Polymer Suspension Insulators & Composite Polymer Insulators: Advantages, Disadvantages, and Specifications

Polymer (composite) insulators – often called polymer suspension insulators or tension insulators – are modern alternatives to traditional porcelain or glass units on overhead lines. Introduced in the 1960s, these insulators use a fiberglass rod core covered by silicone or EPDM polymer sheds.. Compared to bulky ceramic insulators, polymer designs offer major advantages: they are much lighter and more compact, and their silicone surfaces are inherently hydrophobic (water-repellent)​. In practice, this means less weight on towers and durable insulation even when wet. In this guide we’ll explore those advantages, note the downsides, explain key specifications buyers should check, and outline where polymer tension insulators work best.

Advantages of Polymer Suspension & Composite Insulators

Polymer composite insulators excel in weather resistance. The silicone-based housing repels water and dust, helping prevent surface leakage​. In fact, a silicone rubber insulator remains water-repellent even under rain – water beads up and rolls off instead of forming a conductive film​. This “self-cleaning” effect means polymer insulators keep dry surfaces longer than porcelain or glass, greatly improving performance in polluted or coastal environments. The image above illustrates this principle: the silicone resin coating (chemical formula at right) resists UV rays and acid, and its hydrophobicity prevents dust or HNO₃ vapors from creating conductive paths​.

• Lightweight and Compact: A key benefit is weight. Polymer insulators can be 1/7 to 1/10 the weight of equivalent ceramic units​. Engineers can often shorten towers or use lighter hardware because the insulator string weighs far less​. For linemen and crews, the lighter weight also means easier handling and fewer breakages during transport or installation​.
• Hydrophobicity and Pollution Resistance: The low-surface-energy silicone sheds stay extremely water-repellent​. Even after absorbing moisture, the silicone can transfer its hydrophobicity to dirt on its surface​, so contaminants remain dry. In practical terms, polymer insulators tolerate industrial pollution, salt spray or coastal conditions better than porcelain. Utilities in humid or polluted climates report far fewer wet-flashovers on polymer strings, because the insulator stays “dry” under rain​.
• Extreme Weather & UV Durability: Polymers remain tough under temperature and UV stress. Silicone rubber stays flexible down to very low temperatures (often tested to –50 °C)​, whereas porcelain can become brittle in the cold​. It also resists scorching heat and thermal aging, performing reliably in desert or tropical regions​. Importantly, modern silicone formulas include UV stabilizers, so the sheds resist sunlight for decades. This means a Polymer insulator can endure continuous sun and ozone exposure longer than many earlier materials.
• High Strength-to-Weight: Despite being lighter, polymer insulators have high tensile strength. The fiberglass core rod is very strong, so a polymer suspension string supports heavy line tension with a smaller profile. The low weight also reduces seismic and wind loading on towers. In high-wind or earthquake areas, the flexibility and lightness of polymer insulators allow them to better withstand vibration without cracking.

Overall, these features make polymer suspension insulators highly attractive for new lines where performance in contamination and climate is critical.

Disadvantages of Polymer Insulators

Despite the benefits, polymer insulators have known drawbacks that engineers and buyers must consider. The primary concern is moisture ingress. If water ever breaches the silicone housing (through a crack or a faulty seal), it can penetrate into the fiberglass core, causing leakage currents or internal tracking. In other words, polymer designs rely on perfect housing integrity – a damaged shed can create a conductive path. By contrast, a glazed porcelain is impervious to leakage, so utilities remain cautious about any core-insulator system.

