they are outside the scope of national and international technical standards. A technical standard may include compatibility of a socket with more than one form of plug. The Thai dual socket is specified in figure 4 of TIS 166-2549 and is designed to accept Thai plugs, and also Type A, B, and C plugs. Chinese dual sockets have both an unearthed socket complying with figure 5 of GB 1002-2008 (both flat pin and 4.8 mm round pin), and an earthed socket complying with figure 4 of GB 1002-2008. Both Thai and Chinese dual sockets also physically accept plugs normally fitted to 120 V appliances (e.g. 120 V rated NEMA 1-15 ungrounded plugs). This can cause an electrical incompatibility, since both states normally supply residential power only at 220 V. Voltage rating of plugs and power cords Plugs and power cords have a rated voltage and current assigned to them by the manufacturer. Using a plug or power cord that is inappropriate for the load may be a safety hazard. For example, high-current equipment can cause a fire when plugged into an extension cord with a current rating lower than necessary. Sometimes the cords used to plug in dual voltage 120 V / 240 V equipment are rated only for 125 V, so care must be taken by travellers to use only cords with an appropriate voltage rating. Appliance connections and extensions So that manufacturers need not build many similar appliances differing only in the type of plug fitted, a common strategy is to provide an IEC 60320 inlet on the appliance and a detachable power cord (mains flex lead) and appropriate plug. The appliance need only to be tested to the power inlet. Some appliances have a switch for selection of voltage. Extension cords (extension leads) are used for temporary connections when a socket is not within convenient reach of an appliance's power lead. A power strip with multiple sockets may also have a switch, surge voltage protection, or over-current protection. Special purpose plugs and sockets Special purpose sockets may be found in residential, industrial, commercial or institutional buildings. Examples of systems using special purpose sockets include: "Clean" (low electrical noise) earth for use with computer systems, Emergency power supply, Uninterruptible power supply for critical or life-support equipment, Isolated power for medical instruments, tools used in wet conditions, or electric razors, "Balanced" or "technical" power used in audio and video