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IEC 61858-1:2014 — Modifying Established Insulation Systems: Wire-Wound Winding EIS
IEC 61858-1:2014 provides a systematic framework for evaluating modifications to an established Electrical Insulation System (EIS) used in wire-wound winding electrotechnical devices. When a manufacturer needs to substitute a material — whether due to supply chain changes, cost reduction, or performance improvement — this standard specifies the test procedures required to verify that the modified EIS retains its thermal classification.
Core Principle
The standard operates on a comparative principle: the performance of a candidate EIS (with the modification) is compared against a reference EIS that has proven service experience or has been evaluated by the IEC 61857 series. The goal is not to re-run full thermal ageing from scratch, but to demonstrate that the modification does not degrade the thermal performance below the established class.
The standard operates on a comparative principle: the performance of a candidate EIS (with the modification) is compared against a reference EIS that has proven service experience or has been evaluated by the IEC 61857 series. The goal is not to re-run full thermal ageing from scratch, but to demonstrate that the modification does not degrade the thermal performance below the established class.
1. Types of Modifications Covered
The standard addresses five categories of modifications, each with specific evaluation requirements:
| Modification Type | Clause | Evaluation Approach | When Applicable |
|---|---|---|---|
| Substitution of phase/ground insulation | 5 | Generically identical substitution or limited testing per Table 1 | Slot liners, phase separators, slot sticks, tie cords |
| Substitution of winding wire | 6 | Thermal class verification + chemical compatibility test | Magnet wire replacement (round or rectangular) |
| Substitution of impregnating resin/varnish | 7 | Thermal class determination via IEC 61857-21 or compatibility test | Dip and bake, trickle, or vacuum-pressure impregnation (VPI) resins |
| Substitution with other EIM | 8 | Technically equivalent material demonstration or previous evaluation | Sleeving, tape, lead wire insulation |
| Additions to the EIS | 9 | Compatibility test with existing components | Additional varnish coats, tying cords, phase paper |
2. Evaluation Procedures A through D
The standard defines four increasingly rigorous evaluation procedures:
- Procedure A (Clause 5.1): Generically identical substitution — no testing required if the new material is from the same generic family (e.g., replacing one Nomex-brand aramid paper with another aramid paper of equal or better thickness and thermal class).
- Procedure B (Clause 10): Chemical compatibility test using sealed-tube ageing. The candidate and reference materials are aged together in a glass tube at elevated temperature, and changes in dielectric strength, mass, and appearance are compared.
- Procedure C (Clause 11): Single-point thermal ageing test — the candidate EIS is aged at a single temperature and its end-point time is compared to the reference EIS. If the candidate’s time to failure is equal to or greater than the reference at the same temperature, the modification is acceptable.
- Procedure D (Clause 12): Full thermal ageing test — the most rigorous approach, requiring ageing at three or more temperatures to establish the EIS Relative Thermal Endurance Index (RTE). This is necessary when the reference EIS thermal endurance data is not available or when the modification is substantial.
Engineering Insight: Choosing the Right Procedure
Procedure B (chemical compatibility) is the most commonly used for material substitutions because it balances cost and confidence. A typical sealed-tube test requires only 500–1000 hours of ageing at a single elevated temperature. Procedure D (full ageing) is reserved for major modifications such as changing the wire enamel type or the entire impregnation system.
Procedure B (chemical compatibility) is the most commonly used for material substitutions because it balances cost and confidence. A typical sealed-tube test requires only 500–1000 hours of ageing at a single elevated temperature. Procedure D (full ageing) is reserved for major modifications such as changing the wire enamel type or the entire impregnation system.
2.1 Winding Wire Substitution Rules (Clause 6)
Wire substitution is one of the most common EIS modifications. The standard provides detailed guidance:
- Non-bondable wire: Substitution within the same thermal class (e.g., Class 200 from one manufacturer to another) requires chemical compatibility testing (Procedure B). Downgrading in thermal class requires full ageing (Procedure D).
- Bondable wire: More stringent requirements apply because bondable coatings (self-bonding adhesives) add complexity. Both the wire enamel and the bond coat must be evaluated.
- Conductor material substitution: Changing from copper to aluminum (or vice versa) requires Procedure D because thermal expansion differences can mechanically stress the insulation system.
