High-dielectric-strength insulation

Definition (what it is)

High-dielectric-strength insulation is an insulating material or complete insulation system engineered to withstand high electric field intensity without dielectric breakdown. The property is quantified as dielectric strength—breakdown voltage per unit thickness—typically reported in kV/mm and measured under standardized AC, DC, or impulse test conditions (for example, ASTM D149, IEC 60243). “High” is relative to the application’s required withstand levels, and in practice refers to both the intrinsic material properties and the way the insulation is designed, processed, and applied (geometry, interfaces, and defects all matter).

Function and key technical characteristics

Its purpose is to maintain electrical isolation between conductors and between live parts and accessible/grounded structures during normal operation and under transients. Key characteristics include:

• High breakdown field (dielectric strength): the maximum electric field before failure, evaluated with AC, DC, or impulse tests.
• Insulation resistance and low leakage: high volume and surface resistivity to suppress conduction and minimize leakage current.
• Permittivity and dielectric loss: relative permittivity (εr) and dissipation factor (tan δ) that affect capacitive coupling, signal integrity, and dielectric heating.
• Partial discharge performance: high partial discharge inception voltage (PDIV), low discharge magnitude, resistance to corona/erosion; freedom from voids and sharp edges that concentrate fields.
• Tracking and surface performance: resistance to tracking and erosion on contaminated or moist surfaces (often characterized by Comparative Tracking Index, CTI).
• Thermal capability: retention of dielectric and mechanical properties across the operating range and thermal class; adequate thermal conductivity where heat removal is important.
• Environmental and chemical resistance: stability under humidity, coolant/oil exposure, road salt/chemicals, radiation (where relevant), vibration, and thermal cycling.
• Mechanical robustness and processability: sufficient strength, toughness, adhesion (for coatings and potting), and compatibility with forming, molding, lamination, impregnation, or coating processes.
• Insulation coordination and geometry: proper creepage and clearance, field grading, and stress-control features; thickness uniformity and defect control are critical to avoid localized breakdown.

Relevance in modern EV design

High-dielectric-strength insulation underpins safety, reliability, and power density in high-voltage EV architectures (typically 400–1000+ V):

• Power electronics and charging: isolation in traction inverters and on-board chargers (DC-link capacitors, busbars, gate-driver isolation, encapsulated modules), including management of high dv/dt and fast-switching SiC devices.
• Electric machines and e-axles: slot liners, interturn and phase insulation, end-winding encapsulation, and vacuum impregnation to prevent partial discharge and premature aging.
• Battery packs and modules: cell-to-cell and module-to-pack barriers, dielectric films and laminates, busbar insulators, and coatings that prevent shorts and arc propagation while meeting creepage/clearance rules.
• High-voltage wiring and connectors: cable primary insulation and jacketing for elevated voltage, temperature, bending/flex, and fluid exposure; connector housings and seals with high CTI.
• Sensors and control electronics near HV parts: conformal coatings and isolation barriers that maintain signal integrity and resist moisture and contamination.

A higher dielectric strength enables thinner insulation for a given withstand requirement, improving packaging, weight, thermal paths, and efficiency while meeting insulation coordination and EV safety standards (for example, IEC 60664 for creepage/clearance and coordination, ISO 6469 for EV electrical safety, UL 2202/2580 for charging systems and traction batteries).

Typical materials and constructions

• Polymer films and laminates: polyimide (PI), PET, PEN, PEEK, PPS, aramid paper, biaxially oriented polypropylene (BOPP for capacitors); used for slot liners, interturn barriers, battery and busbar laminates.
• Cable and molded thermoplastics: polyethylene (PE), crosslinked polyethylene (XLPE), polypropylene (PP), fluoropolymers (PTFE, FEP, PFA); used in HV cable insulation, molded insulators, and barriers with low loss and high strength.
• Elastomers and gels: silicone rubber and gels, fluorosilicones, EPDM; employed in seals, boots, bushings, and gel encapsulation where flexibility, hydrophobicity, and tracking resistance are needed.
• Thermosets and composites: epoxy and polyurethane potting/encapsulation systems (often ceramic-filled for thermal conductivity and PD resistance), cyanate-ester or BMI blends for higher temperatures; glass- or mica-reinforced laminates for structural insulation.
• Ceramics and glass: alumina, aluminum nitride, silicon nitride substrates (e.g., DBC/AMB for power modules), technical glass and porcelain for feedthroughs and sensors.
• Coatings and surface treatments: conformal coatings (urethane, acrylic, silicone, epoxy), parylene (C, N, F variants) for thin, pinhole-free barriers; ceramic conversion coatings such as plasma electrolytic oxidation (PEO) on aluminum housings to form hard, high-dielectric layers.

Manufacturing and application methods

• Film wrapping and lamination for slot liners and multilayer barriers.
• Vacuum pressure impregnation (VPI) of windings; resin transfer molding (RTM) of complex insulation structures.
• Vacuum potting/encapsulation to minimize voids and enhance PD performance in power modules, connectors, and sensors.
• Extrusion and crosslinking (peroxide or silane) for cable insulation; calendering for films and laminates; injection/compression molding for housings and insulators.
• Surface preparation (cleaning, plasma/corona treatment, primers) to improve adhesion and reduce contamination.
• Field control measures: edge rounding, stress-control fillers, semiconductive layers, and careful interface design to mitigate electric field concentration.

Performance considerations and typical values

• Representative dielectric strength ranges (heavily dependent on thickness, test method, and environment): thin polymer films approximately 100–400 kV/mm (much higher possible in capacitor-grade films), elastomers about 20–35 kV/mm, epoxies about 15–30 kV/mm, technical ceramics roughly 10–30 kV/mm. Real, long-term working fields are significantly derated from short-term test results.
• AC, DC, and impulse withstand differ; fast-rising impulses and high dv/dt stress interfaces and voids, promoting partial discharge.
• Properties degrade with temperature, humidity, contamination, mechanical damage, and radiation; altitude reduces air dielectric strength and increases required clearances.
• Surface phenomena (tracking/erosion) can dominate in polluted or wet environments; select high-CTI materials and design adequate creepage paths.
• Insulation coordination requires appropriate creepage/clearance, PD-free operation at maximum service voltage, and verification by PD testing (e.g., IEC 60270) and dielectric withstand tests.
• Design margins: it is common to target PD-free operation and keep operating fields well below breakdown (often by a factor of several times), considering worst-case tolerances, aging, and manufacturing variability.

Synonyms and related terms; examples

• Synonyms/related: dielectric insulation, electrical insulation, high-voltage insulation, insulating barrier, reinforced/double insulation, insulation coordination, creepage, clearance, tracking resistance, PD/PDIV, breakdown voltage, withstand voltage.
• Example applications: polyimide slot liners and phase separators in traction motors; epoxy or silicone gel potting of SiC inverter modules; BOPP film in DC-link capacitors; multilayer polyester/aramid films in battery modules and busbars; parylene or silicone conformal coatings on HV PCBs; PEO-coated aluminum inverter housings; XLPE-insulated HV cables.