL, FORTRAN, COBOL. These use pre-packaged functions, represented by simple words or symbols descriptive of the function concerned. For example, with C language the symbol & is used for the logic AND operation.

However, the use of these methods to write programs requires some skill in programming and PLCs are intended to be used by engineers without any great knowledge of programming. As a consequence, ladder programming was developed.

Ladder Programming is a means of writing programs which can then be converted into machine code by some software for use by the PLC microprocessor.

This method of writing programs became adopted by most PLC manufacturers, however each tended to have developed their own versions and so an international standard has been adopted for ladder programming and indeed all the methods used for programming PLCs.

Ladder diagrams

As an introduction to ladder diagrams, consider the simple wiring diagram of an electrical circuit shown below,

The diagram shows the circuit for switching on or off an electric motor. We can redraw this diagram in a different way, using two vertical lines to represent the input power rails and stringing the rest of the circuit between them. 

Both circuits have the switch in series with the motor and supplied with electrical power when the switch is closed. The circuit shown above is termed a ladder diagram.

In ladder diagram the power supply for the circuits is always shown as two vertical lines with the rest of the circuit as horizontal lines. 
  • The power lines, or rails as they are often termed, are like the vertical sides of a ladder with the horizontal circuit lines like the rungs of the ladder. 
  • The horizontal rungs show only the control portion of the circuit, in the case shown in the figures above, it is just the switch in series with the motor. 
  • Circuit diagram often show the relative physical location of the circuit components and how they are actually wired. 
  • With ladder diagrams no attempt is made to show the actual physical locations and the emphasis is on clearly showing how the control is exercised.

Ladder Diagram Example

This figure shows an example of a ladder diagram for a circuit that is used to start and stop a motor using push buttons. 
  • In the normal state, push button 1 is open and push button 2 closed. 
  • When button 1 is pressed, the motor circuit is completed and the motor starts. Also, the holding contacts wired in parallel with the motor close and remain closed as long as the motor is running. 
  • Thus when the push button 1 is released, the holding contacts maintain the circuit and hence the power to the motor.
  • To stop the motor, button 2 is pressed. This disconnects the power to the motor and the holding contacts open. 
  • Thus when push button 2 is released, there is still no power to the motor. 
Thus we have a motor which is started by pressing button 1 and stopped by pressing button 2.

Different Types of ROM | PROM, EPROM, EEPROM, Flash EPROM & Mask ROM

What is ROM?

ROM is a type of memory that does not lose its contents when the power is turned off. For this reason, ROM is also called non volatile memory.

Different Types of ROM

There are different types of read-only memory, such as 
  1. PROM (Programmable ROM)
  2. EPROM (Erasable Programmable ROM)
  3. EEPROM (electrically erasable programmable ROM)
  4. Flash EPROM
  5. Mask ROM

PROM (programmable ROM) and OTP

PROM refers to the kind of ROM that the user can burn information into. In other words, PROM is a user-programmable memory. 

For every bit of the PROM, there exists a fuse. PROM is programmed by blowing the fuses. If the information burned into PROM is wrong, that PROM must be discarded since its internal fuses are blown permanently. For this reason, PROM is also referred to as OTP (One Time Programmable).

Programming ROM, also called burning ROM, requires special equipment called a ROM burner or ROM programmer.

EPROM (erasable programmable ROM) and UV-EPROM

EPROM was invented to allow making changes in the contents of PROM after it is burned. 

In EPROM, one can program the memory chip and erase it thousands of times. This is especially necessary during development of the prototype of a microprocessor-based project. 

A widely used EPROM is called UV-EPROM, where UV stands for ultraviolet. The only problem with UV-EPROM is that erasing its contents can take up to 20 minutes. 

All UV-EPROM chips have a window through which the programmer can shine ultraviolet (UV) radiation to erase the chip’s contents. For this reason, EPROM is also referred to as UV-erasable EPROM or simply UV-EPROM. 

