The correct answer is D. All of the above.
When cables are operated too hot, the oil may lose its viscosity and it may start drawing off from higher levels. This can cause the sheath to burst, as the oil is no longer able to provide the necessary support. Unequal expansion may also create voids in the insulation, which will lead to ionization. This can cause the cable to fail.
Here is a more detailed explanation of each option:
- Option A: The oil may loose its viscosity and it may start drawing off from higher levels.
When cables are operated too hot, the oil inside them may start to lose its viscosity. This means that it will become less thick and more runny. As a result, it may start to draw off from higher levels in the cable, towards the lower levels. This can cause the sheath to burst, as the oil is no longer able to provide the necessary support.
- Option B: Expansion of the oil may cause the sheath to burst.
When cables are operated too hot, the oil inside them may also start to expand. This is because heat causes materials to expand. As the oil expands, it may put pressure on the sheath of the cable. If the pressure is too great, it can cause the sheath to burst.
- Option C: Unequal expansion may create voids in the insulation which will lead to ionization.
When cables are operated too hot, the insulation around the wires inside them may also start to expand. However, the insulation may not expand evenly. This can create voids in the insulation. These voids can allow electricity to arc through the insulation, which can cause the cable to fail.
- Option D: All of the above.
All of the above reasons can cause cables to fail if they are operated too hot. Therefore, it is important to ensure that cables are not operated at temperatures that are too high.