Newton's Law of Viscosity gives the relationship between the shear stress and the shear rate or the velocity gradient of a liquid that is subject to a mechanical force.

The law is not universal in its application.  It is applicable to some liquids while other liquids do not obey it.

Newton's Law of Viscosity can be mathematically described as

t =  m dv/dy

where t = shear stress
           dv/dy is the velocity gradient

Liquids which obey Newton's Law of Viscosity are called Newtonian liquids.  Liquids whose response cannot be described by this law are called Non-Newtonian Liquids.

A Newtonian liquid is one in which the viscosity of the liquid is constant regardless of the stress applied.  That is, the viscosity of the liquid is the same if it is left alone or agitated vigorously.

In other words, Newtonian liquids obey Newton's law of viscosity which states that the shear stress is proportional to the rate of strain.

Water is a common example of a Newtonian liquid.  The viscosity of water is same whether it is still or in an agitated state.  In contrast, a solution of water and corn starch will be liquid when still but will become highly viscous when agitated.

Liquids in which the viscosity changes in response to the application of stress are called non-Newtonian liquids.

Rheology is the science of the flow of matter, particularly in the liquid or the semisolid state.  It is a subject of interest when designing pumps.  Rheology can help describe the behaviour of materials such as slurries and pastes.

The term Rheology is formed from two Greek works, "Rheo" for flow and "logia" for study. It is based on the assumption that "everything flows" or "Panta Rei", a statement attributed to Heraclitus, a philosopher in ancient Greece.Rheology can also be described as the study of the behaviour of matter when subject to  a deforming load.

In Rheology, flow is considered to be an irreversible deformation of matter as the original state of the fluid or the semi-solid cannot be attained again.

Rheology has applications in the food industry, in pharmaceuticals and in chemical industries.

Peristaltic pumps work by pushing a fluid through a tube.  The liquid is moved in a tube due to the formation of constrictions.  The impeller of the pump consists of two wheels which press against the tube and move the constrictions and the liquid.

The advantage of this pump is that the medium does not come in contact with the impeller.  Hence, this pump is widely used in the field of medicine.  Many life support systems use this pump as they handle body fluids such as blood.

Peristaltic pumps are also used in the food industry where the purity of the food is to be maintained.

Metering pumps are special pumps which are used to deliver a liquid at a specific flow rate in a specific time.  Metering pumps are used for dosing chemicals in industrial processes at a specific quantity.

The quantity and duration is often controlled by a computer.

Metering pumps are positive displacement pumps which can deliver output at high pressures. The pump is usually driven by a piston.

For sensitive liquids which require sterility and for corrosive liquids, the pump is ideal as it does not come in contact with the liquid.  A diaphragm acts as the interface between the piston and the liquid.

Metering pumps can also be designed as gear pumps.

A diaphragm is a special type of pump which uses a diaphragm to create pressure.  The diaphragm pump consists of a container with an inlet and outlet valves.  The top of the container is covered with the diaphragm.

The diaphragm is operated by an external mechanism which moves the diaphragm up and down.  When the diaphragm is pulled up, low pressure is created and the liquid is drawn inside the pump.  When diaphragm is pushed down, the fluid inside is pressurized and released through the outlet valve.

A salient feature of the diaphragm pump is that the fluid does not come in contact with the pump components.  Diaphragm pumps are used in the field of medicine where they can be used to construct artificial hearts and in dialysis.

They are also used widely in the food processing industry.

Another common application is in aquariums.  Aquarium pumps which provide air to aerate aquariums are a type of diaphragm pumps.

Plunger Pumps and Piston pumps work on the same principle.  They both draw a fixed volume of liquid through the inlet and pressurize the liquid and then release the liquid.

The difference lies in the construction.  In Piston pumps, the seal which prevents leakage moves with the piston.  In plunger, the seal is stationary and the plunger alone moves.

This enables the generation of very high pressure.  Plunger pumps can build pressures of the order of MegaPascals.  Plunger Pumps are used in high pressure applications.

A plunger pump is a positive displacement pump.  In a plunger pump, the plunger moves up and down a cylinder.

When the plunger moves up the cylinder, a low pressure is created in the cylinder.  The fluids is drawn by the low pressure by means of non return valves in the inlet of the pump.

When the plunger moves down, the increased pressure closes in the non-return valves in the inlet.  The liquid is pressurized as the plunger moves downwards.  This high pressure opens the outlet valve and the liquid is ejected at high pressure.

Plunger Pumps can be used at high pressure because of their robust and simple design.

Cavitation refers to the erosive action on the pump impeller and casing by the explosion of bubbles which form in the medium.

When the liquid passes through the impeller, the pressure drops in certain areas, this causes the liquids to drop below the vapor pressure.  As a result, bubbles are formed.  When these bubbles break, the energy released can remove small amounts of materials from the impeller and the casing.

Cavitation can cause the housing of the pump to fail.  In some instances, it can cause damage to the impeller.  A damaged impeller can get unbalanced and cause high vibration.  The outflow of the pump will also be affected

Cavitation in Pumps is classified into two types

Suction Cavitation
In suction cavitation, inadequate flow into the pump from the suction side due to blocked filters can cause low pressure in the eye of the impeller, this causes bubbles to form.  These bubbles pass to the discharge side where they encounter high pressure.  This causes the bubbles to implode releasing energy which can damage the impeller.

Discharge Cavitation
This occurs due to reduced outflow from the pump.  If the output of the pump is reduced, the pressure increases.  The liquid is unable to exit the pump and recirculates within the pump.  This high speed flow causes a vacuum between the liquid and the wall of the pump housing.  This causes bubbles to form and damage the inner surface of the casing.

In Pump design, shear sensitivity is an important factor.  Shear sensitivity refers to the response of a liquid when the shear forces inside a pump increase.  Shear forces increase as the pump speed increases.

Water is a shear insensitive liquid.  That is, even if the water is stirred fast it does not change its viscosity.  There are substances in which the viscosity of the liquid changes when shear forces are applied.

Viscosity is a fundamental property of the liquid.  The change in viscosity can affects other processes.  Hence, when pumps is selected, the shear sensitivity of the liquid should also be kept in mind.

Shear sensitive liquids are pumped using low speed pumps where the shear is minimal.

Centrifugal pumps are used widely to pump liquids such as water.  They cannot be used to pump viscous liquids.
Impeller of a screw pump (positive displacement)

The high viscosity of the liquid will cause increased friction losses between the impeller and the liquid.  The friction losses result in reduced head and the pump efficiency reduces.  There is a sharp reduction in the pump flow as the viscosity of the medium increases.

