One of the basic concepts associated with the string of suspended insulators is efficiency. The electric voltage across the string of suspension insulators is not uniformly distributed, rather the disc nearest to the conductor has mare voltage across it. And the voltage keeps on decreasing as one moves closer to the cross arms. This unequal potential distribution is undesirable as well and technically referred to as string efficiency.
Mathematical equation for String Efficiency
String efficiency is mathematically defined as the ratio of voltage across the whole string to the product of total discs and the voltage across the disc that is located next to the conductor.
The formula for String Efficiency
String efficiency = Voltage across string/(n*Voltage across the disc that is located nearest to the conductor)
In power systems, it is often required to have the high string efficiency. Since the higher, the string efficiency is the more uniform is the voltage distribution.