求翻译：4.油罐车的拖地铁链不许缠绕在大梁上。油料和其它物质一样，当与另一物体相互摩擦时也会产生静电。这种静电跳火，可能将燃料点燃引发火灾。所以在油库上要装有接地的地线，油罐车上也要装有一条接地的铁链，以防止油罐内产生静电火花，此铁链不能随便缠绕在大梁车架上，必须放在地面上，并且拖地段不少于50厘米。是什么意思？

4. tank wagons on subway chain wrapped around the frame is not allowed. As oil and other substances, when mutual friction with another object will generate static. This static Flash, could light the fire of fuel. So in the depot to be fitted with a ground on the ground, also chains to be fitted with a ground tank wagons, to prevent the tank have a static electric spark, the chains just wrapped on the beam frame, must be placed on the ground, and drag-and-lots of not less than 50 cm.