Switch and socket of the underlying framework, increase the compressive properties. It supports generally has two kinds of steel and PC. PC material support although insulation, but there is no steel good compressive capacity. So now the scaffold structure has two layers, three layers, the steel frame and PC. The two layers of support structure, the steel frame directly as chassis, although the surface of the paint, but in the long run once fall off is easy to rust, pose a safety hazard. Three layer structure, the PC as chassis, the steel cover, can effectively prevent the steel frame and the contact wire, insulation effect is better.