Programming a UV-EPROM

To program a UV-EPROM chip, the following steps must be taken:
  1. Its contents must be erased. To erase a chip, remove it from its socket on the system board and place it in EPROM erasure equipment to expose it to UV radiation for 5—20 minutes.
  2. Program the chip. To program a UV-EPROM chip, place it in the ROM burner (programmer). To burn code or data into EPROM, the ROM burner uses 12.5 volts or higher, depending on the EPROM type. This voltage is referred to as Vpp in the UV-EPROM data sheet.
  3. Place the chip back into its socket on the system board.
As can be seen from the above steps, not only is there an EPROM programmer (burner), but there is also separate EPROM erasure equipment. The main problem, and indeed the major disadvantage of UV-EPROM, is that it cannot be erased and programmed while it is in the system board. To provide a solution to this problem, EEPROM was invented.

EEPROM (electrically erasable programmable ROM)

EEPROM has several advantages over EPROM, such as the fact that its method of erasure is electrical and therefore instant. as opposed to the 20-minute erasure time required for UV-EPROM. 

In addition, in EEPROM one can select which byte to be erased, in contrast to UV-EPROM, in which the entire contents of ROM are erased. 

However, the main advantage of EEPROM is that one can program and erase its contents while it is still in the system board. it does not require physical removal of the memory chip from its socket. In other words, unlike UV-EPROM, EEPROM does not require an external erasure and programming device.

To utilize EEPROM fully, the designer must incorporate the circuitry to program the EEPROM into the system board. In general, the cost per bit for EEPROM is much higher than for UV-EPROM.

Flash memory EPROM

Since the early 1990s, Flash EPROM has become a popular user-programmable memory chip. and for good reasons. 
  • First, the erasure of the entire contents takes less than a second, or one might say in a flash, hence its name, Flash memory. 
  • In addition, the erasure method is electrical, and for this reason it is sometimes referred to as Flash EEPROM. To avoid confusion, it is commonly called Flash memory. 

The major difference between EEPROM and Flash memory is that when Flash memory's contents are erased, the entire device is erased, in contrast to EEPROM, where one can erase a desired byte. 

Although in many Flash memories recently made available the contents are divided into blocks and the erasure can be done block by block, unlike EEPROM, Flash memory has no byte erasure option.

Because Flash memory can be programmed while it is in its socket on the system board, it is widely used to upgrade the BIOS ROM of the PC. Some designers believe that Flash memory will replace the hard disk as a mass storage medium.

This would increase the performance of the computer tremendously, since Flash memory is semiconductor memory with access time in the range of 100 ns compared with disk access time in the range of tens of milliseconds. For this to happen, Flash memory's program/erase cycles must become infinite, just like hard disks. 

Program/erase cycle refers to the number of times that a chip can be erased and reprogrammed before it becomes unusable. At this time, the program/erase cycle is 100,000 for Flash and EEPROM, 1000 for UV-EPROM, and infinite for RAM and disks. 

Mask ROM

Mask ROM refers to a kind of ROM in which the contents are programmed by the IC manufacturer. In other words, it is not a user-programmable ROM. 

The term mask is used in IC fabrication. Since the process is costly, mask ROM is used when the needed volume is high (hundreds of thousands) and it is absolutely certain that the contents will not change. 

It is common practice to use UV-EPROM or Flash for the development phase of a project, and only afier the code/data have been finalized is the mask version of the product ordered. 

The main advantage of mask ROM is its cost, since it is significantly cheaper than other kinds of ROM, but if an error is found in the data/code, the entire batch must be thrown away. It must be noted that all ROM memories have 8 bits for data pins; therefore, the organization is x8.

Controlling a Servo Motor with Arduino

A servomotor is also defined as a rotary actuator that allows for very fine control of angular positions.

Many servos are widely available and quite cheap. Servos can drive a great amount of current. This means that you wouldn't be able to use more than one or two on your Arduino board without using an external source of power.

When do we need Servos?

Whenever we need a way to control a position related to a rotation angle, we can use servos.

Servos can not only be used to move small parts and make objects rotate, but can also be used to move the object including them. Robots work in this fashion, and there are many Arduino-related robot projects on the Web that are very interesting.

In the case of robots, the servo device case is fixed to a part of an arm, for instance, and the other part of the arm is fixed to the rotating part of the servo.

How to control servos with Arduino

There is a nice library that should be used at first, named Servo.

This library supports up to 12 motors on most Arduino boards and 48 on the Arduino Mega.

By using other Arduino boards over Mega, we can figure out some software limitations. For instance, pins 9 and 10 cannot be used for PWM's analogWrite() method (

Servos are provided in three-pin packages:
  • 5 V
  • Ground
  • Pulse; that is, control pin
Basically, the power supply can be easily provided by an external battery, and the pulse still remains the Arduino board.

