The correct answer is A. 1 + 0.2 kV mm.
The formula for determining the thickness of insulation between low voltage and high voltage windings is:
$t = \dfrac{E}{K} + d$
where:
- $t$ is the thickness of insulation in millimeters
- $E$ is the rated voltage in kilovolts
- $K$ is the dielectric constant of the insulation material
- $d$ is the creepage distance in millimeters
The dielectric constant of a material is a measure of its ability to store an electric charge. The creepage distance is the minimum distance between two conductors that must be maintained to ensure that there is no breakdown of the insulation.
For a practical formula, we can assume that the dielectric constant of the insulation material is 10 and the creepage distance is 1 mm. This gives us the following formula:
$t = \dfrac{E}{10} + 1$
For a rated voltage of 1 kV, this gives us a thickness of insulation of 1.1 mm. For a rated voltage of 2 kV, this gives us a thickness of insulation of 1.3 mm. And so on.
The other options are incorrect because they do not take into account the dielectric constant of the insulation material.