• Moisture & Leakage: As noted, the main disadvantage is susceptibility to water penetration​. If moisture enters, leakage current on the core can occur, leading to flashovers or hidden internal damage. This is why high-quality sealing (e.g. advanced injection molding) is critical.
• Aging and Tracking: Early-generation polymers (which used epoxy resin for the rod or housing) showed aging issues. In fact, studies report that first-generation epoxy-housed insulators suffered UV damage, moisture ingress and electrical tracking or erosion under discharge​. That taught manufacturers to switch to silicone/EPDM materials. Today’s silicone-insulated units are much better, but under extremely harsh conditions (salt spray, industrial acid fumes, heavy corona) even silicone will gradually erode or chalk over many years​. Continuous dry-band arcing can eventually degrade the shed surface. In short, polymer insulators can age if the stress level is very high.
• Mechanical Vulnerability: Polymer insulators are quite robust, but not indestructible. Severe mechanical damage (e.g. a struck or chewed housing) can expose the core. Rodents or wildlife have been known to chew silicon rubber if climbing on lines. Some polymers slowly develop crazing or micro-cracks under UV, which over decades can let in moisture (this is called “chalking”). Any such damage can degrade performance. In extreme salt or industrial environments, careful selection of housing compound (e.g. peroxide-cured high-purity silicone) is needed to avoid deterioration.
• Cost and Track Record: Polymer units usually cost more per piece than an equivalent Porcelain insulator​. (Glass is cheapest; porcelain is in-between; polymers tend to be highest price.) The higher price reflects advanced materials and processing. However, some organizations argue polymers reduce life-cycle costs via less maintenance. Another factor is that polymer insulators have been widely used only since the 1970s; porcelain has decades more proven field life. This means utilities often demand rigorous testing, certifications and warranty from polymer suppliers.

In summary, polymers must be carefully specified and manufactured. When high-quality materials and processes are used, most of the early disadvantages are mitigated – but moisture-sealing and material aging remain the key issues to acknowledge in any application.

polymer insulator specifications and What to Look For

When choosing a polymer Suspension insulator, check that its electrical and mechanical specifications suit your needs. Important specs and features include:

• Mechanical Rating (SML): The Specified Mechanical Load (SML) or short-time mechanical rating is the ultimate tensile strength the insulator can carry. Buyers should ensure the SML meets or exceeds the line tension it will see. A common guideline is to pick a polymer whose SML is equal to or greater than the M&E (mechanical & electrical) rating of the porcelain string it replaces​. The ANSI/NEMA standard for polymer tension insulators also defines load tests (Cantilever Breaking Load, etc.) – make sure the supplier provides SML (and, if needed, cantilever load) values.
• Electrical Ratings: Verify the power-frequency line-to-line and line-to-ground voltage ratings (kV) plus impulse (lightning/switching) withstand levels. Polymer insulators should have defined dry and wet flashover voltages per IEC/ANSI tests. Higher-quality designs often achieve the same voltage rating with shorter rod length, because their creepage distance is larger (see below). Consult the manufacturer’s impulse and switching values if provided.
• Creepage (Leakage) Distance: This is the total surface path length along the insulator (from line end fitting to tower end fitting). A long creepage distance helps prevent flashover under pollution or wet conditions. Look for compliance with pollution-contamination standards (e.g. IEC 60815), which specify minimum creepage per kV for given environments. As a rule, more mm of creepage is better for dirty conditions. The creepage distance “protects the insulator from the impact of contamination and harsh environment”​.
• Arcing (Dry) Distance: Sometimes called air-gap or flashover distance, this is the straight-line distance metal-to-metal (through air) from conductor pin to tower pin including across the sheds​. This dimension determines the dry and impulse flashover voltage. A larger dry arc distance gives higher basic insulation level. Insulator drawings or catalogs often list the dry and wet flashover values; compare these as well.
• Core & Housing Materials: Check the type of fiberglass and resin in the core. Standard E-Glass FRP is common, but boron-free, corrosion-resistant glass (CR-E or ECR) is preferred for suspension insulators to prevent brittle fracture​. The resin (epoxy vs polyester vs vinyl ester) affects strength and thermal stability. The housing should be silicone rubber (or EPDM/silicone blend) rated for outdoor use. Note that exact silicone formulations vary by vendor; look for manufacturers who specify hydrolytic stability and UV resistance.
• Metal fittings: Ensure the end fittings are made of high-strength galvanized steel or a corrosion-resistant alloy. Often, bodies use ASTM A153/A153M hot-dip galvanizing specs for hardware​. The crimp or flange design should securely anchor the rod without stress risers. Some vendors test each insulator with a routine tensile (50% SML) or cantilever test – this routine test load (RTL) guarantees assembly quality.
• Standards Compliance: The insulator should conform to relevant standards. In North America, look for ANSI C29.12 (for composite suspension type) or C29.17 (post insulators). Internationally, IEC 61109 (composite suspension/tension) and IEC 62217 (composite post) apply. Compliance means the insulator underwent defined type tests (mechanical, impulse, temperature cycle, etc.). Always request test reports or certificates when possible.
• Environmental Ratings: Some manufacturers provide environmental class info, such as minimum working temperature, UV index rating, or salt-fog test results. Confirm the working temperature range (polymer insulators often cover –50°C to +50°C or more). If the line is in a corrosive industrial area, ask about specialized formulations (e.g. peroxide-cured silicone).
• Dimensional Details: Finally, review the physical dimensions: insulator length, shed spacing, and stacking arrangement if multiple units are used. The “section length” (end fitting to end fitting distance) and “rubber length” affect how many units fit under a given clearance. The catalog should give mechanical cantilever load (for dead-end use) and tensile data.

