Power Line Markers are extremely important indicating equipment which are fitted on to power lines to make them visible to low flying Aircraft.  They are usually in the shape of balls in bright colors.  Ultraviolet resistant International Orange is the standard color.  However, they are usually in a combination of colors to make them visible in all seasons.  They are generally made of fiberglass.

Power Line Markers can be found in Power lines which are located near airports. 

Power Line Markers are mounted by means of helicopter as the locations on the power lines are not accessible otherwise. 

In order to make these Power Line Markers visible at night, reflector strips are sometimes attached to them.  These reflective strips reflect light from low flying aircrafts and helicopters making the power lines visible even in the night. 

Self Illuminating Power Line Markers are also available.  These Power Line Markers generate power from the electrostatic field of the Power line and illuminate LEDs in the Markers making them visible in the night. 

Power Line Markers are also necessary in areas where industrial cranes and other tall vehicles move.  If the Power Lines are not clearly visible, there may accidents when these vehicles and equipment come in contact with the high voltage lines. 

Special disc type markers are available which can be attached to these lines by means of insulated hot sticks. 

Corona Rings are metallic rings which are fixed at the end of bushings and insulators.  Corona rings work by distributing the electric field uniformly along the surface and preventing areas of high electric stress concentration which can cause corona.  Usually, sharp edges can cause the formation or corona as the electrical gradient exceeds the critical voltage for the formation of corona. 

The corona ring is connected to the conductor.  Hence, it is at the same potential as the conductor. 

The Disadvantages of Corona in Power Lines are as follows
  1. Corona results in loss of power due to the discharge along the surface of the conductors.  
  2. Corona results in damage to the conductor surface over a period of time.  
  3. Corona results in the production of ozone in the atmosphere.  
  4. This leads to cracking of elastomeric components like O-rings, gaskets, etc due to the chemical action of ozone. 
  5. Insulation of Electric Equipments can be damaged. 
  6. The audible noise from the power lines may be annoying. 

While Corona has many disadvantages, it also has an advantage. 
  1. Since the corona dissipates energy, the magnitude of steep surges caused by lightning or surges is mitigated as they pass through the lines. 

Protection Relay systems are classified into two types.
  • Primary relaying Equipment and
  • Secondary relaying Equipment
The Primary relay protection equipment is the first line of defence.  The secondary relay scheme comes in line when the primary relay system fails to act.

The Primary relay protection scheme can fail due to reasons such as

Failure of DC tripping voltage supply.
Failure of Current or voltage signal to the relays.
Failure of the Circuit Breaker.
Failure of the internal mechanism of the Protection Relays. 

The Secondary relay Protection scheme is intended to operate in the event of a failure of the primary supply.  Hence, the secondary relay protection scheme should be totally independent of the primary.  The current and voltage signals, the power supply of the relay, the output to the breaker should all be independent of the primary protection scheme.  The secondary protection scheme has a time delay greater than that of the primary relay protection scheme. 

Any Protection Relay should fulfill the following functional Characteristics.
  • Reliability
  • Sensitivity
  • Selectivity and
  • Speed


Reliability means that the relay will act when it is required to act.  This is ensured by making sure


Sensitivity refers to the characteristic of the relay to act when the actual fault conditions occur.  Sensitivity is usually represented in terms of the minimum volt-amperes required for the relay operation. 


The relay should act according to the present time delay.  It should neither operate too fast or too slow.  If it is too slow it can cause damage to the equipment, if it operates too fast it may unnecessarily trip equipment for transient faults.


Selectivity refers to the ability of the relay to discriminate between faults.  This is critical as only the smallest possible section of the power system should be taken out of line in the event of a fault.  The relay should be able to discriminate between a transient fault and a through fault.  It should be able to differentiate between downstream faults and immediate faults.
that all the components of the protection from the voltage and current signals to the dc power supply for the trip circuit to the internal components of the relay are checked for for functionality and integrity.  The failure of any one of these components can result in the failure of the relay to act affecting the reliability.

Coupling refers to the transfer of energy from one medium to another.  

AC coupling allows only AC signals to pass through.  An example would be a capacitor connected in series.  DC coupling on the other hand, permits both AC and DC signals to pass through.

Oscilloscopes have a DC/AC coupling button.  If for instance you observe a ripple above a rectified DC waveform and you want to measure it, you will have to activate the AC coupling button.  This blocks the constant DC waveform and displays only the pulsating AC component. 

The magnet emits flux along its entire surface area.  However, only some part of the flux will be used for the intended application, say to develop force inside a motor or to lift the load in the case of a crane.  

This is known as the active flux.  The Active surface of a magnet is the surface which emits the active flux.

The area of the magnet from which this flux emanates is called the active surface of a magnet.

