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IEC 60317-25: Enamelled Winding Wires — How a Micron-Thin Coating Withstands Kilovolts Between Turns
The Enamel Film Is Only Tens of Microns Thick — Why Can It Withstand Thousands of Volts Between Turns?
IEC 60317-25:2010 specifies enamelled round copper winding wire (Class 200, polyester or polyesterimide + polyamideimide composite coating). The inter-turn insulation in every motor and transformer relies on this 20–80 μm coating — it must maintain integrity after conductor bending, stretching, and thermal cycling.
| Test | Condition | Requirement | Defect Detected |
|---|---|---|---|
| Breakdown Voltage | Twisted pair, 50 Hz | ≥2,200V (0.1mm dia.) | Pinholes, uneven thickness, poor cure |
| Pinhole Test | Saline electrolysis | ≤3 pinholes/30m | Micron-scale bare copper spots |
| Scrape Resistance | Incremental needle force | ≥ minimum force | Insufficient adhesion |
The pinhole limit — ≤3 per 30 metres — represents over 99.999% insulation coverage. A single micron-scale pinhole concentrates the inter-turn electric field far beyond the enamel dielectric strength, causing turn-to-turn short-circuit. Pinhole testing is a 100% in-line production test — never a sample check.
TNLab — Winding wire reliability depends not on the average enamel thickness, but on whether even a single pinhole exists.
