Failure modes
Why covers fail, and what each test method is trying to predict
A GOC 4 wrap goes over a bare conductor 38 mm across the strands, and every row on its datasheet is a laboratory stand-in for one specific way that wrap will eventually stop working.
Covers give way at the surface, not through the wall
Dielectric strength is the figure everybody quotes and the one least likely to explain a part that came back in pieces. 20 kV/mm (min., ASTM D149) through a wall of a millimetre or more is a very large margin over anything a feeder or a 36 kV cubicle presents to it. Punctures are rare. Outsides are what go.
Six mechanisms account for most of it. Four have a bench test standing in for years of service, printed on the sheet. Two — condensation inside a closed box, and a cover pressed back out of line after somebody opened it — have no test method anywhere in the range.
Scope: parts fitted over metal already assembled and usually meant to come off again. A sleeve shrunk onto a bar during panel build ages and is removed under different rules; the same publisher's busbarsleeves.com carries that subject.
Six ways a fitted cover stops working
Each named with the test that predicts it, or with an honest blank where there is none.
Tracking and erosion
A carbonised conducting path grows across the surface on contaminated moisture, and the same run reports material loss with it. Tracking draws a line; erosion digs a pit. It is why non-tracking leads every polyolefin and silicone cover sheet here.
Ultraviolet and ozone
5,000 hours with no colour change (ASTM G154) sits on the polyolefin line-cover sheet and nowhere else in this group; the silicone feature list claims UV and ozone resistance with no method beside it. Colour change is a proxy for chalking and lost stretch.
Condensation in a cable entry box
A clearance adequate dry stops being adequate when the box breathes overnight and the surfaces wet. It is why the Right Angle and Straight Boots are internally coated with water-resistant red mastic rather than simply pushed over the bushing; the boot is stated against flashover from high humidity, rodents and surge impulse.
Bridging in a panel or substation
The manufacturer's insulation-enhancement literature describes flashover from accidental bridging of phases, or phase to earth, usually by birds, animals, vegetation or airborne debris. No datasheet row predicts it and no cover stops the animal arriving; what a cover removes is the bare metal it would have landed on.
Specified outside its band
Most reports of a cover cracking are a specification error, not a material failure. The four families do not share a temperature band, and three of the four disagree at the cold end. The table below is the whole of it.
Refitted badly after an inspection
The mode unique to a removable part, and the reason this site asks whether a cover comes off again before anything else. One pressed back crooked holds a pocket of damp against the metal it was fitted to protect, and from ten metres below it looks like a cover doing its job.
ASTM D2303 names two endings, not one
The standard's title is Liquid-Contaminant, Inclined-Plane Tracking and Erosion of Insulating Materials, and the useful part is the plural. Tracking is a carbonised conducting path across the surface. Erosion is material gone from it. One run produces both, so a sheet reporting no tracking, erosion or flame failure is reporting three separate absences, and a cover can arrive at any one of them on its own.
Which one arrives first is set by the contaminant, and that is why this method matters more on a covers page than any dielectric figure. Coastal salt, cement dust, fertiliser drift and the film a monsoon leaves all conduct once wetted, and every one of them attacks a surface whose wall is nowhere near its dielectric limit. The polyolefin wrap and the silicone covers publish a D2303 result; the moulded PVC shroud publishes none, so quoting 3.25 kV against PVC borrows a figure from a different polymer. How the two families lay the result out differs, and that is unpacked on how to read a cover datasheet.
It is not a voltage rating. 3.25 kV is a condition applied to a coupon on a bench, while the same silicone cover is offered from 12 kV on GSOC-M to 765 kV on GSOC-EH. The two numbers are not comparable, and a supplier offering the test figure as a withstand has misread their own sheet.
Temperature bands, and the check either side of each
| Cover | Material | Continuous limit | The check either side of it |
|---|---|---|---|
| Insulcover GOC wrap | Cross-linked polyolefin | −45 to +105 °C (IEC 216) | Heat shock 90 °C for 4 hr, low-temperature flexibility −40 °C for 4 hr, both no cracking (ASTM D2671) |
| GSOC self-locking cover | Silicone rubber | Not printed on the sheet | Air ageing 150 °C for 168 hr (ASTM D2671); installable without difficulty at 0 °C |
| GRB / GSB heat-shrink boot | Cross-linked polyolefin | −40 to +110 °C | Recovers onto the bushing at a shrink temperature of 125 °C |
| BUSBOOT moulded shroud | Flexible PVC | −20 to +115 °C (IEC 216) | Flammability pass (UL 94-V0); no ageing run printed |
IEC 216 is printed beside the continuous limit on the Insulcover and BUSBOOT sheets. The Right Angle / Straight Boot page lists IEC 216 among its standards but not against the temperature figure, so that band is given without a method. The GSOC sheet publishes an ageing run and a cold-install statement and no continuous limit at all.
