throughout. These cables are connected to powerful pumps that pump the oil through them while in operation. Safety switches turn off the oil flow in the event of a leak, in order to limit the effects of a hydrocarbon fire.]] in Sweden, showing rapid fire spread through burning of cable jackets from one cable tray to another.]]A cable is one or more wires or optical fibers bound together, typically in a common protective jacket or sheath. The individual wires or fibers inside the jacket may be covered or insulated. Combination cables may contain both electrical wires and optical fibers. Electrical wire is usually copper because of its excellent conductivity, but aluminum is sometimes used because it costs less. Construction Electrical cables may be made flexible by stranding the wires. In this process, smaller individual wires are twisted or braided together to produce larger wires that are more flexible than solid wires of similar size.