Application issues



Designs of plugs and sockets have gradually developed to reduce the risk of electric shock and fire. Plugs are shaped to prevent finger contact with live parts, and sockets may be recessed. Some types can also include fuses and switches.
Shutters on the socket prevents foreign objects from contacting live contacts. The first shuttered socket was introduced by British manufacturer Crompton, in 1893. Electrical insulation of the pin shanks to reduce live contact exposure was added to some designs, as early as 1905.
The assigned IEC appliance class is governed by the requirement for earthing or equivalent protection. Class I equipment requires an earth contact in the plug and socket, while Class II equipment is unearthed and protects the user with double insulation.
Where a "neutral" conductor exists in supply wiring, polarization of the plug can improve safety by preserving the distinction in the equipment. For example, appliances may ensure that switches interrupt the line side of the circuit, or can connect the shell of a screw-base lampholder to neutral to reduce electric shock hazard. In some designs, polarized plugs cannot be mated with non-polarized sockets. Wiring systems where both circuit conductors have a significant potential with respect to earth, do not benefit from polarized plugs.
Safety advocates, the United States Army, and a manufacturer of sockets point out a number of safety issues with universal socket and adapters, including voltage mismatch, exposure of live pins, lack of proper earth ground connection, or lack of protection from overload or short circuit. Universal sockets may not meet technical standards for durability, plug retention force, temperature rise of components, or other performance requirements, as they are outside the scope of national and international technical standards.
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.
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.
An IEC standard 61558-2-5, adopted by CENELEC and as a national standard in some countries, describes one type of shaver supply unit. Shaver sockets may accept multiple two-pin plug types including Europlug (Type C), Australian (Type I) and BS 4573. The isolation transformer often includes a 115 V output accepting two-pin US plugs (Type A). Shaver supply units must also be current limited, IEC 61558-2-5 specifies a minimum rating of 20 VA and maximum of 50 VA. Sockets are marked with a shaver symbol, and may also say "shavers only" .