PET insulation film breakdown voltage testing is a critical dielectric evaluation used to verify the electrical endurance, safety, and lot-to-lot consistency of polyethylene terephthalate films in motors, transformers, capacitors, flexible circuits, and battery systems. This technical guide explains the applicable standards, sample preparation requirements, electrode configurations, voltage application methods, and typical dielectric strength values for PET films, providing manufacturers and quality engineers with a complete framework for reliable insulation assessment.
Technical Overview
PET (polyethylene terephthalate) insulation film is a biaxially oriented polyester film valued for its high dielectric strength, excellent mechanical toughness, chemical resistance, and stable performance over a wide temperature range, typically from -60°C to +130°C. It serves as a core dielectric barrier in electric motors, transformers, film capacitors, flexible printed circuits, and lithium-ion battery systems.
Breakdown voltage testing measures the voltage at which an electrical discharge punctures the film and the material permanently loses its insulating capability. When the breakdown voltage is normalized by the specimen thickness measured at the puncture point, the result is expressed as dielectric strength in kV/mm or V/µm. For commercial biaxially oriented PET films, dielectric strength typically falls between 150 and 300 kV/mm, with thin films of 12 to 25 µm achieving the highest values and thicker films of 100 µm and above gradually declining toward the lower end of the range due to volume defect statistics.
The primary reference standards are ASTM D149 and IEC 60243-1, which define electrode geometry, voltage application sequences, ambient conditions, and reporting requirements for solid electrical insulating materials tested at commercial power frequencies.
Testing Samples
Sample type, thickness, and conditioning directly influence breakdown results. Submitted specimens must be flat and free of wrinkles, creases, pinholes, and visible contamination, and each specimen must fully cover the electrode footprint with sufficient margin to prevent surface flashover.
Biaxially Oriented PET Film (BOPET)
Plain, uncoated films in standard commercial thicknesses such as 12, 19, 23, 25, 36, 50, 75, 100, 125, 188, and 250 µm. These represent the baseline material grade used for dielectric qualification, material comparison, and lot release testing.
Coated and Treated PET Films
Corona-treated, heat-sealable, flame-retardant, or silicone-coated films used for slot liners, wrapping layers, and barrier applications. Coating chemistry and coating thickness can shift breakdown behavior, so these materials are tested in the as-delivered state rather than with coatings removed.
Metallized PET Films
Films carrying vacuum-deposited aluminum or zinc layers for film capacitor windings. Metallization eliminates interfacial air gaps and modifies breakdown statistics; specimens are oriented so the metallized face contacts one electrode to ensure reproducible field conditions.
PET Laminates and Adhesive Tapes
Composite structures such as PET film laminated with presspaper or aramid paper, and single-sided or double-sided adhesive PET tapes used in battery packs and harness wrapping. Testing evaluates the complete laminate or tape stack rather than the film alone, reflecting the performance of the real insulation system.
Specimen Preparation and Conditioning
- Cut specimens at least 20 mm larger than the electrode diameter on all sides; 100 mm × 100 mm squares are commonly used.
- Condition at 23 ± 2°C and 50 ± 5% relative humidity for a minimum of 24 hours, extended to 96 hours for hygroscopic adhesives and paper-containing laminates.
- Measure thickness at each intended test point using a dead-weight micrometer with 1 µm resolution; thickness data is mandatory for calculating dielectric strength.
- Test a minimum of five specimens per condition; ten or more breakdowns are recommended when Weibull statistics or lot acceptance decisions are involved.
Testing Items
- AC breakdown voltage at power frequency (50/60 Hz), reported as the RMS voltage at which puncture occurs.
- DC breakdown voltage, relevant to HVDC, photovoltaic, and battery-system applications.
- Dielectric strength in kV/mm or V/µm, calculated as breakdown voltage divided by thickness measured at the failure point.
- Statistical characterization including mean value, minimum value, maximum value, standard deviation, and Weibull distribution parameters for reliability assessment.
- Failure site examination to confirm that puncture occurred through the film bulk rather than as a surface flashover or edge tracking event.
Testing Methods
Testing is performed in accordance with ASTM D149 or IEC 60243-1, or the equivalent GB/T 1408 series, using a power-frequency breakdown tester equipped with a regulated voltage source, calibrated voltage measurement, and a fast electronic overcurrent trip.
Electrode System
- The standard IEC 60243-1 configuration uses symmetric cylindrical electrodes 25 mm in diameter with rounded edges; ASTM D149 permits alternative arrangements including unequal cylinders and sphere-plane geometries.
