Example purposes include electromagnetic interference (EMI) filtering in an EMI filter, and the use of the Earth as a return path in a single-wire earth return distribution system. Low-voltage systems In low-voltage networks, which distribute the electric power to the widest class of end users, the main concern for design of earthing systems is safety of consumers who use the electric appliances and their protection against electric shocks. The earthing system, in combination with protective devices such as fuses and residual current devices, must ultimately ensure that a person does not come into contact with a metallic object whose potential relative to the person's potential exceeds a safe threshold, typically set at about 50 V. In most developed countries, 220 V, 230 V, or 240 V sockets with earthed contacts were introduced either just before or soon after World War II, though with considerable national variation. However in the United States and Canada, where the supply voltage is only 120 V power outlets installed before the mid-1960s generally did not include a ground (earth) pin. In the developing world, local wiring practice may or may not provide a connection to an earth. On low voltage electricity networks with a phase to neutral voltage exceeding 240 V to 690 V, which are mostly used in industry, mining equipment and machines rather than publicly accessible networks, the earthing system design is equally important from safety point of view as for domestic users. From 1947 to 1996 for ranges (including separate cooktops and ovens) and 1953 to 1996 for clothes dryers, US National Electrical Code allowed the supply neutral wire to be used as the equipment enclosure connection to ground if the circuit originated in the main service panel. This was permitted for plug-in equipment and permanently connected equipment. Normal imbalances in the circuit would create small equipment to ground voltages, a failure of the neutral conductor or connections would allow the equipment to go to full 120 volts to ground, an easily lethal situation. The 1996 and newer editions of the NEC no longer permit this practice. For similar reasons, most countries have now mandated dedicated protective earth connections in consumer wiring that are now almost universal. In the distribution networks, where connections are fewer and less vulnerable, many countries allow the earth and neutral to share a conductor. If the fault path between accidentally energized objects and the supply connection has