Insulation Coordination

Contamination: Nonceramic Insulators, Coatings & Icing

Part Three of three. How polymeric (composite) insulators and silicone coatings transform contamination performance: the hydrophobic surface and the silicone flashover mechanism, RTV coatings as a porcelain retrofit, hydrophobicity recovery and its impact on reclosing, ageing and strength test methods, the six methods to improve performance, resistive glaze, and two special problems — icing and bird-streamer flashover.

Reading time ≈ 45 min · Part Three of Three

Section 1

Beyond Ceramic — Nonceramic Insulators and Coatings

Parts One and Two dealt mainly with ceramic insulation. This final part turns to nonceramic (polymeric) insulators, silicone coatings, resistive glaze, the methods used to improve contamination performance, and two location-specific contamination problems — icing and bird streamers.

Ceramic insulators (porcelain and glass) have excellent properties: high mechanical strength, high arc and tracking resistance, UV resistance, a non-porous surface and decades of service experience. But they are not always the best choice in pollution. Over recent decades, nonceramic insulators — treated as equivalent to composite and polymer insulators throughout this page — have been developed and widely deployed, offering better contamination performance, lower weight and more design flexibility.

Continuing from Part Two

Part Two explained how contamination severity is converted into insulation-design requirements using the deterministic and probabilistic methods, altitude correction and LI/SI/TOV considerations. This final part focuses on practical improvement methods: composite insulators, silicone rubber, RTV coatings, resistive glaze, washing and greasing, and the special contamination mechanisms of icing and bird streamers.

The central message of Part Three

Silicone rubber, RTV coatings and composite insulators can significantly improve contamination performance, mainly because of hydrophobicity. But they introduce other design questions — ageing, tracking, erosion, quality control, hydrophobicity recovery after flashover, and suitability for reclosing. Improved pollution performance does not mean the design is automatically solved.

What Part Three covers

how composite insulators are built, and why silicone rubber performs well;
RTV silicone coatings as a retrofit for existing porcelain;
the hydrophobic flashover mechanism and hydrophobicity recovery;
ageing and strength test methods for nonceramic insulators;
the six methods to improve contamination performance, and resistive glaze;
icing and bird-streamer flashover as special contamination problems.

Section 2

What Is a Composite Insulator?

A composite insulator has two main parts: a central fibreglass-reinforced resin rod that carries the mechanical load, and external polymeric weather sheds that provide the insulating surface and protect the rod from moisture, UV and contamination. The sheds may be separately moulded and slipped over the rod, or extruded directly onto it, giving a lightweight long-rod insulator.

Unlike cap-and-pin porcelain strings, composite long-rods normally have no intermediate metal electrodes along the insulation length. That helps, because it avoids the local stress concentration at each disc-to-disc metal interface.

Section 3

Shed Materials and Fillers

The main shed materials are EPDM (ethylene propylene diene monomer), EPM (ethylene propylene monomer) — together grouped as EPR, ethylene propylene rubber — and silicone rubber (SiR). Today silicone rubber, or a silicone/EPDM combination, is common; silicone usually gives very good contamination performance because of its hydrophobic surface.

Polymeric materials can be vulnerable to tracking and erosion if poorly formulated, so fillers are added. A common filler is ATH (alumina trihydrate), which improves resistance to tracking and erosion. This matters because contamination flashover involves leakage current, dry-band arcing, local heating and surface discharge — all of which degrade a material that cannot resist tracking.

Section 4

Why Composite Insulators Perform Well

Hydrophobicity

Silicone rubber surfaces are hydrophobic — they repel water, so it beads into droplets instead of forming a continuous film. Since a continuous wet conductive film is a precondition for contamination flashover, keeping water as separated droplets interrupts the leakage path and reduces leakage current.

Hydrophobicity, in one idea

Hydrophobicity means water does not spread into a continuous film — it forms separated droplets. This is the key concept of the whole page: contamination flashover requires a conductive wet path along the surface, so if the water stays as separated droplets, leakage current is reduced and dry-band arcing is delayed.

Hydrophobicity transfer — why a polluted silicone surface still wins

Silicone rubber does not only repel water on a clean surface. Low-molecular-weight (LMW) silicone chains can migrate through or over the pollution layer and make the pollution surface partly hydrophobic. This hydrophobicity transfer is one of the main reasons silicone rubber keeps performing well under contaminated service conditions, where a porcelain surface would already have formed a conductive film.

