Steps to select number of insulator discs

The number of discs is chosen so that the string withstands every voltage stress it will meet: power frequency in dry and wet conditions, lightning impulse and switching surges. Pollution and spare-disc allowances are then added. The steps are:

1. Fix the system voltage. Take the nominal voltage $V$ and the maximum system voltage $V_m$ (about $1.1V$, for example 245 kV for a 220 kV line).
2. Find the required withstand levels from standards or the given table for $V_m$:
   • 1-minute power-frequency dry withstand
   • 1-minute power-frequency wet withstand
   • lightning impulse withstand (BIL)
   • switching impulse withstand (BSL), for $V_m > 245$ kV
3. Correct for site conditions. Divide the required withstand values by the air-density factor $\delta$ (altitude, temperature) and any humidity factor. This gives the equivalent values at standard conditions.
4. Power-frequency dry test: from the disc flashover table, pick the smallest number of discs whose 1-minute dry FOV is at least the required dry withstand.
5. Power-frequency wet test: do the same with the wet FOV column. This usually governs over the dry test.
6. Lightning impulse: choose the discs whose impulse FOV is at least the BIL. Allow for the uneven voltage distribution (string efficiency) along the string.
7. Switching surge: compute the surge $V_{ss} = SSR \times \sqrt{2}V_m/\sqrt{3}$. Use a switching surge ratio of 2–3.5 depending on breaker type and line length. Choose discs whose switching flashover, or equivalent wet FOV, is at least $V_{ss}$.
8. Pollution (creepage) check: the number of discs ≥ (specific creepage in mm/kV × $V_m$) / (creepage distance per disc). Use 16–31 mm/kV from light to very heavy pollution.
9. Take the largest number from steps 4–8.
10. Add allowances: one extra disc for a defective or punctured disc. Use one or two more discs in tension (dead-end) strings than in suspension strings.
11. Check the string length and air clearance against the tower window, the swing angle and the cross-arm length. If the string becomes too long, consider a V-string, anti-fog discs or composite insulators.

Example: for a 220 kV line ($V_m = 255$ kV row of the table: wet 395 kV, BIL 900 kV), 10 discs meet the wet and impulse levels. Adding one spare disc gives 11 discs per suspension string.