RTV pollution-flashover silicone coating (Room Temperature Vulcanizing; called “coating of room-temperature-cured silicone rubber for pollution flashover prevention of insulators” in DL/T 627-2018) is a functional coating based on a room-temperature-cured silicone rubber binder with fillers and additives, sprayed on site onto the air side of porcelain or glass insulators. It does not win by thickening the insulation: it uses hydrophobicity, hydrophobicity transfer and hydrophobicity recovery to stop a polluted surface from forming a continuous conductive water film, so that outdoor insulation withstands operating voltage in fog, dew and drizzle.
TL;DR
- Positioning: an RTV coating is a hydrophobisation retrofit for insulators already in service. It can be applied with a short or no outage window, which makes it the third route alongside replacing composite insulators and replacing long-creepage pollution insulators.
- Mechanism: low surface energy makes water bead up instead of sheeting; more importantly, low-molecular siloxanes migrate out of the coating and hydrophobise the pollution layer itself — the essential difference from ordinary film-forming protective coatings.
- Acceptance: hydrophobicity is graded HC1–HC7 by the IEC/TS 62073:2016 spray method. Published readings of DL/T 627-2018 place a compliant coating between HC1 and HC4; site maintenance commonly treats a fall to HC6 as a re-inspect or recoat trigger.
- Selection: establish the pollution class first (equivalent salt deposit density ESDD and non-soluble deposit density NSDD), then check the reference unified specific creepage distance bands of IEC/TS 60815-2:2025 (16–20 mm/kV for class a up to above 50 mm/kV for class f) and add 10%–15% margin.
- Limits: a coating is not a structural part. UV, arc erosion and dust cover decay hydrophobicity, and film thickness, surface cleanliness plus the temperature/humidity window explain why one product performs differently on different sites.
Key data and standard figures
- Scope (DL/T 627-2018): outdoor insulation of AC systems above 1000 V, ambient −40 °C to +40 °C, insulators operating under pollution; DC systems may refer to it. Effective 2019-05-01, replacing DL/T 627-2012.
- Tack-free time ≤45 min at 23±2 °C and cured density ≤1.28 g/cm³ — visible limits of DL/T 627-2018, as compiled from public standard readings.
- Volume resistivity ≥1.0×10¹² Ω·m (that is 1.0×10¹⁴ Ω·cm); flammability class moved from FV-0 to FV-1; hydrophobicity requirement HC1–HC4 (annex grading) — same source.
- Hydrophobicity test (IEC/TS 62073:2016): spray from about 250 mm at roughly 1 mL/s for 15–20 s, then compare with the HC1–HC7 reference images within 10 s. The spray unit described under DL/T 627 delivers 0.7–1.0 mL per pulse with a 50°–70° spread angle.
- Pollution classes by ESDD (usual Chinese bands, compiled from public training material): class 0 ≤0.03, class Ⅰ 0.03–0.06, class Ⅱ 0.06–0.10, class Ⅲ 0.10–0.25, class Ⅳ 0.25–0.50 mg/cm², matched to creepage distance ratios of 14, 16, 20, 25 and 31 mm/kV with a further 10%–15% design margin.
- Reference unified specific creepage distance (IEC/TS 60815-2:2025 for AC porcelain and glass insulators; second edition issued December 2025 superseding the 2008 first edition): a 16–20, b 20–25, c 25–31.5, d 31.5–40, e 40–50 mm/kV, and above 50 mm/kV for class f, which needs a dedicated study.
- Flashover development: a partial arc that has travelled roughly 70%–80% of the creepage distance can change suddenly into a complete flashover (public training material). Pollution flashovers cluster in fog, dew and drizzle, with 85% relative humidity commonly taken as the critical threshold.
- Service life: public data most often quote 5–8 years for a coating. Vendor literature describes RTV systems hardening after about 5 years and fluorocarbon-resin-based durable systems reaching 8–10 years. Treat the measured hydrophobicity class and coating appearance as the governing evidence.
Four pollution-flashover routes compared
The choice is not which option is better in the abstract, but which one is feasible once the existing insulator type, pollution class, available outage window and whole-life cost are fixed together.
