When designing a high voltage transmission line utilizing toughened glass bell insulators, engineers must consider whether or not to specify a zinc sleeve to accompany the insulator pins. Although the upfront cost is slightly more than the forged steel pin, those who are new to the use of toughened glass bells may ask what justifies a slightly higher expense for the addition of this feature.
The primary function of any insulator is to provide mechanical strength to the insulator assembly by maintaining the conductor’s placement and distance from the pole or lattice structure. In the case of toughened glass bell insulators, this strength is primarily provided by the pin.
Once a line is energized, the pin is subjected to both mechanical and electrical forces. The constant exposure to electrical current causes the pin’s structure to deteriorate over the life of an insulator and is known as electrolytic corrosion.
The presence of the electrical current creates a conductive medium which allows for the movement of ions from the material with a low potential, resulting in the breakdown of this anode. Over an extended period of time, this corrosion can have a detrimental effect on the residual strength of an insulator suspension assembly.
When a sleeve composed of zinc in adherence to the IEC/ANSI standards for purity is applied and bonded to the pin, it acts as a sacrificial anode. A bonding surface of at least 80% ensures there are no gaps and corona discharges are avoided.
The zinc sleeve prevents the removal of ions from the component by providing an even lower potential than the forged steel pin. The ions are removed from the zinc sleeve which has no impact on the overall mechanical strength of a toughened glass bell insulator. The image below shows the placement of the zinc sleeve within the structure of the insulator.