Besides, the flow inside the pump will not be streamlined and this increases the losses and loading in the impeller.

Hence, the centrifugal pump cannot be used to pump viscous liquids.  Viscous fluids are pumped by positive displacement pumps.

Velocity head is the difference in velocity of the liquid at the suction nozzle and the discharge nozzle.  This difference in velocity is caused by difference in the size of the nozzles.

If the discharge nozzle is smaller than the suction nozzle, the water will exit at a higher speed at the discharge nozzle than at the suction nozzle.

This difference in velocities creates a head.  This head is known as the Velocity Head in Pumps.  If the discharge and the suction nozzles are of equal size, the velocity head is zero.

Suction Head
The suction head refers to the difference in level between the water  in the sump to the centre line of the pump.  It is expressed in metres

The Delivery head
The Delivery head is the difference in level from the water level in the tank to the centre line of the pump.  It is expressed in metres

Static head
The difference between the water level of the tank to the water level of the sump when the pump is not running is called the Static head.  Its unit is metres.

The centrifugal pump is not self-priming.  The pump needs to be primed before it can function.  Priming involves filling the pump casing with water and removing the air inside.

When the centrifugal pump is started for the first time, the casing should be filled with water until it is filled completely, the delivery valve should be closed and the pump should be started.  When sufficient pressure is built, the delivery valve can be gradually opened.

Priming can be done manually for the first time by means of special openings provided in the pump to fill water.  For subsequent start ups, special arrangements can be made such as having a footer valve in the suction pipe.  This ensures that the pump remains flooded even when it is not running. Another method is to have a separate smaller pump to remove the air in the main pump.

In some installations, the pump is placed at lower level than the sump so that the pump is always flooded.

A Diffuser pump is on in which stationary vanes called diffusers are fitted radially facing the discharge from the impeller.  These vanes which gradually increase in space as the water moves outwards serve to convert the velocity of the water exiting the impeller into pressure.

Diffuser pumps do not require a volute.  Diffuser pumps provide a more efficient conversion of velocity input pressure.  The flow is more controlled and smoother.

For large capacity pumps, the diffuser model is smaller in size than the volute design.

The diffuser design is more efficient at peak loads while the volute design is efficient at off peak loads.

The Hydraulic Efficiency of a pump is the ratio of the useful hydrodynamic energy in the fluid to the energy supplied to the rotor of the pump.

Hydraulic Efficiency = Useful hydrodynamic Energy / Energy Supplied to the Rotor of the pump

The hydraulic efficiency is related to the number of vanes in centrifugal pumps and the depth of the channels. Optimizing the design of the impeller such that the friction between the liquid and the impeller surface is minimum can also improve hydraulic efficiency.

The Volumetric Efficiency of a Pump refers to the ratio of the total ouput of the pump to the total output of the pump without losses.

It can also be described as the ratio of the theoretical flow of the pump to the actual flow.  The theoretical flow of the pump can be calculated from the pump design data.  The theoretical flow of the pump is the product of the displacement per revolution and the number of revolutions.

The actual flow is measured by a flow meter which is connected to the pump output.

For instance if the displacement per revolution of a pump is 120cc/ rev and the pump has a speed of 1000 rpm, the theoretical flow rate would be 120x1000 = 120000cc or 120 litres per minute.

If the actual flow measured is around 115 litres per minute, the volumetric efficiency of the pump can be calculated as (115/120) x 100 = 95%

The volumetric efficiency can be reduced by reducing the leakages from the pump.

The volute refers to the part of the casing which expands along its curvature around the pump.  The primary function of the volute is to convert the kinetic velocity of the water developed by the impeller into pressure.  

The volute achieves this by gradually increasing in area along the pump as it approaches the discharge port.  This causes a gradual drop in velocity and an increase in pressure.  

The volute also serves to minimize circulation losses and balances the forces around the impeller. 

Water Extraction Pumps are pumps which are specially designed to extract water from sources such as underground wells, lakes and tanks.

These pumps are used in boreholes.  They are generally placed vertically.  They bring the water from the borehole to the surface.

A circulation pump is a pump which is used to circulate a liquid within a closed circuit.  They differ from other pumps in that they do not have to pump the fluid through a head.

Thus they require less power for the same pumping volume.  They are principally used in domestic heating systems,  They are also used in industrial applications where water is used for cooling.

Waterjet cutting is a method of cutting materials using a pressurized jet of water.  The water at high pressure up to 90000 psi.

The primary advantage of waterjet cutting is that not heat is generated in the process.  This the material which is cut does not undergo any transformation in the surface due to the process.  This is different from other mechanical cutting processes where heat is generated and a coolant is used.

The water jet can be incorporated into a CNC system which can cut the material according to a preset pattern.

Sometimes, an abrasive is added to the water in order to cut hard substances such as granite.

Video of a WaterJet

Pumps are manufactured in standard sizes by manufacturers.  The actual requirement for an individual application may be different from the standard size.

Thus, the pump can be further optimized to match the requirements.  This will improve pump efficiency and can save power.  One ways of doing this is by trimming the impeller.

Impeller trimming refers to the reduction in diameter of the impeller to suit the required head.  The impeller vanes and the shrouds are both machined to reduce the diameter.

The pump rotor is balanced at the time of manufacture.  However, during operation, the rotor dynamics can change.

This can be due to a variety of reasons such as corrosion of the impeller to uneven alignment with the motor.  The impeller can get corroded due to the medium and the environment.  Cavitation caused by pressure fluctuations can also damage the impeller.  Consequently, the rotor of the pump can get unbalanced.

This can result in excessive vibration.  If corrective action is not taken, further damage to the impeller and the pump can result. Damage to bearings is a principal result of excessive vibration.   The vibration levels of the pump should be monitored.  The impeller should be balanced periodically to bring the vibrations within the recommended limits.

Related Links

Wet Rotor Pumps are pumps in which the medium, that is, the liquid being pumped is used to cool the rotor.  The rotor is designed such that the water being pumped surrounds the rotor and provides a cooling effect.

The Wet Rotor, thus, does not need any external cooling arrangement such as a fan.  This reduces the power consumption as well as the noise.

The Wet Rotor Pumps also require lesser maintenance than ordinary pumps.

The impeller rotates within a pump casing to develop pressure.  To prevent leakage of the medium within the pump, the clearance between the pump and the impeller should be as small as possible.

This small clearance results in occasional contact between the rotating impeller and the pump casing.  This causes wear of the surfaces of both the pump and the impeller at the point of contact.

This can damage the impeller and the pump resulting in expensive maintenance.  To prevent this, wear rings are mounted on both the impeller and the pump casing.

