In-service safety inspection and testing of electrical equipment

APPENDIX KARC WELDERS(Informative)K1 GeneralA welding power supply is a device that provides an electric current to perform welding operations.Current requirements can be very low, say 5A but the average current required is usually of theorder of 80A and some applications may be considerably higher.Welding machines are usually classified as constant current (CC) or constant voltage (CV). A CCsupply varies its output voltage to maintain a steady current, while a CV supply will vary its outputcurrent to maintain a set voltage.The selection of supply type largely depends on the type of welding expected to be undertaken.Typically shielded metal arc welding will use a CC supply, while gas metal arc and flux-cored arcwelding will use a CV source. With a CC machine the welder can be assured that a fixed level ofcurrent is reaching the material to be welded regardless of minor variations in the arc distance.K2 Machine constructionMost welding machines belong to one of the following types:(a)(b)(c)Transformer – where the mains voltage or output of a generator is transformed into a highcurrent, low voltage output. This type usually allows the welder to vary the output current byeither moving a magnetic shunt in and out of the transformer core, or by selecting from a setof taps on the transformer. These are typically the least expensive.Generator or alternator – where a voltage source is derived from a mechanical energysource. For example, an internal combustion engine is used to drive the generator oralternator. A salient feature of this configuration is that if specified, a DC voltage can begenerated directly, without the need for additional rectifiers.Inverter – with the availability of high power semi-conductors it became possible to build adevice able to accept a DC input from say a battery and use it to feed an inverter based onswitching principles to produce a high power supply capable of coping with high weldingloads. More commonly, a mains or generated supply is switched at high frequency into atransformer. The advantage of this type is that the electrical characteristics of the weldingpower can be changed by software in real time. Typically, the controller software will exhibitfeatures such as current pulsing, variable ratios and current densities.K3 Operational risksThe primary hazards of electric welding are electric shock, burns from hot material, ultravioletradiant energy, toxic fumes, fire and explosion. Inert gas welders may have associated ozoneoxides of nitrogen, fluoride and silicon which are highly toxic and in both the short and long termcause inflammation and congestion of the respiratory track.The welding area should be well ventilated and illuminated, adequately screened to minimize risk tonon participants, and well away from inflammable materials, gases, liquids or their containers.Page 39