| Aspect | RTV silicone coating | Composite insulator | Pollution-type porcelain/glass | Periodic washing |
|---|---|---|---|---|
| How it works | Hydrophobises the surface, suppresses the conductive film | Silicone sheds give hydrophobicity plus creepage | Adds creepage distance to resist the pollution class | Removes the pollution layer and its soluble salts |
| Creepage distance | Essentially unchanged (thin systems) | Increased markedly | Increased markedly | Unchanged |
| Outage window | Live-line or short outage; robot application exists | Outage required for replacement | Outage required for replacement | Live washing possible under dedicated safety conditions |
| Durability | 5–8 years in public data, with periodic hydrophobicity checks | Valid for the unit life | Valid for the unit life | One cleaning cycle, typically months to one or two years |
| Typical failure | Chalking, cracking, delamination, arc erosion, wetting once pollution covers the film | Silicone ageing, erosion, interface breakdown | Pollution build-up still demands cleaning | Water quality and work-safety exposure |
| Best fit | Rising pollution where the structure cannot be changed | New lines or full refurbishment | Heavy-pollution new build with low-maintenance targets | Stations where pollution is salt-dominated and reachable |
How a pollution flashover develops and where RTV intervenes
Stage 1 — pollution deposit
Industrial dust, sea salt, road dust and agricultural dust settle on the insulator under gravity and the electric field, forming a layer of soluble salts plus insoluble ash. Engineering describes it by equivalent salt deposit density (ESDD, the soluble fraction expressed as NaCl per unit area, mg/cm²) and non-soluble deposit density (NSDD).
Stage 2 — a conductive water film
Dry pollution hardly conducts. Fog, dew or drizzle wets the layer, dissolves the salts and creates a conductive path, so leakage current rises. This is what separates a pollution flashover from a lightning overvoltage event: it happens at operating voltage. On a hydrophilic porcelain or glass glaze the water sheets into one continuous film, so whether the surface is hydrophobic decides the current magnitude.
Stage 3 — dry band and partial arc
Joule heating evaporates moisture locally and leaves a high-resistance dry band. Voltage concentrates across that band until it breaks down, striking a partial arc. The arc channel is conductive, which raises current density in the remaining wetted pollution and closes a positive feedback loop. Public training material puts the turning point at an arc covering about 70%–80% of the creepage distance.
Three intervention paths of RTV
(1) Hydrophobicity: the polydimethylsiloxane backbone (Si–O–Si) presents outward methyl groups, so surface energy sits in the 20–24 mN/m range and water contracts into isolated beads. (2) Hydrophobicity transfer: low-molecular siloxanes diffuse into the adjacent pollution layer and even onto neighbouring porcelain, making the polluted surface hydrophobic too — IEC/TS 62073 documents this transfer and recovery, and it is why an RTV coating still works on a fouled old insulator. (3) Recovery: after wetting or arcing, siloxane segments reorient and hydrophobicity returns partly by itself; deep chalking, erosion or mechanical burial under thick dust is not recoverable and calls for recoating.
Selection: pollution class first, coating system second
Read the pollution map, do not guess
A pollution class should combine measured ESDD and NSDD, the system pollution map and service experience. GB/T 26218.1-2010 (modified adoption of IEC/TS 60815-1:2008; issued 2011-01-14, effective 2011-07-01) supplies definitions and general principles, while GB/T 26218.2 and IEC/TS 60815-2:2025 give the class bands for porcelain and glass insulators. Coastal, chemical and cement-plant corridors commonly sit one or two classes above neighbouring farmland, and coating payback is largest when a line crosses a class boundary.
Thin versus durable (thick) systems
Field literature splits systems into thin coatings that rely on hydrophobicity and thick or durable (PRTV) systems that also add some creepage. They differ in film thickness and number of passes, filler loading and arc-erosion resistance, and in the recoat interval. Public readings of DL/T 627-2018 show that the 2018 edition dropped the solids-content requirement, widened the sampling lot from ≤500 kg to ≤2000 kg and adjusted both the product grading and the flammability class from FV-0 to FV-1 — a shift of emphasis from formulation parameters to in-service performance. Nominal film thickness, spreading rate and any primer follow the manufacturer TDS and DL/T 5727-2016.
How to read an artificial pollution test report
Coating performance is qualified by the artificial pollution tests of the GB/T 4585 series (2024 edition, previous 2004): salt fog, solid layer and related procedures deposit a controlled salt and ash density on the specimen, and withstand or stepped-up voltage methods then measure flashover voltage. Check three things in any report — the pollution application method with measured ESDD and NSDD, the water addition rate and spray water conductivity, and whether the quoted figure is a withstand voltage or a 50% flashover voltage. At the same nominal salt density, solid layer and salt fog results can differ by more than ten percent.
Field application and quality control
Surface preparation and the weather window
Remove loose pollution, oil and any chalked old coating; wash and dry fully where needed, because residual moisture and dust cause poor adhesion and bare spots. Cured silicone depends on airborne moisture to crosslink, so very dry air slows curing; at the same time the substrate temperature must stay above the dew point, otherwise condensation blistering and adhesion loss follow. Stop spraying in rain, fog or dew.
