Transmission and other electrical equipment can be exposed to overvoltages.  Overvoltages can be caused by a number of reasons such as lightning strikes, transient surges, sudden load fluctuation, etc.  In the event of an overvoltage, the insulating equipment such as the insulators on a transmission line or bushings in a transformer can be exposed to high voltages which may lead to their failure.

Arcing horns are protective devices that are constructed in the form of projections in the conducting materials on both sides of an insulator. Arcing horns are fitted in pairs.  Thus in transmission lines they are found on the conducting line and the transmission tower across the insulators.  In transmission lines, in the event of a lightning strike on the tower, the tower potential rises to dangerous levels and can result in flashovers across the insulators causing their failure.  Arcing horns prevent this by conducting the arc across the air gap across them.

Arcing horns function by bypassing the high voltage across the insulator using air as a conductive medium.  The small gap between the horns ensures that the air between them breaks down resulting in a flashover and conducts the voltage surge rather than cause damage to the insulator.The horns are constructed in pair so that one horn is on the line side and the other is on the ground side.

Arcing Horns are also used along with air insulated switchgear equipment. Air insulated switchgear are vulnerable to damage due to arcing.  Arcing horns serve to divert the arc towards themselves thus protecting the switching equipment.  The arcing horns serve to move the arc away from the bushings or the insulators.  

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When induction motors are started a very heavy current is drawn.  This current, which can be 5 to 6 times the rated current of the machine, occurs due to lack of back emf in the windings.  These currents  cause sudden loading on power sources such as generators or transformers.  Besides, such heavy currents also cause voltage dips and a momentary drop in frequency in generators which can trip other equipment connected to the power system.

Softstarters are equipment which limit the initial start current of the motors by controlling the voltage.  This is done by means of thyristors.  This "soft" starting prevents the occurence of the high inrush currents.  The downside of using softstarters is that the reduced voltage applied on the stator windings can result in increased slip causing heating of the rotor. 

Another advantage of the softstarter is that it can be used to gradually increase the torque.  In loads such as conveyor belts involving chains and other transmission gear, this avoids jerks which over time can cause wear on the machinery.  The voltage and the torque are gradually increased to the rated value.

While stopping a motor, the softstarter usually provides the "softstop" function which involves doing the starting process in reverse.  The voltage is gradually reduced to zero this gradually stops the system avoiding sudden jerky motions due to system inertia.



The Secondary of the potential Transformers are usually earthed.  There are various reasons for this.  The first reason is concerned with safety.  Since, the primary winding of the potential transformer is connected to High voltage, there are chances for the high voltage, in rare conditions, to get transferred to the secondary.  This can occur due to capacitance, insulation failure or any other damage.  Such a scenario would be extremely dangerous.  For this reason, the secondary is usually earthed at the star point for star connected transformers.







Another reason for earthing the secondary would be functional.  For applications such as synchronization where two PT voltages need to be compared, the synchronizing equipment (dark lamp method and bright lamp methods) would function only if the star point of the PTs are grounded.