Document 7VNabk31KMBawg68XgNKeqLB

method can be used in the distributing cen ter or the tool room to check portable tools before they are issued or upon their return The test should be made between the ground ing prong on the plug and the tool frame Double-insulated tools Maintaining a good ground and making sure a man uses it properly can be, unfortu nately, a big problem Add to this the fact that some companies have not yet installed the three-blade grounded receptacle because of the easy availability of the adapter that allows the use of a two-blade receptacle In the latter case, an operator can use a portable electrical tool without connecting the adapter ground wire Sometimes, if the adapter be misplaced, a workman may even cut off the grounding connection on the port able power tool If an electric light outlet is used as a power source, the third wire may never be connected A completely safe operation, therefore, re quires regular tests to maintain a good ground, and regular checks to make sure workers are grounding electrical equipment To overcome these difficulties, portable tool manufacturers have developed another way to provide safety from electrical shock for tool users One that has gained consider able acceptance is double insulation (See discussion under "Electric tools," in previous chapter ) GROUND CLAMP Fig 33-6 -- A simple low-voltage test unit with an indicating light bulb The diagram shows a con tinuity test between the ground prong of the plug and the ground wire connection to the metal shell Courtesy Self-Insurers Service. Inc Explosion-proof apparatus Standard electrical apparatus considered safe for ordinary application are obviously unfit for installation m locations where flam mable gases, vapors, dusts, and other easily ignitable flammable materials are present Sparks and electric arcs originating in such fittings have been the igniting medium m costly fires and explosions Explosion-proof Apparatus is defined in the National Electrical Code Apparatus enclosed in a case which is capable of withstanding an explosion of a specified gas or vapor which may occur within it and of preventing the ignition of a specified gas or vapor surrounding the enclosure by sparks, flashes, or explosion of the gas or vapor within, and which operates at such an external temperature that a surrounding flam mable atmosphere will not be ignited thereby Before electrical equipment and its asso ciated wiring is selected for a hazardous lo cation, it is necessary to determine the exact nature of the flammable materials present For instance, an electrical fitting or device which is found by test to be safe for installa tion in an atmosphere of combustible dust may be unsafe for operation in an atmos phere containing flammable vapors or gases The National Electrical Code should be checked for these types of hazardous mate rials, which are described in detail m Articles 500 through 503 A summary of the hazard ous location classifications described in this code are given in Fig 33-7 A study should be made of the machines or devices to be used and of the processes in volving liquids, gases, or solid substances, and their ratings should be checked When hazards have been determined and the layout of the building inspected, it can be decided whether one small section of the plant should be classified as hazardous or whether the hazardous conditions extend to all parts The results of this study should be pre sented to a manufacturer of explosion-proof apparatus so that electrical equipment and wiring can be selected for safe installation Leading manufacturers of electrical equip ment often maintain staffs of engineers to guide buyers m the purchase of explosionproof fittings Because of the highly tech- (Text continues on page 994 ) 989