Breakdown Voltage Testing to Chinese National Standards (GB): Dielectric Strength Testing Methods, S

Breakdown voltage testing to Chinese national standards (GB standards) determines the dielectric strength of insulating materials, liquids, and electrical components, verifying that products can withstand the voltages encountered in real service. This technical guide explains the governing GB standards such as GB/T 1408 and GB/T 507, the applicable sample types, standard test methods, industrial applications, and frequently asked questions to help manufacturers plan compliant testing programs.

Technical Overview

Breakdown voltage testing is a fundamental electrical safety and material characterization test that measures the voltage at which an insulating material loses its dielectric capability and allows a destructive conductive current to pass through it. When the applied electric field exceeds the dielectric strength of the material, avalanche ionization occurs internally, resulting in permanent physical damage such as punctures, carbonized tracking channels, or arcing. Because insulation failure is one of the leading causes of electrical equipment accidents, breakdown voltage testing is a mandatory requirement in most Chinese national standards (GB standards) for electrical products, cables, transformers, and insulating materials.

Two closely related quantities are evaluated during testing. Breakdown voltage (Ub) is the absolute voltage, expressed in kilovolts (kV), at which breakdown occurs. Dielectric strength, also called electric strength (Eb), is the breakdown voltage divided by the specimen thickness at the breakdown point, expressed in kilovolts per millimeter (kV/mm). Together, these values define the practical insulation capability of a material and are used for material grading, product design margins, and quality acceptance.

Chinese national standards for breakdown voltage testing are largely harmonized with IEC standards, allowing test results to support both domestic certification (CCC where applicable) and international market access:

• GB/T 1408.1 — Determination of the dielectric breakdown voltage and electric strength of insulating materials, Part 1: Definitions and general requirements (equivalent to IEC 60243-1)
• GB/T 1408.2 — Test methods using power frequency AC voltage (equivalent to IEC 60243-2)
• GB/T 1408.3 — Test methods using direct current (equivalent to IEC 60243-3)
• GB/T 507 — Determination of the power frequency breakdown voltage of insulating liquids (equivalent to IEC 60156)
• GB/T 1695 — Determination of electrical breakdown voltage of vulcanized rubber
• GB/T 1981 — Insulating varnishes and impregnating products for electrical use
• GB/T 3334 and related standards for insulating papers, pressboards, and films

Comparable international methods include IEC 60243, ASTM D149, ASTM D877, ASTM D1816, EN 60243, UL 746A, and JIS C 2110. Accredited third-party laboratories typically perform GB standard breakdown voltage testing under controlled temperature and humidity conditions using calibrated high-voltage test sets with safety-interlocked enclosures, and results are reported with statistical evaluation of multiple specimens.

Testing Samples

Breakdown voltage testing per GB standards applies to a wide range of solid, liquid, and composite insulation systems. Specimen geometry, thickness, and conditioning requirements differ by material family, and the applicable product standard always takes precedence.

Solid Insulating Materials

Engineering plastics, vulcanized rubber, epoxy compounds, phenolic laminates, mica products, ceramic insulators, and glass are tested as flat specimens, typically 1 mm to 3 mm thick. Rigid materials are often tested while immersed in a clean insulating liquid medium, such as transformer oil, to prevent surface flashover so that true internal puncture is achieved.

Insulating Films and Thin Sheets

Polyester (PET), polyimide (PI), polypropylene, PTFE, and capacitor-grade films are tested using small-diameter electrode systems specified in GB/T 1408. Because thin films have intrinsically high electric strength, careful thickness measurement and multi-layer stacking rules are applied, and breakdown statistics often follow a Weibull distribution.

Insulating Liquids

Transformer oil, capacitor oil, cable impregnating compounds, and electro-insulating liquids are tested in a standardized oil cup with two spherical electrodes at a 2.5 mm gap according to GB/T 507. Six breakdowns are performed on each filled cell, with stirring and resting intervals between strikes, and the mean value is reported.

Wires, Cables, and Windings

Enameled magnet wire, insulated cables, motor windings, and coil assemblies are tested using product-specific GB standards with electrodes such as dead-weight metallic cylinders, wrapped foil, or beads. For finished cables, breakdown tests may be conducted in water or air on the insulation between conductors or between conductor and screen.

Components and Finished Products

Insulators, bushings, switchgear parts, printed circuit board laminates, varnished or potted assemblies, and appliance insulation are subjected to breakdown or withstand voltage tests as part of type testing under their respective product safety standards.

