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Rubber insulating mats are used to protect workmen against electric shock.  These mats are placed in front of high voltage panels and bus bar panels.  Rubber Insulation mats provide insulation from the ground.  The Rubber mats should also provide slip protection.  These mats are available in different voltage ratings(classes 0 to 4). 

The classification is as follows.

Class 1  -    7000V
Class 2  -    17000V
Class 3  -    26500V
Class 4  -    36000V

They are also available as Type 1(elastomeric without ozone protection) or Type II(Type IIA is resistant to ozone while Type IIB is resistant to fire).  The design and construction of Rubber Insulation mats is governed by ASTM D178 (ASTM stands for American Society for Testing and Materials, it has now been renamed ASTM International).

The mats are usually provided with an eyelet with which they can be fastened to the floor.  The eyelet is made of a non insulating material.

Rubber mats are widely used in factories, power plants, switch yard.

Electrical Gloves are used for working on equipment which may or may not be live.  These gloves provide protection against electric shock.  They are usually made of rubber.

Wires or sharp edges in equipment may puncture holes in the electrical gloves and seriously compromise insulation.  Hence, a leather protective glove known as the  overglove should always be worn over the electrical gloves for mechanical protection and safety against cuts or punctures  Some electrical gloves have inner lining of cotton for comfort and to prevent irritation to the skin. 

It is important to ensure that the electrical gloves worn fit properly on the worker's hand.  Hence, measurements of the hand need to be taken prior to ordering.
Electrical Gloves have a specific voltage rating.  The voltage rating of the gloves must be checked against the voltage of the equipment where the work is carried out. 

Electrical gloves made of rubber are vulnerable to cracking due to the effects of ozone.  Ozone causes cracking in rubber.  If the gloves are to be used in installations near cities where there the levels of ozone are high due to pollution, it is necessary to check the gloves for ozone resistance

Electrical Gloves are classified as 
Class 00 - up to 500V AC
Class 0   - up to 1000V AC
Class 1   - up to 7500V AC
Class 2   - up to 17000V AC
Class 3   - up to 26500V AC
Class 4   - up to 36000V AC

Ozone resistance in Electrical Gloves is indicated by Type I or Type II.  Type I gloves do not have ozone resistance while Type II gloves are ozone resistant.
The Gloves are provided along with a test certificate.  All the gloves must be tested for insulation integrity once every six months.  Gloves with even minor damages such as small holes or cuts should be immediately discarded.  Gloves are tested for cuts and holes by an air inflation test.  Air is blown into the gloves by a special device.  Cuts and holes in the gloves can thus be detected by means of the leakage of air.

Besides, the gloves should be visually inspected every time before work. 

Gloves are an essential component of working with high voltage.  Some people find gloves cumbersome and inconvenient.  Nevertheless they are an important part of keeping oneself safe from electric shock, flash over or electrocution. 

Contactors fail due to a wide range of reasons.  Some of the common reasons are excess current flowing through the contacts.  High current can be either due to overload or due to short-circuit.  High current can cause the contacts to melt.  The electrodynamic forces during a short circuit can mechanically damage the contactor.

Another cause of contact failure is overvoltage.  Overvoltage causes high current to flow in the coil damaging it.  Chattering can be another reason for failure.  Continual chattering damages the contacts and causes arcing. 

Contactors should be properly sized keeping in mind their current closing and interrupting capacities. 

Only Genuine spare parts should be used.

Age is another reason for contactor failure.  The winding in the coil are bonded together with a varnish (encapsulation).  This prevents movement of the coil windings when current passes through the coil.  Age can cause these coils to crack or move causing the insulation to break.   

Temperature can also cause the contactor to fail.  Hence, adequate provision for ventilation needs to be provided.  The contactor should not be installed in placed which are too hot.

Power Quality too has an impact on the life of the contactor.  Transients, voltage and frequency fluctuations can cause the coil to get damaged. 

A corrosive environment which contains damaging chemicals or vapors can also cause damage to the contactor coil.

Mechanical shock or excessive vibration can also cause contactors to fail.  

