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IEC TR 63017: Thermal Endurance of Flexible Insulating Sleeving
Evaluation methodology and material selection guide for thermal class qualification
1. Introduction and Purpose of IEC TR 63017
IEC TR 63017 is a Technical Report that provides guidance on evaluating the thermal endurance of flexible insulating sleeving used in electrical equipment. Flexible sleeving is a ubiquitous component in electrical machines, transformers, switchgear, and harness assemblies, serving as primary or supplementary insulation for conductors, bus bars, and connection points. The Thermal Endurance Index (TEI) and Thermal Class designation derived from this report allow design engineers to select the appropriate sleeving material for a given operating temperature profile, ensuring reliable long-term performance under continuous thermal stress.
The report complements the product specifications of IEC 60684 (flexible insulating sleeving) by focusing specifically on thermal aging behavior. While IEC 60684 defines the dimensional, mechanical, and electrical properties of sleeving at room temperature, IEC TR 63017 establishes a standardized aging and test protocol to determine how these properties degrade at elevated temperatures. The document covers the most common sleeving material types: silicone rubber (SR), polyvinyl chloride (PVC), polyethylene terephthalate (PET), polytetrafluoroethylene (PTFE), and glass fiber with various impregnating varnishes.
Thermal class is designated by a number corresponding to the maximum continuous operating temperature in degrees Celsius: Class 105 (A), 130 (B), 155 (F), 180 (H), 200 (N), 220 (R), and 240 (S). A sleeving rated Class 180 (H) must demonstrate a TEI of at least 180 °C per the aging protocol in this report.
2. Test Methodology and Thermal Endurance Assessment
2.1 Accelerated Aging Protocol
The thermal endurance assessment follows the Arrhenius methodology described in IEC 60216 (now consolidated as IEC 60216-1 through IEC 60216-8). Sleeving samples are aged in air-circulating ovens at a minimum of three temperatures (e.g., 200 °C, 220 °C, 240 °C for a Class 180 product). At predetermined intervals — typically 250 h, 500 h, 1000 h, 2000 h, 5000 h — samples are removed and subjected to diagnostic tests. The primary diagnostic endpoint for sleeving is the breakdown voltage retention: the sleeving is considered to have reached end of life when its dielectric strength falls below 50 % of the initial value. Table 1 presents a typical aging matrix.
| Aging temperature (°C) | Sampling intervals (h) | Number of samples per interval | Diagnostic test | End-point criterion |
|---|---|---|---|---|
| 200 | 500, 1000, 2000, 5000 | 10 | Dielectric strength (kV/mm) | < 50 % of initial |
| 220 | 250, 500, 1000, 2000 | 10 | Dielectric strength (kV/mm) | < 50 % of initial |
| 240 | 100, 250, 500, 1000 | 10 | Dielectric strength (kV/mm) | < 50 % of initial |
The time to end point at each temperature is plotted on a logarithmic scale against the reciprocal absolute temperature (1/T). A linear regression yields the activation energy Ea and allows extrapolation to the service temperature. The Thermal Endurance Index is defined as the temperature at which the median life reaches 20 000 hours. For Class 180 sleeving, the TEI must equal or exceed 180 °C at 20 000 h median life.
2.2 Supplementary Diagnostic Tests
Beyond dielectric strength, the report recommends monitoring additional properties: tensile strength and elongation at break (mechanical integrity), mass loss (thermogravimetric analysis, TGA), and visual inspection for cracking, embrittlement, or color change. The combination of electrical and mechanical diagnostics provides a holistic view of thermal degradation. For glass-fiber-based sleevings, tensile strength retention is often the more sensitive indicator, as the glass fibers themselves are thermally stable and it is the impregnating varnish that degrades first.
IEC TR 63017 cautions that the diagnostic test conditions themselves can influence the remaining life measurement. Dielectric strength testing, being destructive, must be performed on separate samples from those intended for mechanical testing. The report recommends a minimum initial sample population of 80 units per aging temperature to account for destructive testing losses and statistical outliers.
