Rubber Insulation Mats

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

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. 

Common Reasons for Contactor Failure

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

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

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 in Tranmission Lines

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. 

Shielding in High voltage lines

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.