Hot sticks are hand-held rods made of insulating material.  Hot sticks are used to work on electric equipment and conductors at very high voltages.  They allow the maintenance crew to conduct a wide range of activities on high power lines without the need to de-energize them.  
 
Hotsticks have special sockets on which a range of tools can be mounted and operated.  They can be used for tightening bolts on clamps, rewiring fuses, cutting trees which grow too close to the conductors.  These tools are made of fibreglass.  In earlier times, they were made of wood which had been specially treated with chemicals.

Power tools can also be used with Hot Sticks.  Hydraulic accessories are available which can operate the power tools.  These hydraulic accessories are self insulating.  Telescope models are also available which have poles which slide over one another. 

It is important that Hot sticks should be used only along with the regular personnel protective equipment such as gloves, shoes, etc.



Vibration in overhead conductors is a very serious issue.  Excessive vibration can result in conductor failure which can be catastrophic.  Vibrations should be kept within limits for safe operation of the transmission lines.

Vibration in Power Lines can be categorized into three types.
  1. Aeolian Vibration
  2. Gallop
  3. Simple swinging
Aeolian Vibration

These vibrations are caused by aerodynamic forces generated as the wind blows across the conductor.  The frequency of the vibration may receive a positive feedback from the conductor's natural vibration frequency.  This self-exciting vibration can cause cracks on the conductors due to  fatigue and can cause damage particularly where the conductors is fastened to the insulators by means of clamps

Dampers on conductors near the string insulator
This kind of vibration can be minimized by the use of dampers.  The most widely used damper is the stock bridge damper.  This damper consists of two weights which are fitted on either side of a cable. This shape is known as a "dog bone".   The cable is fastened to the main conductor by means of clamps. 

When the conductor vibrates, the weights fitted on either side vibrate and dissipate the vibrational energy. 

Gallop

These vibrations are low frequency and high amplitudes.  These vibrations are caused by the wind blowing over conductors which are not circular . Hence, conductors are designed to be circular to prevent gallop-type vibrations.  These vibrations can result in breaking of the conductor.  They can also result in flash over if the conductors come too close to each other during oscillations. 

Simple Swinging

Stockbridge Vibration Damper
This kind of vibration occurs in the horizontal direction as the conductors swing under the influence of wind.  This kind of swinging does not have any major impact.  However, it needs to be ensured that the that lines to not come too close to each other or the tower to cause a flash over.

Bundled Conductors are used in transmission lines where the voltage exceeds 230 kV.  At such high voltages, ordinary conductors will result in excessive corona and noise which may affect communication lines.  The increased corona will result in significant power loss.  Bundle conductors consist of three or four conductors for each phase.  The conductors are separated from each other by means of spacers at regular intervals.  Thus, they do not touch each other. 

Bundled conductors have higher ampacity (current carrying capacity see Article) as compared to ordinary conductors for a given weight.  This is due to the reduced influence of the skin effect (see Article). 

The reactance of bundled conductors is also lesser than single conductors.  However, bundled conductors experience greater wind loading than single conductors. 

Notice the guard wire on top of the tower
Shielding is a method of lightning protection used in High voltage Transmission lines.  Overhead transmission lines are particularly vulnerable to lightning strikes.  A lightning strike can cause disruption, damage to transmission equipment and generate transients which may damage substation equipment such as transformers.  This may result in considerable downtime. 

Shielding involves running a grounded wire above the line conductors.  The wire shields the line conductors.  Lightning which reaches the shielding wire are discharged to the ground.  This method of shielding minimizes lightning strikes on the power lines to a large extent, though, it does not eliminate the threat completely. 

As the shield wire may carry extremely high voltage in the event of a lightning strike, the shield wire is guided down the tower maintaining adequate clearance with the line conductors.  The shield wire is then connected to a dedicated earth pit.  The earthing resistance of the earth pit needs to be extremely low to enable the lightning charge to quickly get discharged to the earth.  Otherwise, this may result in backflashovers.

Shield wires are used only in high voltage lines as the small clearances in low voltage lines may result in backflashovers during lightning strikes.

