Polyolefin or silicone line cover? An honest comparison

The fork is the fitting tool, not the dielectric

Both families are non-tracking, both are tested to ASTM D2303 with no tracking, erosion or flame failure up to 3.25 kV for 20 minutes, and both go onto metal that is already strung up in the air. On dielectric strength alone there is almost nothing to choose between them: 20 kV/mm minimum (ASTM D149) for the cross-linked polyolefin GOC, against >210 kV/cm (ASTM D149) for the self-locking silicone GSOC. Those two figures are the same order of magnitude — 210 kV/cm is 21 kV/mm — printed in different units because the two datasheets were drawn up at different times.

What genuinely separates them is how the cover gets onto the conductor. GOC is a wrap-around cover installed with the Split Insulation Tube Tool, and the manufacturer's description is specific that the tool stays in one location for the fit — the conductor is fed through it. GSOC needs no special tool, and its sheet records that it is installable without difficulty at 0 °C. If your crew is on a live-line pole platform partway along a span, that single row decides the job before any electrical figure is read.

The third construction, interlocking silicone GSIOC, sits between them on paper and below both on voltage: it field-installs over bare conductors for phase-to-phase and phase-to-earth insulation up to 33 kV. It is the option to look at when you want silicone's handling but the run is short and the class is medium voltage.

One product, two voltage ceilings — both are printed here

The polyolefin cover carries two different voltage figures in the manufacturer's own material. The Insulcover product page states insulation enhancement up to 400 kV. The Insulation Enhancement Products sheet, and the later revision of it, both state up to 66 kV for the same wrap-around Insulcover and reserve 765 kV for the silicone. Specify against 66 kV. That is the figure the two more recent documents agree on, it is the conservative one, and where two sources of the same manufacturer disagree the lower published number is the only defensible one to design to. If a tender demands the higher figure, get it confirmed in writing against a specific sheet revision before it goes into a schedule.

Where the silicone wins

Two of these are decisive on their own. The rest are margin.

• Voltage headroom, by an order of magnitude. GSOC-EH is published to 765 kV against 66 kV for the polyolefin. Above 66 kV there is no comparison to make — the polyolefin is simply not offered there.
• No installation tool. The Split Insulation Tube Tool has to be set up in one location and the conductor fed through it. GSOC self-locks by hand, and its sheet records installation at 0 °C without difficulty. On an energised retrofit, or anywhere the crew cannot bring a tool to the working position, this is the whole argument.
• More elongation. 300% minimum (ASTM D638) against 250% for the polyolefin and 200% for GSIOC, which is what lets it take an irregular profile — a bundled jumper, a clamp shoulder — without a wrinkle that later holds water.
• Wall thickness published per voltage class. 2.0 mm at 12 kV, 2.7 and 3.0 mm at 36 kV, 5.0 mm at 765 kV. You can see what the class is buying you. The GOC selection chart gives conductor size, diameter and roll length, and no wall figure at all.
• Retained properties after ageing. After 150 °C for 168 hours (ASTM D2671) GSOC still holds 3.5 N/mm² and 150% elongation (ASTM D638). The polyolefin sheet publishes no post-ageing figure to set against it.
• The derating statement — and its limit. The GSOC feature list states that it does not require further derating of conductors. That statement is made for GSOC and only GSOC. Neither the polyolefin sheet nor the GSIOC sheet says it, so do not carry it across, and do not let a supplier carry it across for you.
• An optional tape layer. Self-fusing silicone tape, 63.5 mm × 0.76 mm, applies over the fitted GSOC cover for added insulation; the quantity depends on conductor size.