Document mmj6O533rwgkKRBQKgg0zvg4J

ELECTRICAL HAZARDS For emergency grounding of electnc tools, such as may be used by contractors before regular facilities are available, a flexible ground wire may be fastened direct ly to the noncurrent-carrymg metallic parts of the tools and then fastened to a spring clip that can be clipped onto a good ground (metallic budding framework, water pipe, etc ) Built-in grounding One method of equip ment grounding uses a three-conductor cord (four-conductor for three-phase ctrcuit) with a multicontact polarized plug-in re ceptacle This method ensures use of the ground because one wire forms a continu ous path from the equipment through the receptacle to the ground This method of grounding can be used only where polarized receptacles that admit the polarized plugs have been installed. Otherwise, it would be necessary to use adapters or to change plugs, with resultant loss of tune and possible hazard from wrong connections If portable electnc tools are not properly grounded, accidents such as the following may occur 1 While standing on a wet terrazzo floor, a worker using an ungrounded portable drill was preparing to drill holes m the floor. Insulation within the drill failed and its frame became "hot" Since there was no ground to carry off the current safely, it passed through the workman's body, killing him 2 A workman was drilling holes in a large exhaust hood in a building in which the wiring had polarized receptacles How ever, his tool had only a two-wire cord and was not otherwise grounded A de fect developed in the tool, and its case became energized The man had been perspiring freely and was standing on a conductive tile floor He was electro cuted 3 A worker was using a portable electnc drill while standing on wet ground His dnU was of the three-wire polarized type, but the location did not have polar ized receptacles The workman neglected to attach the ground wire of his adapter and, when the tool case became charged, be was electrocuted. Since an early lack of standardization in the design of plugs and receptacles and in Fig. 31-5. This Is the siondord receptacle and plug recommended by the Notional Electrical Manufacturers Association. The receptacle is designed to receive a plug having three blades, a U-shaped or round grounding blade with two standard parallel polarized blades This type of fixture also permits use of the nonpolar ized standard plug. their application led to confusion, a stand ard* was set up This standard specifies that a receptacle similar m size and shape to the ordinary two-hole type be designed to receive a plug having three blades, a U-shaped or round grounding blade and two standard parallel polarizing blades (Fig 31-5) A receptacle of this type will permit use of the polarized or nonpolarized standard plug for such tools as soldering irons. "American Standards Association, 10 East 40th St, New York City. Outlet Re ceptacles, Attachment Plug Caps, and Appli ance Plugs, with supplement C73 1 (Also NEMA WD 1-1961 ) 31-13