Transient voltage surge Suppressors (TVSS) are protective devices used on the LV side.  They are generally used in installations having sensitive electronic circuit.

Transient Voltage surge voltage suppressors are voltage clamping devices which limit the transient voltage to a safe value.  Transient Voltage Surge suppressors are made of shunting elements which are sensitive to high voltage.  They are made in the form of gas discharge tubes, silicon avalanche diodes or metal oxide varistors.

Transient Voltage Surge Suppressors differ from Surge suppressors in that they are specifically designed for transients and not for normal surges.  Besides, Transient Voltage Surge Suppressors can only be used in the LV side whereas surge suppressors can be used at all voltage levels.

Since, Transient Voltage Surge Suppressors are connected between the line and the ground, the system should have an efficient earthing system for the Transient Voltage Protection to be effective.

Shackle Insulators are used in low voltage distribution lines. They are otherwise known as spool insulators.

Shackle Insulators are used at the end of distribution lines or at sharp
turns where there is excessive tensile load on the lines.  These insulators can be mounted either in the vertical or horizontal position. 

The Shackle Insulator is mounted axially.  The loading is on circumferential grooves in the insulator.  The conductor is secured in the groves by means of soft-bending wires.  The insulators are bolted to the cros-arm of the pole. 

Neodymium Magnets are the strongest of all permanent magnets available today.  Neodymium Magnets are widely used today in hard disk drives, motors and cordless equipment.  Neodymium Magnets are made from an alloy of Neodymium, Iron and Boron.  Neodymium is one of the Rare earth elements in the Periodic Table.  

Neodymium magnets have a high co-ercivity which means that it is difficult to demagnetize them.  They also have high magnetic field strength.  This makes them preferable over other magnets. 

Neodymium magnets are widely used in medical equipment.

However, Neodymium Magnets have a lower Curie Temperature around 80 deg. C, which is the temperature at which a magnet loses its magnetism or a ferromagnetic material becomes paramagnetic.  

Neodymium magnets are brittle and can shatter easily.  They can interfere with cardiac pacemakers. 

Their high strength can cause bodily injury as they can attract materials like nails, iron balls through the human body. When these magnets are broken, they shatter into dust which is combustible. 

They should not be brought near other magnets as they can be attracted with such a force that they shatter on impact.  Fingers caught between two neodymium magnets can get crushed.  These magnets should be kept out of the reach of children.  Children who accidentally swallow these magnets can be seriously injured or even killed.

Neodymium magnets are extremely corrosive and can get easily oxidised when exposed to air.  Hence, they are usually coated with a protective layer of zinc, epoxy or tin.


The Capacity of a battery is the quantity of energy a battery can store and deliver.  The capacity of a battery is usually indicated in terms of Ampere hours or Ah.  This is also known as the rated capacity of a battery.  The total energy stored in a battery can be calculated by multiplying the Ampere-hour rating with the battery voltage.  Thus a 10Ah battery with 110V can store an energy of 1100Wh or 1.1kWh.

The duration of a battery is thus dependent on its discharge rate.  Manufacturers provide discharge charts for their batteries.  Charging and discharging rates have a major impact on the life of a battery.  A battery that is discharged at a faster rate may not deliver the same amount of energy as a battery with a lesser discharge rate.
Some manufacturers specify the capacity of a battery as a function of the discharge rate.

Temperature, too, influences the capacity of a battery.  Batteries at high temperatures will have a better capacity that batteries loaded at low temperatures.  Extremely high temperatures, however, can cause damage to the battery and greatly reduce capacity.

In addition to these factors, age is also a factor that determines the capacity of a battery.  As a battery ages, its capacity reduces.   

With the advent of digital relays, earlier secondary outputs such as 5A and 1A were considered unnecessary for some applications.  Accordingly, current transformers with lower outputs were designed.  These outputs, usually in the mA range, are constructed by increasing the no of turns in the secondary. 

Since, these current transformers have a large number of secondary turns as compared with normal CTs, special voltage suppression devices are incorporated in order to limit the voltage to a safe value if the current transformer is accidentally open circuited. 