Wiring Servo to Arduino

The following diagram is that of a servo wired to an Arduino for both power supply and control:

Firmware controlling one servo using the Servo library

Here is a firmware that provides a cyclic movement from 0 degrees to 180 degrees. 

Servo myServo; // instantiate the Servo object 
 int angle = 0; // store the current angle 

void setup() 

// pin 9 to Servo object myServo 

void loop() 

for(angle = 0; angle < 180; angle += 1) 


for(angle = 180; angle >= 1; angle -=1) 



We first include the Servo library header.

Then we instantiate a Servo object instance named myServo.

In the setup() block, we have to make something special. We attach pin 9 to the myServo object. This explicitly defines the pin as the control pin for the Servo instance myServo.

In the loop() block, we have two for() loops, and it looks like the previous example with the piezoelectric device. We define a cycle, progressively incrementing the angle variable from 0 to 180 and then decrementing it from 180 to 0, and each time we pause for 20 ms.

There is also a function not used here that I want to mention, This function reads the current angle of the servo (that is, the value passed to the lastcall to write()). This can be useful if we are making some dynamic stuff without storing it at each turn.

Electric Potential, Potential Difference and EMF

Just as a body raised above the ground has gravitational potential energy, similarly, a charged body has electric potential energy. 

When a body is charged, work is done in charging the body. This work done is stored in the body in the form of electric potential energy. The charged body has the capacity to do work by moving other charges either by attraction or repulsion. 

Quantitatively, electric potential is defined as under :

The electric potential at a point is the electric potential energy per unit charge.

Electric Potential,  V  =  (Electric potential energy) / Charge = W/Q

The SI unit of energy or work is 1 J and that of charge is 1 C so that SI unit of electric potential is 1 J/C which is also called 1 volt.

Thus when we say that electric potential at a point is 10 V, it means that if we place a charge of 1C at that point, the charge will have electric potential energy of 10 J. 
Similarly. if we place a charge of 2 C at that point, the charge will have electric potential energy of 20 J. Note that potential energy per unit charge (i.e electric potential) is 10 V.

Potential Difference

The difference in the potentials of two charged bodies is called potential difference (pd). 

Consider two bodies A and B having potentials of +5 V and +3 V respectively as shown in figure 1 below. Each coulomb of charge on body A has an energy of 5 Joules while each coulomb of charge on body B has an energy of 3 Joules. Clearly, the body A is at higher potential than body B.

If the two bodies are joined through a conducter [see figure 2 above], then eleetrons will flow from body B to body A ( The conventional electric current will be in opposite direction i.e. from A to B). 

When the two bodies attain the same potential, the flow of current stops. Therefore, we arrive at a very important conclusion that current will flow in a circuit if potential difference exists. No potential difference, no current flow. It may be noted that potential difference is sometimes called voltage.

Unit of Potential Difference

Since the unit of electric potential is volt, one can expect that the unit of potential difference will also be volt. It is defined as :

The potential difference between two points is 1 Volt if one joule of work is done in transferring l C of charge from the point of lower potential to the point of higher potential.

Consider points A and B in an electrical circuit as shown in figure above. Suppose VA = VB = 1 Volt. It means that 1 J of work will be done in transferring l C of charge from point B to point A. 

Alternatively, 1 J of work (or energy) will be released (as heat) if 1 C of charge moves from point A to point B. Note that volt is the unit of energy.

Concept of EMF and Potential Difference

There is a distinct difference between e.m.f. and potential difference. 

The e.m.f. of a device, say a battery, is a measure of the energy the battery gives to each coulomb of charge. Thus if a battery supplies 4 joules of energy per coulomb, we say that it has an e.m.f. of 4 volts. The energy given to each coulomb in a battery is due to the chemical action.

The potential difference between two points, say A and B, is a measure of the energy used by one coulomb in moving from A to B. Thus if potential difference between points A and B is 2 volts, it means that each coulomb will gave up an energy of 2 joules in moving from A to B.

Fundamental Differences between EMF and Potential Difference

The following are the differences between e.m.f. and p.d. :

  • The name EMF at first sight implies that it is a force that causes current to flow. But this is not correct because it is not a force but energy supplied to charge by some active device such as a battery.