Thickness and coverage
Apply the number of passes and wet film thickness stated in the TDS; a single heavy pass sags and cures poorly inside. Verify dry film thickness by the section method described in the DL/T 627 annexes, sampling tower by tower. Shed undersides and ball-and-socket cups need dedicated lance angles and masking; these are the places most often missed.
Inspection and recoat criteria
After curing, compare hydrophobicity against the HC1–HC7 reference; a new coating normally reads HC1–HC2. Acceptance inspection should cover missed areas, ponding, sagging, cracking and poor coverage inside the sheds. In service, schedule verification and plan a recoat when hydrophobicity has decayed to HC5–HC6, or when chalking and arc erosion depth become visible.
Frequently asked questions
Can an RTV coating replace pollution-type insulators?
Where the pollution class has risen only modestly and the existing creepage distance is close to requirement, spraying RTV usually reaches the target within a far shorter outage, which is why it is the standard retrofit route. If the class is far beyond what the structure can carry — for example class e or f above 40 mm/kV — no coating substitutes for creepage distance, and composite or pollution-type insulators must be used.
Is cleaning still needed after coating?
Yes, but at longer intervals. Hydrophobicity transfer suppresses the conductive film, while accumulated pollution steadily buries the coating and brings in soluble salts. Operating practice controls both variables: arrange cleaning or recoating when ESDD approaches the upper bound of the class or when the HC class has visibly decayed.
What do HC1 to HC7 actually mean?
The seven IEC/TS 62073:2016 spray classes: HC1 isolated beads with no film; HC2 beads, slightly distorted; HC3 flattened drops with streaks or wet marks; HC4 irregular shapes with partial coverage; HC5 water linking into wetted zones; HC6 a water layer over most of the surface; HC7 one continuous sheet, equivalent to a hydrophilic surface. Moving from 1 to 7 means hydrophobicity is being lost.
What is the relationship between RTV and PRTV?
RTV names the curing class — room temperature vulcanising. PRTV usually denotes a durable, prepared-on-site composite system that buys a longer recoat interval and some extra creepage through higher filler loading and film thickness, at the cost of more sensitive application and curing conditions and harder local repair. Because governing standards, film thickness and pass criteria differ, compare and accept on the declared standard (DL/T 627 or another stated standard) and TDS values rather than on names.
Is higher humidity always better for curing?
No — the relationship is not monotonic. Condensation-cure RTV silicone needs airborne moisture to crosslink, so low relative humidity slows it markedly; but very high humidity or a condensing substrate traps water at the interface and produces blistering and adhesion loss. A usable window is normally defined as substrate temperature at least 3 °C above dew point, no rain or dew, and controllable wind, with exact values from the product instruction and DL/T 5727-2016.
Is a thicker film safer?
Not necessarily. Thickness balances hydrophobicity against arc-erosion resistance: too thin and coverage is incomplete and the film burns through; too thick and it sags, cures incompletely inside, and may alter field distribution across the sheds. Follow the TDS and design thickness and check by section sampling rather than adding passes by feel.
Can it be used on DC systems?
DL/T 627-2018 is written for AC insulators and notes that DC may refer to it. DC duty carries a constant voltage component that changes pollution accumulation and flashover development, and the ESDD-to-creepage relationship is not transferable (see the IEC/TS 60815-4 series). A DC project should run its own pollution test or follow a DC selection guide instead of borrowing AC conclusions.
Can we spray straight over fouled old insulators?
Clean first. Low-molecular siloxanes do transfer hydrophobicity into the pollution layer, but only if the coating can wet the substrate and form a continuous film; loose thick dust, oil and chalked layers break adhesion and later flake off. Surface preparation requirements are in DL/T 5727-2016 and the product instruction.
Last updated: 2026-10-13 | Technical editorial by Kexin New Materials (Guangdong) Co., Ltd. Supply terms are quoted EXW / FOB. Figures are compiled from published standards and literature; engineering decisions must follow the design documents and the standard texts themselves.
Sources: DL/T 627-2018 (coating of room-temperature-cured silicone rubber for pollution flashover prevention of insulators; replaces DL/T 627-2012; effective 2019-05-01; ICS 29.080.10); DL/T 5727-2016 (field application code); GB/T 4585 (artificial pollution tests on porcelain and glass insulators for AC systems, 2024 edition); GB/T 26218.1-2010 (MOD IEC/TS 60815-1:2008); IEC/TS 60815-2:2025 (second edition, supersedes 2008); IEC/TS 62073:2016 (guidance on measuring hydrophobicity of insulator surfaces); plus public power-network training material and vendor data for pollution bands and service life. Items marked “compiled from public standard readings” come from secondary summaries on standards platforms and should be checked against the standard text before procurement or acceptance.