Square wave high current impulse testing, also known as rectangular impulse current testing, is a critical method for evaluating the surge energy withstand capability of overvoltage protection components such as metal oxide varistors (MOVs), gas discharge tubes (GDTs), and surge protective device (SPD) modules. Performed in accordance with IEC 60060-1, IEC 61051-2, and the IEC 61643 series of standards, this test applies a flat-topped, long-duration current waveform—commonly a 2 ms square wave—to verify energy absorption, voltage stability, and structural integrity. Accredited third-party laboratories rely on this technique to qualify components used in power supply, telecommunications, photovoltaic, and industrial surge protection applications.
Technical Overview
Square wave high current impulse testing is an electrical stress test in which a device under test (DUT) is subjected to a defined number of high-amplitude, long-duration rectangular current impulses. Unlike short lightning-type waveforms such as the 8/20 µs or 10/350 µs impulse, the square wave maintains a substantially constant current plateau—typically 2 ms (2000 µs) or another duration specified in the applicable component standard. Because the current flows for a much longer time, the test deposits significantly greater thermal and electrical energy into the component, making it an effective benchmark for long-duration energy handling capability.
The test is widely applied to nonlinear resistive surge protective components, most notably metal oxide varistors, where it establishes the rectangular impulse current withstand rating and supports the assignment of energy absorption classes. The waveform is generated by a pulse-forming network and verified with a calibrated measurement system to ensure compliance with the tolerances defined in the governing standards.
Key Waveform Parameters
According to IEC 60060-1, a rectangular impulse current is characterized by the following parameters:
- Peak value of the current plateau, with the plateau defined as the region where the current remains above a specified fraction of the peak value.
- Virtual duration of the plateau, commonly 2 ms (2000 µs), with alternatives such as 500 µs or 1000 µs specified by certain product standards.
- Tolerances on peak value and plateau duration, including limits on overshoot, undershoot, and oscillation superimposed on the flat top.
- Total impulse duration, including the rise and decay portions of the waveform.
- Charge transfer, calculated as the integral of current over time, which expresses the electrical charge delivered to the component.
Governing Standards
Testing is typically conducted with reference to the following standards and their national equivalents:
- IEC 60060-1 / IEC 60060-2 – High-voltage test techniques: definitions, waveform requirements, and measurement system verification.
- IEC 61051-1 and IEC 61051-2 – Varistors for use in electronic equipment: general specifications and test methods, including the rectangular impulse current test.
- IEC 61643-311 and IEC 61643-331 – Component specifications for gas discharge tubes and metal oxide varistors used in SPDs.
- GB/T 10193 and GB/T 10194 – Chinese national standards for varistors, harmonized with the IEC 61051 series.
- UL 1449 – Referenced for surge protective device requirements where long-duration current withstand of internal components must be demonstrated.
Testing Samples
Metal Oxide Varistors (MOVs)
Disc-type, strap-type, and chip-type varistors represent the primary sample category. These components are tested to verify their rated rectangular impulse current, which directly determines the maximum surge energy class assigned in the manufacturer's datasheet.
Thermally Protected Varistors
Varistors integrated with thermal disconnect mechanisms are evaluated to confirm that the protective element withstands the specified rectangular impulse without premature or failed operation of the disconnect device.
Gas Discharge Tubes (GDTs)
GDTs may be assessed with long-duration impulse currents where the applicable specification requires verification of charge transfer capability under sustained surge conditions.
Surge Protective Device (SPD) Modules and Sub-Assemblies
Complete SPD blocks and their internal component assemblies can be tested at the module level to demonstrate compliance with end-product performance requirements.
Testing Items
Typical test items evaluated under square wave high current impulse testing include:
- Rated rectangular impulse current withstand (e.g., 2 ms square wave at the specified amplitude).
- Nominal rectangular impulse current verification for classification purposes.
- Energy absorption capability, expressed in joules, delivered during the impulse train.
- Charge transfer capability, expressed in coulombs.
- Residual (clamping) voltage measurement while the impulse current is flowing.
- Varistor voltage drift, measured before and after stressing, to quantify parameter stability.