Smaller diameter

Composite long-rods often have smaller diameters than station porcelain, which changes the surface current path and wetting behaviour and contributes to excellent contamination performance.

Shed shape

Shed material matters, but so does shed shape. The profile affects pollution accumulation, wetting, drainage, dry-band formation, arc development and self-cleaning — so composite performance is geometry-dependent as well as material-dependent.

Section 5

Silicone Rubber: HTV and RTV

Silicone rubber comes in two main forms. HTV (high-temperature vulcanised) silicone is commonly used to manufacture sheds on composite insulators. RTV (room-temperature vulcanised) silicone can be sprayed or brushed onto existing porcelain insulators as a coating, and is also used in some silicone insulator construction.

With the main technologies now introduced, the table below compares them in one place — their advantage, their limitation and their typical contamination performance:

Table 1 — The main outdoor-insulation technologies compared.

Insulation Type	Main Advantage	Main Limitation	Typical Contamination Performance
Porcelain / ceramic	Long service life; arc and UV resistance	Hydrophilic surface — a wet pollution film can form	Baseline
Silicone rubber (SiR)	Hydrophobicity, strong pollution performance, light weight	Ageing; tracking / erosion risk if poorly designed	Highest of the listed options
EPDM / EPR	Lighter than ceramic; better than porcelain in some cases	More material-dependent; field performance can vary	Better than porcelain, below SiR
RTV-coated porcelain	Retrofit solution; improves hydrophobicity	Coating quality and ageing must be managed	Often close to SiR behaviour

Section 6

RTV Coating on Porcelain Insulators

Rather than replacing an insulator, the porcelain surface can be coated with RTV silicone to give it a hydrophobic surface. This reduces the formation of a continuous wet conductive layer, so leakage current and dry-band arcing fall and contamination performance improves — especially valuable for existing substations and lines where wholesale replacement is costly or difficult. The recommended thickness is about 20 mils:

\[ 20\ \text{mils} = 0.020\ \text{in} \times 25.4 = 0.508\ \text{mm} \approx 0.5\ \text{mm} \]

It is usually applied in three to five spray coats, and can also be brushed or applied over existing coatings. Carrier solvents vary (naphtha, anhydrous cyclohexane, 1,1,1-trichloroethane). A safety point: some carriers are flammable or explosive — naphtha, for example, is explosive and unsuitable for live application — so manufacturer instructions and site safety procedures must be followed.

Section 7

Advantages of Nonceramic Insulators

Utilities first adopted nonceramic insulators mainly for their strength-to-weight ratio and resistance to gunshot vandalism. From a contamination viewpoint, the major advantage is improved contamination performance. Other benefits include lighter weight and easier handling, possible tower compaction, mechanical flexibility, reduced radio noise, lower maintenance in some conditions, and a lower hazard from failed surge arresters when polymeric housings are used.

Section 8

Life Expectancy and Ageing

Ceramic insulators may last 40–50 years; second-generation nonceramic insulators may have life expectancies around 20–30 years. This does not make them unsuitable — it means ageing and long-term material performance must be assessed. Nonceramic insulators can be affected by UV radiation, corona, erosion, tracking, chalking, crazing, moisture ingress, interface defects, seal failure, and (in some historical cases) brittle fracture.

Not "better" just because it is polymeric

Early nonceramic insulators saw failures; intensive research produced better second-generation designs. Composite performance depends strongly on material formulation, design and manufacturing quality, sealing and quality control — so a composite insulator must be a proven design from a reliable manufacturer, assessed for both electrical strength and ageing.

Section 9

Field Experience — Not All Polymers Are Equal

Field performance has differed by material: some silicone designs performed very well while some EPDM/EPR designs performed poorly in certain environments. Material selection and service environment must be matched.

Table 2 — Reported field experience with polymeric and coated insulators.

