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IEC 62062: Round Robin Tests for Comparative Tracking Index (CTI) Evaluation
Improving IEC 60112 Through Multi-Laboratory Testing of Insulating Materials
Introduction and Scope
IEC TR 62062:2002, titled “Results of the Round Robin series of tests to evaluate proposed amendments to IEC 60112,” documents a comprehensive inter-laboratory investigation into methods for determining the Comparative Tracking Index (CTI) of solid insulating materials. The tracking phenomenon – the formation of conductive paths across the surface of an insulating material under electrical stress and contamination – is one of the most critical failure mechanisms in electrical and electronic equipment. When an insulating surface is exposed to moisture and contamination, leakage current can cause local heating, carbonization, and ultimately the formation of a conductive tracking path that leads to short-circuit failure.
The CTI test, originally standardized in IEC 60112, measures a material’s resistance to this tracking process. However, by the late 1990s, the test method had accumulated several areas of ambiguity that led to reproducibility concerns among laboratories worldwide. IEC 62062 was commissioned by IEC Technical Committee 15E to conduct a structured Round Robin exercise to generate the empirical data needed to validate proposed amendments and improve the precision of the standard.
The CTI value is one of the most important parameters in electrical insulation design, directly influencing creepage distance requirements in IEC 60950, IEC 60664-1, and IEC 62368-1. Materials with CTI below 175 V are classified as Tracking Index (TI) materials and require significantly larger creepage distances.
Round Robin Test Design and Materials
The experimental program was divided into two parts. Part 1 used equipment conforming to the existing IEC 60112 3rd edition, establishing a baseline for inter-laboratory variability. Part 2 used equipment modified to meet the proposed draft amendments, allowing direct comparison of the new method’s effectiveness in reducing result scatter.
Six different electrical insulating materials were selected to represent a wide range of tracking behaviors and CTI values:
| Material | Expected CTI Range (V) | Primary Application |
|---|---|---|
| Unsaturated polyester | 575 – 600+ | Electrical insulation, molded parts |
| Flame-retarded polyamide | 400 – 550 | Connectors, switch components |
| Phenolic laminate | 175 – 250 | PCB substrates, terminal boards |
| Flame-retarded PBT | 250 – 375 | Automotive connectors, relay bases |
| Standard PBT | 375 – 500 | General-purpose electrical enclosures |
| Standard polycarbonate | 250 – 375 | Transparent covers, insulating barriers |
A total of 11 laboratories, designated A through K, participated in the exercise, with over 1,700 individual tests performed to determine both the 50-drop and 100-drop tracking points. The testing strategy included both ascending voltage (starting low, increasing stepwise) and descending voltage (starting high, decreasing) approaches to capture potential methodological biases.
The 50-drop and 100-drop test points represent different statistical criteria for CTI determination. Some materials exhibit identical values for both, while others show significant differences. IEC 62062 found that materials with a CTI of 600+ particularly need both determinations because of the voltage limitation of test apparatus at 600 V.
Equipment Variations and Their Impact
One of the most revealing aspects of the Round Robin was the documentation of significant equipment differences across participating laboratories. These variations directly impacted measurement consistency and provided the empirical justification for many proposed amendments.
Voltage drop under load: The IEC 60112 test requires a specific voltage source impedance, but laboratories showed wide variation. Laboratory F’s 1975-vintage spring-loaded units exhibited an extreme voltage drop of 440 V at 600 V setting, while Laboratory D’s commercial apparatus achieved less than 30 V drop. The proposed amendments introduced tighter specifications for transformer rating and voltage regulation.
Electrolyte drop size consistency: The standard requires a specific electrolyte drop mass, but the Round Robin revealed that newly designed apparatus from Laboratory F produced drop sizes ranging from 13 mg to 37 mg against the proposed specification of 19 mg to 24 mg. This finding directly led to a new requirement specifying both maximum and minimum allowable mass for individual drops.
Specimen support material: Laboratories used glass, metal, and other materials for the specimen support table. Metal supports were found to influence the electric field distribution and tracking behavior. The proposed amendments standardized on insulating support materials.
