Document zQNNzq5gyGDvrGrrd3RymNndg

Since these welding converters are subject to abuse and are given little expert attention as compared to ordinary electrical equipment, they should be well protected, both mechan ically and electrically Often the machines are moved about, so convenient primary power receptacles should be provided about the shop or construction operation Plugs and receptacles should meet the requirements of Underwriters Laboratories Inc AC transformers for welding are usually air-cooled The transformers should be built in accordance with the high standards of elec trical construction used on power or distribu tion transformers A typical industnal-rated AC transformertype welding unit should be equipped with a voltage-reducing control which automati cally reduces the open-circuit secondary volt age to 38 volts during idling This reduction of the voltage is delayed two to three sec onds from the instant the arc is broken to prevent interference with normal welding op erations Contact of the electrode to the work automatically restores normal welding volt age Other safety features are (a) circuit breaker, integrally mounted on the unit, for providing thermal overload protection and serving as a convenient disconnect switch, (b) circuit breaker of the automatic type or a fused safety switch mounted on the wall and rated for use with the unit and the power supply circuit, (c) three-conductor power supply cable, one conductor of which is for grounding purposes, between the wall mounted breaker and the unit Large welding machines are usually in stalled as stationary units Since primary line switches are not normally provided as part of the equipment, they should be installed by the person making the installation The smaller machines, which may be used as portable units, customarily have primary switches built into them and are provided with primary cables and attachment plugs of proper capacity Welding transformers should not be attached to lighting circuits under any circumstance If a gasoline-powered welding generator is used inside a building or in a confined area, the engine exhaust should lead to the outside atmosphere Otherwise, carbon monoxide and other toxic gases may accumulate Voltages The voltage across the welding arc vanes from 15 to 40 volts, depending on the type and size of electrode use The welding circuit must supply somewhat higher voltage to stnke the arc This voltage is called the open cir cuit or "no load" voltage After the arc is established, the open circuit voltage drops to a value about equal to the arc voltage plus the lead voltage drop The open circuit volt ages on DC welding machines should be less than 100 volts Constant voltage power sup plies (welder or converter) are now also being widely used For AC transformer welding machines, ANSI Z49 1, Safety m Welding and Cutting, prescnbes a maximum open circuit voltage of 80 volts on manuals, 100 volts on automatics Some standard heavy duty industrial AC welding machines have open circuit voltages of 65 volts Reliable control equipment is available which automatically reduces^ the open circuit voltage to about 30 volts during the "off" arc period, at which time the welder may be exposed to the open circuit voltage This control equipment is commonly used where welders are working outdoors on wet steel or in confined quarters or under other severe conditions where the probability of con tact with the open circuit voltage may be greater Heavy duty AC welding machines (ratings usually over 500 amp for automatic machine welding) are also built with open circuit voltages of 75 to 80 volts with a special tap to provide 100 volts where necessary The tap may be needed to obtain rated output from the machine if the line voltage is low or if the voltage of the secondary circuit drops The tap should not be accessible to the welder for current adjustment but should be under the control of a responsible electrician or supervisor Open circuit voltages should be as low as 50 volts on small AC welding machines used without expert supervision Since these ma chines are often used with low arc voltage electrodes and with leads not more than 20 ft long, the probability of a large voltage drop m the welding circuit is small For other manual and automatic welding and cutting processes in which the work 961