- Leakage current at the specified DC operating voltage, compared before and after the test.
- Visual and structural inspection for cracking, chipping, flashover, tracking, or package damage.
Testing Methods
Sample Preparation and Initial Characterization
Prior to stressing, samples are conditioned according to the standard atmosphere requirements and subjected to initial electrical characterization. This normally includes measurement of the varistor voltage at a specified DC test current (for example, 1 mA) and, where required, leakage current at rated DC voltage. These baseline values serve as the reference for post-test drift assessment.
Generator Configuration and Calibration
The impulse generator—typically an LC pulse-forming network charged from a high-voltage source—is configured to deliver the required peak current and plateau duration for the sample under test. The output is verified using a calibrated measurement chain consisting of a current viewing resistor or Rogowski coil and a calibrated digitizer, in accordance with the measurement uncertainty requirements of IEC 60060-2. Waveform parameters such as peak value, plateau duration, and oscillation content must fall within the tolerances permitted by the applicable component standard.
Impulse Application Sequence
The DUT is connected to the generator output with short, low-inductance connections to minimize waveform distortion. A defined number of consecutive impulses—commonly ten—is then applied at the specified amplitude, with intervals between impulses (typically no more than one impulse per minute) as prescribed by the standard. Where the standard requires assessment at multiple current levels or durations, the sequence is repeated on separate sample groups. Residual voltage is captured during each impulse using a calibrated voltage probe or divider.
Post-Test Evaluation and Acceptance Criteria
After the impulse sequence, samples are re-measured under the same conditions as the initial characterization. Typical acceptance criteria include:
- No visible damage such as cracks, punctures, burn marks, or external flashover.
- Change in varistor voltage within the specified limit, commonly not exceeding ±10% of the initial value.
- No open-circuit or short-circuit failure of the component.
- Leakage current remaining within the limit defined by the product specification.
- No unintended operation of integrated thermal or disconnect protection elements.
Applications
Square wave high current impulse testing supports a broad range of industries and use cases:
- Component qualification for MOV manufacturers establishing energy absorption classes and rectangular impulse current ratings.
- Incoming quality control and batch acceptance testing for procurement of surge protective components.
- Certification support for SPDs installed in low-voltage power distribution, photovoltaic systems, and electric vehicle charging infrastructure.
- Telecommunications network protection, where GDT and varistor modules must withstand long-duration power fault and induction surges.
- Railway and industrial electrification systems requiring verified energy withstand of protective components.
- Comparative evaluation and failure analysis during product development and design validation.
- Compliance demonstration for components referenced in end-product standards published by IEC, EN, GB, UL, and JIS bodies.
FAQ
Why use a square wave instead of an 8/20 µs impulse?
The 8/20 µs impulse simulates short-duration lightning-induced surges, while the square wave delivers current over a much longer plateau, typically 2 ms. This subjects the component to far higher energy and charge transfer, revealing thermal weaknesses and long-duration energy limitations that a short impulse cannot expose. The two tests are therefore complementary rather than interchangeable.
Is the square wave impulse test destructive?
The test is generally considered a stress or qualification test. Properly rated samples should pass without functional degradation, but the applied energy is significant, and tested samples are usually not returned to production use. Testing at levels beyond the rated value, conducted to determine failure thresholds, is destructive by design.
What sample information should be submitted with a test request?
Clients should provide the component datasheet, the applicable standard and clause to be verified, the required peak current and plateau duration, the number of impulses and interval, and any special conditioning or measurement requirements. Sample quantity should cover initial and final measurements plus any statistical replication specified by the standard.
How long does testing take?
For a standard qualification sequence on a defined sample group, testing—including initial characterization, impulse stressing, and post-test measurement—is typically completed within a few working days. Additional time should be allowed for generator calibration at non-standard waveform parameters or for extended multi-level assessment.
Which rating on a varistor datasheet corresponds to this test?
The relevant rating is usually published as the "rectangular impulse current," "impulse current at long duration," or "2 ms square wave current" rating, expressed in amperes for a stated duration and number of impulses. The corresponding energy rating in joules is derived from the test current, plateau duration, and measured clamping voltage.