String insulator configuration and advantages

A string (suspension) insulator is a chain of porcelain or glass discs joined by metal caps and pins. It hangs from the tower cross-arm, and the conductor is clamped at the bottom. It is used for all lines above about 33 kV.

Basic configuration

     cross-arm ===========
                  |  <- hanger / clevis
                [===]  disc 1
                  |
                [===]  disc 2
                  :
                [===]  disc n
                  |
           ------(o)------ conductor clamp

• Each disc is designed for about 11 kV. The number of discs is chosen to match the line voltage.
• The common arrangements are I-string, V-string, tension (strain) string, and double or multiple strings for heavy conductors.

Advantages

1. Each disc works at a low voltage (about 11 kV), so a string can be built for any voltage by adding discs. One large pin insulator would not be practical.
2. It is cheaper than a pin insulator above 33 kV, because the cost of a pin insulator rises steeply with voltage.
3. A damaged disc can be replaced alone. The whole string need not be changed.
4. The string is flexible. It swings with the wind, so the mechanical stress on it is lower.
5. The conductor hangs below the cross-arm, so the cross-arm and tower partly shield it from lightning.
6. Line voltage can later be raised just by adding discs.
7. Strings can be used in parallel to get higher mechanical strength.