Site / Condition	Reported Result
Florida Power & Light (9-day, 172 outages)	Porcelain line posts 23 outages/100 mile-years; silicone posts none; one EPDM far worse than porcelain (withdrawn from inventory)
Brighton, SW England (severe sea salt; 34.5/230/500 kV)	Silicone best; 230 kV rubber-shed units no flashovers; 500 kV nonceramic beat porcelain fog-type; EPR ≈ porcelain 20% longer
Western Swedish coast (severe salt spray, 5–9 yr)	HTV silicone insulators and RTV-coated porcelain: no flashovers
PG&E Moss Landing (66 kV, 5-unit strings)	Uncoated string flashed in 1–3 months; RTV-coated string no flashovers in 6.5 years

The lesson: silicone rubber can outperform even a longer porcelain fog-type string, and both HTV silicone and RTV-coated porcelain give strong performance in severe coastal environments.

Section 10

RTV Coatings in the Field

RTV coatings significantly improve existing porcelain without replacement — valuable for substations, coastal and industrial sites, and retrofits where outages are hard to arrange. By suppressing leakage current they may even help eliminate wood-pole fires and flashovers.

At the Milestone power station (345 kV, near Long Island Sound), sea spray caused line and station flashovers; increasing leakage distance to 1041 cm on switchgear was not enough, and a 1985 hurricane caused another complete 345 kV outage. RTV coating was then investigated: laboratory tests showed RTV-coated insulators raised flashover voltage by about 20% to 50%, provided the insulators were allowed to rest 2–3 days after flashover so hydrophobicity could recover.

Section 11

Hydrophobicity Recovery and Reclosing

Hydrophobicity is the key property of silicone surfaces — but flashover can temporarily damage it. After a flashover the surface may behave more like ceramic, and needs time to recover; reported recovery periods range from 8–10 hours to 24–72 hours, depending on material and conditions.

Implication for reclosing

If a line trips on contamination flashover and is reclosed quickly, the surface may not have recovered its hydrophobicity — so a second flashover may occur. Fast reclosing after a contamination flashover may be unsuccessful for silicone-based insulation. This does not mean reclosing should never be used; it means the repeated-flashover risk must be recognised in protection and operating philosophy.

Section 12

Flashover Mechanism on Silicone — The Hydrophobic Stage

When new, silicone rubber is hydrophobic: water beads rather than spreading, so no continuous conductive layer forms and leakage current stays low. Industrial or salt pollution then deposits on the surface. At first the hydrophobic surface still helps, but the crucial difference from porcelain is that silicone rubber can transfer hydrophobicity to the pollution layer.

Low-molecular-weight (LMW) chains

Silicone rubber contains LMW chains that migrate from inside the material to the surface. Within about 10–12 arc-free hours they can form a hydrophobic layer on top of the pollution — so even a polluted surface can recover hydrophobic behaviour. This is one of the main reasons silicone performs well in pollution.

Section 13

Wetting, Filaments and Spot Discharges

Under dew, fog or high humidity, salt pollutants dissolve in water droplets and a resistive layer forms around each droplet, so leakage currents begin to flow — though the surface is not yet a continuous film. As wetting continues, droplet density rises and the gaps shrink; close droplets join into filaments that create more continuous conductive paths.

Voltage then appears between filaments, and small spot discharges occur between droplets or filaments. These discharges destroy local hydrophobicity, creating wet regions and more conductive paths. If the process continues, it develops into flashover.

Section 14

The Silicone Flashover Sequence

Flashover on a silicone surface

the new silicone surface is hydrophobic — water beads;
pollution deposits on the surface;
LMW chains migrate and make the pollution layer hydrophobic;
moisture forms droplets, salt dissolves inside them;
leakage current flows through resistive droplet paths;
droplets densify into filaments; spot discharges ignite;
discharges destroy local hydrophobicity; a wet conductive region forms and extends — flashover.

The key point: silicone delays the development of a continuous conductive wet layer — it does not make flashover impossible.

Section 15

Two Kinds of Test — Strength and Ageing

Two different questions must be answered for composite insulators. Strength tests ask: what voltage can the insulator withstand under a defined contaminated condition? Ageing tests ask: how does the material perform after years of UV, rain, humidity, salt mist, heat, dry periods and electrical stress? Ageing tests are especially important for polymers, because the material can degrade over time.

The CIGRE ageing concept applies several environmental stresses while the insulator is energised at maximum line-to-ground voltage: solar/UV radiation, artificial rain, dry heat, damp heat near saturation, high humidity at room temperature, and slightly polluted fog. Conditions may change every 2 hours, one cycle lasting 24 hours, with a total of 5000 hours suggested to represent about 8 years of service.

