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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.

Relay Timing Characteristics | Instantaneous, Inverse Time and Definite Time Lag Relays

An important characteristic of a relay is its time of operation. By ‘the time of operation’ is meant length of the time from the instant when the actuating element is energised to the instant when the relay contacts are closed. 

Sometimes it is desirable and necessary to control the operating time of a relay. For this purpose, mechanical accessories are used with relays.

Instantaneous Relay

An instantaneous relay is one in which no intentional time delay is provided. 

In this case, the relay contacts are closed immediately after current in the relay coil exceeds
the minimum calibrated value. 

Figure shows an instantaneous solenoid type of relay. Although there will be a short time interval between the instant of pickup and the closing of relay contacts, no intentional time delay has been added. 

The instantaneous relays have operating time less than 0·1 second. The instantaneous relay is effective only where the impedance between the relay and source is small compared to the protected section impedance.

The operating time of instantaneous relay is sometimes expressed in cycles based on the power system frequency.
e.g. one-cycle would be 1/50 second in a 50-cycle system.

Inverse Time Relay

An inverse-time relay is one in which the operating time is approximately inversely proportional to the magnitude of the actuating quantity. 

Figure shows the time-current characteristics of an inverse current relay. At values of current less than pickup, the relay never operates. At higher values, the time of operation of the relay decreases steadily with the increase of current. 

The inverse-time delay can be achieved by associating mechanical accessories with relays.

  • In an induction relay, the inverse-time delay can be achieved by positioning a permanent magnet (known as a drag magnet) in such a way that relay disc cuts the flux between the poles of the magnet. When the disc moves, currents set up in it produce a drag on the disc which slows its motion.

  • In other types of relays, the inverse time delay can be introduced by oil dashpot or a timelimit fuse. Figure shows an inverse time solenoid relay using oil dashpot. The piston in the oil dashpot  attached to the moving plunger slows its upward motion. At a current value just equal to the pickup, the plunger moves slowly and time delay is at a maximum. At higher values of relay current, the delay time is shortened due to greater pull on the plunger.

The inverse-time characteristic can also be obtained by connecting a time-limit fuse in parallel with the trip coil terminals as shown in figure below. 

The shunt path formed by time-limit fuse is of negligible impedance as compared with the relatively high impedance of the trip coil. Therefore, so long as the fuse remains intact, it will divert practically the whole secondary current of CT from the trip coil. 

When the secondary current exceeds the current carrying capacity of the fuse, the fuse will blow and the whole current will pass through the trip coil, thus opening the circuit breaker. The timelag between the incidence of excess current and the tripping of the breaker is governed by the characteristics of the fuse. Careful selection of the fuse can give the desired inverse-time characteristics, although necessity for replacement after operation is a disadvantage.

Definite Time Lag Relay

In this type of relay, there is a definite time elapse between the instant of pickup and the closing of relay contacts. This particular time setting is independent of the amount of current through the relay coil ; being the same for all values of current in excess of the pickup value. 

It may be worthwhile to mention here that practically all inverse-time relays are also provided with definite minimum time feature in order that the relay may never become instantaneous in its action for very long overloads.

Substation Earthing System Design

We have already given an introduction to substation earthing system in an article published earlier. Today we are going to discuss about the design of substation earthing system.

Before 1960s the design criterion of substation earthing system was "low earth resistance.” (Earth Resistance< 0.5 ohms for High Voltage installations). 
During 1960s, the new criteria for the design and evaluation of Substation Earthing System were evolved particularly for EHV AC and HVDC Substations. The new criteria are :
  1. Low Step Potential 
  2. Low Touch Potential 
  3. Low Earth Resistance.
The conventional “Low earth resistance criterion” and Low Current Earth Resistance Measurement continues to be in practice for Substations and Power Station upto and including 220 kV.

The parts of the Earthing System include the entire solid metallic conductor system between various earthed points and the underground earth-mat. The earthed points are held near-earth potential by low resistance conductor connections with earth-mat.

Substation Earthing System Design

Underground Horizontal Earth Mesh (Mat/Grid)

The mesh is formed by placing mild steel bars placed in X and Y directions in mesh formation in the soil at a depth of about 0.5 m below the surface of substation floor in the entire substation area except the foundations. 

The crossings of the horizontal bars in X and Y directions are welded.

The earthing rods are also placed the border of the fence, surrounding building foundations, surrounding the transformer foundations, inside fenced areas etc. 

The mesh ensures uniform and zero potential distribution on horizontal surface of the floor of the substation hence low “step potential" in the event of flow of earth fault current.

Earthing Electrodes (Spikes)

Several identical earth electrode are driven vertically into the soil and are welded to the earthing rods of the underground Mesh. Larger number of earth electrodes gives lower earth resistance.

  • The number of Earth-Electrodes (Spikes) Ns for soil resistivity 500 ohm meter and earth fault current Is is :
Ns = Is / 250 Amperes
i.e., approximately 250 Amp per spike, for soil resistivity of 500 ohm-meter.

  • The number of Earth-Electrodes (Spikes) Ns for soil resistivity 5000 ohm meter is 
Ns = Is / 500 Amperes
i.e., approximately 500 Amp per spike, for soil resistivity of < 5000 ohm-meter.
Is = Short Circuit level of the substation, A
    33 kV sub stations     : 25000 to 31000 A
    400 kV Substations   : 40000 A

    Earthing Risers

    These are generally mild steel rods bent in vertical and horizontal shapes and welded to the earthing mesh at one end and brought directly upto equipment / structure foundation.

    Earthing Connection

    Galvanized Steel Strips or Electrolytic Copper Flats or Strips/Stranded Wires (Cables) /Flexibles. 
    These are used for final connection  (bolted/welded/clamped) between the Earthing Riser and the points to be earthed. 

    For Transformer Neutral/High Current Discharge paths copper strips/stranded wires are preferred. 
    Galvanised Iron Strips/stranded wires are more common for all other earthing connections. The earthing strips are finally welded or bolted or clamped to the Earthed Point.
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