PET high-temperature insulation film power frequency withstand voltage testing is a critical dielectric assessment that verifies the electrical endurance of PET (polyethylene terephthalate) films used in motors, transformers, and power electronics under 50/60 Hz AC stress. This guide covers test standards, sample preparation, electrode configurations, voltage parameters, and acceptance criteria for ensuring reliable insulation performance at elevated operating temperatures. Understanding these testing procedures helps manufacturers and quality engineers select the right film grade and validate compliance with international safety requirements.
Technical Overview
PET (polyethylene terephthalate) insulation film is a biaxially oriented polyester film widely used as a dielectric barrier in electrical and electronic equipment. Standard PET film offers a long-term operating temperature of approximately 120 to 130 degrees Celsius, placing it in Class B (130°C) thermal insulation classifications, while specially stabilized high-temperature PET grades can withstand intermittent exposure above 130°C. The material combines high tensile strength, excellent dimensional stability, low moisture absorption, and outstanding dielectric properties, with intrinsic dielectric strength values typically ranging from 120 to 180 kV/mm depending on film thickness.
The power frequency withstand voltage test, also known as the AC hipot or dielectric withstand test, evaluates whether the film can sustain a specified alternating current voltage at 50 Hz or 60 Hz for a defined period without breakdown. Unlike a breakdown strength measurement that determines the failure point, the withstand test applies a pass/fail criterion: the specimen must survive a predetermined stress level, typically with leakage current remaining below a defined threshold. Because dielectric performance degrades as temperature rises, high-temperature PET film is frequently tested both at ambient conditions and at elevated temperatures to simulate real operating environments in motor slots, transformer windings, and power module packaging.
Testing is conducted in accordance with recognized standards including IEC 60674-2 (plastic films for electrical purposes), IEC 60243-1 (electric strength of insulating materials at power frequencies), ASTM D149, GB/T 13542 series, and GB/T 1408.1. These standards define electrode geometry, voltage application methods, conditioning requirements, and reporting formats to ensure reproducible and comparable results across laboratories.
Testing Samples
Plain Biaxially Oriented PET Film
Standard BOPET insulation film in common thicknesses of 23 μm, 36 μm, 50 μm, 75 μm, 100 μm, 125 μm, 188 μm, and 250 μm is the baseline sample type. Specimens are cut to squares or rectangles of at least 100 mm by 100 mm so that the active electrode region is fully covered and creepage paths to the sample edge are eliminated. Surfaces must be clean, free of folds, wrinkles, pinholes, and particulate contamination, as minor defects dramatically reduce breakdown voltage.
High-Temperature Stabilized PET Film
Heat-treated or chemically stabilized PET grades designed for sustained operation at 130°C to 155°C represent the core sample category for high-temperature qualification. These films may incorporate stabilizer packages that resist thermal oxidation and hydrolysis. Specimens from these grades are tested in the as-received condition and after thermal aging exposure to verify that dielectric withstand capability is retained throughout the service life.
Coated and Composite PET Films
PET film coated with FEP or PTFE for heat sealing, corona-resistant nanoparticle coatings, flame-retardant formulations, or PET laminated with polyimide layers constitutes a third sample group. Because coatings alter surface conductivity and heat dissipation, these samples are tested in the finished laminate configuration to reflect actual product performance rather than the base film alone.
Sample Conditioning
Prior to testing, specimens are conditioned according to the governing material standard, typically 23 ± 2°C and 50 ± 5% relative humidity for 24 hours unless otherwise specified. For hot-state testing, samples are preheated in an oven or heated electrode assembly at the target temperature, commonly 130°C or 155°C, until thermal equilibrium is reached, usually requiring 15 to 30 minutes of dwell time before voltage application.
Testing Items
A comprehensive power frequency evaluation program for PET high-temperature insulation film includes the following test items:
- AC withstand voltage test at power frequency (50/60 Hz): application of a specified test voltage for 60 seconds with no breakdown or flashover permitted.
- Dielectric breakdown voltage determination: progressive voltage increase until failure to characterize the ultimate withstand margin.
- Dielectric strength calculation: breakdown voltage divided by specimen thickness, expressed in kV/mm or V/μm.
- Leakage current measurement: monitoring current throughout the withstand period, with typical acceptance limits of 0.5 mA to 5 mA depending on application requirements.
- Hot-state withstand voltage test: repetition of the withstand test at elevated ambient temperatures of 130°C, 155°C, or application-specific levels.
- Statistical breakdown analysis: Weibull distribution fitting of multiple breakdown data points to report characteristic strength and shape parameters.
- Post-aging withstand verification: withstand testing after thermal aging at 155°C to 180°C for defined durations to assess retention of dielectric integrity.
Testing Methods
Electrode System and Test Fixture
Testing follows the short-time or withstand configurations of IEC 60243-1 and ASTM D149. The standard electrode system consists of two opposing cylindrical metal electrodes, typically 25 mm in diameter with edges rounded to a 2.5 mm radius, made of brass or stainless steel with polished surfaces. The upper electrode weighs approximately 50 g or applies a defined contact pressure to ensure uniform interface contact. For voltage levels above approximately 10 kV, specimens and electrodes are immersed in clean insulating oil (such as transformer oil per IEC 60296) with a breakdown voltage above 40 kV to prevent surface flashover and corona discharges that would otherwise invalidate the measurement.
