Arcing horns are an inexpensive form of overvoltage protection. They are employed mainly as lightning protection. Their function is to prevent damage to equipment that would be caused by high levels of voltage by providing an independent path for that voltage to earth. Furthermore, they must allow the equipment to resume normal operation once the high voltage event is dissipated.
Hey! That sounds like the definition of a surge arrester. Yes, arcing horns are surge arresters. They are also called rod gap arresters.
On a step-down transformer arcing horn are found on the primary side connections (bushings). When a high voltage event (i.e., lightning) occurs the air in the air gap breaks down (ionizes), and the energy arcs to earth.
Arcing horns are used as protection against overvoltage mainly due to lightning. These are named horn gaps due to their shape. Space between horn gaps increases from bottom to top. During high voltages, a discharge is maintained in between the gap. As the space increase thus due to discharge air gets hot and discharge travels from bottom to top and will not be able to maintain in between large spaces. Secondly, the high-frequency transient is retarded by reactance connected and discharge cannot occur at normal voltages due to the high resistance of nonlinear resistance connected with horn gaps.
Transformer arcing horns (also known as protective air gaps) are used to limit the crest of a traveling wave when it reflects off the transformer impedance. Such traveling waves can be caused by switching surges and lightning strikes and will cause insulation failure in the transformer if the crest voltage is not limited.
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