Now, the wear occurs only in the wear rings and not in the actual surface of the impeller and the pump.   These wear rings are relatively inexpensive and are replaced at periodic intervals. 

Galling refers to a very severe type of wear in which microscopic particles of the material of one component is transferred to the other component.  This occurs due to severe mechanical wear which can cause the materials to fuse due to heating. 

This type of wear, generally,  occurs in components which have high load and which operate at low speed.  Poor lubrication is also a reason for galling.  Galling if normally found in components such as bearings, pistons of IC engines and in shafts. 

Hardened materials have a lower tendency to tall.  Soft materials are more vulnerable to galling.

Galling can be prevented by ensuring adequate lubricating and by choosing the materials of the mating surfaces carefully.  The materials can be from materials with different hardness levels such as different grades of steel. 

Bolts are particularly vulnerable to galling action.  Due to improper tightening techniques heat can be generated between the bolt and the thread.  Ensuring proper tightening techniques which prevent heat generation can prevent galling. 

Wear is the loss of material due to friction with another surface caused by motion.  Wear causes weakening of components and ultimately breakdown of materials. 

Wear resistance, therefore, is a highly desirable property in materials.  Wear can be minimized by selecting proper materials which can withstand the expected wear during the design stage itself.

Nevertheless, no matter how good the material selection is, there will always be some wear during operation.

Wear resistant coatings increase the wear resistance by forming a protective layer over the surface experiencing friction.  These coating can be applied as a spray or by means of a special forming process.

There are different types of wear resistant coatings. Ceramic Coatings, Phenolic coatings, Polymer and Epoxy coatings are some of the types of coatings available. 

The type of coating is chosen based on the working environment and the degree of protection desired.

In engineering, wear resistance refers to the property of materials to resist wear and tear during normal operation.  Wear resistance is a desirable property in moving parts such as bearings, wheels and tyres etc.

There are different mechanisms of wear.  Some of them are adhesive, erosive, cavitation and fretting.  Wear generally occurs as a combination of different mechanisms. 

All materials have wear resistance as a standard specification of materials.

Wear resistance is evaluated by specialized tests which simulate normal operation. 

By understanding the level of wear expected during normal operation, materials can be made wear resistant.  Wear resistance can be achieved by special manufacturing process and appropriate choice of materials. 

There are also other types of wear resistance methods such as special coatings and sprays.

The losses in the pump can be categorized into three categories

Mechanical Losses
The first type of losses is the mechanical loss.  This loss is due to friction between the pump components such as the impeller and the bearings.

Hydraulic Losses
These losses occur due to the energy expended in overcoming the friction between the fluid and the impeller surface

Volumetric Losses
Volumetric Losses occur due to re-circulation of the fluid within the pump.  This does not contribute towards the development of pressure.

Leakage Losses
Leakage Loss is caused due to leakage of the liquid from the pump due to reasons such as malfunctioning of seals, etc.

The Best Efficiency Point is the point at which the pump operates at the maximum flow efficiency.  The Best Efficiency Point is mentioned in the efficiency curves of the pump mentioned by the manufacturer. 

The pump should be operated at or near the Best Efficiency Point for maximum Efficiency.  The Best Efficiency Point in the pump is chiefly dependent on the design of the impeller and its speed.

Operating the pump at the BEP means that the prime mover has to spend less power to drive the  pump.  This improves the energy efficiency

Wire to Water Efficiency refers to the the overall efficiency of the pumping system.  The pumping system has a prime mover, usually a motor and the pump.  The Wire to Water efficiency is calculated as the energy supplied through the wire to the final quantity of water pumped by the system.

The Wire to Water Efficiency is obtained by multiplying the efficiency of the motor with the Best Efficiency point of the pump.

Consider a motor which has an efficiency of 95%.
Let us assume that the motor drives a pump which has a Best Efficiency point of .84, then the Wire to Water ratio of the pump will be .95x.84 = .978

Turndown Ratio is the ratio of the maximum output of the boiler to the minimum output at which the boiler can be operated efficiently.  For example, a boiler which has a maximum output of 30 Horsepower and minimum efficient output of 6 horsepower will have a turndown ratio of 5.

The higher the turndown ratio, the more flexibility one gets in the operation. 
If the turndown ratio is too low, the boiler may need to operate at a higher than desired output, switch off for a while and then switch on.  This increases the cycling frequency. 

Boilers operated on gas will have a turndown ratio of around 10-15.  Oil fired boilers can have a high turndown ratio.  The turndown ratio can be around 20.

Coal can also have a high turndown ratio. 

Electrically operated boilers have the lowest turndown ratio.  This is because, it is difficult to control the electrical input to a heater. 

Feed water is the water that is fed to the Boiler.  The feed water is a mixture of the condensed water also known as return water and fresh water (makeup water) to compensate for its losses.  The make up water is treated to remove its minerals. 

Chemical treatments are carried out to bring the conductivity, pH and alkalinity within specific limits.  The water is deaerated to remove dissolved oxygen and carbon dioxide.

The makeup water that is fed into the boiler depends on the total water losses.  Water leaks from the boiler and the pipelines as steam.  Some of the water is deliberately released via blowdown to remove the impurities which have settled at the bottom of the tank.

The Fusible Plug is a safety device in a boiler.  When the temperature of the boiler reaches abnormal levels, the plug fuses (melts) and provides an exit for the steam.  The jet of steam which exits the boiler will alert the operator to the high temperature.

A typical scenario is when the water level in the boiler falls below the safe limit.  This can cause the boiler to overheat and ultimately fail due to the metal walls softening. 

The plug is placed below the water level.  If the water level falls below the plug, the boiler starts to heat and the plug operates, releasing steam.

Boiler Mountings are the accessories which are mounted on the boiler for the effective control and safety of the boiler.  There are many accessories available today.

The mandatory boiler mountings are
1. Safety Valves - 2 Nos
2.  Water level Indicators - 2 Nos
3. Steam Stop Valve
4. Fusible plug
5. Blow Off Cock
6. Manholes and Mudholes

Mobile Boilers are boilers which are not fixed to any particular location.  They can be moved from place to place.  A common example is the locomotive boiler in railway engines.  These boilers move along with the Engine. 

Mobile boilers  are available in a variety of capacities.  They are also available in a variety of pressure ranges.  The Boilers can be used to provide both steam and hot water.

Mobile Boilers are mounted on a trawler which can be towed from one place to another.  These boilers are handy and can be installed at the locations where steam is required on a temporary basis.  They can be used for heating, sterilizing in plants and in hotels.  Many mobile boilers are powered by electricity.