3. Engineering Design Insights and Material Selection
3.1 Comparing Sleeving Material Families
The thermal endurance data produced under IEC TR 63017 enables engineers to make informed material choices. Silicone rubber sleeving typically achieves Class 180–220, combining excellent flexibility with high-temperature stability, but its mechanical tear strength is lower than that of PET or glass-fiber alternatives. PVC sleeving, limited to Class 105 (105 °C), is cost-effective for consumer appliances but unsuitable for traction motors or industrial drives. PTFE sleeving offers the highest thermal class (240–260 °C) along with exceptional chemical resistance, making it the standard choice for aerospace wiring and high-temperature sensor leads — though its cost is 5–10 times that of silicone rubber alternatives.
3.2 Thermal Aging and System-Level Reliability
In practical applications, the thermal class of the sleeving must be coordinated with the thermal class of the surrounding insulation system. A Class 180 sleeving used in a Class 130 (B) transformer winding offers substantial margin, while a Class 130 sleeving used in a Class 155 (F) motor would become the life-limiting component. The “weakest link” principle dictates that sleeving should be rated at or above the thermal class of the adjacent magnet wire insulation, slot liner, and impregnating varnish. Many premature motor failures attributed to “winding failure” are, upon post-mortem analysis, found to originate at sleeving degradation at the coil connection points — precisely where the thermal stress is highest due to poor heat dissipation in the end-turn region.
Advancements in mica-reinforced silicone sleeving now allow Class 220 (220 °C) rated products with wall thicknesses as low as 0.3 mm, enabling significant space savings in high-power-density traction motors for EVs and hybrid-electric aircraft. IEC TR 63017 provides the thermal validation framework necessary to qualify these next-generation materials.
4. Frequently Asked Questions
Q1: How does the Thermal Endurance Index (TEI) differ from the “temperature index” used in IEC 60216?
The TEI in IEC TR 63017 is the direct application of the IEC 60216 temperature index methodology to flexible sleeving products specifically. The primary difference is the diagnostic endpoint: sleeving uses dielectric strength retention, whereas IEC 60216 often uses tensile strength or flexural modulus depending on the material family.
The TEI in IEC TR 63017 is the direct application of the IEC 60216 temperature index methodology to flexible sleeving products specifically. The primary difference is the diagnostic endpoint: sleeving uses dielectric strength retention, whereas IEC 60216 often uses tensile strength or flexural modulus depending on the material family.
Q2: Can the report’s methodology be applied to heat-shrinkable sleeving?
Yes, with the important caveat that heat-shrinkable sleeving must be tested in its fully recovered (shrunk) state. The aging behavior of the crosslinked polymer in the shrunk state can differ significantly from the expanded state, so preconditioning at the recovery temperature (typically 125–175 °C for 5 min) is mandatory before beginning the aging exposure.
Yes, with the important caveat that heat-shrinkable sleeving must be tested in its fully recovered (shrunk) state. The aging behavior of the crosslinked polymer in the shrunk state can differ significantly from the expanded state, so preconditioning at the recovery temperature (typically 125–175 °C for 5 min) is mandatory before beginning the aging exposure.
Q3: What is the effect of humidity and vibration on thermal endurance?
IEC TR 63017 addresses dry heat aging only. Combined stress factors (temperature + humidity + vibration) are covered in IEC 60068 (environmental testing) and should be considered separately. In practice, adding 10–20 °C of thermal margin is advisable when the sleeving is exposed to simultaneous moisture and vibration, as hydrolysis can accelerate the embrittlement of polyester-based materials.
IEC TR 63017 addresses dry heat aging only. Combined stress factors (temperature + humidity + vibration) are covered in IEC 60068 (environmental testing) and should be considered separately. In practice, adding 10–20 °C of thermal margin is advisable when the sleeving is exposed to simultaneous moisture and vibration, as hydrolysis can accelerate the embrittlement of polyester-based materials.
Q4: How frequently should the TEI be reassessed for a given product line?
The report recommends re-evaluation whenever the base resin formulation, filler content, or manufacturing process (curing temperature, draw ratio, wall thickness) changes by more than 10 %. A full TEI determination requires 12–18 months of aging; a simplified screening using a single temperature (20 °C above the rated class) with two diagnostic intervals can provide a reasonable confirmation in 4–6 months.
The report recommends re-evaluation whenever the base resin formulation, filler content, or manufacturing process (curing temperature, draw ratio, wall thickness) changes by more than 10 %. A full TEI determination requires 12–18 months of aging; a simplified screening using a single temperature (20 °C above the rated class) with two diagnostic intervals can provide a reasonable confirmation in 4–6 months.
