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IEC TS 62993:2017 – Guidance for Clearances, Creepage Distances and Solid Insulation (1000-2000 V AC / 1500-3000 V DC)
💡 Key Insight: IEC TS 62993 fills a critical gap in the IEC 60664 series by extending insulation coordination guidance to the voltage range above 1000 V AC and 1500 V DC. This range — used in industrial drives, renewable energy systems, and EV infrastructure — was previously a “grey zone” where designers had to extrapolate from lower-voltage rules or rely on fragmented guidance from various product standards.
1. Scope and Basic Principles of Insulation Coordination
IEC TS 62993 provides guidance for determining clearances (shortest distance through air between conductive parts), creepage distances (shortest distance along an insulating surface), and requirements for solid insulation for equipment with rated voltages above 1000 V AC or 1500 V DC, up to 2000 V AC or 3000 V DC. This voltage range covers a growing segment of electrical equipment including industrial motor drives, large power converters, photovoltaic inverters, energy storage systems, and electric vehicle charging infrastructure.
Insulation coordination is the systematic process of selecting insulation dimensions that provide the required performance under both normal operating conditions and expected overvoltage events. The key principle is to ensure that the insulation system can withstand all voltage stresses that may occur during the equipment’s service life, including continuous operating voltage, temporary overvoltages, transient overvoltages (surges), and recurring peak voltages.
⚠ Design Challenge: As voltage increases, the relationship between clearance and dielectric strength becomes nonlinear due to field effects. At lower voltages (below 1000 V), empirical tables are well-established. In the extended range covered by this specification, factors such as altitude correction, non-uniform electric fields, and partial discharge inception become increasingly significant. Designers must carefully consider these factors to avoid both under-insulation (safety risk) and over-insulation (excessive cost and size).
2. Dimensioning Rules for Clearances and Creepage Distances
2.1 Clearance Dimensioning
The specification provides comprehensive tables and methods for determining clearances based on several key parameters:
| Parameter | Considerations | Impact on Clearance |
|---|---|---|
| Rated impulse voltage | Determined by overvoltage category (I-IV) and system voltage | Primary determinant for transient withstand |
| Working voltage | Maximum continuous voltage across the insulation | Determines steady-state stress |
| Temporary overvoltage | Short-duration voltage increase (e.g., during faults) | May exceed working voltage significantly |
| Recurring peak voltage | Periodic voltage peaks from power electronics switching | Critical for PWM drive applications |
| Pollution degree | PD 1 (clean) to PD 4 (conductive pollution) | Higher pollution requires larger clearances |
| Altitude | Correction factor for altitudes above 2000 m | 1% per 100 m above 2000 m typically |
| Electric field configuration | Uniform vs. non-uniform field distribution | Non-uniform fields require higher impulse withstand |
Clearances are dimensioned to withstand transient overvoltages (impulse voltages) as the primary design criterion, with verification that the clearance is also adequate for the working voltage, temporary overvoltages, and recurring peak voltages. The larger of the values determined by each stress type governs the final clearance.
2.2 Creepage Distance Dimensioning
Creepage distances prevent gradual surface degradation (tracking) and flashover along insulating surfaces. The specification provides creepage distance tables based on:
- Working voltage: The RMS or DC voltage that appears continuously across the insulation
- Pollution degree: The expected conductivity and dryness of the pollution in the micro-environment
- Material group: Classification of insulating materials by their comparative tracking index (CTI):
- Group I: CTI ≥ 600 V
- Group II: 400 V ≤ CTI < 600 V
- Group IIIa: 175 V ≤ CTI < 400 V
- Group IIIb: 100 V ≤ CTI < 175 V
✅ Engineering Insight: For DC applications, the creepage distance requirements differ from AC due to the different electrochemical degradation mechanisms. DC voltage tends to promote electrolytic corrosion and material migration more aggressively than AC. The specification provides specific creepage distance tables for DC applications, which typically require larger distances than equivalent AC voltages. This is particularly relevant for photovoltaic systems, battery storage, and HVDC equipment operating in this voltage range.
3. Solid Insulation and Test Verification
3.1 Solid Insulation Requirements
Solid insulation — including insulating materials used in printed circuit boards, insulation barriers, encapsulating compounds, and wire enamel — must withstand both the continuous voltage stress and any transient overvoltages without breakdown. The specification requires that solid insulation be verified by one of the following methods:
- Withstand test: Application of a specified test voltage for a defined duration (typically 1 minute)
- Partial discharge measurement: Verification that partial discharge does not occur at or below a specified voltage level
- Design-based verification: Using established data for known material systems and thicknesses
3.2 Test Methods and Severities
The specification defines detailed test procedures including conditioning requirements (temperature, humidity) and test voltage levels:
| Test Type | Test Voltage Determination | Acceptance Criteria |
|---|---|---|
| Dielectric strength test | Based on clearance distance (Table 6 of the spec) with altitude correction | No flashover or breakdown during test duration |
| Partial discharge test | Pre-stress voltage then reduced to specified PD extinction voltage | PD level below specified limit (typically 5-10 pC) |
| Solid insulation conditioning | Temperature and humidity exposure per severity level (Table 8) | No reduction in dielectric performance after conditioning |
🚨 Critical Safety Consideration: The specification emphasizes that test voltages must be corrected for test site altitude. A clearance that passes the dielectric test at sea level may fail when the equipment is installed at high altitude due to reduced air density. The altitude correction factors in the specification ensure that the test voltage applied at the manufacturer’s facility adequately represents the conditions at the installation site. This is particularly important for equipment that may be installed in mountainous regions or high-altitude solar installations.
Frequently Asked Questions
Q1: What is the difference between clearance and creepage distance?
Clearance is the shortest distance through air between two conductive parts, measured along a straight line. Creepage distance is the shortest distance along the surface of an insulating material between two conductive parts. Clearance primarily determines withstand against transient overvoltages, while creepage distance determines resistance to tracking and flashover under polluted conditions.
Q2: Why is this specification needed if IEC 60664-1 already covers insulation coordination?
IEC 60664-1 covers insulation coordination for equipment with rated voltages up to 1000 V AC and 1500 V DC. IEC TS 62993 extends the same principles and methodology to the range above these limits, up to 2000 V AC and 3000 V DC, which is not covered by IEC 60664-1.
Q3: How does pollution degree affect creepage distance requirements?
Higher pollution degrees require significantly larger creepage distances. For example, a given working voltage may require a creepage distance that is approximately 2x larger for Pollution Degree 3 compared to Pollution Degree 1. The specification provides separate tables for each pollution degree to ensure adequate performance in the expected installation environment.
Q4: What is the significance of the Comparative Tracking Index (CTI)?
CTI measures a material’s resistance to tracking — the formation of conductive paths on an insulating surface due to electrical stress and contamination. Materials with higher CTI values can tolerate higher voltages at a given creepage distance, or require smaller creepage distances for a given voltage. CTI is determined by standardized test methods specified in IEC 60112.
