Date | 2026-06-29 11:35:18
Engineers designing high-voltage insulation parts face the same question repeatedly: the material is right, the wall thickness is sufficient — so why does breakdown or surface discharge still occur?
Whether in a circuit breaker arc chute, a contactor base or an insulating divider in an energy storage system, insulation failure is often not because the material is insufficiently insulating, but because the electric field distribution is wrong — local field strength exceeds what the material can withstand.
BMC/SMC, as thermoset composites, are widely used in high-voltage insulation. This article starts from basic electromagnetic concepts, explains the physical meaning behind the dielectric parameters of BMC/SMC, and shows how those parameters can be used to optimise insulation design.
Electric field strength describes the magnitude and direction of an electric field, normally expressed in V/m or kV/mm. In an insulation structure the field is not uniformly distributed — at sharp corners, electrode edges and air gaps the field strength is markedly above the average. The phenomenon is called field concentration.
When local field strength exceeds the dielectric strength of the material — its breakdown strength — the material loses its insulating property and breakdown occurs. This is why the same material can show breakdown voltages differing by several times under different geometries and electrode arrangements.
Permittivity, or dielectric constant, measures how strongly a material polarises in an electric field. The higher it is, the more charge the material can store, but it also affects how the field is distributed through the insulation structure. In a multi-layer insulation system, materials of different permittivity compete for voltage — the material with the lower permittivity takes the higher field strength.
The dielectric dissipation factor reflects how much electrical energy the insulation consumes in an alternating field. The higher the factor, the more heat the material generates under AC voltage, and over long operation that can accelerate thermal ageing.
Dielectric strength is the core index of a material's voltage withstand capability. Through formulation optimisation, BMC/SMC reach 18–20 kV/mm and above. The company standard Q/JTJ0001-2025 requires electrical strength ≥20 kV/mm.
The implication: a 2 mm thick BMC insulating plate can theoretically withstand more than 40 kV without breakdown. For conventional low-voltage apparatus in the 690 V to 1500 V range, that index provides ample safety margin.
Volume resistivity reflects the ability of a material to resist the passage of current. BMC reaches 10¹²–10¹⁴ Ω·cm and SMC above 10¹⁴ Ω·cm. The company standard requires insulation resistance ≥10¹³ Ω in the normal state. For high-voltage insulation parts, volume resistivity governs the magnitude of leakage current — the higher the resistivity, the lower the leakage current and the better the long-term reliability.
Permittivity affects field distribution. The company BMC standard requires a dielectric constant (1 MHz) ≤4.5. In AC high-voltage applications a lower permittivity helps reduce capacitive current and dielectric heating.
The dissipation factor directly affects heating under AC voltage. The company standard requires a dissipation factor (1 MHz) ≤0.02. For power equipment in continuous service, low dielectric loss means less heat accumulation and a longer service life.
In high-voltage insulation design, creepage distance and clearance are two parameters explicitly regulated by international safety standards.
Clearance is the shortest distance through air between two conductive parts. If the gap is too small or the voltage too high, the air ionises and an arc forms. Clearance design depends mainly on peak working voltage and environmental conditions.
Creepage distance is the shortest path along the surface of the insulating material. When the surface is contaminated or damp, the contamination forms a conductive path and current creeps across the surface. Creepage design depends on RMS working voltage, pollution degree and material group.
Material groups are classified by comparative tracking index (CTI):
Group I material — CTI ≥ 600
Group II material — 400 ≤ CTI < 600
Group III material — CTI < 400
The company's BMC/SMC has PTI ≥600 V and belongs to Group I material. At the same pollution degree and working voltage, that means a smaller creepage distance is required than for Group II or Group III material. At the same creepage distance, insulation reliability is higher; at the same insulation requirement, a more compact design becomes possible.
Engineering significance: when designing a high-voltage insulation part, it is not enough to look at dielectric strength alone — clearance and creepage distance must be calculated and guaranteed to the requirements of the relevant standard. The higher the CTI/PTI of the material, the greater the design margin on creepage distance.