These current transformers have high accuracy.  They are also cheaper in cost and smaller in size.  The disadvantages of these Current transformers are that they poor phase shift performance and can perform only with specific dimensions of wires. 

mA Current Transformers are different from Current Transducers which employ current sensors to sense current and generate a 4..20 mA

The cores of Current Transformers are usually constructed of Iron.  Some CTs have an air core.  Iron Core CTs have higher power output. However, they have low accuracy.  They are vulnerable to both static and transient errors.

Air core CTs have high accuracy and exhibit linear characteristics.  Hence, they are also known as linear couplers. They also cannot get saturated.

However, the downside of Air core Current Transformers is that they have low capacity.  Hence, the cannot be used to power electromagnetic relays.  They can only be used for static relays.

Leakage Transformers are transformers where the magnetic flux of the secondary is loosely coupled to the flux of the primary.  This loose coupling ensures that the transformer can withstand short circuit conditions.

This is particularly required in the case of welding transformers, where high currents may occur if the welding electrode gets fused with the job.  Thus these transformers have high leakage inductance.

Leakage Transformers are also used in extra low voltage applications where short circuit conditions are expected.  The primary and secondary windings of a leakage transformers are adequately sized to withstand heavy currents in short circuit conditions.

Leakage Transformers are also known as Stray field Transformers.

Toroidal Transformers are transformers with a core that is constructed in the form of a toroid.  A toroid is a geometric shape which resembles a doughnut. 

This form of transformer construction has the advantage of having a higher efficiency in comparison with an ordinary transformer.  They also have lesser size and weight.  These transformers are widely used in audio amplifier circuits as they do not have very less mechanical hum. 

These transformers have very low leakage flux as the core, being circular eliminates airgaps.  Another advantage of the toroidal transformer is the low Electromagnetic Interference.  Since the windings are located outside the core, heat dissipation is also easier. 

An important precaution should be taken while fastening the Toroidal Transformer.  The fastening bolt, which usually passes through the centre of the toroidal transformer should only be fastened at one end.  If the bolt is fastened at two ends to metallic objects.  It may act as a secondary causing a huge current in the primary and can even cause fire as the bolts melts under the high induced current. 

The downside of Toroidal Transformers is its high manufacturing cost.  Hence, they are usually made only upto a few kVA. 

Toroidal Current transformers are widely used for low and medium voltage applications.

Ferrites are ceramic compounds which are composed of Iron (III) oxide (Fe2O3).  Ferrite cores are used in Transformers where the supply voltage has a high frequency, such as in electronic circuits.

Steel, contain silicon,  which is used for low frequency transformers cannot be used in high frequency transformers as the core loss due to hysteresis and eddy current loss will be phenomenal.

Ferrite has a high permeability which makes it suitable for use as a core.  It also has very high resistivity.  This ensures that eddy currents and hysteresis losses are kept to a minimum. 

The combination of high permeability and high resistivity makes Ferrite ideal for high frequency transformer core design. The only disadvantage is that being a ceramic it is brittle and is vulnerable to cracks.

It is also used for high frequency filters such as harmonic filters. 

Ferrites are classified into hard and soft ferrites.  While soft ferrites are used to make transformer cores, hard ferrites are used to make permanent magnets.

Brushes are components which are used to transfer electric power from a rotating component to a fixed circuit or vice versa.  They are used widely in rotating electric machines such as motors, generators, etc.  In DC motors, brushes are used along with commutators to change the direction of the current.  In AC synchronous machines, brushes are used to transfer DC power to the rotating poles for excitation. 

The brushes are made of carbon or copper.  Copper has a higher conductivity than carbon and thus is preferred for low voltage and high current applications.  It also has high mechanical strength and longer life.  However, the downside of copper is that it wears in a un-uniform manner.  The dust caused by the wear can cause arcing and can damage the commutator surface.

Carbon is soft on the commutator surfaces and it is smoother as it has good lubrication properties.  However, it introduces a large voltage drop across the contacts.   
Most brushes today have a mixture of copper and carbon.