There are relevant calculations in Code for Design of Power Cable Engineering or Code for Design of Urban Cables. Or buy a book about cable calculation compiled by Ma Guodong or others, which contains the calculation of sheath overvoltage, which is very simple. For your project, the length of the cable is not long and the running current is not large. Direct grounding can be adopted, and the other end is grounded through the protector. Be sure to ensure the reliability of grounding. Many problems at joints are caused by faulty grounding wires or poor grounding. The other end of your existing grounding method is suspended, which is more dangerous. Short-circuit is easy to cause overvoltage in the suspension section and puncture the outer sheath.

As for your cable head, it is either the quality problem of the cable head, or the thermal burn-out or electric breakdown. Without relevant information, it is not easy to analyze. Generally, the grounding method is not good, and many problems will occur.

That's all for now.