By examining these specs – and comparing across vendors – buyers can ensure the polymer insulator truly meets the project’s needs. Remember that polymer tension insulators are usually quoted per 12-inch/305mm section (especially in ANSI specs), so verify exactly what voltage or load each unit covers.

Application Areas and Selection Criteria

Polymer suspension insulators find favor in many special-use cases, but they must be chosen with care. Here are some guidelines for application and selection:

• Polluted or Coastal Environments: Where salt spray, industrial dust or chemical pollutants occur, polymer insulators often outperform porcelain. The hydrophobic silicone sheds keep surfaces clean, and the material transfers hydrophobicity to any deposited particles​. In practice, this means polymer strings can run longer without maintenance in dirty areas. For example, in coastal regions or power plants, utilities often specify silicone insulators because they resist salt-fog flashovers that would plague porcelain.
• Extreme Climates (Hot or Cold): Polymers excel in temperature extremes. In freezing climates, the flexibility of silicone prevents cracking under ice loads or cold shocks​. (Porcelain can crack in severe cold or under ice buildup.) In very hot, sunny areas, the UV-stable polymer can last longer than ceramic. Many polymer insulators are rated to –50 °C and +250 °C​. If your line goes through mountain passes or deserts, polymer may be a safer bet.
• Weight-Sensitive Designs: Since they are much lighter, polymers are ideal when structure or transportation weight is a concern. For ultra-high-voltage lines or very tall towers, saving even a few hundred kilograms per string can reduce foundation and material costs. Lighter insulators also simplify helicopter or long-haul construction. If you are upgrading an existing route to a higher voltage but cannot redesign towers, swapping a porcelain string for a shorter/lighter polymer string (with equal or better creepage) is often done.
• Voltage Level & Configuration: Polymer “suspension” insulators are used for both transmission (tension) and distribution (light to mid-voltage) lines. For very high voltages, polymer insulators are typically used in strings of several units (just like porcelain discs). When selecting, ensure enough sections in series to meet the target BIL and creepage. For distribution pole-top or dead-end uses, polymer post-type or dead-end polymer insulators are also available. Always match the polymer insulator’s voltage class to the line’s nominal voltage. Many manufacturers supply polymer sections up to 345 kV or more in special designs.
• Cost vs. Maintenance Trade-off: Although polymer insulators cost more upfront, in many cases their reduced maintenance pays off. If a polymer line avoids repeated insulator washes or replacements, the life-cycle cost can be lower. Procurement managers should weigh initial price against expected maintenance savings in the specific environment.
• Standards and Compatibility: Finally, ensure the polymer insulator fits system standards. If other lines use ANSI/IEC-tested hardware, keep consistency in testing and performance. For example, if your utility has an ANSI standard string in use, the polymer string should at least meet those tests. And be mindful of accessories: make sure you have compatible clamps or end fittings for the polymer string.

In short, polymer tension insulators are especially compelling when environmental conditions or engineering constraints (weight, space, weather) favor their strengths​. They can be used almost anywhere traditional insulators are, but their benefits shine in the toughest service conditions.

Conclusion

Polymer (composite) suspension insulators bring many advantages – lightweight construction, excellent pollution and weather resistance, and modern materials – making them an attractive option for today’s power grids. Electrical engineers will appreciate their compact design and hydrophobic silicone sheds, while procurement teams benefit from lower long-term maintenance. At the same time, it’s important to be aware of the downsides: polymers require pristine housing and careful spec’ing to avoid moisture-related failures. By checking critical specifications (load ratings, creepage, materials, standards) and matching products to the line’s environment and voltage, users can harness the strengths of polymer insulators. With the right selection, polymer suspension insulators offer a robust solution for overhead lines – especially in harsh climates or where weight savings and reliability under contamination are key​