IEC 61145 — Thermal Endurance of Electrical Insulating Materials by the Fixed Time Method: A Technical Analysis

The long-term thermal endurance of electrical insulating materials is a cornerstone parameter in the design and life-cycle management of virtually all electrical power equipment — rotating machines, power transformers, cables, switchgear, and printed circuit boards. Among the various degradation mechanisms that insulation systems experience in service, thermal aging is arguably the most universal and irreversible. IEC 61145, though now withdrawn and superseded by IEC 60216-7, established the methodological foundation for one of the two principal approaches to thermal endurance testing: the fixed time method. This standard provided a rigorous framework for multi-temperature aging experiments in which specimens are exposed to elevated temperatures for predetermined time intervals, followed by diagnostic property measurement to determine the time required to reach a defined end-point criterion. The engineering principles embedded in IEC 61145 remain directly relevant to modern insulation evaluation practice and form an essential part of any materials engineer’s analytical toolkit.

This article delivers a deep technical examination of IEC 61145, covering the experimental architecture of the fixed time method, the Arrhenius-based statistical framework for lifetime inference, the derivation and interpretation of the Temperature Index (TI) and Halving Interval (HIC), practical guidelines for diagnostic parameter selection, and the relationship between this historical standard and its replacement IEC 60216-7. The discussion is oriented toward practicing engineers and material scientists who design, specify, or qualify electrical insulation systems.

1. Experimental Architecture of the Fixed Time Method 🔬

The fixed time method prescribed by IEC 61145 differs fundamentally from the conventional continuous-aging approach (IEC 60216-1). In continuous aging, each specimen is aged until it fails, yielding an exact failure-time data point. In the fixed time method, by contrast, specimens at each temperature level are removed from the oven at pre-selected time intervals and tested to determine whether the diagnostic property has degraded to or beyond the end-point criterion. The principal advantage of this approach is experimental manageability: test duration at each temperature is known in advance, and the test program can be planned with predictable resource allocation across multiple temperatures simultaneously.

1.1 Temperature Level Selection and Experimental Design

IEC 61145 mandates a minimum of three temperature levels and strongly recommends four. The temperature increment between adjacent levels should lie between 10 K and 20 K. Two boundary conditions govern the choice of temperatures:

• Lowest temperature (T_min): Must produce a thermal life of at least 5,000 hours. This ensures that the test captures the long-term aging behavior relevant to service conditions. In practical terms, T_min is typically chosen such that the extrapolation required to reach 20,000 hours (the standard reference life for TI determination) does not exceed 25 K below T_min — a constraint designed to limit the statistical uncertainty associated with extrapolation.
• Highest temperature (T_max): Should accelerate aging to produce failure within approximately 100 hours. This upper bound is deliberately set to avoid excessively high temperatures that could alter the fundamental degradation mechanism — for instance, crossing the glass transition temperature (Tg) of a polymeric matrix, or triggering thermal decomposition pathways (pyrolysis rather than thermo-oxidation) that are not present at service temperatures.

The fixed exposure intervals should follow an approximately logarithmic progression. Typical sequences employed in practice are: 24 h, 48 h, 96 h, 168 h, 336 h, 672 h, 1,344 h, and 2,688 h. The critical experimental requirement is that at least three to four exposure intervals at each temperature level must yield specimens that have reached the end-point criterion, providing sufficient data for regression analysis.

1.2 Diagnostic Properties and End-Point Criteria

The choice of diagnostic property — the measurable characteristic that tracks material degradation — is perhaps the single most consequential decision in the test design. IEC 61145 permits any property that changes monotonically and measurably with thermal aging, provided it is relevant to the material’s end-use function. The most commonly employed properties include:

• Tensile strength retention: Widely used for thin films, tapes, laminates, and sleevings. The conventional end-point is 50% of the initial value, based on the observation that many insulating materials retain adequate short-term electrical strength even after substantial mechanical degradation, making the mechanical criterion more conservative and hence safer for design purposes.
• Elongation at break retention: Particularly sensitive for elastomeric and rubber-like materials (e.g., silicone rubber, EPDM, heat-shrink tubing). Elongation often degrades more rapidly than tensile strength, making it the more discriminating parameter for these material classes.
• Dielectric strength retention: The most directly relevant parameter for insulation function. However, dielectric strength measurements exhibit inherently higher scatter than mechanical tests, requiring larger specimen populations for equivalent statistical confidence. The standard end-point is again 50% of the initial value.
• Mass loss: Applicable when thermo-oxidative or pyrolytic degradation dominates, but less suitable when crosslinking (which may cause initial mass stability) is the primary aging mechanism.
• Tan delta (dissipation factor), volume resistivity, and surface resistivity: Electrical diagnostic parameters that may be more sensitive to early-stage degradation than breakdown voltage, but require careful control of measurement conditions (temperature, humidity, electrode configuration) for reproducible results.

