dpst

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1. Ground Checker
An appliance contained in a plastic housing and which has no exposed metal parts should not be dangerous to handle from the standpoint of electrical shock. On the other hand, a portable power tool such as an electric drill can be dangerous to handle, especially if the user is standing on a moist surface or touches a grounded object, unless the metal case of the tool is grounded through a three-prong power plug. It is now standard practice to equip hand tools with three-wire cords terminated in three-prong safety plugs. Power is transmitted through two of the prongs and two of the wires. The third prong is connected to ground through the electric outlet and its wire is connected to the metal frame of the tool. When there is no three-slot 115V outlet available, the tool can only be connected to the power line through a two-prong to three-prong adapter. The adapter has a pigtail lead which is supposed to be connected to the screw that secures the outlet faceplate. If the outlet box (and faceplate) are actually rounded, the tool case is grounded, making it safe to use. However, when the device has a two-prong plug, it may be dangerous to use if it has ail internal ground. Consider the circuitry of a ground checker that can be assembled in a small minibox. It consists of a small 120V lamp, a DPST toggle switch, a three-hole 115V receptacle (Jl), a Fahnstock clip or binding post (J2), and a two-wire power cable equipped with a standard two-prong plug (P). One side of the 115V ac power line is grounded (by the power company) and the other side of the line is "hot" with respect to ground. To use the ground checker, the appliance is plugged into Jl which will accept either a two-prong or three-prong plug. With P plugged into an ac outlet, either terminal X or Y of Jl will be grounded through the power line, depending upon which way P is plugged into the outlet. When checking an appliance or tool that has a three-prong safety plug, flip switch S from one position to the other. If the third wire of the power cable is connected internally to the metal case of the device, lamp I should light when S is in one position, but not in the other. When checking a device that has a two-prong power plug, connect J2 to the metal case of the device (or other exposed metal part) through a clip lead. The lamp should not light when set to either position. If the lamp does light, there is an internal ground in the device.
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