However, all these designs had limitations, particularly in terms of flexibility, cost, and ease of termination. Consequently, 25 years ago we saw the introduction of cables with single or double layers of mica–glass tapes on the conductor, insulation, and outer jackets of very good fire-retardant compounds based on polyolefins (PE, PP, and so on); it was found that compounds containing halogens created conductive by-products during combustion, which caused circuits to fail in fire-performance tests