Date | 2026-05-13 09:31:53
In both low-voltage electrical equipment and high-voltage switchgear, the failure of an insulating component is rarely a simple binary outcome of "breakdown or no breakdown." From an engineering perspective, a more meaningful question is: how much dielectric strength margin should be reserved at a given wall thickness during the design phase? This directly determines whether the component can pass type testing and maintain reliable insulation throughout years of operation.
For Bulk Molding Compound (BMC)—a fiber-reinforced unsaturated polyester thermoset—the dielectric strength is influenced by formulation, molding process, and structural design. Relying solely on a supplier-provided nominal value, such as "≥20 kV/mm," is insufficient for precise insulation design. Wall thickness variations affect electric field distribution, and fiber orientation during molding impacts local dielectric performance—both must be accounted for in design.
GB/T 1408.1-2016 “Insulating Materials – Electrical Strength Test Methods, Part 1: AC Test” specifies short-term breakdown testing under power-frequency AC voltage. The rapid voltage ramp method (2 kV/s) is commonly used for testing BMC materials.
According to Wenzhou Jintong corporate standard Q/JTJ0001-2025, the electrical strength test setup for BMC/SMC materials uses:
Transformer oil as the dielectric medium
20 mm spherical electrodes
Rapid voltage ramp mode
A material passes if ≥20 kV/mm is achieved. Importantly, this value is obtained on standard test samples (typically 3–4 mm thick) and does not directly represent the withstand voltage for components of arbitrary thickness.
Breakdown in solid dielectrics does not strictly follow a linear “Voltage = Electric Field × Thickness” relationship. In heterogeneous materials like BMC:
Fiber-matrix interfaces can create localized electric field concentration points.
Thin walls amplify the impact of local fiber orientation variations.
Thick walls may increase the probability of voids or filler settlement, potentially reducing breakdown strength relative to linear expectation.
Internal testing on BMC 16XX series samples (3–5 replicates per formulation, rapid ramp per GB/T 1408.1-2016) yielded the following typical results:
| Sample Wall Thickness | Breakdown Voltage Range | Average Dielectric Strength | Engineering Notes |
|---|---|---|---|
| 1.0 mm | 12–16 kV | ~12–16 kV/mm | Thin-wall designs should assess fiber distribution uniformity |
| 2.0 mm | 28–36 kV | ~14–18 kV/mm | Common wall thickness range for insulating parts |
| 3.0 mm | 48–60 kV | ~16–20 kV/mm | Intrinsic material performance fully realized |
| 4.0 mm | 58–72 kV | ~14.5–18 kV/mm | Increasing thickness yields diminishing gains |
| 6.0 mm | 72–90 kV | ~12–15 kV/mm | Thick-wall components should verify internal defect control |
Note: Values vary depending on glass fiber content, filler type, and color batch.
1–3 mm wall thickness: Breakdown voltage increases almost linearly with thickness. For regions where creepage distance is sufficient, 3 mm represents a cost-effective design reference.
Above 4 mm wall thickness: Incremental breakdown voltage gains diminish. In thick-wall structures, optimizing venting in the mold and using high filler loading can be more effective than simply increasing thickness.
Consistent breakdown voltage is critical for mass production. Factors affecting variability include:
Molding parameters: Maintaining 50–70% peak pressure for 15–30 s during holding reduces internal voids, improving batch consistency.
Fiber distribution: Symmetric feeding ensures 80–90% coverage of cavity projection, minimizing localized dielectric weak spots.
Post-curing: 30–60 min at 80–100°C reduces residual stress and potential breakdown initiation points.
| Application | Recommended Minimum Wall Thickness | Typical Breakdown Voltage | Safety Margin Basis |
|---|---|---|---|
| Low-voltage breaker housing (400 V) | 1.5 mm | ≥20 kV | Basic insulation per GB/T 16935.1, withstand voltage category |
| Contactor insulation plates (690 V) | 2.0 mm | ≥28 kV | Overvoltage Category III, test voltage 2500 V/60 s |
| Medium-voltage switchgear insulation supports (12 kV system) | 4.0 mm | ≥58 kV | GB/T 11022-2020, power-frequency withstand voltage 42 kV/1 min, ≥1.3× margin |
| High-voltage insulator rods (40.5 kV system) | ≥6.0 mm | ≥72 kV | Power-frequency withstand voltage 95 kV/1 min; combined creepage distance design required |
These recommendations apply to solid insulation only; creepage and clearance must be verified per applicable standards.
Assuming “≥20 kV/mm” ensures safety for all wall thicknesses is a common misconception. Engineers should:
Reference actual breakdown voltage vs. wall thickness measurements
Use batch variability data as production acceptance criteria
For detailed BMC insulation data and design support, contact Wenzhou Jintong:
📧 wendy.qiu@smcbmc.com
📞 +86 138 6830 5300
