3FL Silicone Long Rod Insulators for Overhead Power Lines

Why 3FL — Performance Meets Durability

The Siemens 3FL silicone long rod insulators combine very high electrical insulation and mechanical tensile strength in a compact, lightweight package. The low weight and slim design make them particularly well suited to overhead compact-line applications with low tower designs and short line spans, and they are more economical to transport and install than conventional insulators. They can be used either as suspension or as tension insulators.

Every production step — from incoming raw-material inspection through component assembly to routine testing of the finished product — is monitored under Siemens' long-established high-voltage quality regime.

Design: One-Piece HTV Housing, ECR-Glass Core, Crimped End Fittings

The insulator housing is a one-piece HTV (high-temperature vulcanizing) silicone rubber housing produced in a one-shot injection molding process. The HTV silicone is molded directly onto the core rod, overlapping the triple junction point and part of the metal end fittings. This totally encloses the most sensitive region of a composite insulator — the junction zone where metal end fitting, FRP rod, and silicone housing meet — eliminating traditional sealing systems and preventing any moisture ingress.

The core rod is a boron-free, corrosion-resistant ECR (electrical- and corrosion-resistant) glass-fiber-reinforced plastic rod. Its extremely high hydrolysis and acid resistance completely eliminates the risk of so-called brittle fracture in 3FL insulators.

End fittings of hot-dip galvanized forged steel or ductile cast iron are attached directly to the FRP core rod by a circumferential crimping process, with each crimp monitored by a special control system. A complete range of end fittings to the latest IEC and ANSI standards is available up to 210 kN SML, and the 3FL is 100% exchangeable and compatible with existing insulators and line hardware of all types.

The special end-fitting design in the junction incorporates an integrated grading ring that minimizes electrical field strength and partial discharge inside the junction zone and on the silicone housing surface, reliably preventing corrosion of the insulating material.

Pollution Performance of the HTV Silicone Housing

Thanks to the pollution-layer behavior of HTV silicone rubber, the 3FL remains reliable even under extreme service conditions. The strongly hydrophobic housing stops a conductive film from forming on its surface: even severe ambient conditions such as salt fog in coastal regions or dust-laden industrial air cannot impair the material's intrinsic hydrophobicity. Surface currents and discharges are ruled out, and neither water nor dirt on the housing surface can cause insulator flashover — a significant factor for insulator performance.

Maximized Service Life

The one-piece housing (weathersheds plus core rod sheath) has only one internal interface in the entire insulator — the boundary between housing and FRP core rod. This eliminates all internal interfaces between weathersheds and rod coating, which are normally highly sensitive to tangential electrical field stress and, in worst cases, suffer erosion damage of the polymer interfaces and of the bonding between sheds and rod sheath.

The junction point in the connection zone, where FRP rod, metal end fitting, and silicone housing meet, is water- and air-tight sealed during manufacturing by the overmolding housing system, which totally encloses it with the HTV silicone of the housing itself. The very high bonding strength of the one-piece HTV housing to the FRP core, combined with the overmolding system, prevents moisture ingress at the connection zone.

Following extensive E-field calculations along the insulator — and at the high-voltage connection zone in particular — the 3FL design was optimized for maximum reduction of electrical field stress, reduced corona effect, and minimized RIV value. Two design keys reduce field stress at the triple point and on the silicone surface: (1) the spherical-shaped rim of the end fitting inside the housing homogenizes the E-field distribution on the high-voltage side, acting as an integrated grading ring effective up to 170 kV; (2) the overmolded design and the housing shape at the connection zone displace higher field strength outside the housing into the surrounding air, exploiting the higher relative permittivity of silicone. As a result, 3FL insulators can be applied on systems up to 170 kV without additional grading or corona rings.

Standards and Tests

All 3FL long rod insulators are designed and tested in compliance with the latest IEC standards. Every insulator leaving the factory undergoes a routine mechanical tensile test at no less than 50 percent of its defined SML, held for at least ten seconds.

End Fitting Couplings and Dimensions

Couplings are available as Socket and Ball (IEC 60120), Clevis and Tongue (IEC 60471 / IEC 61466-1), Y-Clevis and Eye (IEC 61466-1). The table consolidates the coupling dimension data by type and SML rating (dimensions A–D in mm as per the standard drawings).

Accessories and Recommended Corona Rings

Arc protection devices such as arcing horns and corona rings for reduction of electrical field stress and corona effect are designed on the basis of extensive electrical field simulations. For system voltages above 170 kV, corona rings are included as a standard feature of the 3FL insulator application. Customer-specific solutions as well as other connection and cable clamps are available on request.