Accelerated ageing is comparison, not a life prediction

Accelerated ageing tests (5000-hour, 1000-hour and similar) are useful for comparison and qualification, but they do not reproduce every site-specific ageing mechanism. They should not be read as exact life predictions — field experience, manufacturer quality control and inspection history remain important.

Section 16

Salt-Mist Ageing Test and Pass Criteria

A simpler ageing test energises the insulator in salt mist (salinity around 10 kg/m³) for about 1000 hours, with the mist not directly impinging on the insulator and a trip relay set at 1 A. It passes if there are no more than three tripouts, no shed punctures, the core cannot be seen, no major holes or slits, no severe housing degradation, no tracking, and erosion does not reach the core — criteria that focus on physical degradation, not only flashover. The applied voltage is set from the leakage distance and the IEC medium-contamination value:

\[ V_{\text{test}} = \frac{L_{\text{creep,total}}}{34.6} \]

\(V_{\text{test}}\): applied rms test voltage (kV)
\(L_{\text{creep,total}}\): total leakage distance (mm)
\(34.6\): IEC withstand specific creepage for medium contamination (mm/kV)

Section 17

Accelerated Weather Cycle for Line Posts

Another accelerated approach uses separate winter and summer cycles combining salt mist, rain, UV and applied voltage, with a 5-hour cycle time. The interpretation: 10 laboratory days of summer cycle plus 11 laboratory days of winter cycle represent about one calendar year of service, and the insulators were subjected to roughly 6 years of simulated ageing.

Section 18

Strength Tests Without Destroying Hydrophobicity

For new silicone, applying a uniform contamination layer is difficult: the hydrophobic surface resists water-based mixtures, and scrubbing, sandblasting or chemical treatment would damage the very property being tested. Special methods form a contamination layer while preserving hydrophobicity — for example applying kaolin with a swab and dipping in a kaolin/NaCl slurry; or spraying droplets, sprinkling Tonoko powder, washing, then immersing in a Tonoko/NaCl slurry; or first artificially ageing the insulator (to mimic service) before dipping.

Ideally the strength test follows ceramic practice (three-out-of-four withstand), but that is slow, so some use a flashover method: apply a voltage below flashover for ~30 minutes, then raise in ~5% steps held ~5 minutes each until flashover. This gives an approximate flashover voltage — not necessarily a true withstand — so results need careful interpretation. Note again that after flashover the surface temporarily loses hydrophobicity and behaves more like ceramic until it recovers (8–10 h or 24–72 h).

Section 19

Withstand Voltage versus Salt Deposit Density

Tests on EPDM, silicone and porcelain long-rod insulators showed the withstand voltage falls with severity as a power law, with an exponent close to that of porcelain:

\[ V_{\text{withstand}} \propto SDD^{-0.20} \]

\(V_{\text{withstand}}\): contamination withstand voltage
\(SDD\): salt deposit density

The relationship is not linear. For a tenfold increase in SDD:

\[ \frac{V_2}{V_1} = \left(\frac{SDD_2}{SDD_1}\right)^{-0.20} = 10^{-0.20} \approx 0.63 \]

So a tenfold rise in SDD cuts the withstand voltage to about 63% — which is why contamination severity has such a strong effect on outdoor insulation.

Section 20

How Much Better Are Silicone and EPDM?

Relative to porcelain, silicone rubber gave withstand voltages about 50–60% higher; EPDM about 20–25% higher. In accelerated ageing of line posts, silicone units generally exceeded EPR — improvements over ceramic posts of roughly 29–48% for EPR and 55–83% for silicone. The ranking is:

\[ \text{SiR} \;>\; \text{EPDM/EPR} \;>\; \text{Porcelain} \]

Table 3 — Approximate withstand improvement versus porcelain.

Solution	Improvement vs Porcelain
Silicone rubber (SiR)	≈ 50–60% higher (×1.5–1.6)
EPDM / EPR	≈ 20–25% higher (×1.2–1.25)
RTV-coated porcelain	≈ 20–50% higher (vs uncoated)

Reported ranges, not universal factors

These figures are reported test and field-performance ranges, not universal design factors. The actual improvement depends on the material formulation, the shed profile, the contamination type, NSDD, ageing, surface condition, the wetting process and the test method — so use them as guidance, and confirm against data relevant to the specific design.

