Date | 2026-05-13 09:31:53
In low-voltage apparatus and high-voltage switchgear, insulation failure is rarely a simple pass-or-fail event. The question that matters during design is narrower: at a given wall thickness, how much margin does the part actually have? The answer decides whether the product passes type testing and whether it still insulates reliably after years in service.
For fibre-reinforced unsaturated polyester molding compounds such as BMC, dielectric strength depends on formulation, molding process and part geometry together. A nominal “dielectric strength ≥20 kV/mm” from a material supplier is not enough to design a part against — field distribution changes with thickness, and fibre orientation during molding affects local dielectric behaviour. Both have to be accounted for.
GB/T 1408.1-2016 covers short-time breakdown testing of solid insulating materials under power-frequency AC voltage. The rapid-rise method at 2 kV/s is the route commonly used for BMC.
Under Q/JTJ0001-2025, BMC and SMC dielectric strength is tested in transformer oil at room temperature with a Φ20 mm spherical electrode in rapid-rise mode, and the material-level acceptance criterion is ≥20 kV/mm. That figure describes the intrinsic property of a standard plaque, usually 3–4 mm thick. It is not the voltage a part of any thickness can simply be expected to withstand.
Breakdown in a solid dielectric does not follow voltage = field strength × thickness as a rule. In a heterogeneous material like BMC, the interface between glass fibre and resin is where the field concentrates. As wall thickness falls, local unevenness in fibre orientation has a larger effect on overall dielectric behaviour. As wall thickness rises, internal voids and filler settling become more likely, and breakdown strength can fall away from the linear expectation.
Measured across BMC 16 series plaques to GB/T 1408.1-2016 rapid-rise, three formulations with five specimens each:
Wall thickness — Typical breakdown voltage — Average dielectric strength — Design reading
1.0 mm — 12–16 kV — approx. 12–16 kV/mm — Thin walls need close attention to fibre distribution
2.0 mm — 28–36 kV — approx. 14–18 kV/mm — The most common wall thickness for insulation parts
3.0 mm — 48–60 kV — approx. 16–20 kV/mm — Intrinsic material performance fully developed
4.0 mm — 58–72 kV — approx. 14.5–18 kV/mm — Gains flatten as thickness keeps rising
6.0 mm — 72–90 kV — approx. 12–15 kV/mm — Thick parts should verify internal defect control
Two conclusions follow, both directly usable in design.
First, going from 1 mm to 3 mm raises breakdown voltage substantially, close to a proportional relationship. Where creepage distance is already satisfied structurally and only solid insulation strength has to be assured, 3 mm is a good cost-performance reference point.
Second, beyond 4 mm the return on additional thickness falls away. In a thick-wall part, better insulation margin comes from optimizing mold venting — the multi-stage venting strategy in Q/JTJ0002-2025 — to reduce internal micro-voids, and from a higher filler ratio to lower the partial-discharge tendency of the matrix, rather than from adding thickness.
The scatter in breakdown voltage is what decides production consistency. For one formulation, the coefficient of variation is driven mainly by three process factors.
Molding parameters: holding 50–70% of peak pressure for 15–30 seconds in the packing stage, as specified in Q/JTJ0002-2025, measurably reduces internal micro-voids. Void count and size set the partial-discharge inception voltage, and through it the batch-to-batch consistency of breakdown voltage.
Fibre distribution uniformity: charging by the centre-symmetric method (Q/JTJ0002-2025 clause 4.2) so the charge covers 80–90% of the cavity projection area reduces local dielectric weakening caused by orientation differences.
Post-cure: 30–60 minutes at 80–100 °C after demolding relieves residual stress and reduces the chance of a stress concentration acting as a breakdown path.
Application — Minimum wall — Expected breakdown voltage — Basis of margin
LV circuit breaker housing (400 V) — 1.5 mm — ≥20 kV — Insulation coordination to GB/T 16935.1, basic insulation impulse category
Contactor insulation barrier (690 V) — 2.0 mm — ≥28 kV — Overvoltage category III; dielectric test usually 2500 V / 60 s
MV switchgear support (12 kV system) — 4.0 mm — ≥58 kV — GB/T 11022-2020, power-frequency withstand 42 kV / 1 min, ≥1.3× margin
HV insulating pull rod (40.5 kV system) — 6.0 mm+ — ≥72 kV — Withstand 95 kV / 1 min, coordinated with creepage distance
These wall thicknesses cover solid insulation only. A real part must also satisfy creepage distance and clearance, to whichever insulation coordination standard applies to the product.
Reading “dielectric strength ≥20 kV/mm” as “safe at any thickness” is one of the most common mistakes in insulation part selection. The workable approach is to combine the actual wall thickness, operating voltage class and insulation coordination standard, ask the material supplier for measured breakdown voltage ranges at that thickness, and accept batches on consistency data rather than on a single best result.
For technical data sheets, grade recommendations, or a discussion about your specific requirements:
📧 wendy.qiu@smcbmc.com 📞 +86-13868305300