Maximum Values by Type (3FL2–3FL6)

The 3FL family spans five mechanical classes. Maximum section length is quoted for versions with Socket and Ball end fittings, with length increments of 52 mm.

3FL2 — Long Rod Insulators for Distribution, SML 70 kN

3FL2 long rod insulators meet the highest requirements in distribution power systems up to 72 kV, with high lightning-impulse and power-frequency withstand voltages and a long creepage class (> 31 mm/kV). Available end fittings for SML = 70 kN: Ball 16 (IEC 60120, connection length V = 75 mm), Socket 16A (IEC 60120, 79 mm), Clevis 13L (IEC 60471, 87 mm), Tongue 13L (IEC 60741, 87 mm), Y-clevis 16 (IEC 61466-1, 94 mm), Eye 17 (IEC 61466-1, 93 mm).

Section length is the reference value for the Socket-and-Ball size-16 version per IEC 60120; for other end fittings, add the housing length and the connection lengths of both fittings.

3FL3 and 3FL4 — Technical Data (Selected Lengths)

3FL3 (SML 100 kN, routine test load RTL 50 kN) and 3FL4 (SML 120 kN, RTL 60 kN) silicone long rod insulators serve suspension and tension applications for 60 kV through 550 kV. Length increments are 52 mm; intermediate, shorter, or longer lengths are available on request. The table lists one representative catalog length per Um class from the document's full 52 mm-increment listing (the source table contains every increment). Um is based on 25 mm/kV; standard creepage distance is catalog code 3, extra-high creepage is code 4. Section length is for Socket and Ball end fittings; grading ring column gives earth-side/HV-side diameters (x = none). Catalog code pattern: 3FL_-___-3SB11-1__1, where the SML digit is 3 for 100 kN and 4 for 120 kN, and the code's three-digit length field states the nominal housing length in mm/10 (H ≈ field × 10).

For other end-fitting combinations (base: Socket and Ball, catalog code SB), section-length adjustments apply, e.g. Clevis 16L/Tongue 16L (CT) +30 mm, Clevis/Clevis (CC) +31 mm, Clevis/Eye 24 (CE) +40 mm, Clevis/Ball 16 (CB) +20 mm, Tongue/Tongue (TT) +29 mm, Eye 24/Ball 16 (EB) +29 mm, Eye/Eye (EE) +49 mm, Y-clevis 19/Eye 24 (YE) +48 mm, Y-clevis 19/Ball 16 (YB) +28 mm. End fitting connection lengths V: Ball 16 = 108 mm (code B), Socket 16A = 99 mm (S), Socket 16B = 103 mm (R), Clevis 16L = 119 mm (C), Tongue 16L = 118 mm (T), Y-clevis 19 = 127 mm (Y), Eye 24 = 128 mm (E).

3FL5 and 3FL6 — Technical Data (Selected Lengths)

3FL5 (SML 160 kN, RTL 80 kN) and 3FL6 (SML 210 kN, RTL 105 kN) cover the same 60 kV through 550 kV application range with heavier mechanical ratings and size-20/19L/22L fittings. Length increments are 52 mm; intermediate, shorter, or longer lengths on request. The table lists one representative length per Um class from the full listing; catalog codes follow the pattern 3FLx-___-3SB21-1__1. Electrical withstand values match the 3FL3/3FL4 series at equal housing lengths; section lengths and weights are higher due to the larger end fittings.

End fitting connection lengths V for these classes: Ball 20 = 135 mm (code B), Socket 20 = 129 mm (S), Clevis 19L = 145 mm (C), Clevis 22L = 154 mm (C), Tongue 19L = 144 mm (T), Tongue 22L = 153 mm (T), Y-clevis 22 = 156 mm (Y), Eye 25 = 153 mm (E). Section-length adjustments vs. Socket and Ball base: Clevis 19L/Tongue 19L (CT) +25 mm, Clevis/Clevis (CC) +26 mm, Clevis 19L/Eye 25 (CE) +34 mm, Clevis 19L/Ball 20 (CB) +16 mm, Tongue/Tongue (TT) +24 mm, Eye 25/Ball 20 (EB) +24 mm, Eye/Eye (EE) +42 mm, Y-clevis 22/Eye 25 (YE) +45 mm, Y-clevis 22/Ball 20 (YB) +27 mm.

Figures & drawings

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