How does IEC 60228 classify insulated cable conductors, and what factors contribute to the difference between AC and DC resistance in these conductors?

IEC 60228 categorizes insulated cable conductors into distinct classes based on construction and flexibility: solid conductors (Class 1) for rigid installations, stranded configurations (Class 2) balancing durability and bendability, and highly flexible variants (Classes 5-6) designed for dynamic applications. Regardless of material—copper or aluminum—each conductor size must adhere to strict DC resistance limits, though performance diverges significantly under alternating current.   AC resistance consistently exceeds DC values due to two electromagnetic phenomena: the skin effect, where higher frequencies concentrate current flow near the conductor surface, reducing effective cross-sectional area; and the proximity effect, arising from magnetic field interactions between adjacent current-carrying conductors that distort current distribution. While DC resistance depends solely on material resistivity and geometry, AC systems introduce frequency-dependent losses—even at standard 50-60Hz power frequencies—necessitating specialized design considerations for high-frequency applications such as hollow conductors to mitigate skin effect inefficiencies.