  • EMF maintains p.d. while p.d. causes current to flow.

  • When we say that EMF of a device (e.g., a cell) is 2 V, it means that the device supplies an energy of 2 joules to each coulomb of charge. When we say that a p.d. between points A and B of a circuit (suppose point A is at higher potential) is 2 V. it means that each coulomb of charge will give up an energy of 2 joules in moving from A to B.

Cables for Three Phase Service

In practice, underground cables are generally required to deliver 3-phase power. For the purpose, either three-core cable or three single core cables may be used. 

For voltages upto 66 kV, 3-core cable (i.e., multi-core construction) is preferred due to economic reasons. However, for voltages beyond 66 kV, 3-core-cables become too large and unwieldy and, therefore, single-core cables are used. 

The following types of cables are generally used for 3-phase service :

  1. Belted cables — upto 11 kV
  2. Screened cables — from 22 kV to 66 kV
  3. Pressure cables — beyond 66 kV.

Belted Cables

These cables are used for voltages upto 11kV but in extraordinary cases, their use may be extended upto 22kV. Figure shows the constructional details of a 3-core belted cable. The cores are insulated from each other by layers of impregnated paper. Another layer of impregnated paper tape, called paper belt is wound round the grouped insulated cores. The gap between the insulated cores is filled with fibrous insulating material (jute etc.) so as to give circular cross-section to the cable.

The cores are generally stranded and may be of noncircular shape to make better use of available space. The belt is covered with lead sheath to protect the cable against ingress of moisture and mechanical injury. The lead sheath is covered with one or more layers of armouring with an outer serving (not shown in the figure).

The belted type construction is suitable only for low and medium voltages as the electrostatic stresses developed in the cables for these voltages are more or less radial i.e., across the insulation. However, for high voltages (beyond 22 kV), the tangential stresses also become important. These stresses act along the layers of paper insulation. 

As the insulation resistance of paper is quite small along the layers, therefore, tangential stresses set up leakage current along the layers of paper insulation (It is infact a leakage current but should not be confused with the capacitance current). The leakage current causes local heating, resulting in the risk of breakdown of insulation at any moment. In order to overcome this difficulty, screened cables are used where leakage currents are conducted to earth through metallic screens.

Screened Cables

These cables are meant for use upto 33 kV, but in particular cases their use may be extended to operating voltages upto 66 kV. Two principal types of screened cables are 
  1. H type cables 
  2. S.L. type cables.

H type cables

This type of cable was first designed by H. Hochstadter and hence the name.  Figure shows the constructional details of a typical 3-core, H-type cable. Each core is insulated by layers of impregnated paper. 

The insulation on each core is covered with a metallic screen which usually consists of a
perforated aluminium foil. The cores are laid in such a way that metallic screens make contact with one another. An additional conducting belt (copper woven fabric tape) is wrapped round the three cores. 

The cable has no insulating belt but lead sheath, bedding, armouring and serving follow as usual. It is easy to see that each core screen is in electrical contact with the conducting belt and the lead sheath. 

As all the four screens (3 core screens and one conducting belt) and the lead sheath are at earth potential, therefore, the electrical stresses are purely radial and consequently dielectric losses are reduced.

Advantages of H-type Cables
Two principal advantages are claimed for H-type cables. 
  • Firstly, the perforations in the metallic screens assist in the complete impregnation of the cable with the compound and thus the possibility of air pockets or voids (vacuous spaces) in the dielectric is eliminated. The voids if present tend to reduce the breakdown strength of the cable and may cause considerable damage to the paper insulation. 
  • Secondly, the metallic screens increase the heat dissipating power of the cable.

S.L. type cables

It is basically H-type cable but the screen round each core insulation is covered by its own lead sheath. There is no overall lead sheath but only armouring and serving are provided. 

The S.L. type cables have two main advantages over H-type cables. 
  • Firstly, the separate sheaths minimise the possibility of core-to-core breakdown. 
  • Secondly, bending of cables becomes easy due to the elimination of overall lead sheath. 
However, the disadvantage is  that the three lead sheaths of S.L. cable are much thinner than the single sheath of H-cable and, therefore, call for greater care in manufacture.

