What do you mean by grading of cables?

The process of obtaining a uniform distribution of dielectric stress or voltage gradient in a dielectric is called grading of cable. The dielectric stress is minimum at the outermost sheath of the conductor whereas it is maximum at the surface. This non-uniform distribution of the stress causes insulation to break down and increases the thickness of the cable. So, to avoid this, the uniform distribution of dielectric stress is achieved with the grading of cables.