IEC 61109 is the cornerstone standard for composite insulators for AC overhead lines with nominal voltage above 1000 V. If you are buying composite insulators and your supplier cannot show you a current (within 5 years) IEC 61109 type test certificate issued by an accredited lab, do not buy. This guide walks through every type test the standard requires, what each one proves, why it matters, and how to verify the manufacturer’s claims before you wire the deposit.

An insulator datasheet is a contract between the manufacturer and the buyer. Every number on it has a physical meaning, an associated standard, and a test that proves it. This guide walks through every parameter on a typical composite, glass or porcelain insulator datasheet, explains the formulas, gives worked examples, and shows you which values are negotiable (creepage, color, markings) versus which are absolutely fixed (SML, BIL, type test reports).

“Composite insulator” and “silicone insulator” are often used interchangeably in the power industry, but they refer to different levels of

Composite suspension insulator engineering guide: design, FRP core, silicone housing, creepage, IEC 61109 tests, and how composite long rod insulators stop pollution flashover on HV lines.

Medium-voltage overhead lines are expected to operate for decades under wind, rain, UV exposure, pollution, lightning impulses, and mechanical stress.