Mobile Boilers come in both fire tube and water tube versions.

A boiler, as the name suggests, boils the water before turning it into steam at subcritical pressure - the pressure at which bubbles can form.  Steam generators, on the other hand, convert water into steam into steam without boiling at a super-critical pressure.

There are also constructional differences between a boiler and a steam generator.  A boiler contains many tubes which carry the water.  A steam generator has, generally,  only one tube in which the heating occurs. 

A waste heat recovery boiler is a device which recovers the heat produced by another industrial
process or equipment such as a genset, incinerator, furnaces, etc.  Many industrial processes produce heat which is rejected into the environment as waste.  This heat can be captured by the boiler and used to generate steam. 

The steam which flows through the exhaust is diverted by means of a diverter to pass through the boiler and then to the exhaust. 

This steam generated by the boiler can be used for a variety of functions such as to generate power, for other application such as in the case of a textile mill, for heating, etc. 

Other benefits of Waste Heat recovery Boilers are that they reduce pollution and the temperature of the exhaust gas.  This reduces the maintenance requirements of the exhaust systems. 

According to the standards of the ASME (American Society of Mechanical Engineers), Boilers can be classified on the basis of pressure into the following types. 

Low Pressure Boilers

Boilers with operating steam pressure not exceeding 1.021 atmosphere and a temperature of 394 K. 

Power Boilers

Power Boilers are those boilers whose pressure rating and temperature are above those of Low pressure boilers.

Miniature Boilers

These are boilers with very small capacity with a pressure less than 6.8 atmospheres and a gross volume less than 0.1415 cubic metres.

Steam Traps are devices which are used to release condensate which may form in steam lines.  Steam traps allow only the condensate and prevent the useful steam from escaping. 

Condensate should be removed from steam lines.  Condensates can cause hammering in the pipelines and corrosion.

In addition to Steam, Steam traps also release air and other gases. 

There are many different types of steam traps.  The most simple can be a nipple in a pipeline.  Since condensate (water) is heavier than steam, it will collect at the lowest point.  The trap opens once sufficient amount of condensate has collected. 

Water present in a steam system moves at a very high velocity driven by the steam pressure.  In some cases, the acceleration is even greater than that of steam.  When this water at high speed hits a fitting such as valve or a bend, violent impact resulting in noise or a pressure shock which travels through the system is produced.  This is called water hammering

Mild cases of water hammering can cause noise or vibration.  Extremely severe cases can result in fracture of the pipe. 

Condensate Hammering is more damaging than water hammering.  In condensate hammering, a pocket of steam surrounded by cooler water condenses into water.  This causes a rapid reduction in volume.  The water surrounding the pocket are thus drawn inwards and a collision occurs.  This results in a severe a rapid over pressurization which can easily damage gaskets, valves and other components.  The consequences can be very disastrous even fatal.

Hammering can be removed by preventing water from entering the steam lines.  Reducing Carryover, ensuring that the steam traps are functioning properly are some of the precautions.  The steam velocity in a boiler should not be allowed to exceed limits.  Steam velocity has a direct bearing on the intensity of the hammering. 

The piping should be as per design.  Sagging of the pipeline can cause water hammering.  Damaged insulation can cause condensate formation in the pipelines.  Insulation should be checked and replaced if damaged. 

Vacuum Pumps are used in Boiler Systems to evacuate the air from the piping.  Air reduces the heat transfer and acts as a thermal insulator.  It allows impedes the flow of steam.  The Vacuum pump is used to evacuate air from the pipelines. 

The gases present in the air such as oxygen and carbon dioxide can dissolve with air and cause corrosion.  Using a Vacuum pump can remove these gases and lower the dissolved gas level in the system preventing corrosion. 

Once air is removed from a system and a vacuum is created, steam requires a very low pressure to flow across the system. 

Since the pressure of the system decreases, the boiling point of water also decreases.  This results in a reduction of the fuel consumption.  Vacuum pumps can also be used for lifting the condensate  to the receiver.  This is necessary to prevent water hammering.

Vacuum pumps are specified in the amount of air they can move at a given vacuum. They can produce vacuums of the range of 5, 10 and 15 inches of Hg.

The Stroboscopic Effect in Fluorescent lamp is a phenomenon which causes running or moving equipment to appear stationary or appear to be operating slower than they actually are.

In an AC supply, the voltage drops 100 times a second to zero volts as the supply frequency is 50 Hz.  When a Fluorescent lamp is operating with an AC supply, the light intensity drops 100 times a second.  This flicker is not noticeable to the human eye due to the persistence of vision. 

When a worker in a factory observes a running machine, say a flywheel under the illumination of a fluorescent light, the flywheel may appear to be stationary or to be operating at reduced speed.  This can result in accidents and is highly dangerous.

A sewing machine whose needle moves up and down may appear to be stationary and the operator can prick the fingers.  These are some examples where the stroboscopic effect in the Fluorescent lamps can prove to be dangerous.  When using fluorescent lamps around rotating or moving machinery, two lamps powered by two different phases should be used.  This ensures that both the lamps do not flicker due to the zero crossing at the same time. 

If another phase is not available, a capacitor can be added in series to one lamp.  This ensures that there is a phase lag between the two lamps. The Stroboscopic effect can be eliminated by  using electronic ballasts where the supply to the lamps is of a very high frequency of the order of kiloHertz.

Optical Cables are used extensively in the field of telecommunication.  They have numerous advantages over conventional communication on wires.  They are efficient, quick and secure.  Optical Communication Cables are designed to provide high efficiency of transmission.  They are also designed to withstand the challenges of the external environment such as corrosion, heat and physical stress.
The Optical Cable has the following main components. 
The Core provides the pathway for the light to travel.  It is made of glass or transparent plastic material. 
The cladding is the layer that covers the core.  The function of the Cladding is to reflect the light which may come out of the core back into the core.  This results in Total Internal Reflection which ensures that there is no loss of the light signal. 
The Buffer is a coating which is outside the Cladding.  The Buffer serves to protect the optical fibre. 
Aramid Yarn Protection
The Aramid yarn which surrounds the buffer provides crush protection to the cable.
Protective Jacket
The protective jacket offers mechanical protection to the cable. 

A Ballast in a fluorescent light is necessary to get the light glowing.  The Ballasts generates a high voltage by means of the would coil which acts as an inductance.  When the starter interrupts the supply to the inductance, a powerful voltage is generated.  These are called magnetic ballasts. 

However, magnetic ballasts take time to get the lamps to start.  They also have an undesirable hum.  They also produce a flicker before the tube lights up continually.