Section 21

Nonsoluble Deposit, Layer Thickness and Timing

NSDD means non-soluble deposit density — non-soluble material such as dust, clay or inert particles (Tonoko, kaolin). Even though this material is not highly conductive by itself, it can hold moisture, increase surface roughness, reduce hydrophobic behaviour and so lower the withstand voltage. Large NSDD reduces withstand because the thicker layer reduces surface hydrophobicity and supports more leakage paths — so non-soluble deposits matter too, not only soluble salt.

Increasing the contamination-layer thickness reduces the hydrophobic benefit: a thick pollution layer masks the silicone surface, and if hydrophobicity cannot transfer through it the surface becomes vulnerable — thicker layer ⇒ lower withstand. This is important in dusty desert or industrial areas. Finally, withstand voltage may be slightly lower if testing follows contamination too soon, because the LMW chains need time to migrate and restore hydrophobicity.

Section 22

Resistive (Semiconducting) Glaze

A resistive glaze is a special coating on porcelain that permits a small continuous current to ground:

\[ I_{\text{glaze}} \approx 1\ \text{mA} \;\text{to}\; 8\ \text{mA} \]

Resistive glaze is not RTV

The two work differently. RTV improves performance mainly by hydrophobicity. Resistive (semiconducting) glaze improves it by allowing a small controlled current to flow — the current grades the voltage along the insulator and produces surface heating, which helps keep the contamination dry.

This intentional current improves performance two ways: voltage grading (it distributes voltage more uniformly, reducing local high-stress regions) and surface heating (it keeps the contaminant dry, and a dry layer is less conductive). Dry bands may still form, but the controlled current and heating can eliminate them before they develop into flashover. Field tests have been successful and the glaze is arc-resistant — though its life expectancy has been questioned.

Cold switch-on concern

Cold switch-on means energising a line or item of equipment after it has been de-energised for some time. If the insulators became wet and contaminated while de-energised, there was no leakage-current heating to keep them dry — so energisation can occur under a severe wet-polluted condition. For resistive glaze this is the critical case, because the protective heating current only exists when the line is live.

Section 23

Performance Ranking and the Key Limitation

For the same length, silicone rubber has the best contamination performance, EPDM/EPR is better than porcelain but below silicone, and porcelain has excellent durability but poorer pollution performance than silicone surfaces — \(\text{SiR} > \text{EPDM/EPR} > \text{Porcelain}\). Approximate improvements: silicone ~50% (50–60% higher withstand), EPDM/EPR ~25% (20–25% higher), RTV-coated porcelain 20–50% depending on recovery time and conditions.

The key limitation of silicone-based insulation

Hydrophobicity recovery after flashover. The hydrophobic surface can be temporarily damaged and needs time to recover, so quick reclosing after a contamination flashover may cause repeated flashover — this must be reflected in protection and operating philosophy.

Section 24

Methods to Improve Contamination Performance

The goal is acceptable contamination performance while keeping the connected length as short as possible — ideally no longer than the length needed for lightning, switching, mechanical and clearance requirements. In severe pollution, the main methods are:

Six improvement methods

Nonceramic insulators — especially silicone rubber; best where contamination performance is the main driver.
RTV silicone coating — line or station; silicone-like performance as a practical retrofit, usually better than greasing/washing.
High-leakage / fog-type insulators — more creepage per unit length (creep-to-length ratio ≈ 2.9–4.5); profile must allow effective use.
V-string configuration — better washing and wetting than vertical I-strings; needs suitable tower geometry.
Semiconducting glaze — voltage grading and surface heating; RTV generally considered superior.
Greasing and washing — traditional but maintenance-heavy (grease ≈ 3 mm in GB practice); RTV usually more cost-effective long term.

Which method to choose

A simple decision view: for new lines, consider silicone-rubber composite insulators where service experience supports them; for existing porcelain substations, RTV coating is often an effective retrofit; where ceramic must be retained, high-creepage or fog-type insulators may be used; where line geometry allows, V-strings may help; and washing and greasing should usually be treated as maintenance-based or last-resort solutions.

More creepage is not always enough

More creepage distance usually improves contamination performance, but it is not sufficient by itself. Poor shed design can trap pollution, wet poorly, prevent natural washing or encourage dry-band formation. Creepage distance, shed profile and material must be considered together.

