Spray Method Hydrophobicity Test for Composite Insulator Surfaces (IEC/TS 62073 / GB/T 24622)

The spray method hydrophobicity test is a rapid, standardized visual technique for evaluating the water-repellent behavior of composite insulator surfaces, particularly silicone rubber housings and sheds. Based on IEC/TS 62073 and its Chinese equivalent GB/T 24622, the test assigns a hydrophobicity class from HC1 (strongly hydrophobic) to HC7 (fully wetted) by spraying a fine water mist onto the surface and comparing the droplet pattern with reference photographs. This article explains the test principle, applicable samples, procedure, classification criteria, engineering applications, and frequently asked questions.

Technical Overview

The spray method hydrophobicity test, also known as the water spray classification test or HC spray test, is a qualitative-visual method used to assess the water-repellent (hydrophobic) characteristics of insulator surfaces. In this test, a fine mist of water is sprayed onto the surface under test, and the resulting appearance—whether the water forms discrete, rounded droplets or spreads into continuous films—is compared against standardized reference images. The surface is then assigned a hydrophobicity class ranging from HC1 (strongly hydrophobic) to HC7 (hydrophilic, fully wetted).

Hydrophobicity is the decisive property that gives composite insulators their excellent pollution flashover performance. When water remains as discrete droplets instead of forming a continuous conductive film, leakage currents and dry-band arcing are suppressed, dramatically improving performance in polluted environments. The spray method is specified in IEC/TS 62073 (Guidance on the measurement of hydrophobicity of insulator surfaces) and its equivalent Chinese standard GB/T 24622, and it is referenced in product standards such as IEC 61109, IEC 62217, and GB/T 19519 for composite insulators. Because it requires only a simple spray device and water, the method is suitable for both laboratory testing and on-site condition assessment of energized-line hardware by accredited third-party laboratories and utility maintenance teams.

Testing Samples

Composite Insulators

Complete or sectioned composite (polymeric) insulators with silicone rubber housings and sheds, including suspension, tension, line post, station post, and pin-type insulators used on overhead transmission and distribution lines and in substations. Both new production units and insulators removed from service can be tested.

Silicone Rubber Material Specimens

Molded plates, slabs, or plaques of housing material taken from the production compound. These specimens are commonly used during type tests and formulation qualification to verify the intrinsic hydrophobicity of the material, including its hydrophobicity transfer capability toward pollution layers.

RTV Anti-Pollution Coatings

Room-temperature-vulcanizing (RTV) silicone coatings applied to glass or porcelain insulators and bushings. Coated witness plates, factory-coated samples, and field-coated equipment surfaces are tested to verify coating quality at application and to monitor hydrophobicity retention over the service life.

Other Polymeric Insulating Components

Cable accessories, transformer bushings, surge arrester housings, interphase spacers, and components made from EPDM, silicone-EPDM blends, or other elastomers, which may be evaluated for comparative material assessment.

Field-Aged and Contaminated Insulators

Insulators retrieved from heavily polluted service environments or after long-term exposure. Testing these samples provides valuable data on aging, chalking, surface degradation, and the recovery behavior of hydrophobicity after environmental stressing.

Testing Items

The core deliverable of the test is the hydrophobicity classification of the insulator surface. Typical testing items include:

• Spray method hydrophobicity class (HC1 to HC7) per IEC/TS 62073 and GB/T 24622, determined at multiple representative locations on shed and housing surfaces.
• Acceptance verification of new silicone rubber insulators, for which most utility and product specifications require HC1 to HC2 out of the factory.
• Hydrophobicity transfer evaluation, in which the material specimen is covered with an artificial pollution layer (such as kieselguhr or salt deposit) and the transfer of hydrophobicity into the pollution layer is classified after conditioning.
• Hydrophobicity recovery assessment, tracking the return of hydrophobicity after wiping, corona exposure, or artificial aging.
• Complementary static contact angle measurement (Method B of IEC/TS 62073) to provide quantitative supporting data where required.

Testing Methods

Apparatus and Test Conditions

The test requires a hand-operated spray bottle or atomizer capable of producing a fine, uniform mist of discrete droplets, together with clean deionized water (low conductivity, typically not exceeding about 10 µS/cm, is recommended) and a camera for documentation. Tests are performed at ambient temperature on dry surfaces, shielded from direct sunlight, strong wind, and rain. The surface should be tested in its as-received condition unless otherwise specified, since wiping or cleaning can alter the result. A set of standard reference photographs for classes HC1 through HC7 must be available for comparison.

Test Procedure

• Select several representative test areas, typically the upper surfaces of sheds at different positions along the insulator and at different orientations around the circumference.
• Hold the spray nozzle at a distance of approximately 25 cm (with a tolerance of about ±10 cm) from the surface to be tested.
• Spray a fine, uniform mist of water onto the surface for a short duration, typically on the order of 10 to 25 seconds, so that the surface becomes evenly covered with discrete water droplets without excessive run-off or artificially formed continuous films caused by over-spraying.
• Immediately after spraying, generally within about 10 seconds, observe and photograph the droplet pattern, since droplet shapes can evolve over time.
• Compare the photographed pattern with the reference images and assign the corresponding hydrophobicity class to each tested location.
• Repeat the test at all selected locations; the reported result is commonly the prevailing class together with the worst (highest) class observed, as defined by the applicable product specification.