Important Caveat
The standard notes that “generically identical” does NOT mean “commercially identical.” Two materials from different suppliers may have different additive packages, curing profiles, or processing characteristics that affect long-term thermal performance. When in doubt, perform at least Procedure B testing.
The standard notes that “generically identical” does NOT mean “commercially identical.” Two materials from different suppliers may have different additive packages, curing profiles, or processing characteristics that affect long-term thermal performance. When in doubt, perform at least Procedure B testing.
3. Chemical Compatibility Test (Procedure B — Clause 10)
The sealed-tube chemical compatibility test is a distinctive feature of IEC 61858-1. Its principle is elegantly simple: place specimens of both the reference and candidate materials in a glass tube, seal it, and age it at an elevated temperature. The confined environment accelerates chemical interactions that would occur over years of normal service.
3.1 Test Setup and Key Parameters
- Test temperature: Typically 20 K above the thermal class temperature (e.g., 180 C for a Class 155 (F) system).
- Ageing duration: 500–1000 hours, depending on the materials and temperature.
- Evaluation criteria: Dielectric strength retention (must remain above 50% of initial), mass change, visual appearance (cracking, discoloration, delamination).
3.2 Interpretation
The candidate combination is considered compatible if its retained dielectric strength after ageing is at least equal to that of the reference combination. If the candidate performs worse, the modification is not acceptable without further testing (Procedure C or D).
Common Failure: Catalytic Degradation
A well-known failure mechanism in EIS modifications is catalytic degradation — where one material (e.g., a new polyester-imide wire enamel) accelerates the decomposition of another material (e.g., an existing phenolic-based impregnating varnish). This phenomenon is only detectable through system-level compatibility testing and is the primary reason why single-material data sheets are insufficient for EIS evaluation.
A well-known failure mechanism in EIS modifications is catalytic degradation — where one material (e.g., a new polyester-imide wire enamel) accelerates the decomposition of another material (e.g., an existing phenolic-based impregnating varnish). This phenomenon is only detectable through system-level compatibility testing and is the primary reason why single-material data sheets are insufficient for EIS evaluation.
4. Engineering Design Insights for EIS Modification
- Document the Reference EIS Thoroughly: Before any modification, fully characterize the reference EIS including material specifications, processing parameters (cure time/temperature), and thermal endurance data. Without a well-documented reference, modification evaluation per IEC 61858-1 is not possible.
- Processing Matters: A material substitution may require changes in processing (e.g., higher cure temperature for a new varnish). The standard expects that the modified EIS is processed according to the new material’s recommendations, not the old material’s process.
- Winding Wire Types (Annex A): The standard provides a comprehensive classification of winding wire types (round copper or aluminum conductors). When substituting wire, ensure the new wire falls within the same type classification — Type 1 (single coating), Type 2 (dual coating), or Type 3 (triple coating).
- Cost-Benefit of Full Ageing: While Procedure D is expensive and time-consuming (12–18 months), it provides the highest confidence and may enable up-rating of the thermal class — a potential competitive advantage for motor and transformer manufacturers.
Frequently Asked Questions
Q1: Can IEC 61858-1 be used for form-wound (shaped conductor) windings?
No. IEC 61858-1 is specifically for wire-wound winding EIS. For form-wound EIS, use IEC 61858-2. The companion technical report IEC TR 61857-2 provides guidance on selecting the correct part of the standard based on winding construction.
Q2: How many samples are required for the chemical compatibility test (Procedure B)?
A minimum of 5 specimens each of the reference and candidate combinations are typically required. Additional specimens may be needed for intermediate inspections. The standard recommends including at least one “control” tube containing only the individual materials to distinguish between material ageing and interaction effects.
Q3: What happens if a modified EIS fails the compatibility test?
If the candidate EIS fails Procedure B, the modification may still be acceptable if it passes Procedure C (single-point ageing) or Procedure D (full ageing). The procedures are hierarchical in rigor — failure at a lower level does not preclude success at a higher level, though the cost and time increase substantially.
Q4: Does a change in wire diameter require re-evaluation of the EIS?
A change in wire diameter alone (same wire type and thermal class) is generally not considered a modification requiring full evaluation, provided the impregnating varnish can adequately penetrate the winding. However, a significant change (e.g., from 0.5 mm to 1.0 mm) that affects the thermal mass or resin flow should be assessed on a case-by-case basis.