Electronic eliminate the problem of initial flicker and hum.  They also reduce power consumption.
Electronic Ballasts work by converting the AC supply into DC first.  This rectified DC is then chopped by a chopper circuit to generate high voltages to start the discharge in the lamp.  The chopped AC waveform is t a very high frequency of the order of kHz.  This ensures that the flicker is reduced to a minimum.

The efficiency of the lamp is also increased.  Electronic Ballasts are 10% more efficient than magnetic ballasts. Electronic Ballasts can generate harmonics.  This, however, is insignificant as the amount is very small.

Conductivity in metals is due to the presence of free electrons in the atomic lattice.  When the metal is heated, the atoms in the lattice vibrate.  This results in reduced movement of the electrons as they hit against the vibrating atoms.

This results in an increase in resistance of the metals. 

Most Metals have positive temperature coefficient of resistance. There are , however, exceptions such as carbon and semiconductor metals such as Silicon and Germanium.  Some Alloys have zero temperature coefficient of temperature which means that the resistance does not change with increase of temperature.  Manganin is an example.

When a semiconductor is heated, the conductance increases and the resistance decreases.  Semiconductors, thus, have a negative temperature coefficient of resistance.
When heat is applied to a semiconductor material, the outermost electrons in the atom gain energy.  These electrons are able to overcome the attraction of the nucleus and leave the atom.  Thus they become free electrons which can conduct. 
The number of electrons increase exponentially and this results in a large drop in the resistance. 
Electronic devices will behave erratically above a certain temperatures.  Hence, all electronic devices such as laptops will have a safe temperature beyond which they cannot function. 
The effect of vibration of the atomic lattice on the mobility of the electrons is offset by the large numbers of electrons which enter the conduction band. 

In many instances, optical cables are needed to be jointed with one another.  Jointing in optical cables is different from jointing in electrical wires and cables.  The connection should be made such that there is minimum loss of the light energy. 
There are different methods of jointing optical cables
Fusion Jointing
In the method, the surface of the two optical fibres to be connected are heated and fused together.  This ensures that the light is conveyed from one fibre to another efficiently
Mechanical jointing
In this method, the surfaces of the ends are held firmly to make proper contact.  An gel or epoxy is used to match the different reflective indices of the materials.  The fibres are held together by mechanical splices. 

A cleaver is a tool used to cleave optical fibres prior to splicing.  Optical fibres should have a plain and clear surface when they are cut.  The cut should be perpendicular to the longitudinal axis of the fibre.  This is essential to avoid loss of light or distortion.    The cleaving tool is used to cleave the fibre. 
Cleaving Tools generally use diamond tips and blades to cleave the fibres.
Certain mechanical cleaving tools use a diamond blade to make a wedge in the fibre and the twist the fibre to produce a clean break.  The cleave angle should be perfectly 90 degrees. 
The blade in the cleaver may have to be replaced after a certain number of splices.  Most manufacturers provide replacement blades. 
The cleave made is examined by the splicing machine prior to fusion

Optical fusion Splicers are used to join two optical fibres using fusion.  The splicing is done by first heating the ends to be joined.

The fibres after being cleaved are fed into the splicer.  The two fibres to be joined are held against each other and the alignment is checked after which the fusion is done.  The heating is done by means of an electric arc or a laser.  The heating is done in about 15 seconds and the fusion is then carried out.  The device is battery operated. 

Splicers can be programmed for different types of fibre optic cables.  Most Splicers are portable and have a rugged construction.  The life of the heating electrodes is specified after which the electrode may have to be replaced. 

An optical time domain reflectometer is a device which is used to check the integrity of an optical fibre system detect and locate faults in optical cables. 
The optical time domain reflector sends out a series of optical pulses from one end of the cable.  The light which is reflected is analyzed.  This gives information about the state of the optical cable and its terminations.  If there is a drop in the quality of the light of if the distorted, it can indicate a problem such as a cut or an improper splice joint. 
The optical time domain reflectometer can also measure the attenuation of signals through the optical fibre.

The Q factor of an inductor is a very important parameter.  The Q factor tell us how close the inductor to an ideal inductor. 

An ideal inductor is an inductor which has no losses.  That is, its series resistance is zero.  It is not possible to construct an ideal inductor as all inductors are made of wires which have resistances. 

Q factor is the ratio of the inductive reactance to the series resistance of the inductor  at a given frequency.

Q= XL/ R

The Q factor is an crucial parameter when designing resonant circuits as it will affect the damping.  Higher the Q factor, higher is the efficiency of the inductor. 

Moulded chokes are chokes which are moulded in a polymer or synthetic material.  These chokes are used in applications such as Led lighting, automotive electronic components, mobile  phones etc. 
Moulded chokes are small in size and highly compact. 
These chokes have very small inductance values from 10 microhenries to 1000 micro henries.

A choke is an inductor which is used to block AC voltages from a circuit.  Thus, it "chokes off" the AC currents. Chokes usually have fixed values.

Chokes can be classified into
  • Audio Frequency chokes which function at the power and audio frequency and
  • Radio Frequency chokes which function at high frequencies.
Audio frequency chokes  have toroidal cores made of ferrite material. Radio frequency chokes have iron powder or ferrite materials.

Optical Fibre Metallic Wire
Not a lightning hazard as it is non conducting Can attract and transmit lightning
Lighter in weight Heavier in weight
Not affected by Interference Affected by interference
High data bandwidth Lower data bandwidth
Lower data loss Data loss is more
Faster data transmission Relatively slower data transmission
Unauthorised tapping of data is difficult Easier to tap data without authorization.
Difficult to terminate Easier to terminate
High initial cost Lower initial cost
Less affected by chemicals and pollution More prone to effects of pollution
No risk of sparking.  Hence, can be used in petroleum and chemical industries. Risk of sparking and fire.  Hence, cannot be used in hazardous environments. 

Terminating resistors are used in communication cables to prevent reflection of the transmitted signal.  The reflected signal can cause interference which may affect data transmission.  Hence, to prevent this resistances are connected in parallel.

The value of the impedance will match the wave impedance of the line.  Thus a communication line with an impedance of 120 ohms will have a 120 ohm resistor connected across it.  Short cables can function without terminating resistors. 

A Band Stop Filter is a filter which blocks a set of frequencies in a specific band and permits frequencies above and below that range to pass through. 

Functionally, the Band Stop filter does the opposite of the Band Pass Filter. 

The series LC circuit is connected in parallel.  At the resonant frequency of the LC circuit, the reactance is minimum.  Thus a specific frequency is shorted across the input and prevented from reaching the output.