Pressure Cables

For voltages beyond 66 kV, solid type cables are unreliable because there is a danger of breakdown of insulation due to the presence of voids. When the operating voltages are greater than 66 kV, pressure cables are used. In such cables, voids are eliminated by increasing the pressure of compound and for this reason they are called pressure cables. 

Two types of pressure cables  are commonly used.
  1. Oil-filled cables 
  2. Gas pressure cables

Oil-filled cables 

In such types of cables, channels or ducts are provided in the cable for oil circulation. The oil under pressure (it is the same oil used for impregnation) is kept constantly supplied to the channel by means of external reservoirs placed at suitable distances (say 500 m) along the route of the cable. 

Oil under pressure compresses the layers of paper insulation and is forced into any voids that may have formed between the layers. Due to the elimination of voids, oil-filled cables can be used for higher voltages, the range being from 66 kV upto 230 kV. 

Oil-filled cables are of three types viz., 
  • Single-core conductor channel, 
  • Single-core sheath channel and 
  • Three-core filler-space channels.

Figure shows the constructional details of a single-core conductor channel, oil filled cable.
The oil channel is formed at the centre by stranding the conductor wire around a hollow cylindrical steel spiral tape. The oil under pressure is supplied to the channel by means of external reservoir. As the channel is made of spiral steel tape, it allows the oil to percolate between copper strands to the wrapped insulation. The oil pressure compresses the layers of paper insulation and prevents the possibility of void formation. 

The system is so designed that when the oil gets expanded due to increase in cable temperature, the extra oil collects in the reservoir. However, when the cable temperature falls during light load conditions, the oil from the reservoir flows to the channel.

The disadvantage of this type of cable is that the channel is at the middle of the cable and is at full voltage w.r.t. earth, so that a very complicated system of joints is necessary.

Figure given below shows the constructional details of a single core sheath channel oil-filled cable. In this type of cable, the conductor is solid similar to that of solid cable and is paper insulated. However, oil ducts are provided in the metallic sheath as shown. In the 3-core oil-filler cable shown in Fig. 11.8, the oil ducts are located in the filler spaces. These channels are composed of perforated metal-ribbon tubing and are at earth potential.

The oil-filled cables have three principal advantages. 
  1. Firstly, formation of voids and ionisationare avoided. 
  2. Secondly, allowable temperature range and dielectric strength are increased. 
  3. Thirdly, ifthere is leakage, the defect in the lead sheath is at once indicated and the possibility of earth faults is decreased. 
However, their major disadvantages are the high initial cost and complicated system of laying.

Gas pressure cables

The voltage required to set up ionisation inside a void increases as the pressure is increased. Therefore, if ordinary cable is subjected to a sufficiently high pressure, the ionisation can be altogether eliminated. At the same time, the increased pressure produces radial compression which tends to close any voids. This is the underlying principle of gas pressure cables.

The construction of the cable is similar to that of an ordinary solid type except that it is of triangular shape and thickness of lead sheath is 75% that of solid cable. 

The triangular section reduces the weight and gives low thermal resistance but the main reason for triangular shape is that the lead sheath acts as a pressure membrane. The sheath is protected by
a thin metal tape. The cable is laid in a gas-tight steel pipe. The pipe is filled with dry nitrogen gas at 12 to 15 atmospheres. The gas pressure produces radial compression and closes the voids that may have formed between the layers of paper insulation. 

Such cables can carry more load current and operate at higher voltages than a normal cable. Moreover, maintenance cost is small and the nitrogen gas helps in quenching any flame. However, it has the disadvantage that the overall cost is very high.

Video: How Stepper Motor Works?

How does a robotic arm in a production plant repeat exactly the same movements over and over again? How can you move an automatic milling machine so precisely? It is available with stepper motor. 
What is special about the stepper motor. It is that it can control the angular position the rotor without closed loop feedback.
Let us see the working of a variable reluctance motor, that is the kind of step by simple step motor. Then we turn to hybrid stepper motor, a type of more accurate and used motor.  

How Star Delta Starter Works?

Star-Delta starting is frequently referred to as “Soft-starting” a motor. But what is soft about his starting method? Why is it used? What are the advantages? What are the disadvantages?

Let’s first analyse what Star Delta starting is! It will be explained by using an example motor.

What is Star Delta starting?