Voltage Application Procedures
Three standard voltage application methods are recognized. In the withstand method, the test voltage is raised rapidly to the specified level, typically within 10 seconds, and then held constant for 60 seconds; the sample passes if no breakdown occurs. In the slow rate-of-rise method, voltage increases uniformly, commonly at 100 V/s to 500 V/s, until breakdown, with the rate selected depending on expected breakdown voltage. In the step-by-step method, voltage starts at an initial level equal to 40 percent of the anticipated breakdown voltage and increases in discrete steps, typically 5 to 10 percent increments held for 10 to 20 seconds each, until failure. For routine quality control of PET film, the 60-second withstand method at a specified multiple of the rated stress is the most widely applied procedure.
Typical Test Voltage Levels
Test voltage selection depends on film thickness and application specification. Representative withstand levels for PET insulation film include 4 kV for 50 μm film, 6 kV for 100 μm film, and 8 to 10 kV for 188 μm to 250 μm film applied for 60 seconds. Some motor and transformer specifications require withstand values of 5 to 7 kV regardless of thickness when the film serves as primary insulation. When determined by breakdown testing, thin PET films of 23 μm to 50 μm commonly exhibit dielectric strengths of 150 kV/mm or higher, while thicker films of 250 μm show reduced values in the range of 90 to 120 kV/mm due to the well-documented thickness dependency of polymer dielectric strength.
High-Temperature Test Procedure
For hot-state evaluation, the electrode assembly with specimen is placed inside a forced-convection oven or a purpose-built heated test chamber. The system is allowed to stabilize at the target temperature, and temperature is verified with a calibrated sensor positioned adjacent to the specimen. Once equilibrium is confirmed, the withstand voltage is applied for the standard 60-second duration while leakage current is continuously recorded. Because PET dielectric strength decreases measurably with temperature, hot-state acceptance criteria are typically specified at 75 to 85 percent of the room-temperature withstand level, and test reports must clearly state the test temperature.
Failure Detection and Reporting
Breakdown is detected by the collapse of test voltage accompanied by a surge in current, which trips the instrument breaker or arc detection circuit within milliseconds. Visible puncture points are confirmed by microscopic examination. Each test series should include a statistically sufficient number of specimens, with a minimum of five samples for withstand verification and ten or more breakdown points for Weibull analysis. Test reports document film grade and thickness, conditioning parameters, electrode configuration, immersion medium, voltage application method, rise rate, test voltage and duration, test temperature, individual and statistical results, and any observed failure modes such as edge tracking or surface flashover.
Equipment Requirements
The test apparatus comprises an AC high-voltage generator with adjustable output up to at least 50 kV, voltage regulation with better than 3 percent stability, a ramp rate controller, current monitoring with adjustable trip thresholds, safety interlocks, and a grounded shielded test enclosure. Calibration of voltage measurement systems against traceable reference standards must be performed at regular intervals, and the waveform distortion of the output should remain below 5 percent to ensure a true sinusoidal power frequency stress on the specimen.
Applications
Power frequency withstand voltage testing of PET high-temperature insulation film directly supports quality assurance and safety certification across multiple industries:
- Motor and generator manufacturing: slot liners, phase insulation, and turn-to-turn barriers where PET film must survive winding overvoltages and hot-spot temperatures in Class B and Class F systems.
- Transformer and coil production: interlayer insulation in bobbin-wound transformers, ballast coils, and relays requiring verified dielectric barriers between windings.
- Flexible printed circuits: PET base films and coverlayers in FPC applications where dielectric withstand validates insulation between conductive layers.
- Wire and cable processing: wrapping tapes for magnet wire bundles and harness insulation requiring confirmed withstand margins.
- Power electronics and battery systems: barrier films and insulating tapes in inverters, converters, and lithium-ion battery packs where high-voltage isolation is safety-critical.
- Certification and compliance: supporting documentation for UL, IEC, and GB product approvals where material-level dielectric data is a prerequisite for system-level recognition.
FAQ
What test voltage should be applied to PET insulation film?
The test voltage depends on film thickness and the end-use specification. Common practice applies 6 kV for 100 μm film and up to 10 kV for 250 μm film for 60 seconds. Manufacturers often specify withstand values as a multiple of operating voltage, and the breakdown margin should remain at least two to three times the required withstand level to account for temperature, aging, and manufacturing variability.
Why is testing performed at high temperature instead of only at room temperature?
Dielectric strength of PET declines as temperature increases because thermal energy accelerates charge injection and mobilizes polymer chain segments. Since the film in a motor slot or transformer may operate continuously at 130°C or above, hot-state testing verifies that the insulation margin exists under actual service conditions rather than only under favorable laboratory ambient conditions.
Which standards govern PET film power frequency withstand testing?
The principal standards are IEC 60674-2 for film specifications, IEC 60243-1 and ASTM D149 for power frequency electric strength test methods, and GB/T 13542 and GB/T 1408.1 as corresponding Chinese national standards. Application-specific documents, such as motor insulation system standards, may impose additional requirements.
How does film thickness affect test results?
Polymer films exhibit inverse thickness dependency: thinner PET films demonstrate higher dielectric strength per unit thickness, while thicker films show lower kV/mm values due to thermal and space charge effects. For this reason, dielectric requirements are always stated together with the nominal thickness, and comparisons between films of different thicknesses must be normalized accordingly.
What constitutes test failure, and how are false failures avoided?
Failure is defined as dielectric breakdown, evidenced by voltage collapse and a current surge through the punctured specimen. False failures from surface flashover are avoided by immersing specimens in insulating oil at higher voltages, maintaining adequate specimen dimensions beyond the electrode footprint, and verifying that failure points are located under the electrode area rather than at the edges.
How many samples should be tested for reliable results?
For pass/fail withstand verification, a minimum of five specimens per condition is recommended. For breakdown strength characterization and Weibull statistical analysis, ten or more valid breakdown points per condition provide acceptable confidence intervals for the characteristic breakdown voltage and distribution parameters.