The Band Stop Filter can be made by connecting a Low Pass Filter and a High Pass Filter in parallel. 

A Band pass filter combines the characteristics of the High Pass and Low Pass Filters. 

The Band pass Filter, as the name suggests, allows only signals of a particular band or range of frequencies to pass through.  All other signals are blocked or shorted.  Band Pass Filter

Band pass Filters can be made by connecting a high pass filter in series to a low pass filter or vice versa. 

A Band pass filter can be made by connecting an inductor in parallel to filter the low frequency components which lie below the desired frequency.  Another inductor in series will then block the high frequency components which lie above the desired frequency.

Likewise, the Band pass filter can also be made using capacitors as in the second figure.  Here, the first capacitor filters the high frequency components and the second capacitor in series blocks the low frequency components. 

High Pass FilterA High Pass Filter is a filter which permits high frequency signals to pass through and blocks only low frequency signals.  The high pass filter has a relatively simple construction.  The filter can be constructed by either providing a low impedance path to high frequency signals from the input to the output or by providing a low impedance path to low frequency

If a capacitor is connected in series between the input and the output, it will provide low impedance to high frequency signals and high impedance to low frequency signals.  High frequency signals alone will be able to pass the capacitor.

Alternatively, if an inductor is connected in parallel to the input, it will offer low impedance to low frequency signals which will get shorted across the input.  High frequency signals will alone reach the output.

A Low pass filter is a filter which permits only low frequency signals to pass through.  High frequency signals are blocked or shorted across the input.  The low pass filter offers low impedance to low frequencies and high impedance to high frequencies.Low Pass Filter

There are two ways of constructing a Low Pass Filter. 

The first method is to connect an inductor in series to the output.  The reactance of the inductor is so chosen that it offers high reactance to high frequency signals.  Thus, high frequency voltages are blocked.  At low frequency, the reactance is low and thus low frequency signals are allowed to pass.

Another method is to connect a capacitor in parallel to the input.  The capacitor provides low reactance to high frequency signals which are shorted across the input.  The low frequency signals see a high reactance in the capacitor and they alone reach the output.  The series resistance serves to limit the current.

Air Termination Rods are used to provide lightning protection to parts of a building which protrude from the superstructure.  Air termination rods can fixed to terraces, beams, etc.  Air termination rods consist of a rod or any pointed surface which is connected to the lightning protection system of the building. 

Air Termination Rods are also used for equipment which are kept in exposed flat surfaces such as solar panels and pipelines. 

Air Termination Rods may need to be supported against winds by means of suitable support structures.

Bonded Connections are used to connect a conductor to another body such as a pipe, gate, door or a railing.  All metallic objects in a building or a facility need to be connected to the earth.  This is necessary to prevent accidental voltage getting induced in them due to contact

Bonded Connectors are usually galvanized to prevent corrosion.  They have flat surfaces which increase the contact area with the object to be bonded.  They are made of a material similar to the conductor material such as copper or aluminium. 

Voltage limiting devices are used in Electric Traction systems, to prevent dangerous voltages from appearing the insulated tracks and the earthed components of the installation. 

Overvoltages can occur due to lightning or due to short circuits.  Voltage limiting devices typically use a MOV (metal oxide varistor) and and air gap mechanism to conduct the high voltage impulse to the ground.

If case of minor overvoltages, the MOV operates and diverts the surge.  It then returns to its normal non-conducting state.  In case of severe overvoltages, a permanent short circuit occurs between the protective electrodes and the device has to be replaced.

Recurrent Surge Oscillation is done on the windings of large generators such as turbo alternators.  The Recurrent Surge Oscillation Test (RSO) helps identify shorts in the winding.

Shorts in the winding occur as the insulation between turns deteriorates and fails.  Shorts can cause localised heating and arcing which can further damage the alternators.  Shorts can also become earth faults in course of time.  Multi-turn shorts can also result in a drop in the voltage.

The Recurrent surge oscillation tests is done by sending voltages of low voltage and high frequency through the winding and checking the waveform at the other terminal.  If the waveform has suffered any distortion, it may indicate an abnormality.  The waveform can give information such as the location of the fault and its severity.

Some short circuits may not be obvious when the rotor is at rest.  The conductors will come in contact with each other only during the running condition, due to the centrifugal force.  To identify such faults, the rotor is made to rotate and the test is conducted.

The Half Wave Rectifier functions using a single diode.  It rectifies only half of the sine wave.  Half Wave Rectifier

During the positive half cycle, the diode D1 is in forward bias and current flows to the load.  In the negative half cycle, when the voltage is applied in the opposite direction, the diode is in reverse bias and no current flows.

The Half wave rectifier is a simple device which requires only one diode to be put in series with the load.

The half wave rectifier has a low power output. 

Besides, the ripple content of the rectified DC supply is very high.  This can damage the loads.

It has very few applications and is only used in emergencies as a temporary measure.

Full Wave Rectifier with Centre Tapped Transformer

This Full Wave rectifier has only two diodes.  Each diode conducts during one half cycle.  In the first half cycle, diode D1 is forward bias and the current flows into the load and returns through the centre tap of the transformer. 

During the negative half cycle, diode D2 is in conduction.  The current flows through the load in the same direction and returns through the centre tap.  Thus the current is in the same direction through the load.

This means of rectifier has generally been replaced by the bridge rectifier as the centre tap is not always available.

Controlled Rectifiers are rectifiers which have thyristors in place of a diode.  A thyristor is a three terminal device which can be switched on by applying a suitable gate voltage.  Controlled Bridge Rectifier

In a Controlled Rectifier, generally two of the diodes are replaced with a thyristor.  The thyristor enables the enables switching on the output of the rectifier at the desired time.  This allows the output voltage and current to be controlled.

A rectifier with two thyristors is called a half controlled rectifier while a rectifier where all the diodes have been replaced with thyristors is called a fully controlled rectifier.

Controlled Rectifiers are also known as converters.

A Bridge Rectifier is a very popular and widely used circuit.  The Rectifier converts an AC supply into a DC supply.  The circuit requires only four diodes. 

The four diodes operate two at a time.  That is, during the positive half cycle, diodes D1 and D4 are in forward bias and conduct.   Diodes D2 and D3 are in reverse bias. Circuit Diagram Bridge Rectifier

In the negative half cycle of the AC supply, diodes D2 and D3 are in conduction while D1 and D4 are in reverse bias. 

Thus the current from the rectifier flows in only one direction.

Bridge rectifiers are used in Alternators for excitation of the field.  They are also used in welding machines for DC welding and in battery Chargers

Bridge Rectifiers are also used in measurement circuits to measure the amplitude of an alternating signal. 