Overcurrent trip characteristics: The existing standard required the overcurrent relay to operate when “a current of 0.5 A has passed for 2 s,” but lacked specification of RMS measurement and precise trip thresholds. Some disconnections occurred at currents as low as 0.49 A, while others required significantly higher currents.
| Laboratory | Apparatus Type | Key Issue Identified | Part 2 Improvement |
|---|---|---|---|
| A, B, C | Identical commercial | High voltage drop (52 V at 600 V) | Used same equipment (non-conforming) |
| D | 1.0 kVA, low drop | Minimal – best baseline equipment | Upgraded overcurrent trip |
| F | 1975 vintage / new design | Extreme voltage drop + variable drop sizes | New apparatus built (but drop size issue remained) |
| G | In-house built | Drop mass marginally high (4.97 g / 200 drops) | Upgraded to conform |
| J | 1975 vintage / new design | Electrode penetration issues | New apparatus, improved results |
Key Findings and Statistical Results
The Round Robin produced several statistically significant findings that directly shaped the amendments to IEC 60112. In Part 1 (existing standard), 56% of 50-drop CTI results across 11 laboratories were identical, and 93% fell within ±1 unit (±25 V) of the reference values. Seven laboratories achieved 100% of their results within this ±1 unit range – a remarkably good result considering the diversity of equipment in use.
The three laboratories (A, B, C) that used identical equipment achieved 83% identical results and 95% within ±1 unit on repeated measurements, demonstrating that the test method itself had acceptable intrinsic reproducibility when equipment was properly standardized.
However, the results from equipment upgrades were mixed. While Laboratory G’s upgraded apparatus achieved a narrower result distribution, and Laboratory J’s new design significantly outperformed its vintage equipment, the high-tech new apparatus from Laboratory F performed worse than its older counterparts due to the drop size uniformity problem. This outcome underscored a crucial lesson: technical sophistication does not automatically translate to measurement quality if fundamental parameters (such as drop mass control) are not adequately specified.
The proposed amendments reduced result scatter without shifting mean values, confirming that the changes improved precision while maintaining continuity with historical data. This is the ideal outcome for any test method revision – better consistency without invalidating legacy qualification data.
Engineering Design Implications
The findings of IEC 62062 have direct engineering consequences. For designers of electrical equipment, the improved precision of CTI testing means that material selection decisions can be made with greater confidence. The recommended additional requirement for individual drop mass limits (19-24 mg per drop) and the clarification of the overcurrent trip specification ensure that CTI values reported by different laboratories are more comparable.
The standard also recommended that the phenomenon of hole formation in test specimens should not invalidate results, as numerically identical CTI values were obtained from specimens with and without hole formation. This practical insight helps avoid unnecessary retesting. However, the standard required that hole formation be reported as part of the test documentation to allow users to assess potential correlations with material behavior in real-world applications.
Furthermore, the introduction of the “no flame persisting for more than 8 s” criterion for failure by burning addressed a previously ambiguous area where test termination conditions were not clearly defined.
Frequently Asked Questions
Q1: What is the practical meaning of a CTI value of, say, 400 V?
A CTI of 400 V means that the material can withstand 50 drops of electrolyte at 400 V without forming a conductive tracking path. This does not mean the material is safe to use at 400 V operating voltage – CTI is a material property indicator, not a direct withstand voltage rating. In practice, CTI influences creepage distance requirements per IEC 60664-1, with higher CTI materials requiring shorter creepage distances for the same voltage.
Q2: Why were both 50-drop and 100-drop points measured?
The 50-drop point determines the CTI per the standard method. The 100-drop point provides the Proof Tracking Index (PTI), which tests sustained tracking resistance. Some materials show identical 50-drop and 100-drop values, while others degrade significantly between 50 and 100 drops. The distinction helps designers select materials appropriate for their specific reliability requirements.
Q3: Does the Round Robin data from 2002 still apply today?
The fundamental physics of tracking has not changed, and the CTI test method (IEC 60112) remains essentially the same with the amendments validated by this TR. The equipment specifications clarified by this study (transformer rating, voltage drop, drop mass limits, overcurrent trip) remain relevant for current testing. However, new insulating materials developed since 2002 (e.g., halogen-free flame retardants, bio-based polymers) may exhibit different tracking behavior and are covered by the standard scope.
Q4: How does CTI relate to PCB quality and reliability?
For PCBs, CTI directly affects the minimum allowable creepage distance between conductive traces, particularly in high-voltage designs and environments with condensation or pollution. PCBs made from materials with lower CTI require wider trace spacing for the same voltage rating. This is especially important in power supplies, motor drives, and industrial electronics where PCB contamination from dust and humidity is common.