Three of those bands do not overlap at the cold end, so a shroud lifted off an indoor panel schedule and fitted to an outdoor terminal box in a hill station is outside its published range before it is energised. The silicone ageing row is the most useful in the group because it prints an answer rather than a pass mark: after 168 hours at 150 °C the cover holds 3.5 N/mm² and 150% elongation (ASTM D638) against unaged figures of 7 N/mm² and 300%. It halves, then holds. No polyolefin cover here publishes a retained value to set beside that.
Two manufacturer sheets, two sets of Insulcover mechanical figures
The wrap-around polyolefin cover is published twice with different numbers against the same GOC 1 to GOC 4 chart. The Polyolefin Overhead Line Cover page gives tensile strength 9 N/mm² (min.), ultimate elongation 250% (min.) and volume resistivity 1 × 10¹³ Ω·cm (min.). The Insulation Enhancement Products sheet gives 10 N/mm², 400% and 1 × 10¹⁴ Ω·cm — and carries a row the product page omits: accelerated ageing at 90 °C for 7 days, tensile and elongation both inside ±25% variation. Specify against the lower pair, and ask for the ageing row in writing against a named sheet revision, because only one document has it.
Which published test predicts which failure
- ASTM D2303 — tracking and erosion
- The surface mechanism, reported as a voltage held for a time. It predicts behaviour on a wet, dirty surface and nothing else.
- ASTM G154 — ultraviolet weathering
- 5,000 hours with no colour change, on the polyolefin line cover. Accelerated UV exposure ranks compounds against each other; it is not a count of years on a pole, and nothing published converts one into the other.
- ASTM D2671 — heat shock, cold flexibility, air ageing
- Three questions from one standard. 90 °C for 4 hr and −40 °C for 4 hr are short soaks bracketing transport, storage and the fit; 150 °C for 168 hr is the ageing run.
- IEC 216 — continuous temperature limit
- The band the polymer may sit in permanently, measured on the material. It is not a rating for the conductor or joint underneath, and none of these sheets carries a conversion between the two.
- ASTM D570 — water absorption
- 0.5% (max.) on the polyolefin wrap and the silicone cover alike. Read it beside volume resistivity, which is measured on a dry coupon and says little about a compound that swells through a monsoon.
- ASTM D638 and ASTM D2240 — tensile, elongation, hardness
- The handling predictors, and the nearest proxy for whether a part survives being taken off and put back. 40 ± 5 Shore D on the polyolefin wrap against 65 ± 5 Shore A on the moulded PVC shroud separates a part that shrugs off a knock from one that yields under a thumb.
- ASTM D149 — dielectric strength
- The figure that predicts least here. 20 kV/mm on the polyolefin wrap, 16 kV/mm on the PVC shroud, 15 kV/mm on the heat-shrink boot — all far above the duty, and not one a touch rating.
What no cover sheet in this range publishes
No allowable erosion depth, no inspection interval, no end-of-life criterion. No abrasion or cut-through result anywhere in the group. No continuous temperature limit on the GSOC sheet and no UV figure with a method beside it. And nothing describing what a cover is worth after being removed and refitted once. Put each of those to the manufacturer against a named sheet revision rather than borrowing a number from a neighbouring product because the polymer sounds similar.
Where each of these is decided
- How to read a cover datasheetEvery method above decoded, and which columns are missing.
- Fitting covers, shrouds and capsEarly failures are fitting defects. Includes the removal half.
- Wrap-around polyolefin line cover, GOCThe fullest test column in the range, and the tool it needs.
- Silicone line covers, GSOC and GSIOCThe 150 °C ageing run and the 0 °C install, in full.
- Shrouding a switchgear panelWhere condensation and bridging happen, point by point.
- Covered conductor against a retrofit coverThe one failure that is never the cover's.
Send the failure, not the part number
A failure question is answerable only against the part actually fitted. Send the cover code or a photograph with a rule laid in it, the voltage class, indoors or outdoors, and what the surface looks like — carbon tracks, a pit, chalking, a split along the closure, or nothing visible at all.