PET insulation film impulse voltage withstand testing evaluates the ability of polyethylene terephthalate films to survive the lightning and switching transient overvoltages encountered in motors, transformers, capacitors, and power electronics. Using standard 1.2/50 microsecond impulse waves in accordance with IEC 60243-3, IEC 60060-1, and ASTM D3426, the test determines impulse breakdown voltage, impulse dielectric strength, and verified withstand levels for film thicknesses ranging from a few micrometers up to 350 micrometers. This article explains the sample types, test items, test procedures, and typical applications of impulse withstand testing for PET insulation films.
Technical Overview
Polyethylene terephthalate (PET) film, most commonly produced as biaxially oriented BOPET, is one of the most widely used solid insulation materials in electrical engineering. It offers a dielectric constant of approximately 3.2 to 3.4 at 1 kHz, a low dissipation factor, high volume resistivity, and stable mechanical and dielectric performance up to roughly 120 to 130 degrees Celsius. Available in thicknesses from about 3.5 micrometers to 350 micrometers, PET films serve as motor slot liners, transformer interlayer insulation, capacitor dielectrics, cable wraps, and base films for flexible printed circuits.
In actual service, these films are often exposed not to steady power-frequency voltage but to short-duration transient overvoltages caused by lightning strikes, switching operations, and the steep-fronted voltage pulses generated by inverter-fed drives. Impulse voltage withstand testing reproduces these conditions in the laboratory by applying standard impulse waves, most commonly the 1.2/50 microsecond lightning impulse defined in IEC 60060-1, directly to film specimens placed between defined electrodes.
Because the electrical stress lasts only microseconds, breakdown behavior differs fundamentally from 50 or 60 Hz testing. The measured impulse breakdown voltage is governed by the volt-time characteristic of the material, and the resulting impulse dielectric strength is typically higher than the corresponding power-frequency value. Testing is generally performed in accordance with IEC 60243-3, IEC 60060-1, and ASTM D3426, while equipment-level insulation coordination requirements are covered by IEC 60664-1.
Testing Samples
Pure Biaxially Oriented PET Films
Standard commercial grades in common thicknesses such as 23, 36, 50, 75, 100, 125, 188, and 250 micrometers are tested as square or circular specimens with dimensions large enough, typically at least 100 by 100 millimeters, to prevent flashover around the electrode edges. Both plain films and films with corona-treated or flame-treated surfaces can be evaluated, since surface treatment can influence breakdown behavior.
Coated, Metallized, and Modified Films
Heat-sealable coated films, flame-retardant grades, antistatic-coated films, and metallized films used in capacitor construction are tested in their as-supplied condition. The presence of coatings or metallization changes the field distribution within the dielectric and must therefore be reflected in the test configuration and reporting.
PET-Based Composite Insulation Materials
Laminated structures such as DMD and NMN composites, in which PET film is bonded to nonwoven polyester or aramid layers, PET-based adhesive tapes, and other flexible laminates are tested to verify the impulse performance of the complete insulation system rather than the film alone.
Specimen Preparation and Conditioning
Specimens are visually inspected for pinholes, wrinkles, and contamination, cleaned if necessary with a lint-free cloth, and measured for thickness at several points in the test area according to ASTM D374. Before testing, specimens are conditioned at 23 plus or minus 2 degrees Celsius and 50 plus or minus 5 percent relative humidity for at least 24 hours in line with ISO 291 or ASTM D618. A minimum of five specimens per test condition is recommended, and twenty or more specimens are preferred when statistical breakdown analysis is required.
Testing Items
Depending on the purpose of the evaluation, a PET insulation film impulse test program can include the following deliverables:
- Impulse breakdown voltage in kilovolts, defined as the peak value of a 1.2/50 microsecond impulse that causes complete puncture of the specimen
- Impulse dielectric strength in kilovolts per millimeter, obtained by normalizing the breakdown voltage to the minimum measured thickness of the stressed area
- Verified impulse withstand level, confirming that a specified peak voltage can be applied a defined number of times without breakdown
- Polarity dependence assessment, comparing breakdown behavior under positive and negative impulse polarity
- Volt-time or time-lag characteristics, relating breakdown probability to peak voltage and time to breakdown
- Statistical analysis using Weibull distribution parameters, including the scale parameter and shape parameter, and estimation of the 50 percent and 63.2 percent breakdown voltages
- Conditioned testing after heat aging, humidity exposure, or thermal cycling to assess changes in impulse performance
Testing Methods
Impulse Generator and Waveform
A multi-stage Marx-type impulse generator produces the test voltage. The standard lightning impulse waveform has a virtual front time of 1.2 microseconds with a tolerance of plus or minus 30 percent and a time to half-value of 50 microseconds with a tolerance of plus or minus 20 percent, with the peak voltage held within plus or minus 3 percent of the specified value as required by IEC 60060-1. The waveform is measured through a calibrated damped capacitive or resistive voltage divider and a high-bandwidth digital recorder, and the measuring system is verified before testing begins.
