Arcing Horn is basically projected conductors used to protect the Insulators in High Voltage Transmission Line from damage during flashover. Over voltage on Transmission Line may occur due to various reasons like lightning strike, sudden load variation, fault etc. Due to this high voltage a flash over may take place which will shatter the Insulator. To prevent Insulator from such an occurrence, it is very important that flashover do not take place through the Insulator. Arcing Horn serves this purpose by providing a bypass flashover the high voltage across the insulator using air as a conductive medium.
As shown in figure above, Arcing Horns are normally paired on either side of the Insulator, one connected to the high voltage side of Insulator and the other to ground.
Working Principle of Arcing Horn:
Arcing Horns bypasses the high voltage across the Insulator using air as a conductive medium between the Horns. 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.
Arcing Horns basically form a Spark Gap across the Insulator with a lower breakdown voltage than the air path along the insulator surface, so an overvoltage will cause the air to break down and the arc to form between the arcing horns, diverting it away from the surface of the insulator. An arc between the Horns is more tolerable for the equipment because it provides more time for the fault to be detected and the arc to be safely cleared by remote Circuit Breakers.
At medium voltages, one of the two Horns may be omitted as the Horn to Horn gap can otherwise be small enough to be bridged by a bird.
The presence of the arcing horns necessarily disturbs the normal electric field distribution across the insulator due to their small but significant capacitance. More importantly, a flashover across arcing horns produces an earth fault resulting in a circuit outage until the fault is cleared by circuit breaker operation. For this reason, non-linear resistors known as varistors can replace arcing horns at critical locations.