Frequently asked questions

Four. Project scale, meaning line length, tower or pole count and the number of circuits, sets the base demand. Installation configuration multiplies it, through the number of insulators per string and the number of strings per tower, counting suspension, tension and jumper strings separately. Spare ratio adds 3 to 5 % by industry practice, to cover loss and damage in transport and installation and future maintenance replacement. And type differentiation splits the count, because different insulator types for different scenarios have to be calculated separately.

Basic quantity equals towers multiplied by strings per tower multiplied by insulators per string, plus spares. For a 110 kV single-circuit line with 50 towers, 2 suspension strings per tower, 10 insulators per string and a 3 % spare ratio: 50 by 2 by 10 gives 1,000 units, spares are 30 units, total 1,030 units.

Yes, and it varies by type and specification. Standard insulators at common voltage classes such as 10 kV, 35 kV and 110 kV are usually 50 to 200 units per type. Customised insulators for polluted areas or high altitude are generally 200 to 500 units per type, because of mould opening and line adjustment costs. Minimums can be waived for large projects, in the region of 10,000 units and above.

Yes. For long-term operation, reserve 1 to 2 % of the total installed quantity per year. These cover aging, broken and faulty units found during inspection, including self-exploded glass units and composite units with sheath aging. Buy them in batches every two to three years so they stay consistent with the model and performance already installed.

On porcelain and glass, inspect visually for cracks, self-explosion and loose cement bonding between body and fittings, then use a spark gap tester or insulation resistance meter to detect zero-value and low-value units. A zero-value insulator cannot bear voltage and needs immediate replacement. On composite units, check for sheath aging, cracking or shedding, core rod exposure and end fitting corrosion, then use an ultrasonic detector or infrared thermal imager for internal defects.

Porcelain runs 20 to 30 years under normal operating conditions. Glass runs 25 to 35 years, with self-exploded units replaced individually. Composite runs 15 to 20 years, limited by sheath aging and core rod degradation.

When the failure rate in a line exceeds 5 %, or when testing shows that the insulation performance of most units has dropped below the design standard. Below that, individual replacement is the better route.