Section 25

CIGRE 1996 Practical Comments

Practical conclusions from a CIGRE session: strength tests for composite insulators are mainly necessary when hydrophobicity is completely lost; in heavy pollution, creepage should be selected as for ceramic insulators; and in light pollution, composites may allow a substantial creepage reduction — though the exact reduction was not clearly established.

Conservative interpretation

Do not reduce creepage distance aggressively unless service experience or validated testing supports it.

Section 26

Ceramic versus Nonceramic — A Balanced View

Table 4 — Ceramic and nonceramic insulators compared.

Aspect	Ceramic (Porcelain / Glass)	Nonceramic (Composite)
Contamination surface	Hydrophilic — wets to a continuous film	Hydrophobic — beads water, transfers hydrophobicity
Strengths	Arc-resistant, nonporous, nontracking, UV-resistant, proven, long-lived	Lighter, flexible, compact, low radio noise, vandal-resistant
Life	40–50 years, decades of experience	20–30 years (2nd gen); ageing-dependent
Weaknesses	Poorer pollution performance	Tracking/erosion/UV; formulation and quality dependent; hydrophobicity loss after flashover

Ceramic is not obsolete and nonceramic will not replace it everywhere — but because of their contamination and mechanical advantages, nonceramic insulators are likely to be used increasingly. The best choice depends on pollution severity, voltage, mechanical loading, maintenance strategy, cost, utility experience, ageing performance and manufacturer quality.

Section 27

Icing as a Special Contamination Problem

A different mechanism from salt or industrial pollution

Icing is not the same mechanism as normal salt or industrial contamination. Ice can bridge sheds, shorten the effective surface path, distort the electric field, and create conductive paths during melting. So ice is both an electrical insulation problem and a mechanical loading problem.

An ice thickness of about 1.5 cm can reduce the power-frequency withstand below the maximum line-to-ground voltage for 230 kV to 765 kV lines — making ice a critical design and operational issue.

Ice affects line and station, ceramic and nonceramic insulators. For nonceramic, shed spacing matters: if it is too small, ice bridges the sheds more easily, reducing the effective creepage and lowering flashover voltage. V-strings performed better than I-strings under ice — because ice accumulation, drainage and bridging between sheds differ from a vertical string, so the geometry can reduce the chance that ice forms a continuous path along the insulation. One study found AC flashover voltage under snow and ice was 25–35% lower than under light contamination, with SI and LI strength also reduced. So icing can affect AC, SI and LI withstand — and must be considered alongside shed spacing, configuration and creepage bridging.

Section 28

Bird-Streamer Flashover

A sudden event, not gradual accumulation

Bird-streamer flashover is a sudden conductive-path event, not a gradual pollution-accumulation process. A bird perched above the insulator can release a conductive stream that temporarily bridges part of the insulation distance and initiates flashover — so it is treated separately from conventional contamination flashover.

Birds can cause line flashover through a streamer — a stream of defecation that forms a conductive path in air near the insulator. If it bridges enough of the insulation or creates a high-stress path, flashover can occur. Unlike normal pollution accumulation, this is a sudden contamination event. A classic laboratory investigation simulated the chemical composition and streaming process and confirmed that bird streamers cause line flashovers — explaining some previously unexplained outages.

To reduce these outages, bird guards (one type called a “crown of thorns”) discourage perching where streamers can fall across the critical path — preventing the bird from occupying a dangerous position rather than improving the insulator electrically.

A warning for compact polymer designs

Composite insulators often allow shorter connected lengths, which is useful for compact design — but it may increase bird-streamer risk, because a shorter insulation length makes it easier for a streamer to bridge the critical path. Compact insulation design should therefore also consider bird perching positions and bird guards.

Section 29

When to Use Each Solution

Nonceramic (silicone) — when pollution is severe, weight reduction or compact design is valuable, vandalism resistance is needed, maintenance access is difficult, or porcelain strings would become too long. RTV coating — when existing porcelain flashes over, replacement is expensive, station equipment is hard to replace, or a retrofit is required (especially in substations). High-creepage / fog-type — when ceramic is preferred, pollution is moderate to severe and extra creepage is needed without over-lengthening the string (check the shed profile). Washing or greasing — for seasonal, known, manageable pollution where recurring maintenance is acceptable; RTV or nonceramic solutions are usually more attractive long term.

