Correct Answer - Option 1 : reduced
Concept:
Unequal stress distribution:
- The electrostatic stress in a single core cable has a maximum value (gmax) at the conductor surface and goes on decreasing as we move towards the sheath.
- The maximum voltage that can be safely applied to a cable depends upon gmax i.e., electrostatic stress at the conductor surface.
- For safe working of a cable having homogeneous dielectric, the strength of dielectric must be more than gmax.
- If a dielectric of high strength is used for a cable, it is useful only near the conductor where stress is maximum. But as we move away from the conductor, the electrostatic stress decreases, so the dielectric will be unnecessarily overstrong.
Disadvantages:
The unequal stress distribution in a cable is undesirable for two reasons. Firstly, insulation of greater thickness is required which increases the cable size. Secondly, it may lead to a breakdown of insulation.
Methods to achieve uniform stress distribution:
The process of achieving uniform electrostatic stress in the dielectric of cables is known as the grading of cables.
In order to overcome the above disadvantages, it is necessary to have a uniform stress distribution in cables. This can be achieved by distributing the stress in such a way that its value is increased in the outer layers of the dielectric. This is known as the grading of cables. The following are the two main methods of grading of cables:
- Capacitance grading
- Inter-sheath grading
Results of grading of cables:
- Uniform stress distribution
- Reduction in the quantity of insulation
- Increase in quality of insulation
Therefore, grading of cable means the distribution of dielectric material in such a way that the difference between Emax and Emin is reduced to very minimum.
