The thickness of insulation layer on the conductor in cables dependent upon

A. Reactive power
B. Power factor
C. Operating Voltage
D. Current to be carried 

Answer: C. Operating Voltage

Explanation:
The thickness of the insulation layer on the conductor in cables is dependent upon voltage.

In a conductor, the voltage gradient is developed along the length of the conductor which decreases radially outwards from the conductor. 

If no insulation is being used then air behaves as dielectric to absorb that voltage gradient and may take a few meters to completely absorb the voltage gradient. 

For example overhead bare conductors, that's why they have separated apart with a certain distance. 

In distance between the bare conductor increases with the increase in voltage.

But in the case of underground cables insulation covers the bare conductor. 

The material used in insulation has greater dielectric strength and absorbs all the voltage gradient just within a few mm or cm distance. As the voltage increases the thickness of insulation increases.

Thus insulation absorbs the voltage gradient. 

Also, insulation is made of such material so that it can dissipate the heat generated in the conductor inside. 

As the thickness of the conductor depends on current, so proportional thickness of the conductor allows optimal heating in a conductor, and heat generated dissipates through the insulation although this is very small.

Thus insulation is independent of current and depends only on voltage.



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