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Breakdown voltage of a cable depends upon
Presence of moisture
Operating temperature
Time of application of voltage
All of above
Presence of moisture
The breakdown voltage of an underground cable is not a fixed constant but is influenced by several external and internal factors. Moisture ingress, high operating temperatures, and the duration for which the electrical stress is applied all degrade the dielectric strength of the insulation, thereby lowering the breakdown voltage.
The breakdown voltage of an underground cable is not a fixed constant but is influenced by several external and internal factors. Moisture ingress, high operating temperatures, and the duration for which the electrical stress is applied all degrade the dielectric strength of the insulation, thereby lowering the breakdown voltage.
VbdтАЛ=kтЛЕtтИТn тАФ where VbdтАЛ is breakdown voltage, t is time, and n is a material-dependent exponent (Inverse Power Law).
Dielectric failure in cables is a progressive process. Moisture increases the dielectric loss tangent (tan ╬┤)and causes chemical degradation. Increased temperature lowers the viscosity of impregnating oils in paper-insulated cables, reducing their dielectric integrity. Furthermore, the insulation behaves according to time-dependent failure models; prolonged exposure to high voltage initiates partial discharge activity which eventually leads to complete insulation breakdown.
Moisture creates ionic paths within the insulation, reducing effective resistivity.
Higher temperatures increase dielectric losses, leading to thermal instability.
Breakdown voltage typically follows an inverse relationship with the time of voltage application.
Cables are rated based on their ability to withstand these variables under continuous operation.
Power cable design and insulation coordination
Preventive maintenance and health index estimation of cables
Option A is correct because water ingress significantly lowers insulation resistance.
Option B is correct as thermal degradation is a primary cause of insulation life reduction.
Option C is correct because electrical stress accumulation (partial discharges) over time leads to dielectric punch-through.
D is correct тАФ The breakdown voltage of a cable is a complex variable dependent on environmental factors, thermal states, and the duration of electrical stress application.
Always remember that cable insulation acts as a capacitor; its dielectric strength is highly sensitive to the impurities (like moisture) which increase the permittivity and dissipation factor.