In insulation design, sharp corners, electrode edges and air gaps are the places where field concentration arises. Remedies include fillet transitions — the company mould standard Q/JTJ0003-2025 requires a root radius ≥0.5 mm at rib bases — optimised electrode shape, and grading rings. In BMC/SMC part design, sensible geometry can substantially reduce local field strength.
Voids, inclusions and delamination inside a BMC/SMC part become hot spots for field concentration and reduce local dielectric strength. This is why the company process standard Q/JTJ0002-2025 emphasises multi-stage venting and optimised dwell timing — process control reduces internal defects and keeps insulation performance consistent.
Humidity lowers the surface resistance and dielectric strength of a material. The company standard requires insulation resistance ≥1×10¹² Ω after 24 h immersion in water. BMC/SMC insulation parts used outdoors or in high-humidity environments must take environmental effects on insulation performance into account at the design stage.
In a multi-layer insulation structure — a BMC housing combined with an internal air gap, for example — materials of different permittivity redistribute the field. The design must ensure that the field strength across each layer stays below that layer's dielectric strength.
Principles for grade choice: dielectric strength and PTI are non-negotiable for high-voltage insulation; hot and humid environments require additional attention to insulation resistance after water immersion; high-frequency applications favour grades with low permittivity and low dielectric loss.
Insulation design is not simply a matter of choosing an insulating material and making it thick enough. From the basic physics of field distribution, through material parameters such as dielectric strength and permittivity, to standard requirements such as creepage distance and clearance — every step affects the final reliability of the part.
BMC/SMC reach dielectric strength above 20 kV/mm, volume resistivity above 10¹³ Ω and PTI ≥600 V as a Group I material. Those figures underpin their wide use in high-voltage insulation. But only by understanding what the numbers mean physically, and applying them correctly to structural design, process control and standards compliance, can the insulation potential of the material actually be realised.
Through formulation optimisation, BMC/SMC reach dielectric strength of 18–20 kV/mm and above, and the company standard Q/JTJ0001-2025 requires electrical strength ≥20 kV/mm. That means a 2 mm thick BMC insulating plate can theoretically withstand more than 40 kV without breakdown, giving ample safety margin for conventional low-voltage apparatus in the 690 V to 1500 V range.
Insulation failure is often not because the material is insufficiently insulating, but because the field distribution is wrong: at sharp corners, electrode edges and air gaps the field strength is markedly above the average, which is field concentration. When local field strength exceeds the dielectric strength of the material, breakdown occurs. This is also why the same material can show breakdown voltages differing by several times under different geometries and electrode arrangements.
Clearance is the shortest distance through air between two conductive parts; if the gap is too small or the voltage too high, the air ionises and an arc forms, and its design depends mainly on peak working voltage and environmental conditions. Creepage distance is the shortest path along the surface of the insulating material; when the surface is contaminated or damp, contamination forms a conductive path and current creeps across the surface, and its design also depends on pollution degree and material group.
Material groups are classified by comparative tracking index: Group I CTI ≥600, Group II 400 ≤ CTI < 600, Group III CTI < 400. The company's BMC/SMC has PTI ≥600 V and belongs to Group I, so at the same pollution degree and working voltage the creepage distance required is smaller than for Group II or III material; at the same creepage distance insulation reliability is higher, and at the same insulation requirement a more compact design can be achieved.
Field concentration can be reduced with fillet transitions — the company mould standard Q/JTJ0003-2025 requires a root radius ≥0.5 mm at rib bases — optimised electrode shape and grading rings. Internal defects are controlled through process: the company process standard Q/JTJ0002-2025 emphasises multi-stage venting and optimised dwell timing, to reduce voids, inclusions and delamination, the hot spots where the field concentrates, and to keep insulation performance consistent.
For technical data sheets, grade recommendations, or a discussion about your specific requirements:
📧 wendy.qiu@smcbmc.com 📞 +86-13868305300
