Introduction
The silicone rubber outer sheath of composite insulators is critical for long‑term electrical performance. Thickness variations caused by manufacturing tolerances, abrasion, or aging can affect creepage distance and flashover resistance. Reliable thickness measurement is therefore required during factory acceptance and in‑service inspection. However, conventional contact methods (e.g., callipers, micrometers) are destructive and unsuitable for curved sheds. Ultrasonic thickness gauging offers a non‑destructive alternative, but soft rubber materials present specific challenges: low acoustic impedance, high attenuation, and complex surface geometry.
This article describes a practical ultrasonic approach using a portable thickness gauge (YUSHI PM5 Gen3) equipped with a low‑frequency contact probe, optimised for silicone rubber measurement on insulator sheds.
Acoustic Considerations for Silicone Rubber
Silicone rubber has a nominal sound velocity of approximately 1000–1100 m/s, significantly lower than metals or ceramics. This requires the instrument to have a wide velocity range and appropriate gain settings. The material’s high attenuation coefficient favours the use of lower‑frequency transducers (typically 2–5 MHz) to obtain a usable back‑wall echo. Additionally, the curved shed profile can cause beam scattering and contact instability, demanding a small‑footprint probe with a flexible wear face.
The PM5 Gen3 allows manual or automatic gain adjustment and provides a real‑time A‑scan display, enabling the operator to verify the echo from the rubber‑to‑core interface and distinguish it from spurious reflections caused by surface roughness or contamination.
Field Test Results
Measurements were performed on a set of 15 service‑aged insulators with nominal rubber thicknesses ranging from 3.0 mm to 5.0 mm. For each shed, three readings were taken at different angular positions. The repeatability was within ±0.02 mm across all measurements. Typical values recorded were 3.40 mm and 3.42 mm on adjacent sheds, consistent with factory specifications.
The A‑scan waveform clearly resolved the first back‑wall echo from the rubber layer and a second echo from the epoxy core interface, confirming that the gauge was measuring the rubber thickness alone, without including the substrate.
Advantages over Destructive Methods
Non‑destructive: No cutting or sampling required, preserving the insulator for continued service or further testing.
Adaptable to curved surfaces: The small probe and low‑frequency beam cope well with shed curvature, even on small‑diameter insulators.
Low operator dependence: The one‑key calibration and pre‑set velocity library reduce setup errors, making the technique suitable for routine patrol use.
Data traceability: Built‑in memory stores readings for export and reporting, supporting life‑cycle documentation.
Conclusion
Ultrasonic thickness measurement of silicone rubber layers on power insulators is technically feasible using a portable gauge with appropriate probe selection and waveform verification. The PM5 Gen3 provides reliable, repeatable data that can be used for quality control and condition assessment. The method described here has been validated on service‑aged insulators and offers a practical alternative to destructive testing.
For further technical details or application support, please contact our team.
