Quick answer
SS-EN 50397-1 is the standard that sets out requirements and tests for covered conductors used in overhead lines. It deals with the conductor and its covering, including how the covering must perform under electrical, mechanical and environmental conditions.
APAR designs its medium voltage covered conductors to this standard, along with BS EN 50182, IS 398 Part 2 and Part 4, and IEC 61089. Accessories for covered conductor lines, such as clamps and piercing connectors, are covered by the companion standard EN 50397-2. When writing a tender, name the standard and the voltage class together, and ask for type test reports against it.