IEC 62631-1: Dielectric and Resistive Properties of Solid Insulating Materials

IEC 62631-1:2011 establishes the general framework for measuring and specifying the dielectric and resistive properties of solid electrical insulating materials. As the foundational document of the IEC 62631 series, it defines the terminology, influencing factors, electrode systems, and test procedures used across all subsequent parts covering specific materials and conditions.

Series Structure: IEC 62631 is organized in multiple parts: Part 1 (General), Parts 2-x (Resistive properties), and Parts 3-x (Dielectric properties), each addressing specific measurement methods and material types.

1. Key Definitions and Fundamental Concepts

1.1 Resistive Properties

Volume resistance: The DC resistance measured between opposite faces of a material specimen, characterizing bulk conduction
Surface resistance: The DC resistance measured between two electrodes on the same surface, characterizing surface leakage
Volume resistivity: Intrinsic material property, independent of geometry (ohm-m)
Surface resistivity: Ratio of DC voltage to current per unit width (ohm per square)

1.2 Dielectric Properties

Property	Symbol	Definition	Typical Range
Relative permittivity	epsilon_r	Ratio of material capacitance to vacuum capacitance	2 – 10 (most polymers)
Dissipation factor	tan delta	Ratio of loss current to charging current	0.0001 – 0.1
Loss index	epsilon_r“	Imaginary part of complex permittivity	0.001 – 1.0
Dielectric constant	K	Synonym for epsilon_r	2 – 10

Key Insight: While volume resistivity measures DC conduction through the material bulk, the dissipation factor captures AC losses from dipole relaxation and interfacial polarization — both are needed for a complete characterization of insulating materials.

2. Factors Influencing Dielectric and Resistive Properties

2.1 Time and Frequency Effects

Both resistive and dielectric properties exhibit significant time and frequency dependence. At DC, the measured resistance increases with electrification time as polarization currents decay. At AC frequencies, permittivity decreases and dissipation factor peaks at relaxation frequencies characteristic of specific polarization mechanisms (dipolar, ionic, interfacial).

2.2 Temperature and Moisture

Temperature affects dielectric properties through two competing mechanisms: increased molecular mobility accelerates dipole orientation (increasing permittivity) while also increasing ionic conductivity (increasing losses). Moisture absorption typically increases both permittivity and conductivity, often by orders of magnitude at high humidity levels.

Influencing Factor	Effect on Resistivity	Effect on Permittivity	Effect on tan delta
Increasing temperature	Decreases exponentially	Increases slightly	Increases (conductivity losses)
Increasing frequency	Not applicable (DC)	Decreases step-wise	Peaks at relaxation frequencies
Moisture absorption	Decreases significantly	Increases	Increases
Electric field strength	Decreases at high fields	May increase at high fields	Increases at high fields

3. Electrode Systems and Test Procedures

The standard specifies three primary electrode configurations:

Two-electrode system: Simple parallel-plate configuration for basic measurements
Three-electrode system: Includes guard electrode to eliminate surface leakage currents, essential for accurate volume resistivity measurements above 10¹² ohm-m
Concentric ring electrodes: Specifically for surface resistivity measurements on flat specimens

Critical Engineering Note: The three-electrode system with guard ring is not optional for high-resistivity materials (>10¹² ohm-m). Without a guard electrode, measured values may be dominated by surface leakage rather than the intended volume conduction, leading to errors of several orders of magnitude.

Engineering Design Insights

Test conditioning protocol matters — polymers absorb moisture differently; standard conditioning at 23 C/50% RH for 24 h minimum is essential for reproducible results
Electrification time selection — for DC resistivity measurements, a 1-minute electrification time is standard, but 10-minute values are preferred for materials with slow polarization (e.g., filled epoxies)
Frequency sweep for diagnosis — measuring permittivity and tan delta from 50 Hz to 1 MHz provides a “fingerprint” of material condition; deviations indicate water ingress, curing state changes, or contamination
Temperature correction — resistivity typically follows Arrhenius behaviour; engineers should measure at multiple temperatures to establish the activation energy for predicting performance at operating conditions
Material selection trade-offs — low permittivity (e.g., PTFE, epsilon_r=2.1) minimizes capacitive coupling in high-frequency designs, while higher permittivity materials (e.g., polyimide, epsilon_r=3.5) offer better mechanical and thermal properties

FAQs

Q: What is the difference between volume resistivity and surface resistivity?

A: Volume resistivity measures the resistance to current flow through the bulk of the material and is expressed in ohm-m. Surface resistivity measures resistance to current flow along the surface and is expressed in ohms per square. A material can have high volume resistivity but low surface resistivity due to surface contamination or moisture films.

Q: Why does the dissipation factor (tan delta) peak at certain frequencies?

A: Each polarization mechanism (dipolar, interfacial, ionic) has a characteristic relaxation time. When the applied frequency matches this relaxation time, maximum energy is absorbed, creating a peak in tan delta. This is described by Debye relaxation theory and is useful for identifying specific molecular processes in materials.

Q: How is IEC 62631 structured across its parts?

A: Part 1 (this document) provides general definitions and guidance. Parts 2-1 through 2-3 cover DC resistive properties. Parts 3-1 through 3-3 cover AC dielectric properties including permittivity and dissipation factor measurements across different frequency ranges and temperature conditions.