Final answer:
The correct safety factor for DC cable insulation is between 2 to 4, and insulation effectiveness is measured by the R factor, which indicates its resistance to conductive heat transfer and effectiveness in preventing electrical and thermal hazards.
Step-by-step explanation:
DC cable insulation must be thick enough to provide a safety factor of 2 to 4. When developing insulation, it is critical to balance material and thickness to enhance insulation properties. The desired ratio of thickness to conductivity (d/k), known as the R factor, should be high for better insulation, where a small conductivity parameter 'k' and a larger thickness 'd' are preferable. This R factor corresponds to the resistance to conductive heat transfer; thus, the greater the R factor, the more effective the insulation. As insulation thickness increases, so does the R factor, which relates to the insulation's capacity to prevent heat loss or gain.
For example, household insulation is commonly measured by the R factor, with representative values being an R factor of 11 for 3.5-inch thick fiberglass batts and an R factor of 19 for 6.5-inch thick fiberglass batts. These values indicate how insulation's effectiveness can be quantified and compared. In the context of DC cables, the insulation must be sufficient not only to prevent conductive heat transfer but also to ensure electrical safety by preventing shock hazards and thermal hazards.