Classification Criteria (HC1 to HC7)

• HC1: Only discrete water droplets are present on the surface; the droplets are well rounded or nearly spherical, indicating strong hydrophobicity.
• HC2: Mostly discrete water droplets are present; the majority of droplets are round, with only minor deviation from the spherical shape.
• HC3: Discrete droplets are still dominant, but a noticeable fraction of droplets are elongated or irregular rather than round.
• HC4: Both discrete droplets and visible wetted water paths appear; droplets begin to coalesce and form elongated wetted traces.
• HC5: Some completely wetted areas and continuous water paths exist between discrete droplet regions.
• HC6: Large continuous water films cover most of the surface, with only isolated dry or droplet-covered areas remaining.
• HC7: A continuous water film covers the entire surface with no discrete droplets, indicating a hydrophilic condition.

Evaluation and Acceptance Criteria

For new silicone rubber composite insulators and RTV coatings, acceptance criteria typically require a hydrophobicity class of HC1 or HC2 at all tested locations. For in-service insulators, the test result is interpreted as part of a condition assessment: classes up to HC3 may be acceptable depending on the pollution environment, while classes of HC4 and above indicate a significant loss of hydrophobicity and may trigger follow-up actions such as intensified monitoring, coating application, cleaning, or replacement. Because silicone rubber can recover hydrophobicity through migration of low-molecular-weight siloxanes, trending of results over successive inspection cycles is more meaningful than a single measurement. The test report should document the sampling locations, test conditions, photographic records, the class assigned to each location, and the overall worst-case classification.

Complementary Contact Angle Method

Where a quantitative result is required, the static contact angle method described in IEC/TS 62073 may be performed in parallel. A small droplet of deionized water is placed on the surface and the contact angle is measured with a goniometer or drop-shape analyzer; static contact angles greater than 90° are generally regarded as hydrophobic. The spray method remains preferred for routine and field work because it is fast, non-destructive, and representative of a large surface area, whereas the contact angle method provides higher precision on small, localized areas under laboratory conditions.

Applications

The spray method hydrophobicity test is applied throughout the life cycle of composite insulation systems:

• Factory type tests, sample tests, and incoming inspection of composite insulators for transmission and distribution projects, verifying compliance with IEC 61109, IEC 62217, GB/T 19519, and related utility specifications.
• Quality control of RTV anti-pollution flashover coatings immediately after application and during periodic maintenance inspections.
• Condition-based maintenance of overhead lines and substations, where field measurements of hydrophobicity class support decisions on cleaning, recoating, or replacement of insulators in heavily polluted areas.
• Aging and degradation studies of polymeric housing materials after accelerated weathering, UV exposure, corona stressing, or long-term natural exposure test stations.
• Research and development of silicone rubber formulations, including filler content optimization, tracking of hydrophobicity transfer and recovery mechanisms, and comparison of competing elastomer systems.
• Failure analysis and pollution flashover risk assessments, providing diagnostic evidence alongside pollution severity measurements and leakage current monitoring.

FAQ

What is the difference between the spray method and the contact angle method?

The spray method is a qualitative-visual technique that classifies a large surface area into hydrophobicity classes HC1 to HC7 by comparing sprayed droplet patterns with reference photographs; it is fast, portable, and ideal for field inspection. The contact angle method is a quantitative laboratory measurement of the angle formed by a single static droplet on the surface; it offers precision but evaluates only a small localized spot. IEC/TS 62073 recognizes both approaches, and they are often used together.

Which hydrophobicity class must a new composite insulator achieve?

Most product standards and utility acceptance specifications require new silicone rubber composite insulators and RTV coatings to exhibit hydrophobicity class HC1 or HC2. Some specifications tolerate localized HC3 results, provided the prevailing class at the majority of tested locations is HC1 to HC2 and no continuous wetted paths are observed.

Can the test be performed in the field on in-service equipment?

Yes. The spray method was deliberately designed for simple field application, requiring only a portable spray bottle, water, and reference images. Precautions include testing during calm, dry weather; avoiding direct sunlight that distorts visual assessment; allowing hot insulator surfaces to cool; and never spraying energized equipment without proper live-work procedures and safety clearances.

Does surface contamination affect the test result?

Yes. A thick or soluble pollution layer can mask the underlying hydrophobicity, and greasy contamination may artificially enhance droplet formation. The pollution state of the surface should always be recorded in the report. For aged insulators, hydrophobicity transfer into the pollution layer is itself an important characteristic and can be deliberately evaluated using artificially polluted specimens.

Is the spray test destructive, and how often should it be repeated?

The test is completely non-destructive: it applies only a light water mist and leaves the insulator functional after natural drying. For in-service composite insulators, utilities commonly repeat the measurement on a periodic basis, for example every one to three years depending on pollution severity, and evaluate the trend of the hydrophobicity class over time rather than relying on a single reading.