Knob-and-tube wiring is an old fashioned type of electrical system that has largely been replaced by circuit breakers and fuses. Knob-and-tube electrical systems were commonly used in North American homes and commercial buildings from the 1880s to the 1930s. The type of wiring is different from today's systems because most wires did not have a rubber insulation sleeve around the copper cables. To protect the circuits, porcelain insulators and tubes are used where wiring passes through joists or wall cavities. The system has mostly been replaced by new technologies and is only recommended for certain installations by the National Electric Code (NEC). Today's home electrical systems are designed to meet current demand, and knob-and-tube wiring has several disadvantages. There is a safety concern because knob-and-tubes do not include a grounding conductor. They do not confine electrical switching, and some wires are exposed because junction boxes are not always used.