Electrode System and Ambient Medium
Electrodes follow IEC 60243-1 for sheet materials: an upper cylindrical electrode of 25 millimeters diameter and a lower cylindrical electrode of 75 millimeters diameter, both with rounded edges of approximately 3 millimeters radius. The specimen is centered under the electrodes with light contact pressure that does not damage the film. Tests may be performed in conditioned air or with the specimen immersed in insulating oil conforming to IEC 60296; oil immersion suppresses surface flashover and is preferred for thin, high-strength films at elevated voltage levels. The ambient medium must be reported because it measurably influences the result.
Impulse Withstand Test Procedure
For a withstand verification, a specified number of consecutive impulses, commonly three per polarity and in some specifications up to ten or fifteen, is applied at the specified peak voltage with an interval of at least five seconds between impulses. Both positive and negative polarities are tested. If no breakdown or damaging partial flashover occurs on any specimen, the withstand level is considered verified; collapse of the voltage waveform on any impulse constitutes failure.
Impulse Breakdown Test Procedure
Impulse breakdown voltage is commonly determined by the step-stress method: testing begins at approximately 50 to 70 percent of the expected breakdown voltage, and the peak is increased in steps of about 5 to 10 percent, with one impulse applied per step, until puncture occurs. Alternatively, the up-and-down method may be used to estimate the 50 percent breakdown voltage. For each breakdown event, the peak voltage, time to breakdown, and puncture location are recorded.
Breakdown Detection, Data Evaluation, and Safety
Breakdown is detected by the sudden collapse of the impulse waveshape on the recording and confirmed by visual inspection of the puncture channel in the specimen. Events caused by surface flashover or edge tracking are discarded and the test is repeated. Breakdown data from multiple specimens are evaluated statistically, typically using two-parameter Weibull analysis, and reported together with test conditions. All testing is conducted within a grounded, interlocked enclosure, and the generator and specimen are automatically discharged and grounded before any operator access.
Applications
Impulse withstand testing of PET insulation films supports design, qualification, and quality control across many electrical and electronic products where transient overvoltages determine insulation reliability:
- Slot insulation, slot liners, and phase insulation in rotating machines, particularly inverter-fed motors subjected to steep-fronted surges
- Interlayer and wrap insulation in distribution and instrument transformers
- Dielectric films for metallized film capacitors, where impulse capability affects surge ratings
- Base films and coverlay materials for flexible printed circuits
- Insulation films and tapes used in lithium-ion battery packs for cell-to-cell and cell-to-busbar separation
- PET-based cores of photovoltaic module backsheets exposed to system transients
- Insulation coordination studies and clearance verification for low-voltage equipment in accordance with IEC 60664-1
- Incoming inspection and supplier comparison of film lots, and research and development of new film formulations and coatings
FAQ
Which standards govern PET film impulse withstand testing?
The principal standards are IEC 60243-3 for impulse dielectric strength testing of solid insulating materials, IEC 60060-1 for high-voltage test techniques and waveform definitions, ASTM D3426 for impulse breakdown testing of solid insulation, and IEC 60664-1 for insulation coordination at the equipment level.
Why must both polarities be tested?
The asymmetric electrode geometry and possible space charge injection cause PET films to exhibit measurable differences in breakdown voltage between positive and negative impulses. Equipment-level specifications therefore require withstand verification under both polarities, and the lower of the two results is used for design.
How many impulses are applied for a withstand verification?
Most specifications call for three consecutive impulses per polarity at the specified level, while more demanding specifications require up to ten or fifteen. The number of impulses, their interval, and the polarity sequence should always be stated in the test report.
Should the test be performed in air or in oil?
Air testing better represents many service environments, but for thin films with very high dielectric strength, surface flashover in air can occur before puncture. Immersion in insulating oil suppresses flashover and allows the true breakdown strength of the film to be measured. The chosen medium must be reported with the results.
What impulse dielectric strength is typical for PET film?
For thin biaxially oriented PET films, impulse dielectric strength commonly falls in the approximate range of 250 to 400 kilovolts per millimeter and decreases as thickness increases, consistent with the classic thickness dependence of dielectric strength. Actual values depend strongly on thickness, electrode geometry, ambient medium, temperature, and humidity, so project-specific testing is always required.
How many specimens are needed and how long does testing take?
A minimum of five specimens per condition is acceptable for screening, while twenty to thirty specimens are recommended for reliable Weibull statistics. Including conditioning, most test programs are completed within a few working days to two weeks, depending on the number of thicknesses, voltage levels, and environmental conditions involved.