Electric field strength units — V/m and kV/mm
Field strength is what breaks down insulators. Voltage matters only in context of thickness.
Dielectric breakdown is a field problem, not a voltage problem. A 1 kV signal is fine on 1 mm of Mylar and catastrophic on 1 µm of the same film. Field strength captures this.
The exact factors
1 kV/mm = 10⁶ V/m = 1 MV/m > 1 V/mil = 39.37 V/mm = 39 370 V/m
Dielectric strength of common materials
Dry air (STP): 3 kV/mm. Distilled water: ~65 kV/mm short term. Mineral oil: 12–15 kV/mm. FR-4: 20 kV/mm. Kapton: 300 kV/mm. SF₆ at 1 bar: 89 kV/mm. Vacuum: geometry-dependent, often 20 kV/mm.
Worked example — HV cable insulation
A 33 kV cable with 8 mm insulation runs at 4.1 kV/mm average — well below XLPE's 50 kV/mm short-term strength, but partial discharge inception starts at 3–5 kV/mm over decades. Design margins live in the field strength number, not the voltage.
Recap
kV/mm is the language. Check both peak and continuous field strength against material limits.
Try the converters mentioned in this article
FAQ
Does frequency matter?+
Yes — AC breakdown is 30–70% of DC breakdown for most solids because heating and partial discharge accumulate.
Is 'BIL' the same as dielectric strength?+
No — BIL (basic insulation level) is a system-level voltage rating standardised across a device, not a per-material field strength.
This article was written by The Turbo Unit Converter engineering desk (Applied metrology & engineering units) and last reviewed on 2026-07-07 against NIST SP 811 and the BIPM SI Brochure. Read our full editorial policy.