New Field Study Shows Alumina Content and Microstructure — Not Just Age — Decide How Long Porcelain Insulators Really Last

ITG research presented at the 2025 INMR World Congress applies the Cigré TB 306 framework to long rod insulators — and finds that the routine thermal-cycling test used across the industry can reveal sub-critical aging, but only laboratory bars with big enough built-in flaws show it

PANAMA CITY, Panama — Insulation Technology Group (ITG) has presented new laboratory and field research at the 2025 INMR World Congress examining how porcelain long rod insulators age on overhead power lines — and why some units manufactured decades ago still perform like new. The study, authored by Markku Ruokanen, ITG's R&D and Quality Director, and Marek Vrabec, ITG R&D Engineer, was carried out under the umbrella of Cigré Technical Brochure 306 (TB 306), the industry's standard framework for assessing aging risk in ceramic and glass insulators.

Grid operators around the world face a quiet but costly risk: insulator aging. Unlike storms or cyberattacks, aging rarely makes headlines, yet it follows the same “bathtub curve” familiar to reliability engineers — failures stay low for years, then climb sharply once end-of-life approaches. Replacing lines too early wastes money; replacing them too late risks outages. ITG's research set out to sharpen the tools operators use to judge which is which.

TB 306 assesses aging by tensile-testing one batch of field-collected insulators as received, then subjecting a second batch to an accelerated-aging Thermo-Mechanical Performance (TMP) test before breaking them, plotting both results on a probability-of-failure chart. In TB 306's Scenario F3 example, artificial aging shifts the failing load left enough to cross the Specific Failing Load at roughly 35% probability — meaning advanced aging can be present even in insulators that tested well as received.

The catch: a full TMP test takes 96 hours and typically room for only one or two insulators at a time. Faced with thousands of kilometres of line to evaluate, many transmission operators substitute a much faster routine thermal-cycling test — a 70K thermal shock that takes about 3 hours and can process 60 long rods a week, versus one with TMP. What nobody had confirmed is whether that faster test creates the same kind of sub-critical crack growth the slower one does.

To find out, ITG's team deliberately built “calibrated defects” into laboratory test bars — a classic root-cause-analysis technique — by mixing 1% of 64-micron quartz sand into two porcelain recipes: Body A (a high-alumina C-130 recipe, isostatically pressed) and Body B (a C-120 recipe, plastic-processed). Bars were left unglazed, since glaze can add up to 30% extra strength and mask the very cracking the study was designed to detect.

Fired bars then went through the IEC 60672 thermal-shock test — heated to 120°C and quickly quenched in 20°C water — repeated 3, 10, and 30 times (the standard specifies only 3 cycles; ITG added the extra rounds to see whether more shocks meant more damage). Batches were then broken on a 3-point bending machine and the fracture surfaces examined by electron microscopy at the University of Aveiro, Portugal.

Adding quartz clearly weakened the ceramic — but repeated thermal shocks did not weaken it further. Body A's strength dropped 14% with added quartz; Body B's dropped a smaller 4%. In both bodies, breakage values stayed essentially flat across 3, 10, and 30 thermal-shock cycles.

Chemical and mineralogical analysis explained the gap between the two recipes. Body A carried roughly 8% more alumina than Body B, with a healthier corundum-to-mullite balance and far less microporosity — and came out 35% stronger overall.

Scanning electron microscopy and energy-dispersive spectrometry (ESD) confirmed the story the strength numbers told. Body A's alumina was evenly dispersed through the body; in Body B it tended to clump into agglomerated clusters — a byproduct of Body B's classical plastic manufacturing process versus Body A's isostatic pressing. Even with early cracking observed around quartz particles in Body B, the team could not confirm that repeated thermal shocks pushed those cracks any further.

Their working explanation: at just 10mm in diameter, the lab bars are too small to build up the temperature gradient — and the resulting stress — that would drive sub-critical crack growth. Full-size insulators are a different story: their sheds cool much faster than their thermally-massive core, concentrating stress exactly where routine-tested insulators are known to break, at the shed-to-core junction.

Field Evidence Points the Same Direction

The lab results were reinforced by two field studies. A joint ITG/50Hertz assessment of long rods manufactured 33 years ago found no measurable aging; where a material defect was present, three routine thermal shocks caused only a small, statistically insignificant strength loss. Separately, a non-disclosed assessment of old insulators under TB 306 found that porcelain long rods made in the early 1990s — with high alumina content and corundum dominating over mullite — nearly always resisted sub-critical crack growth.

That finding runs counter to the lifetime curves Freese and Pohlman proposed back in 1999, which projected roughly 20 years of service life for a typical long rod. Insulators with high alumina content and an optimized corundum-to-mullite ratio, the new analysis suggests, can perform like new insulators even after 35 years of service, with the underlying field data pointing to possible lifetimes of 50 years or more for the best-performing recipes.

ITG's conclusions carry a practical message for the industry: material chemistry and microstructure — alumina content, grain size, porosity, and the corundum-to-mullite ratio — control an insulator's resistance to sub-critical crack growth as much as, or more than, age alone. Yet IEC 60672-3:1997 currently defines C-120 and C-130 porcelain only in general terms, without specifying alumina content or microstructure.

ITG is calling for that standard to be revisited, and recommends that any TB 306 assessment of an insulator population be paired with a failure and mineralogical analysis — particularly given how much manufacturing practices, and insulator performance, have shifted since the 1990s.

Insulation Technology Group (ITG) is the parent group of PPC Insulators, LAPP Insulators, and Cerisol, manufacturers of porcelain, glass, and composite insulators for the global power industry. ITG's R&D and Quality organization conducts ongoing materials and field-aging research to support transmission system operators worldwide in managing the lifecycle of their insulation assets.

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This release is based on “Field Ageing Behaviour of Long Rods Analysed According Cigré TB 306” by Markku Ruokanen and Marek Vrabec (ITG), presented at the 2025 INMR World Congress, Panama City, Panama. The full technical paper, including all figures, micrographs, and complete bibliography, is available on request.