Impulse Withstand Voltage Test: Standards, Procedures, and Applications

The impulse withstand voltage test verifies the ability of electrical insulation to withstand short-duration, high-magnitude transient overvoltages caused by lightning strikes and switching operations. Performed in accordance with IEC 60060-1, IEC 60664-1, IEC 61010-1 and related standards, the test applies a standardized 1.2/50 µs impulse waveform to confirm insulation coordination and product safety. This article explains the test principle, applicable sample types, test items, procedures, acceptance criteria, and typical applications.

Technical Overview

The impulse withstand voltage test, also known as the lightning impulse withstand voltage test, is a dielectric test that evaluates whether the insulation system of electrical equipment can endure transient overvoltages without flashover, puncture, or disruptive discharge. Transient overvoltages arise mainly from atmospheric lightning discharges and from switching operations such as breaker opening, capacitor bank energization, and fault clearance. These surges can reach several kilovolts or more within microseconds, stressing insulation in a manner fundamentally different from continuous power-frequency voltage.

The standard lightning impulse waveform specified in IEC 60060-1 is the 1.2/50 µs impulse, in which the virtual front time is 1.2 µs and the virtual time to half value on the tail is 50 µs. The test voltage is applied between live parts and accessible conductive parts, between circuits that require isolation, or across insulation gaps, verifying the basic insulation level (BIL) of the equipment. For high-voltage apparatus above approximately 300 kV, a 250/2500 µs switching impulse is additionally specified.

The impulse withstand voltage test is a core element of insulation coordination as defined in IEC 60664-1, and it is required by numerous product safety and equipment standards, including:

• IEC 60060-1 and GB/T 16927.1: High-voltage test techniques, general definitions and test requirements
• IEC 60664-1: Insulation coordination for equipment within low-voltage supply systems
• IEC 61010-1 and UL 61010-1: Safety requirements for measurement, control, and laboratory equipment
• IEC 62368-1 and IEC 60950-1: Safety of information technology and audio/video equipment
• IEC 61439-1: Low-voltage switchgear and controlgear assemblies
• IEC 60076-3: Insulation levels, dielectric tests, and external clearances in air for power transformers
• IEC 61180 and GB/T 17627: Impulse test techniques for low-voltage equipment

Testing Samples

High-Voltage Power Equipment

Power transformers, instrument transformers, gas-insulated switchgear, circuit breakers, disconnectors, bushings, post insulators, surge arresters, and cable accessories routinely undergo impulse withstand voltage testing as type tests and, in some cases, routine tests. For these products, the test confirms the rated lightning impulse withstand voltage assigned to the specified highest system voltage.

Low-Voltage Electrical and Electronic Equipment

Household appliances, information technology equipment, audio/video products, laboratory instruments, industrial control panels, and medical electrical devices are tested to verify clearances and insulation coordination against the transient overvoltage levels associated with their installation category (overvoltage category I to IV).

Renewable Energy and New Energy Products

Photovoltaic inverters, wind power converters, energy storage systems, electric vehicle supply equipment, and on-board high-voltage components for electric vehicles are exposed to significant surge environments and are verified through impulse withstand voltage tests referenced in standards such as IEC 62109-1 and IEC 61851-1.

Components and Materials

Relays, contactors, switches, terminal blocks, connectors, printed circuit board assemblies, insulation materials, and cable assemblies can be evaluated individually to characterize their impulse withstand capability and to support clearance and creepage design decisions.

Testing Items

Depending on the product standard and the intended installation environment, the following test items are typically performed:

• Rated lightning impulse withstand voltage (Uimp) verification using a 1.2/50 µs waveform
• Switching impulse withstand voltage verification using a 250/2500 µs waveform for extra-high-voltage apparatus
• Chopped-wave impulse test, in which the impulse tail is deliberately cut by a chopping gap, mainly for transformer insulation verification
• Wet and dry impulse tests for external insulation of outdoor equipment such as insulators and bushings
• Positive and negative polarity impulse application to account for polarity effects on insulation
• Basic insulation level (BIL) verification for high-voltage equipment insulation coordination

For a 230/400 V low-voltage supply system, IEC 60664-1 assigns typical rated impulse withstand voltages according to overvoltage category:

• Overvoltage Category I (signal-level equipment): 1.5 kV
• Overvoltage Category II (pluggable consumer equipment): 2.5 kV
• Overvoltage Category III (fixed installation equipment): 4.0 kV
• Overvoltage Category IV (service entrance equipment): 6.0 kV

Testing Methods

Test Setup and Equipment

The test circuit consists of an impulse voltage generator, a voltage measuring system, and a digital recorder or oscilloscope compliant with IEC 61083. The impulse generator, typically based on the Marx circuit principle, charges capacitor stages in parallel and discharges them in series to produce a single high-voltage impulse with the required peak value and waveform shape. The output is measured through a calibrated damped capacitive or resistive voltage divider, and the recorded waveform is evaluated for peak value, front time, and time to half value within the tolerances of IEC 60060-1, namely ±3% on the peak value, ±30% on the front time, and ±20% on the time to half value.

