with a slab of concrete between them. In southern Switzerland along various lakes, telephone poles are made of granite. Starting in the early 1900s, these 18-foot (5 m) poles were originally used for telegraph wires and later for telephone wires. Because they are made of granite, the poles last indefinitely. Power distribution wires and equipment On poles carrying both electrical and communications wiring, the electric power distribution lines and associated equipment are mounted at the top of the pole above the communication cables, for safety. The vertical space on the pole reserved for this equipment is called the supply space. The wires themselves are usually uninsulated, and supported by insulators, commonly mounted on a horizontal crossarm. Power is transmitted using the three-phase system, with three wires, or phases, labeled "A", "B", and "C". Sub transmission lines comprise only these 3 wires, plus sometimes an overhead ground wire (OGW), also called a "static line" or a "neutral", suspended above them. The OGW acts like a lightning rod, providing a low resistance path to ground thus protecting the phase conductors from lightning. Distribution lines use two systems, either grounded-wye ("Y" on electrical schematics) or delta (Greek letter "Δ" on electrical schematics). A delta system requires only a conductor for each of the three phases. A grounded-wye system requires a fourth conductor, the neutral, whose source is the center of the "Y" and is grounded. However, "spur lines" branching off the main line to provide power to side streets often carry only one or two phase wires, plus the neutral. A wide range of standard distribution voltages are used, from 2,400 V to 34,500 V. On poles near a service drop, there is a pole-mounted step-down distribution transformer to transform the high distribution voltage to the lower secondary voltage provided to the customer. In North America, service drops provide 240/120 V split-phase power for residential and light commercial service, using cylindrical single-phase transformers. In Europe and most other countries, 230 V three phase (230Y400) service drops are used. The transformer's primary is connected to the distribution line through protective devices called fuse cutouts. In the event of an overload, the fuse melts and the device pivots open to provide a visual indication of the problem. They can also be opened manually by linemen using a long insulated rod called a hot stick to disconnect the transformer from the line. The pole may be