Basic configurations of insulator string

An insulator string can be arranged in different ways, depending on the voltage, the conductor load, the tower type and the allowable swing.

1. Single I-string (suspension): one vertical string at each phase of a straight-line (suspension) tower.
   • Simple, light and cheap.
   • It swings freely with the wind (up to about 45°), so a longer cross-arm and a larger clearance are needed.
2. Double I-string: two parallel I-strings joined by a yoke plate.
   • Twice the mechanical strength, and it stays secure if one string fails.
   • Used for bundled or heavy conductors and for crossings of roads, rivers and railways.
3. V-string: two strings fixed at separate points on the cross-arm and meeting at the conductor.
   • The conductor cannot swing transversely, so the tower window, phase spacing and right-of-way are smaller.
   • Each leg carries part of the load, which gives higher strength.
   • Common at 400 kV and above, and for compact lines.
4. Tension (strain) string: horizontal, single or double, at angle, section and dead-end towers. It takes the full conductor tension and usually has one or two more discs than the suspension string.
5. Jumper (pilot) string: a short I-string at tension towers. It holds the jumper loop so that the loop keeps its clearance from the tower.
6. Y-string or inverted-V, and long-rod / composite: used in EHV and compact designs to control swing and to save weight.

  I-string        V-string        Tension string
  ===+===       ===+---+===      tower|=[]=[]=[]=o== cond.
     |              \ /
    [ ]             [ ]
    [ ]              o  cond.
     o  cond.