Sample Preparation and Ambient Conditions

The sample is de-energized and mounted as in normal service or according to the condition of worst-case stress defined by the product standard. Components that could be damaged by the impulse or that would intentionally clamp the surge, such as surge protective devices, may be disconnected when the standard permits. Standard laboratory ambient conditions are a temperature of 15 °C to 35 °C, relative humidity of 45% to 75%, and atmospheric pressure of 86 kPa to 106 kPa, unless otherwise specified. Atmospheric correction factors are applied to external insulation tests, and altitude corrections are required for equipment intended for use above 1000 m.

Test Procedure

The typical test sequence is as follows:

• Identify the insulation under test (basic, supplementary, reinforced, or double insulation) and determine the required impulse voltage from the applicable standard, considering the rated voltage, overvoltage category, and insulation type.
• Connect the generator output between the short-circuited live parts and the bonded accessible parts or reference earth, and ground the measuring system correctly.
• Calibrate the waveform at a reduced voltage, commonly around 50% of the specified test level, and verify the waveshape parameters.
• Apply the specified number of impulses at the full test voltage. For most equipment safety standards, such as IEC 61010-1, this consists of three positive and three negative impulses at intervals of not less than 1 second. High-voltage transformer standards define their own sequences, such as reduced full waves followed by full-value and chopped waves.
• Record each impulse waveform and monitor the sample for flashover, breakdown, or disruptive discharge during and after the application.

Acceptance Criteria

The sample passes the test if no disruptive discharge, flashover, or insulation puncture occurs during the impulse applications, and no damage affecting safety is observed afterward. For transformer tests, waveform comparison using the normalized residual deviation (k-factor) method may be applied to detect internal partial breakdown even without external flashover. Any indication of breakdown constitutes a failure and requires design or manufacturing corrective action.

Applications

The impulse withstand voltage test is applied throughout the product life cycle and across the power industry:

• Product safety certification: demonstrating compliance with IEC, EN, UL, and GB based standards in accredited third-party laboratories as part of market access and conformity assessment
• Insulation coordination design: verifying that clearances and solid insulation in new designs withstand the transient levels of the intended overvoltage category
• Type testing and routine testing: confirming manufacturing quality and consistency for transformers, switchgear, and controlgear assemblies
• Acceptance and commissioning: validating high-voltage equipment insulation condition prior to energization in substations and industrial plants
• Research and failure analysis: characterizing new insulation materials, studying aging effects, and diagnosing the root cause of in-service insulation failures

FAQ

What waveform is used in the impulse withstand voltage test?

The standard lightning impulse waveform is 1.2/50 µs as defined in IEC 60060-1, meaning a virtual front time of 1.2 µs and a virtual time to half value of 50 µs. For extra-high-voltage equipment, a 250/2500 µs switching impulse is also specified.

How many impulses are applied during the test?

Most equipment safety standards, including IEC 61010-1 and IEC 62368-1, require three positive and three negative impulses at intervals of at least 1 second. High-voltage apparatus standards such as IEC 60076-3 define different sequences involving reduced full waves, full waves, and chopped waves.

How does the impulse test differ from a power-frequency dielectric strength test?

A power-frequency (AC or DC) dielectric test applies a sustained voltage to verify bulk insulation integrity and leakage behavior, while the impulse test simulates fast transient surges that stress clearances and insulation coordination. The two tests are complementary and are usually both required for comprehensive safety verification.

How is the required test voltage determined?

The test voltage is derived from the product standard based on the nominal supply voltage, the overvoltage category of the installation environment, the type of insulation (basic, supplementary, or reinforced), and the altitude of intended use, following the coordination tables of IEC 60664-1.

Is the impulse withstand voltage test destructive?

The test is intended to be non-destructive for compliant products. If the insulation is inadequate, the impulse may cause flashover or permanent dielectric breakdown, which constitutes a test failure and identifies a design or manufacturing deficiency.

Does altitude affect test results?

Yes. Air insulation strength decreases with altitude, so standards require correction factors for equipment designed for use above 1000 m, either by increasing the test voltage or by enlarging the required clearances at the design stage.