prong plug and provides a ground connection. Two-prong receptacles should be replaced as soon as feasible, and a separate ground wire should be added so that each receptacle is wired as shown in Figure 6.1. The ground wire should be on top so that anything falling onto the plug will not fall onto either the hot or the neutral line.
It is also possible to fit a receptacle with a ground fault circuit interrupter (GFCI), which disconnects the current if a ground fault is detected. GFCI devices are required by local electrical codes for outdoor receptacles and for selected laboratory receptacles located less than 6 feet (1.83 meters) from sinks if maintenance of a good ground connection is essential for safe operation. These devices differ in operation and purpose from fuses and circuit breakers, which are designed primarily to protect equipment and prevent electrical fires due to short circuits or other abnormally high current draw situations. Certain types of GFCIs can cause equipment shutdowns at unexpected and inappropriate times; hence, their selection and use need careful planning.
Receptacles that provide electric power for operations in hoods should be located outside the hood. This location prevents the production of electrical sparks inside the hood when a device is plugged in or disconnected, and it also allows a laboratory worker to disconnect electrical devices from outside the hood in case of an accident. Cords should not dangle outside the hood in such a way that they can accidentally be pulled out of their receptacles or tripped over. Simple, inexpensive plastic retaining strips and ties can be used to route cords safely. For fume hoods with airfoils, the electrical cords should be routed under the bottom airfoil so that the sash can be closed completely. Most airfoils can be easily removed and replaced with a screwdriver.
Laboratory equipment plugged into a 110-V (or higher) receptacle should be fitted with a standard three-conductor line cord that provides an independent ground connection to the chassis of the apparatus (see Figure 6.2). All electrical equipment should be grounded unless it is "double-insulated." This type of equipment has a two-conductor line cord that meets national codes and standards. The use of two-pronged "cheaters" to connect equipment with three-prong grounded plugs to old-fashioned two-wire outlets should be prohibited.
The use of extension cords should be limited to temporary (less than one day) setups, if they are permitted at all. A standard three-conductor extension cord of sufficient rating for the connected equipment with an independent ground connection should be used. Electrical cables should be installed properly, even if only for temporary use, and should be kept out of aisles and other traffic areas. Overhead racks and floor channel covers should be installed if wires must pass over or under walking areas. Signal and power cables should not be intermingled in cable trays or panels. Special care is needed when installing and placing water lines (used, for example, to cool such equipment as flash lamps for lasers) so that they do not leak or produce condensation, which can dampen power cables nearby.
Equipment plugged into an electrical receptacle should include a fuse or other overload protection device to disconnect the circuit if the apparatus fails or is overloaded. This overload protection is particularly useful for equipment likely to be left on and unattended for a long time, such as variable autotransformers (e.g., Variacs and powerstats), vacuum pumps, drying ovens, stirring motors, and electronic instruments. Equipment that does not contain its own built-in overload protection should be modified to provide such protection or replaced with equipment that provides it. Overload protection does not protect the worker from electrocution, but it does reduce the risk of fire.
Laboratory personnel should be certain that all electrical equipment is maintained well, properly located, and safely used. In order to do this, the following precautions should be reviewed and the necessary adjustments made prior to working in the laboratory:
Insulate all electrical equipment properly. Visually inspect all electrical cords monthly, especially in any laboratory where flooding can occur. Keep in mind that rubber-covered cords can be eroded by organic solvents and by ozone (produced by ultraviolet lamps).
Replace all frayed or damaged cords before any