Electric Field Converter
Same value in every unit
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Field strength, not voltage
Insulation is rated by how much field it can stand, because voltage alone says nothing without a thickness. A material rated 20 kV/mm holds off about 20 kV across one millimetre and about 40 kV across two. That is why a datasheet gives you volts per something rather than a voltage.
Two systems that look nothing alike
Metric work uses kV/mm or kV/cm. American datasheets use volts per mil, a mil being a thousandth of an inch. One V/mil is about 39,370 V/m, so the same material reads as roughly 500 V/mil or 20 kV/mm — the numbers give no clue that they describe the same thing.
Why real designs leave a wide margin
- Sharp edges concentrate the field far above the average.
- Moisture and contamination lower the breakdown point sharply.
- Altitude thins the air, and air is often part of the insulation path.
- Time. A material that survives a brief test can fail under continuous stress.
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Frequently Asked Questions
The field an insulator can withstand before it breaks down and conducts. It is quoted as a field strength, not a voltage, because the thickness matters as much as the volts — the same material twice as thick withstands roughly twice the voltage.
Because insulation thickness is specified in mils there — a mil being one thousandth of an inch. One volt per mil is about 39,370 volts per metre, so the numbers look very different from the metric ones for the same material.
Multiply the field strength by the thickness. A material rated at 20 kV/mm, one millimetre thick, should hold off about 20 kV. Real designs leave a wide margin, because edges, contamination and moisture all lower it.
Up to a point. Air breaks down at roughly 3 kV per millimetre at sea level in dry conditions, which is why a spark jumps a small gap. That figure falls with humidity, altitude and any sharp point that concentrates the field.
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