11 ControlOn-gun extraction can be used withcontinuous wire welding processes(MIG/MAG and FCAW). This hasthe advantage that the extractionpoint is always close to the sourceof fume. Disadvantages include theincreased weight of the gun, reducedmanoeuvrability for the welder, andreduced extraction efficiency whenwelding in positions other than flat.Correct adjustment of the inlet isdifficult but important. If the inlet is tooclose to the arc, it removes the shieldinggas and weld porosity occurs.If it is too far away, it does not extractthe fume properly.To reduce background levels of fume inthe workshop, general ventilation mayalso be needed. This is not a substitutefor proper local ventilation. There aretwo basic types of general ventilation:dilution ventilation and displacementventilation.RPE is necessary. RPE protects only theperson wearing it, so it must be used inaddition to, not instead of, extraction.Types of RPE range from simple orinasal respirators for low levels ofparticulate fume to air-fed helmetsfor use in situations where there mayalso be a shortage of oxygen. Takegreat care when choosing RPE. Consultboth manufacturers and the HSE guideRespiratory protective equipment:A practical guide for users (www.hsebooks.co.uk).When extraction equipment or RPE isrequired to control emissions, employersmust ensure that it is properly used andmaintained, with records kept for fiveyears. Employees must make proper useof the equipment provided and reportany faults to the employer.Dilution ventilation directs air towardsthe source of the fume, mixing itwith the workshop atmosphere andultimately removing it using extractionfans. Dilution ventilation is not veryeffective and should be restricted tosituations where the amount of fume issmall.Displacement ventilation introducesclean air into the workshop near floorlevel and extracts fume-laden air atceiling level. This technique is suitedto high workshops containing a largenumber of small sources of fume.If ventilation or extraction cannotprovide adequate control on its own,41
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