Controlled rectifiers contain thyristors instead of diodes.  This enables control of the output supply.

The Class B Chopper is typically used in applications which require transfer of power from the load to the source.  An example would be regenerative braking in trains, where the power from the driving motor is sent to the power mains.  This is also known as inverting operation. Class B Chopper_Circuit_Diagram

In the Class B chopper, the output voltage is positive while the output current is negative.

In a class B chopper, a diode, in reverse bias,  blocks power from the source to the load.  The chopper is connected parallel to the load and the source.  The load voltage is the back-emf of the winding of a DC motor.  When the chopper is in the ON condition, the current due to the back-emf flows through the inductance and the resistance through the chopper.  The diode does not conduct as the voltage across the chopper is zero as the chopper is 'ON'. No Current flows into the source.Chopper B Graph

When the chopper is switched OFF, the voltage across the chopper increases and this biases the diode in the forward direction.  The diode conducts and the power reaches the source.  The source may be a battery or any other power source. 

There are wide range of choppers which are used for different applications.  These circuits differ in the voltage level, method of functioning and the output waveform.

Choppers can be classified into the following types

Step Up or Step Down Choppers

Step up Choppers, as the name suggests, step up the voltage.  These choppers are used when the voltage has to increased to a higher level.

AC and DC Choppers

Choppers can be classified into AC and DC choppers depending on the supply

Circuit Operation

On the basis of Circuit Operation, Choppers can be classified into

  • First Quadrant
  • Second Quadrant and
  • Fourth Quadrant

On the Basis of Commutation

  • Impulse Commutated Choppers
  • Voltage Commutated Choppers
  • Current Commutated Choppers
  • Load Commutated Choppers

Depending on the Direction of Current

Class A

Class B

Class C

Class D and

Class E

In recent times, Choppers are usually classified based on their application such as switched mode power supplies, Class D Electronic Amplfiers, etc.

A Chopper is an electronic circuit which controls or reduces a dc supply.  Their function can be compared to an ac transformer.  In an AC transformer, voltage is controlled by changing the turns ratio of the transformer.  In a chopper, voltage is varied by connecting and disconnecting the load from the source many times in a second. 

The chopper is essentially a switching circuit which switches off and on many times.  The output of a chopper is a square wave form while the input is a unidirectional dc waveform. 

Choppers can be used in motor speed controls.  They are increasingly being used in electric automobile technology. 

Choppers are used widely in Electronics in circuits in solar power conversion, speed control of motors in the industry.  They are used to reduce DC voltage to different levels in machines and other electronic equipments

Choppers have high efficiency and can be designed to have very fine control.

Electrical conduction in materials occurs due to the free electrons which drift about the atomic lattice.  In an atom, the electrons in the outer most orbit are called the valence electrons.  If the electrons have sufficient energy , they can break free of the atom and flow through the lattice when a voltage is applied.

If the energy levels are graphically represented, we will get a band diagram.

In the Band Diagram, there is the box representing the Conduction band and the box representing the valence band. 

Valence Band

The Valence band is the range of energy levels of the electrons in the outermost orbit of the atom. 

Conduction Band

The conduction band is the range of energy levels all electrons which are involved in conduction. 

In conductors, the valence and the conduction bands overlap.  In  Insulators, the valence and conduction bands are far apart. 

In semiconductors, the distance between the valance and the conduction bands are small.  When external energy in the form of heat or light is applied to the semiconductors, the electrons get excited and jump from the valence to the conduction band. 

The difference between the valence and the conduction band is called the energy gap.

Germanium is used in specific applications such as communication, spectroscopy, etc.  They have largely been replaced with silicon.  Germanium diodes are more expensive compared to silicon.

Germanium diodes have a lower forward bias voltage compared to silicon 0.15 volts.  This enables the use of the Germanium diode at low voltages where silicon cannot be used.

Germanium is also used in photoelectronics application.  Germanium diodes have a smaller band gap  0.66 eV.  This means that the electrons can be excited even by near-infrared radiation. 

Germanium is used in solar cells to capture the energy in near-infrared regions of the light spectrum.  Germanium based sensors are used spectroscopy to detect light radiation at low frequencies.

Silicon diodes are diodes in which the P and N materials are made of silicon.  Silicon Diode have a a forward bias voltage of 0.7 volts.  That is, the diode conducts when the voltage across the it in the forward bias is 0.7 volts or greater. 

They are the most widely used diodes in the industry.  Other diodes such as Germanium diodes are used at voltages below 0.7 volts.

The diode can withstand a voltage of 50V or more in the reverse direction.  This is known as the peak inverse Voltage.

Silicon is the most popular and widely used of the semiconductors.    There are many factors which have made silicon the material of choice in the world of electronics.

Some of the advantages are

  1. Silicon is abundant.  Hence, it is also economical.  The extraction process form its ore is cheaper when compared to other materials. 
  2. It is strong and easy to handle.
  3. It forms a nice stable oxide.
  4. Doping is easy.  Both P type materials and N type materials can be formed.
  5. It can be easily cut into wafers.
  6. It has good mechanical strength. Hence, designing circuits in silicon is easy.
  7. Silicon has fewer free electrons in room temperature.  This means that the collector cut-off current in transistor is lower than in other semiconductor materials, such as Germanium.

When a P type material and a N type material are brought in contact with each other, some of the holes in the P material migrate to the N region and combine with electrons.  Similarly, some of the electrons of  N material migrate to the P region and combine with holes. 

Thus, at the point of contact of the P and N materials, a layer is formed which has no majority charge carriers such as holes or electrons.  This region is called the depletion region as the region has been depleted of its charge carriers. 

The depletion region behaves almost like an insulator.  When a voltage exceeding the barrier potential is applied across the PN junction, current starts to flow.

Barrier Potential in a PN junction refers to the potential required to overcome the barrier at the PN junction.

When a P material and N material are brought in contact in a junction, some of the electrons of the N material near the junction cross over to the P material.  These electrons combine with the holes in the P material.  Similarly, the some of the holes of the P material near the Junction cross over to the N material and combine with the electrons.

The region in the contact area is thus depleted of holes and electrons.  This region is called the Depletion Layer.    The majority charge carriers are absent in this region.  This region almost becomes like an insulator.  Thus, there is no conduction after the depletion layer is formed.

For current to flow through this layer, a specific voltage has to be exceeded.  This is known as the barrier potential.  When an external voltage greater than the barrier potential is applied, the PN junction conducts.  

