Document ZLbOJR0y4nL5koqDZ7L23ka7

Welding and Cutting air-cooled with two-winding primaries The transformers should be built in accordance with the high standards of electrical construc tion used on power or distribution trans formers 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 30 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 operations Contact of the electrode to the work automatically restores normal welding voltage Other safety features are (a) cir cuit 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 ofthe 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 mon oxide and other toxic gases may accumulate Voltages The voltage across the welding arc varies from 15 to 40 volts, depending on the type and size of electrode use The welding circuit must supply somewhat higher voltage to strike the arc This voltage is called the open circuit 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 cir cuit voltages on DC welding machines should be less than 100 volts For AC transformer welding machines, ASI Standard Z49 1, Safety in Welding and Cuttmg, prescribes 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 m confined quarters or under other severe conditions where the probability of contact 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 m which the work metal is connected electrically to one side of the circuit Open circuit voltage of 150 volts may be allowed provided the following conditions are present 1 All equipment and circuiting are fully insulated and the operator cannot make electrical contact other than through the arc itself, while the arc is maintained 2 Disconnecting or voltage reducing de vices operate within a time limit not ex ceeding one second after breaking the arc 934