2. Lifetime Modeling and Statistical Data Processing 📊

The analytical core of IEC 61145 is the Arrhenius reaction rate model, which relates the time to reach the end-point criterion (t) to the absolute temperature (T):

log₁₀ t = A + B / T

where A is a material-specific constant reflecting the intrinsic durability at infinite temperature (intercept), and B is proportional to the activation energy Ea: B = Ea / (2.303 R), with R = 8.314 J/(mol·K). The parameters A and B are estimated by ordinary least-squares linear regression of log₁₀(t) against 1/T. The table below summarizes the complete test design workflow and data-processing pipeline defined in the standard:

2.1 Temperature Index (TI) and Halving Interval (HIC)

Two critical thermal endurance parameters are derived from the Arrhenius regression line:

• Temperature Index (TI): The temperature in degrees Celsius at which the material reaches a specified life — conventionally 20,000 hours. TI is the primary metric for thermal class assignment per IEC 60085 (Class E: 120 °C, B: 130 °C, F: 155 °C, H: 180 °C, N: 200 °C, R: 220 °C, S: 240 °C, C: > 240 °C). An insulating material with TI = 158 °C qualifies for Class F application, albeit with minimal residual thermal margin.
• Halving Interval (HIC): The temperature increase required to reduce the material’s life by half, expressed in kelvins. HIC is mathematically related to the Arrhenius slope: HIC = ln(2) / (B / (T₁T₂)) where T₁ and T₂ bracket the temperature range of interest. Physically, HIC quantifies the sensitivity of the material’s life to temperature excursions: a small HIC (e.g., 5–6 K) means that even modest overheating drastically shortens lifetime, whereas a large HIC (e.g., 12–15 K) indicates substantial thermal robustness. This parameter is of direct practical importance for designing thermal protection schemes and determining permissible overload durations.

IEC 61145 requires that TI be reported together with its 95% lower tolerance limit (LTL), which accounts for both the uncertainty in the regression and the scatter of individual data points. A practical rule of thumb: if TI – LTL exceeds 5 K, the test data are insufficiently precise, and additional specimens or temperature levels are needed. The standard also notes that the LTL, not the TI point estimate, should be used as the basis for thermal class assignment to ensure adequate safety margins.

3. Engineering Applications, Standard Evolution, and Best Practices ⚙️

3.1 Practical Applications of the Fixed Time Method

The fixed time method is particularly well suited to three engineering scenarios:

• New material development and formulation screening: When a materials laboratory needs to compare the thermal performance of multiple candidate formulations (e.g., five different epoxy-anhydride systems with varying accelerator concentrations), the fixed time method allows all formulations to be tested in parallel with a single, synchronized withdrawal schedule. The exact failure time of each specimen need not be determined — only whether it survived each inspection interval — which simplifies data collection and reduces the required number of test ovens.
• Insulation system qualification (IEC 60216-2): For materials with projected TI values above 155 °C (Class F and above), the continuous aging method can require prohibitively long test durations at the lowest temperature. The fixed time method enables the test engineer to plan a duration-bounded program, ensuring that results are obtained within a commercially acceptable timeline while still meeting the statistical requirements of the standard.
• Production quality assurance: A simplified variant of the fixed time method — using one or two judiciously chosen temperatures and a single exposure interval — serves as an effective lot-acceptance screening test. If a production batch of, say, polyester film exhibits a tensile strength retention below the historical baseline after 336 h at 180 °C, it can be flagged for further investigation before being released into the manufacturing supply chain.

