TO-3 at Motorola around 1955. The lead spacing was originally intended to allow plugging the device into a then-common tube socket. The metal package can be attached to a heat sink, making it suitable for devices dissipating several watts of heat. Thermal compound is used to improve heat transfer between the device case and the heat sink. Since the device case is one of the electrical connections, an insulator may be required to electrically isolate the component from the heatsink. Insulating washers may be made of mica or other materials with good thermal conductivity. The case is used with high-power and
Electrical wiring in North America only on the wiring code but also on the technical standards for wiring devices, materials, and equipment. Grounding (earthing) of distribution circuits is a notable difference in practice between wiring systems of the United States and those of other regions. Since the early 1960s, wiring in new construction has required a separate grounding conductor used to bond (electrically connect) all normally non-current carrying parts of an electrical installation. Portable appliances with metal cases also have a bonding conductor in the flexible cable and plug connecting them to the distribution system. The circuit return conductor ("neutral") is also connected to ground