P type Materials

The P type material is obtained when a semiconductor is doped with a trivalent impurity such as Aluminium or Boron. P type material is a material which has holes as its majority carriers.  Electrons are the minority Charge Carriers in P type materials.  When a trivalent impurity is added to the crystal lattice of a semiconductor, there is a vacancy for every impurity atom added.  This vacancy is called a hole.

N type Materials

N type materials are made when a semiconductor is doped with a pentavalent impurity.  A pentavalent impurity is one whose atom has five electrons in its outermost orbit (valence electrons).   Examples of pentavalent impurities are Phosphorous, Antimony, Bismuth.  In an N type Material, electrons are the majority charge carriers while holes are the minority charge carriers.

When a semiconductor is doped with a pentavalent impurity for every impurity atom added, there is a free electron.  These electrons are responsible for conduction.

When the PN junction is formed, there is movement of the charge carriers across the junction.  The electrons move from the N material across the junction into the P material.  The holes from the P material cross into the N junction.

This movement of charge carriers results in a current across the junction. 

This current is known as diffusion current.  This current occurs in the absence of potential.

Cable Sleeves are used to enclose a single wire or a group of wires.  Cable sleeves are used to arrange a set of wires going through a panel or to an equipment like a motor.  Cable sleeves are made of different materials from braided metals to rubber and even kevlar. 

Sleeves serve to protect the wires and their insulation from sharp edges.  They can protect wires from UV radiation, moisture, oil and temperature.  cable sleeve

In automobiles, special heat resistant sleeves are used to protect the wires from hot surfaces. 

Teflon sleeves have great cut resistance.  Nylon sleeves have great abrasion resistance. 

Sleeves are available in two broad categories depending on the method of installation. 

The first is the slit sleeve which is cut and the wires are inserted into the sleeve.  The second is the wrap-around, side entry type of sleeve. 

Heat Shrinkable Sleeves

Some sleeves are heat shrinkable.  Heat shrinkable sleeves are made of a polymer which contracts when heat is applied. A stream of hot air from a blower is passed on the sleeve.  This results in the sleeves shrinking and wrapping the wires.  

Electrical Safety Mats are important safety equipment.  Safety mats protect personnel from electric shock by providing an insulated surface to work on.  If the worker comes in contact with a live conductor by mistake, he will not get an electric shock as his feet are insulated from the ground by the safety mat.Electrical Insulation Mats

Safety mats come in different colours.  They are usually made of rubber.  The surface of the mats is ribbed to provide an anti-slip surface to workers. 

Safety mats are also designed to resist aging and ozone. 

Safety mats should also be age and fire resistant.

Safety mats come in different voltage ratings.   They should also be resistant to acids and oils. 

The following is the list of mats and their working voltages


Class of Mats Working Voltage Colour
0 1000 Red
1 7500 White
2 17000 Yellow
3 26500 Green
4 36000 Orange


The most common class of safety mats is the type 0 which is rated for a working voltage of 1000 V.

A synchronous phase modifier is a synchronous motor which is not connected to any load.  The motor can be made to behave like an inductor or a capacitor by decreasing or increasing the excitation. 

Synchronous Phase modifiers are used to regulate the voltage in transmission lines.

Series Capacitors Shunt Capacitors
Used to reduce the line reactance Has no impact on the line reactance
Voltage rise only occurs across terminals.  Can be used to raise the line voltage
No effect on system stability Improves system stability
Does not affect the power factor Used to improve power factor

The ratio of capacitive reactance to the inductive reactance in a transmission line is called the percentage compensation of the Transmission Line.  The inductive reactance is mainly caused by the nature of the load.  The capacitive reactance is caused by the line capacitances and by series capacitors, if any.

Submarine Power Cables are used to carry power underwater.  They are used to connect small islands with the mainland.  Sometimes, the route through the water is shorter than through land.  Submarine Cables are used for both AC as well as DC transmission. 

AC is used up to 80 km while DC is used for long distance transmission.  Submarine cables are mostly gas filled cables.  The cable laying is done by special ships.  XLPE cables fpr submarine transmission are also available.

AC cables can be laid as three separate cables or as a single 3 phase cable.

Offshore wind farms are connected to the mainland by submarine cables.   

Damage to these cables by shipping is a matter of concern.  The routes which contain submarine cables are specially marked in nautical maps.

Gas filled cables are used in many applications.  In these cables, the insulation is filled with gas, usually nitrogen.  The inert gas fills up the voids which may form in the insulation.  There are low pressure and high pressure variants in this type of cable. 

The gas is filled in the insulation and is enclosed by means of a sheath.  Since Nitrogen is easily available, any losses can be easily replaced during installation. 

However, the gas has a low dielectric strength.  It cannot be used at very high voltages.  Gas filled cables can be used up to 275 kV.

Oil Filled cables have almost replaced Gas filled cables in recent times.  However, there are situations where oil filled cables cannot be used such as undersea applications and in hilly terrain.

High Voltage Dielectrics are used for insulation of cables at high voltages.  These dielectrics are subjected to very high stresses and temperatures.  They also have very demanding reliability requirements. 

Some of the common properties of High Voltage Dielectrics are

  1. Dielectrics should be flexible as this reduces the bending radius.  A lower bending radius enables cables to take turns in small spaces. 
  2. The Permittivity should be low so that the charging current is as low as possible
  3. The power factor of the dielectric should be low so that the heat generated is minimum.
  4. The Impulse strength of the insulation should be high.

Impregnated Cables are cables which have insulation which is impregnated with an insulating material.  For instance Paper insulation can be impregnated with oil or synthetic fluid with good dielectric properties.  The impregnated insulation is enclosed in a lead sheath to prevent the ingress of water. 

Some  cables have insulation which requires the insulating compound to be maintained at a particular pressure.  This may require the installation of pumps at periodic locations. 

Oil impregnated cables have special channels for the flow of oil.  The oil which is pressurized fills up any void which may develop in the insulation. 

Modern Impregnated Cables do not require pumps or any pressurizing devices. 

Oil impregnated cables can have their own cooling systems if used at voltages above 525 kV

Dust in electrical Equipment can affect the operation of the equipment.  Dust can result in tracking which causes current to flow over the insulation eventually leading to failure.  Dust on the surfaces of conductive parts such as breaker poles or relay contacts can affect the conductivity and cause heating and voltage drops

Dust when combined with oil or water in the atmosphere can form acids which can corrode and weaken the insulation. 

A layer of dust on the surface of an equipment can affect the heat transfer and prevent adequate cooling. 

Hence, equipment which are to be installed in dust prone environments should be adequately dust proof.  Panels should have filters which prevent the entry of dust.