Star Delta starting is when the motor is connected (normally externally from the motor) in STAR during the starting sequence. When the motor has accelerated to close to the normal running speed, the motor is connected in DELTA. Pictures 1 and 2 show the two connections for a series connected, three phase motor.

The change of the external connection of the motor from Star to Delta is normally achieved by what is commonly referred to a soft starter or a Star Delta starter. This starter is simply a number of contactors (switches) that connect the different leads together to form the required connection, i.e. Star or Delta.

These starters are normally set to a specific starting sequence, mostly using a time setting to switch between Star and Delta. There can be extensive protection on these starters, monitoring the starting time, current, Voltage, motor speed etc.

The cost of the soft starter will depend on the number of starts required per hour, run-up time, Voltage, power rating, and protection devices required.

Video - Understanding STAR-DELTA Starter

If a picture is worth a thousand words, then a video is worth a million. Watch the video below for better understanding of Star-Delta Starter.

 Advantages of using Star Delta starting

The most significant advantage is the reduction in starting current. The starting current will to a large extent determine the size of the cables used, the size of the circuit breakers, the size of the fuses, as well as the transformers.

Requiring 67% less starting current can have a tremendous cost saving implication!

The most significant advantage of using Star-Delta starting is the huge reduction in the starting current of the motor, which will result in a significant cost saving on cables, transformers and switch gear.

Insulating Materials for Power Cables

We have already discussed about the construction of underground cables in the previous article. In this article we will discuss about the insulating materials used in underground cables.

Insulation is a non-conductive material, or a material resistant to the flow of electric current. It is often called a dielectric in radio frequency cables. Insulation resists electrical leakage, prevents the wire’s current from coming into contact with other conductors, and preserves the material integrity of the wire by protecting against environmental threats such as water and heat. Both the safety and effectiveness of the wire depend on its insulation.

The satisfactory operation of a cable depends to a great extent upon the characteristics of insulation used. Therefore, the proper choice of insulating material for cables is of considerable importance.

Properties Required for Insulating Materials

In general, the insulating materials used in cables should have the following properties :

  1. High insulation resistance to avoid leakage current.
  2. High dielectric strength to avoid electrical breakdown of the cable.
  3. High mechanical strength to withstand the mechanical handling of cables.
  4. Non-hygroscopic i.e., it should not absorb moisture from air or soil. The moisture tends to decrease the insulation resistance and hastens the breakdown of the cable. In case the insulating material is hygroscopic, it must be enclosed in a waterproof covering like lead sheath.
  5. Non-inflammable.
  6. Low cost so as to make the underground system a viable proposition.
  7. Unaffected by acids and alkalies to avoid any chemical action

No one insulating material possesses all the above mentioned properties. Therefore, the type of insulating material to be used depends upon the purpose for which the cable is required and the quality of insulation to be aimed at. 

The principal insulating materials used in cables are rubber, vulcanised India rubber, impregnated paper, varnished cambric and polyvinyl chloride.


Rubber may be obtained from milky sap of tropical trees or it may be produced from oil products. It has relative permittivity varying between 2 and 3, dielectric strength is about 30 kV/mm and resistivity of insulation is 1017 Ω cm. 

Although pure rubber has reasonably high insulating properties, it suffers form some major drawbacks viz., readily absorbs moisture, maximum safe temperature is low (about 38ºC), soft and liable to damage due to rough handling and ages when exposed to light. Therefore, pure rubber cannot be used as an insulating material.

Vulcanised India Rubber (V.I.R.)

It is prepared by mixing pure rubber with mineral matter such as zine oxide, red lead etc., and 3 to 5% of sulphur. The compound so formed is rolled into thin sheets and cut into strips. The rubber compound is then applied to the conductor and is heated to a temperature of about 150ºC. The whole process is called vulcanisation and the product obtained is known as vulcanised India rubber.

Vulcanised India rubber has greater mechanical strength, durability and wear resistant property than pure rubber. 

Its main drawback is that sulphur reacts very quickly with copper and for this reason, cables using VIR insulation have tinned copper conductor. The VIR insulation is generally used for low and moderate voltage cables.

Impregnated Paper

It consists of chemically pulped paper made from wood chippings and impregnated with some compound such as paraffinic or napthenic material. 

This type of insulation has almost superseded the rubber insulation. It is because it has the advantages of low cost, low capacitance, high dielectric strength and high insulation resistance. 

