Factors affecting air clearance

Air clearance is the minimum distance that must be kept in air between a live conductor (or its fittings) and earthed parts such as the tower body, cross-arm or earth wire, and between phases. Its purpose is to prevent flashover. For a given line voltage it depends on:

1. Maximum system voltage and overvoltages: clearance is set by the highest stress, not by the nominal voltage. This means the power-frequency maximum voltage, the lightning impulse and, at EHV, the switching surge level.
2. Switching surge ratio: lines with higher switching overvoltages (no closing resistors, long lines, lossless lines) need larger gaps. Above 300 kV this usually governs the clearance.
3. Insulator string swing: wind blows the suspension string through an angle $\theta$ (typically 15°–45°). The clearance must still hold in the swung position. So both the swing angle and the string length affect the cross-arm length. V-strings restrict swing.
4. Wind pressure and conductor size: a larger diameter and a higher design wind give a larger swing angle.
5. Altitude and atmospheric conditions: air density, humidity and temperature change the breakdown strength of air. Clearances are increased at high altitude.
6. Gap geometry: rod-plane gaps (conductor to tower leg) and conductor-to-cross-arm gaps have different breakdown strengths (gap factor). Sharp fittings reduce the breakdown voltage.
7. Rain, fog, pollution and birds: these effectively reduce the gap. A safety margin is added (for example 8 inch or 30 cm).
8. Factor of safety and standards used: for example, 6.5 inch per 10 kV (phase rms) plus 8 inch, or 1 cm per kV (phase peak) plus 30 cm. The IS 5613 or IEC clearances are another source.
9. Live-line maintenance: if hot-line work is planned, a larger clearance is kept for workers and tools.
10. Conductor bundling and corona rings: these change the field distribution and the effective gap.