- Electrode surfaces must be polished and free of pitting or arc damage; damaged electrodes are re-lapped or replaced before further testing.
- Controlled electrode pressure is applied to ensure intimate contact without deforming or stressing the specimen.
Test Environment
- Specimens and electrodes are immersed in clean transformer oil or another specified insulating medium to suppress surface flashover and corona discharge at electrode edges.
- Ambient conditions are maintained at 23 ± 2°C and 50 ± 5% relative humidity unless otherwise specified.
- Elevated-temperature evaluations use a preheated oil bath or environmental chamber, with the specimen soaked at temperature before voltage application.
Voltage Application Sequences
- Short-time (rapid-rise) method: voltage is raised uniformly from zero at a fixed rate, commonly 0.5, 1, 2, or 5 kV/s, until breakdown occurs; this is the preferred method for material comparison.
- Slow rate-of-rise method: testing begins at a specified fraction of the expected breakdown voltage, typically 50%, followed by a slow uniform rise to failure.
- Step-by-step method: the initial voltage, typically 50% of the expected breakdown value, is held for a fixed dwell time and then increased in equal steps with equal dwell periods, commonly 60 seconds, until breakdown; this approach ranks materials under longer electrical stress.
Breakdown Detection and Data Handling
- Breakdown is defined as puncture of the specimen with collapse of the test voltage, detected by the overcurrent trip within milliseconds.
- Each specimen is punctured only once; multiple specimens provide the sample statistics.
- Dielectric strength is reported as breakdown voltage divided by local thickness, together with mean value, standard deviation, and lowest single value where required.
- Results associated with flashover, tracking, or puncture outside the electrode footprint are rejected and the test is repeated with fresh specimens.
Applications
- Motor and generator insulation: qualification of slot liners, phase barriers, wedge films, and turn-to-turn insulation layers.
- Transformer and coil insulation: verification of layer, interwinding, and wrap-around barrier films.
- Film capacitor manufacturing: dielectric lot acceptance and quality control of plain and metallized films.
- Flexible printed circuits: dielectric qualification of base films and coverlay substrates.
- Cable and harness engineering: dielectric screening of wrapping tapes and protective barriers.
- Lithium-ion battery systems: evaluation of insulation films, barrier layers, and adhesive PET tapes used in modules and packs.
- Incoming inspection and supplier qualification: comparing film lots, thickness series, or alternative suppliers on a consistent statistical basis.
FAQ
What is the typical breakdown voltage of PET insulation film?
A 23 to 25 µm biaxially oriented PET film typically breaks down between 5 and 8 kV AC, while 100 µm film may withstand approximately 15 to 20 kV. Normalized dielectric strength generally ranges from 150 to 300 kV/mm and decreases as thickness increases.
Why does dielectric strength decrease as film thickness increases?
Thicker films contain a larger population of defects, and the electric field distribution within the volume becomes less uniform. Because the weakest defect governs failure, the statistical minimum breakdown level drops as thickness increases.
Which standards govern breakdown voltage testing of PET film?
ASTM D149 and IEC 60243-1 are the primary standards for power-frequency breakdown of solid insulating materials, with the GB/T 1408 series as the commonly referenced equivalent. DC evaluations reference IEC 60243-2.
Why are specimens tested while immersed in insulating oil?
Oil prevents premature air flashover and surface tracking at electrode edges, forcing failure to occur as a true puncture through the film bulk, which yields valid and reproducible dielectric strength data.
How many specimens should be tested for a valid result?
At least five breakdowns per condition are needed for a basic mean value, while ten or more breakdowns are recommended when Weibull analysis or lot acceptance decisions are involved.
Do temperature and humidity affect test results?
Yes. Breakdown voltage decreases at elevated temperature, and moisture uptake in hygroscopic adhesives or laminated structures reduces dielectric performance. Conditioning requirements and, where relevant, elevated-temperature test conditions must be clearly specified.
Can AC and DC breakdown results be compared directly?
No. DC breakdown voltages of PET film are typically two to three times higher than AC RMS values because DC stress does not involve cyclic field reversals and space charge effects differ. AC and DC results must be reported as separate data sets.
What safety precautions apply to breakdown testing?
Test equipment generates lethal voltages. Interlocked test enclosures, verified grounding, discharge of specimens and oil after each breakdown, insulated tools, and trained operators are mandatory safety requirements.