The only disadvantage is that paper is hygroscopic and even if it is impregnated with suitable compound, it absorbs moisture and thus lowers the insulation resistance of the cable. For this reason, paper insulated cables are always provided with some protective covering and are never left unsealed. If it is required to be left unused on the site during laying, its ends are temporarily covered with wax or tar. 

Since the paper insulated cables have the tendency to absorb moisture, they are used where the cable route has a few joints. [ Special precautions have to be taken to preclude moisture at joints. If the number of joints is more, the installation cost increases rapidly and prohibits the use of paper insulated cables.]

For instance, they can be profitably used for distribution at low voltages in congested areas where the joints are generally provided only at the terminal apparatus. 

However, for smaller installations, where the lengths are small and joints are required at a number of places, VIR cables will be cheaper and durable than paper insulated cables.

Varnished Cambric

It is a cotton cloth impregnated and coated with varnish. This type of insulation is also known as empire tape. 

The cambric is lapped on to the conductor in the form of a tape and its surfaces are coated with petroleum jelly compound to allow for the sliding of one turn over another as the cable is bent. 

As the varnished cambric is hygroscopic, therefore, such cables are always provided with metallic sheath. Its dielectric strength is about 4 kV/mm and permittivity is 2.5 to 3.8.

Polyvinyl chloride (PVC)

This insulating material is a synthetic compound. It is obtained from the polymerisation of acetylene and is in the form of white powder. 

For obtaining this material as a cable insulation, it is compounded with certain materials known as plasticizers which are liquids with high boiling point. The plasticizer forms a gell and renders the material plastic over the desired range of temperature.

Polyvinyl chloride has high insulation resistance, good dielectric strength and mechanical toughness over a wide range of temperatures. 

It is inert to oxygen and almost inert to many alkalies and acids. Therefore, this type of insulation is preferred over VIR in extreme enviormental conditions  such as in cement factory or chemical factory. 

As the mechanical properties (i.e., elasticity etc.) of PVC are not so good as those of rubber, therefore, PVC insulated cables are generally used for low and medium domestic lights and power installations.

It is cheap, durable and widely available. However, the chlorine in PVC (a halogen) causes the production of thick, toxic, black smoke when burnt and can be a health hazard in areas where low smoke and toxicity are required (e.g. confined areas such as tunnels). Normal operating temperatures are typically between 75ºC and 105ºC (depending on PVC type). Temperature limit is 160ºC (<300mm2) and 140ºC (>300mm2).

PolyEthylene (PE)

PVC and polyethylene are thermoplastic materials which means that they go soft when heated and harden when cooled. Most common thermoplastic material is PVC. Another one is polyethylene.

PE is part of a class of polymers called polyolefins. Polyethylene has lower dielectric losses than PVC and is sensitive to moisture under voltage stress (i.e. for high voltages only).

This compound is used most in coaxial and low capacitance cables because of its exemplary electric qualities. Many times it is used in these applications because it is affordable and can be foamed to reduce the dielectric constant to 1.50, making it an attractive option for cables requiring high-speed transmission. 

Polyethylene can also be cross-linked to produce high resistance to cracking, cut-through, soldering, and solvents. Polyethylene can be used in temperatures ranging from -65°C to 80°C. 

All densities of Polyethylene are stiff, hard, and inflexible. The material is also flammable. Additives can be used to make it flame retardant, but this will sacrifice the dielectric constant and increase power loss.


Thermosetting compounds are polymer resins that are irreversibly cured (e.g. by heat in the vulcanization process) to form a plastic or rubber:

XLPE (Cross-Linked Polyethylene)

XLPE has different polyethylene chains linked together (“cross-linking”) which helps prevent the polymer from melting or separating at elevated temperatures. Therefore XLPE is useful for higher temperature applications. 

XLPE has higher dielectric losses than PE, but has better ageing characteristics and resistance to water treeing. Normal operating temperatures are typically between 90ºC and 110ºC. Temperature limit is 250ºC.

EPR (Ethylene Propylene Rubber)

EPR is a copolymer of ethylene and propylene, and commonly called an “elastomer”. 

EPR is more flexible than PE and XLPE, but has higher dielectric losses than both. Normal operating temperatures are typically between 90C and 110C. Temperature limit is 250C.
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