Full definition
The maximum electric field strength (voltage per unit thickness) that a material can withstand before electrical breakdown (insulation failure) occurs, measured in kV/mm (or V/mil in US units). Dielectric strength is the fundamental property determining the voltage rating of electrical insulation materials: rubber sheets, gloves, mats, cable insulation, and motor winding insulation. Testing per ASTM D149 (short-time and step-by-step methods): a voltage is applied across a specimen of known thickness and increased until breakdown occurs. Typical values for elastomers: natural rubber NR 20-25 kV/mm, EPDM 20-30 kV/mm, silicone VMQ 15-25 kV/mm, neoprene CR 15-20 kV/mm, polyurethane PU 15-25 kV/mm. For comparison: PTFE 15-25 kV/mm, glass 10-15 kV/mm, air 3 kV/mm. Factors reducing dielectric strength: moisture contamination, elevated temperature, mechanical damage (cuts, punctures), aging (oxidation creates conductive pathways), embedded foreign particles, and increased thickness (dielectric strength per mm decreases with thicker specimens). For dielectric mats (ASTM D178) and gloves (ASTM D120), the product is proof-tested at 2.5-3x the rated working voltage to ensure a safety margin. Conductive fillers (carbon black) drastically reduce dielectric strength — hence dielectric products use white/light fillers (kaolin, silica, CaCO₃).