Silicone Rubber Insulators: Hydrophobicity and Creepage Selection
Review pollution severity, hydrophobicity, tracking resistance and creepage; confirm material and test evidence for the specified insulator.
Selection note: use this guide to compare technical requirements. Confirm catalog ranges, typical values and standard clauses against the model, edition and agreed test conditions; project acceptance follows the approved drawings and technical agreement.
On this page
- 01How does pollution flashover happen on an insulator?
- 02Why is silicone rubber hydrophobic and what is hydrophobicity transfer?
- 03What makes an HTV silicone compound resistant to tracking and erosion?
- 04How much creepage distance does a silicone insulator need?
- 05Frequently asked questions
- 06Standards & sources
In short
HTV (high-temperature vulcanised) silicone rubber prevents pollution flashover because its surface is hydrophobic: water forms isolated droplets instead of a continuous conductive film, which suppresses leakage current and dry-band arcing. Its unique advantage over porcelain and glass is hydrophobicity transfer, in which low molecular weight siloxanes diffuse into the pollution layer and make it water-repellent within hours to days. Correctly formulated with ATH filler, HTV silicone achieves tracking class 1A 4.5 to IEC 60587.
- Flashover starts when pollution plus moisture forms a conductive film, leakage current dries bands and arcs bridge them.
- Silicone keeps hydrophobicity class HC1 to HC3 in service and recovers after temporary loss.
- Composite insulators can use less creepage than porcelain in the same pollution class (IEC TS 60815-3).
- ATH filler content and dispersion decide tracking and erosion resistance, not the silicone polymer alone.
- Composite and porcelain housings have different fracture behaviour; compare assembly failure tests and protection for the proposed design.
Pollution flashover is one of the most common causes of unplanned outages in coastal, desert and industrial substations, and it often occurs at normal operating voltage rather than during overvoltages. The selection framework in IEC TS 60815 classifies site pollution severity from very light (a) to very heavy (e) and links each class to a minimum creepage distance. This article explains why HTV silicone rubber housings outperform porcelain in classes d and e, and which material properties buyers should verify.
01How does pollution flashover happen on an insulator?
Pollution flashover develops in four steps: conductive deposits (salt, cement dust, industrial ash) accumulate on the surface; fog, dew or light rain wets the layer and leakage current flows; the current heats and dries narrow bands where voltage concentrates; and partial arcs across the dry bands elongate until they bridge the insulator. On hydrophilic porcelain or glass the wet film is continuous, so this sequence proceeds easily.
The critical parameter is the equivalent salt deposit density (ESDD) together with the non-soluble deposit density (NSDD). Coastal sites within a few kilometres of the sea and desert sites with saline dust regularly reach the heavy and very heavy classes, where porcelain requires either very long creepage or frequent washing and RTV coating.
02Why is silicone rubber hydrophobic and what is hydrophobicity transfer?
Silicone rubber is hydrophobic because its polydimethylsiloxane backbone carries methyl groups that give the surface very low surface energy, so water contact angles typically exceed 100 degrees. Hydrophobicity transfer is the diffusion of free low molecular weight (LMW) siloxane chains from the bulk into the pollution layer, which coats the dirt particles and makes the polluted surface water-repellent as well, typically within hours to a few days.
Hydrophobicity can be temporarily lost after prolonged wetting, corona or heavy discharge activity. The surface then recovers as LMW species migrate back, which is why field inspections use the HC classification (HC1 fully hydrophobic to HC7 fully hydrophilic) defined in IEC TS 62073 and re-check after a dry period rather than immediately after rain.
- HC1 to HC2: discrete droplets, high contact angle, ideal service condition.
- HC3 to HC4: some elongated droplets, acceptable in service.
- HC5 and above: wetted areas forming, investigate cause and re-test after recovery time.
| Property | HTV silicone composite | Glazed porcelain |
|---|---|---|
| Surface behaviour | Hydrophobic, HC1 to HC3 | Hydrophilic, continuous film |
| Hydrophobicity transfer to pollution | Yes, hours to days | No |
| Creepage requirement (IEC TS 60815) | Reduction often permitted (part 3) | Full reference USCD (part 2) |
| Washing or RTV coating in class e | Normally not required | Often required |
| Tracking and erosion (IEC 60587) | Class 1A 4.5 with ATH filler | Not applicable (inorganic) |
| Failure mode under internal pressure | Review tested assembly and fracture behaviour | Review brittle-fracture protection |
| Weight (550 kV class) | Compare actual model mass | Reference |
| Seismic performance | High, elastic tube | Limited by brittle material |
03What makes an HTV silicone compound resistant to tracking and erosion?
Tracking and erosion resistance comes mainly from the alumina trihydrate (ATH) filler. Under dry-band arcing, ATH releases bound water at about 200 degrees C, cooling the surface and preventing carbonised tracks. A well-dispersed, high ATH loading combined with a stable polymer lets HTV silicone reach class 1A 4.5 in the IEC 60587 inclined plane test.
Compare the proposed silicone compound, filler and shed design with its tracking and erosion test evidence; material family alone does not establish performance.
“Buyer check: define pollution severity and creepage, then request material and tracking-resistance evidence for the specified housing.”
04How much creepage distance does a silicone insulator need?
IEC TS 60815-2 gives reference unified specific creepage distances (USCD, relative to phase-to-earth voltage) of about 22.0, 27.8, 34.7, 43.3 and 53.7 mm/kV for pollution classes a to e on porcelain and glass. IEC TS 60815-3 allows polymeric insulators to use a reduced value in many cases because hydrophobicity improves pollution performance. The final value should consider pollution type, insulator diameter and site test experience.
Set creepage from site pollution and insulation coordination requirements, then verify the offered shed profile and actual path length on the drawing.
Applicable EONOVA Equipment
Conclusion
Choose silicone housing by site pollution, shed profile, creepage and material test evidence; inspect hydrophobicity and erosion during service.
Talk to an EONOVA engineerFrequently Asked Questions
Does silicone rubber lose its hydrophobicity permanently?
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Hydrophobicity can fall after wetting or discharge activity and may recover as mobile silicone species migrate to the surface. Retest after a documented drying period; persistent loss calls for inspection of erosion, contamination and compound condition.
Do composite insulators need washing in coastal substations?
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Cleaning depends on site pollution, measured hydrophobicity and the operator maintenance plan. Inspect the sheds and contamination first, then decide whether cleaning or other action is needed for the installed model.
What is the service life of an HTV silicone composite hollow insulator?
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Service life cannot be set from the HTV material alone. Ask for the proposed design, test and maintenance documentation; during service inspect shed cuts or erosion and the flange seals under the site plan.
Standards & Sources
- [1]IEC TS 60815-1/-2/-3: Selection and dimensioning of HV insulators intended for use in polluted conditions
International Electrotechnical Commission (TC 36)
Supports: Pollution classes a to e, reference USCD values and polymer creepage guidance.
- [2]IEC 61462: Composite hollow insulators: Pressurized and unpressurized insulators for use in electrical equipment
International Electrotechnical Commission (TC 36)
Supports: Design and test requirements for composite hollow insulators.
- [3]IEC 60587: Methods for evaluating resistance to tracking and erosion (inclined plane test)
International Electrotechnical Commission (TC 112)
Supports: Class 1A 4.5 tracking and erosion rating.
- [4]IEC TS 62073: Guidance on the measurement of hydrophobicity of insulator surfaces
International Electrotechnical Commission (TC 36)
Supports: HC1 to HC7 wettability classification used in field inspection.
Selection guidance for buyers. Confirm specifications and test evidence for the quoted model and supplying entity.