The Air core coil is a coil which does not use a ferromagnetic material as the core.  The core is empty and is therefore filled with air.  The inductance of the air core coil is not affected by the current unlike coils with Ferromagnetic cores in which coils have a tendency to saturate.  Hence, the inductance value changes as the current is increased. 

Because, the coil does not have a metallic core, the hysteresis losses are negligible.  This results in more efficiency, less distortion and increased power capacity. 

Air cores coils can be used at very high frequencies up to 1 GHz in contrast to ferromagnetic core coils which can be used up to a maximum of 100 MHz.

However, Air core coils also have some disadvantages. For the same value of inductance, the air core coil needs to have more number of turns.  This results in higher copper loss.  The air core inductance is also susceptible to greater interference from stray fields. 

Oil Leakages are a frequent issue in Transformers which if not properly addressed can result in serious accidents such as fire or explosions.  Oil can also cause accidents due to slipping. 

Besides, Transformer oil is an environmental pollutant.  Spillage of Transformer oil can result in severe penalty for environmental violations. 

Oil Leakages can be detected visually most of the times.  However, some minor leakages are not always detectable by the naked eye.  Special techniques such as the application of special fluids which indicate oil leakage by changing colour.

Once an oil leakage is detected, it needs to be arrested and the point of leakage should be plugged.  Welding the leakage would require switching off the transformer and draining the oil.

Transformer oil leaks can be rectified by the application of special polymeric compounds and putties which cure and seal off the leakage. 

Leakages can be prevented by ensuring that the elastomeric components of the transformer such as the gaskets, O rings, etc are replaced at the scheduled replacement intervals.  The Transformer should be properly painted and any damage to the painting should be properly rectified. 

In addition to oil leakages, in transformers filled with Nitrogen the leakage above the oil surface can result in the leakage of nitrogen.  The leakage of the inert gas can be confirmed by applying a soapy solution and observing the bubbles. 

Air Capacitors are capacitors which use air as the dielectric medium.  Air capacitors are used in tuning circuits.  

An Air capacitor is made of two banks of semicircular plates which are mounted on a single shaft.  The rotation of the shaft controls the amount of overlap between the two plates.  This determines the capacitance value of the capacitor. 

Since air has a low dielectric constant, it cannot be used for high voltages.  The capacitance of air capacitors ranges between 100pF to 1 nF. 

Air capacitors are not polar which means that they can be connected in any polarity.

There can be a shift in the capacitance value due to deposition of dust on the plate surfaces and the change in air parameters such as temperature and humidity.  Small insects can also get caught between the plates and create a short circuit. 

SF6 gas detection is important in Circuit Breakers, Gas Insulated Switchgear and other equipment.  SF6 gas leakages can be expensive, can cause equipment tripping.  In addition, they can also result in damage to the environment as SF6 is a green house gas. 

SF6 gas is usually detected by hand held detectors which work on the ultrasonic principle.  These detectors are vulnerable to interference from noise and wind.  Another technique which has gained popularity in recent years is the use of thermographic cameras for SF6 detection.  These cameras detect the leakage of a gas by sensing the difference in temperature between the gas and the surrounding atmosphere.  This technique can be used to detect SF6 leakages from large distances. 

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Resistors are passive components in electric and electronic circuits which are used to introduce a resistance value.  While resistors have their resistive value, they sometimes also have a small inductive value. 

This is due to the fact that the resistance elements are wound.  This winding of the elements introduces an inductance.  This small value may not be significant at low frequencies.  However at higher frequencies and during surges which have steep fronts, the inductance value may become significant and introduce oscillations and other disturbances. 

In order to eliminate the inductance, non-inductive resistors are used.  Non inductive resistors are designed in a manner that there is no inductance in the assembly.  For instance, ceramic resistors which do not have a wound structure are considered non-inductive. 

Wound resistors can also be designed to be non-inductive by having half the winding in one direction and the other half in the other direction.  This ensures that the net inductance is nil.  Film type resistors are also designed to be non-inductive. 

Rectifiers Transformers are Transformers which have an in-built Rectifier in the secondary side.  The Diodes are mounted inside the Transformer Assembly itself.    The Rectifier Transformer is used in applications which require high DC power such as in DC Traction Systems and in processes such as electrolysis and smelting. 

Voltage regulation is achieved using Tap Changers in the HV side. 

Rectifier Transformers are sometimes also known as Rectiformers. 

Rectifier Transformers are usually connected to a bank.  Each Rectifier-Transformer is designed with an inherent phase shift such that the transformers attain the peak voltage one after another.  This ensures a steady supply at peak voltage.  Thus a system with five banks of rectifier transformers with twelve pulses at a phase shift of +12°, – 6°, 0°, + 6° and +12° will result in a system  with 60 pulses .