It is enough to avoid overvoltage damages on insulation materials is creating a low insulated circuit parallel to insulation material. Arc horns are simple protectjon methods which working by this principle.
When overvoltage reached to bushings, it causes cracks while arcing on surface of bushings and also droping-down on insulations on windings. In order to prevent this, horn-shaped electrodes are connected paralel to the bushings on HV lines.
Once the system started arcing because of overvoltage it goes on arcing under operating voltage too; this is the disadvantage of the system. This causes phase ground short circuit, It can stop by opening breaker.
When altitude increases, air density drops. Thus, after 1000 m for every 100 m dielectric of air drops %1.5. The distance between two electrodes for the altitudes between 1000-1500 m should be chosen %15 above the table below, after 1500 m for each 500 m the distance should be increased %7.5
Here is the arcing horn gap distance:
| Insulation Voltage Class (kV) | Maximum Allowable Working Voltage (kV) | Arc Horns Distance VDE 0111/261 (mm) | Safety Distances Between Terminals | |
|---|---|---|---|---|
| Phase to Phase (mm) | Phase to Ground (mm) | |||
| 0,5 | 0,6 | - | 15 | 20 |
| 1 | 115 | - | 25 | 40 |
| 3 | 3,6 | - | 50 | 65 |
| 10 | 12 | 86 | 110 | 115 |
| 20 | 24 | 155 | 210 | 215 |
| 30 | 36 | 220 | 280 | 325 |
| 60 | 72,5 | 400 | 500 | 580 |
| 110 | 123 | 750 | 900 | 1000 |
| 150 | 170 | 830 | 1100 | 1150 |
| 220 | 250 | 1210 | 1600 | 1600 |
| 300 | 330 | 1600 | 2100 | 2100 |
| 380 | 420 | 2300 | 2900 | 2900 |