Testing Items

Depending on the product specification and end-use environment, the following test items are commonly performed under GB national standards:

• Power frequency AC breakdown voltage test (50 Hz, the most common requirement)
• DC breakdown voltage test for HVDC insulation systems and capacitor dielectrics
• Impulse breakdown voltage test using standard lightning impulse waveforms (1.2/50 μs)
• Dielectric strength (electric strength) determination in kV/mm
• Breakdown voltage after conditioning at elevated temperature, humidity, or thermal aging
• Withstand voltage verification, in which a specified voltage is held for a defined time without breakdown occurring
• Breakdown voltage of insulating liquids, including checks after moisture and contamination exposure

Testing Methods

GB/T 1408 defines three standardized voltage application procedures. Selection depends on the material behavior, the intent of the test, and the referencing product standard.

Short-Time (Rapid-Rise) Method

The voltage is applied uniformly across the electrodes and increased from zero at a constant rate, typically 2 kV/s or a rate specified by the product standard, until breakdown occurs. This is the fastest method and is widely used for routine quality control.

Slow Rate-of-Rise Method

The test begins at a specified fraction of the expected breakdown voltage, then the voltage is raised slowly at a controlled rate until breakdown. This method reduces the time at sub-critical stress and improves reproducibility for materials with slow degradation mechanisms.

Step-by-Step Method

The initial voltage is held constant for a specified period, then increased in equal voltage steps with defined dwell times until breakdown occurs. This approach estimates breakdown voltage under sustained stress conditions and is closer to service behavior, though it is more time-consuming.

General Test Procedure

Regardless of the voltage program, the standard workflow includes specimen conditioning, electrode selection, medium control, breakdown detection, and statistical reporting:

• Condition specimens, typically at 23 ± 2 °C and 50 ± 5 % relative humidity for at least 24 hours unless otherwise specified
• Measure specimen thickness at each test point with appropriate precision instruments
• Select the electrode configuration defined by the material standard, such as 25 mm cylindrical electrodes for rigid solids, 6 mm electrodes or 12.5 mm spheres for films, or the 12.5 mm sphere gap oil cup for liquids
• Immerse thick or high-strength specimens in insulating oil or gas to suppress flashover and surface tracking
• Apply voltage using the selected method and record the breakdown value at the moment of puncture, confirmed by a visible breakdown indication and a permanent conduction path
• Repeat on the required number of specimens, commonly five for solids and six strikes per cell for liquids
• Report the mean, standard deviation, minimum and maximum values, breakdown locations, electrode details, medium, temperature, humidity, and voltage rate of rise

Strict safety measures are essential. High-voltage test areas must be interlocked, grounded, and operated only by trained personnel, and all capacitive elements must be discharged after each test. Valid breakdown is distinguished from surface flashover, and flashover events are recorded but treated separately in data evaluation.

Applications

GB standard breakdown voltage testing supports design, certification, and quality assurance across the entire electrical and electronics industry:

• Power transmission and distribution: transformer oil quality assessment, bushings, insulators, and switchgear insulation verification
• Wire and cable manufacturing: insulation and sheath dielectric strength for power cables, enameled wires, and winding wires
• Motor and generator production: slot insulation, impregnated windings, mica tapes, and varnish systems
• Electronics and components: capacitor films, PCB laminates, connectors, and semiconductor packaging materials
• New energy sector: photovoltaic module insulation, electric vehicle high-voltage systems, and battery pack dielectric verification
• Material research and development: comparative evaluation of new polymers, nanocomposite dielectrics, and thermal aging studies
• Incoming inspection and supplier qualification for insulating raw materials and purchased components

FAQ

What is the difference between breakdown voltage and dielectric strength? Breakdown voltage is the absolute voltage in kV at which a specific specimen fails, while dielectric strength is the breakdown voltage divided by the specimen thickness, expressed in kV/mm. Dielectric strength allows fair comparison between materials of different thicknesses.

Should the test be performed with AC or DC voltage? Most GB product standards require power frequency AC (50 Hz) breakdown testing because it represents normal service stress. DC testing per GB/T 1408.3 is specified for HVDC applications, capacitor dielectrics, and space-charge-sensitive materials, since DC breakdown values often differ significantly from AC values.

How many specimens are required? For solid materials, a minimum of five valid breakdowns is typical, though film standards may require more for Weibull statistics. For insulating liquids, GB/T 507 requires six breakdowns per filled cell, and the mean is reported as the breakdown voltage of the oil sample.

Why do thick specimens flash over instead of breaking down? When the surrounding air ionizes at a lower voltage than the internal puncture value, surface flashover occurs first. Testing in a clean insulating liquid medium with properly submerged electrodes suppresses flashover and forces true internal breakdown.

What factors influence the test result? Specimen thickness and uniformity, electrode geometry, voltage rate of rise, temperature, humidity, moisture content, voids or contamination in the material, and the surrounding medium all affect the measured breakdown voltage, which is why standardized conditioning and reporting are mandatory.

What does a typical test report include? A compliant report from an accredited third-party laboratory includes the tested standard, specimen description and thickness, electrode system, conditioning environment, voltage method and rate of rise, individual and statistical breakdown values, dielectric strength, and a clear pass or fail conclusion against the specified limit.