Section 30

Summary of Key Quantities

Equation Summary

RTV coating thickness

\( 20\,\text{mils} \approx 0.5\,\text{mm} \)

Withstand vs SDD

\( V_{\text{withstand}} \propto SDD^{-0.20} \)

Tenfold SDD increase

\( 10^{-0.20} \approx 0.63 \)

Silicone vs porcelain

\( V_{\text{SiR}} \approx 1.5\text{–}1.6\,V_{\text{porc}} \)

EPDM/EPR vs porcelain

\( V_{\text{EPDM}} \approx 1.2\text{–}1.25\,V_{\text{porc}} \)

RTV-coated vs uncoated

\( V_{\text{RTV}} \approx 1.2\text{–}1.5\,V_{\text{unc}} \)

Salt-mist test voltage

\( V_{\text{test}} = \dfrac{L_{\text{creep,total}}}{34.6} \)

Resistive-glaze current

\( I_{\text{glaze}} \approx 1\text{–}8\,\text{mA} \)

Snow/ice flashover

\( \approx 25\text{–}35\%\ \text{lower} \)

Section 31

Final Conclusions

Nonceramic insulators and silicone-based coatings provide major improvements in contamination performance, and the most important reason is hydrophobicity: a hydrophobic surface prevents a continuous wet conductive film, reduces leakage current and delays dry-band arcing. Silicone rubber has the best reported performance, with the approximate ranking \(\text{SiR} > \text{EPDM/EPR} > \text{Porcelain}\) for equal lengths. RTV coating on porcelain can match silicone sheds and is an effective retrofit for existing lines and substations.

But nonceramic insulators must be assessed for ageing, tracking, erosion, corona, UV exposure, sealing and long-term service — not electrical strength alone. After flashover, silicone surfaces temporarily lose hydrophobicity and need recovery time, with implications for reclosing. Resistive glaze improves performance by voltage grading and heating, though RTV is generally considered superior. And icing and bird streamers can cause flashover too — ice reducing AC, SI and LI withstand, streamers flashing over even without normal pollution.

Reader should remember

Silicone rubber and RTV coatings improve contamination performance mainly through hydrophobicity, which reduces the formation of a continuous wet conductive film and delays dry-band arcing. But composite insulation must still be checked for ageing, tracking, erosion, hydrophobicity recovery and quality control. Practical mitigation is not only about increasing creepage distance — material, shed profile, coating condition, maintenance, and local environmental risks such as icing and bird streamers must all be considered.

This completes the three-part contamination series. Part One explained the flashover mechanism and the severity quantities; Part Two converted those quantities into insulation-design calculations; and Part Three has shown how material choice, coatings, maintenance and local environmental effects shape the final practical solution.

This is Part Three — the final part of the three-part self-study series on the contamination of outdoor insulation. Part One covers the mechanism, severity and test methods; Part Two covers insulation coordination, altitude and impulse effects.

Section 32

Glossary

Table 5 — Key terms used on this page.

Term	Meaning
Composite insulator	Fibreglass rod with polymeric weather sheds; lightweight long-rod insulator.
Hydrophobicity	Surface property that makes water bead rather than form a continuous film.
SiR	Silicone rubber — best reported contamination performance.
EPDM / EPR	Ethylene propylene (diene) rubber shed materials.
HTV / RTV	High-temperature / room-temperature vulcanised silicone (sheds / coating).
ATH	Alumina trihydrate filler — improves tracking and erosion resistance.
LMW chains	Low-molecular-weight silicone chains that migrate to restore hydrophobicity.
NSDD	Nonsoluble Deposit Density — inert deposit (clay, dust) that lowers withstand.
Resistive glaze	Semiconducting porcelain coating giving voltage grading and surface heating (1–8 mA).
Cold switch-on	Re-energising a de-energised, wet, contaminated insulator — a severe condition.
Bird streamer	Conductive stream of bird defecation that can bridge insulation and flash over.

Part Three Reading now

Nonceramic Insulators, Coatings & Icing

Composite insulators and the hydrophobic flashover mechanism, silicone rubber and RTV coatings, hydrophobicity recovery, ageing and strength tests, the six improvement methods, resistive glaze, icing and bird streamers.

Series progress 3 of 3

Part Two

Coordination, Altitude & Impulse Effects

Turning severity into insulator length — the deterministic and probabilistic routes, station BIL, altitude correction, and the effect of contamination on LI, SI and TOV withstand.