3.2 Transition from IEC 61145 to IEC 60216-7

IEC 61145 has been formally withdrawn and replaced by IEC 60216-7: “Electrical insulating materials — Thermal endurance properties — Part 7: Determination of the relative thermal endurance index (RTE) of insulating materials using the fixed time method.” The key technical changes introduced in the replacement standard include:

• Introduction of the Relative Thermal Endurance Index (RTE): Unlike IEC 61145, which focused on determining the absolute TI of a single material, IEC 60216-7 emphasizes comparative testing against a reference material of known TI. This approach significantly reduces testing effort — by testing the candidate and reference materials simultaneously at a reduced number of temperature levels, the RTE can be determined in approximately half the time required for an absolute TI determination. The RTE is defined as the temperature at which the candidate material exhibits the same lifetime as the reference material at its TI temperature.
• Enhanced statistical rigor: The replacement standard mandates formal outlier detection tests (Cochran’s maximum variance test for between-temperature homogeneity and Grubbs’ test for extreme observations within temperature groups). It also introduces non-parametric ranking procedures as a supplementary analysis method when the assumptions of ordinary least-squares regression are not met.
• Expanded diagnostic property guidance: IEC 60216-7 provides updated guidance on modern diagnostic techniques, including Fourier-transform infrared (FTIR) spectroscopy aging indices (e.g., carbonyl index for oxidation), DSC oxidation induction time (OIT), and dynamic mechanical analysis (DMA) storage modulus change. These techniques offer the advantage of requiring smaller specimens and providing mechanistic insight that single-point mechanical or electrical tests cannot.
• Standardized uncertainty evaluation: The new standard explicitly references IEC 60216-3 (Statistical methods) for the computation of confidence and tolerance intervals, ensuring consistency across all thermal endurance evaluations within the IEC 60216 family.

Despite these modifications, the core technical principles laid down in IEC 61145 — multi-temperature fixed-interval aging, Arrhenius regression, and TI/HIC derivation — remain fully intact in IEC 60216-7. Data generated under IEC 61145 are still considered valid and may be used as supporting evidence in insulation system evaluations, provided that the test procedures and data analysis are documented with sufficient detail to demonstrate compliance with the essential requirements of the current standard.

3.3 Common Pitfalls and Engineering Recommendations

Drawing on extensive practical experience with thermal endurance testing, the following recommendations are offered to engineers conducting or specifying fixed-time-method evaluations:

• Specimen population: A minimum of 5–10 replicate specimens per temperature per exposure interval is required. Fewer specimens inflate the confidence interval to the point where the TI estimate becomes practically meaningless. For materials with inherently high property scatter (e.g., short-fiber reinforced composites), 15 or more replicates per condition may be necessary.
• Oven characterization: The aging oven must be characterized for temperature uniformity and stability before commencing the test. Temperature mapping at each planned aging temperature should demonstrate that all specimen locations remain within ±1 K of the set-point over the complete aging period. Air circulation rate and fresh-air exchange (for oxidative aging) also require specification — stagnant air conditions can lead to localized oxygen depletion and artificially prolonged life.
• Multi-stress awareness: The fixed time method evaluates thermal stress in isolation. Real insulation systems in rotating machines, transformers, or cables experience combined thermal, electrical, mechanical, and environmental stresses. When using IEC 61145 (or 60216-7) data for system design, apply appropriate safety factors or supplement the evaluation with multi-stress aging tests (e.g., IEEE 1310 for form-wound machine coils, or IEC 60544 for radiation-thermal combined aging in nuclear environments).
• Extrapolation discipline: The standard permits extrapolation up to 25 K below T_min. In practice, extrapolating from 5,000–20,000 hour test data to a 30-year service life (~263,000 hours) involves an extrapolation factor of 13–50 times. The engineer must critically assess whether the degradation mechanism can reasonably be assumed constant over such an extended extrapolation. Any evidence of mechanism change — even if the regression statistics appear satisfactory — should prompt a more conservative design approach or additional long-term verification testing at the service temperature.
• Documentation completeness: The test report should include, as a minimum: full material identification (supplier, grade, lot number, specimen preparation method); complete aging schedule (temperatures, exposure intervals, number of specimens per condition); diagnostic property data (individual values, not just means); regression coefficients (A, B) with standard errors; TI and HIC with 95% confidence and tolerance limits; and residual diagnostic plots. This level of documentation enables independent review and ensures traceability throughout the equipment design life.

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