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Please read this before using presentation

Please read this before using presentation. This presentation is based on content presented at the 2008 Mines Safety Roadshow held in October 2008

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  1. Please read this before using presentation • This presentation is based on content presented at the 2008 Mines Safety Roadshow held in October 2008 • It is made available for non-commercial use (e.g. toolbox meetings) subject to the condition that the PowerPoint file is not altered without permission from Resources Safety • Supporting resources, such as brochures and posters, are available from Resources Safety • For resources, information or clarification, please contact: ResourcesSafety@docep.wa.gov.au or visit www.docep.wa.gov.au/ResourcesSafety

  2. Toolbox presentation: Arc welding hazards and safeguards – electrical

  3. Electric arc welding 3

  4. Welding hazards Electric shock Radiation Burns Heat stress Fire Explosions Asphyxiation Fumes and gases 4

  5. Electric shock Breakage of rope Railways Presence of gas Gas or dust ignition Explosives Light vehicle incidents Unconsciousness/fuming Cranes Drill/power shovels Fixed plant Rock falls Wall failures Not categorized Electrical Truck/ mobile plant Outbreak of fire Third highest category of all reportable occurrences 12% of all shocks involved welding 5

  6. Physiological effect of current 6

  7. Factors affecting electric shock severity Body impedance Current path through body Shock current magnitude Duration of shock AC or DC supply 7

  8. Body impedance Determines the current that flows Can be relatively high ( > 1000 ohms) when the skin is cool and dry Reduces significantly (200-500 ohms) if the skin becomes hot, wet or moist Lowers with increased contact area or pressure 8

  9. Current path through body Body impedance variations Hand to hand = 100% Hand to foot = 100% One hand to neck = 40% Both hands to neck = 20% 9

  10. Shock current magnitude Assume worst conditions: 80 volts AC and 260 ohms resistance Ohm’s Law I = V/R = 80/260 = 300 mA AC current above 30 mA can prove fatal 10

  11. Duration of shock If not rapidly isolated, electric current through the body will cause: muscular contraction (no “let go”) and then fibrillation (and likely death) 11

  12. AC or DC supply AC current is 3-4 times more harmful than DC Although thresholds of physiological effects are higher for DC, electrocution is still possible 12

  13. Incidents 98% of people walk away unharmed from an electric shock 13

  14. Welding incidents Welder was leaning against a boiler wall when the electrode slipped and touched his neck I = V/R = 65/250 = 260 mA Result = Death 14

  15. Welding incidents (continued) Welder made contact through chest to boiler wall with faulty electrode cable I = V/R = 70/200 = 350 mA Result = Death 15

  16. Welding incidents (continued) Assistant hands welder metal object. Resultant current is 35 mA I = V/R = 67/1900 = 35 mA Result = Assistant survived but welder died 16

  17. Recognised safety devices Sound welding practicewith manual trigger switches Involves fitting a switch on the electrode holder that cuts off the welding circuit voltage unless it is held closed Already incorporated into MIG wire feeder machines or voltage reducing devices (VRDs) VRDs do not switch on the welding voltage before a genuine attempt is made to strike an arc, and automatically cut off power when welding ceases Work automatically – welding voltage will not be established if someone has inadvertently become part of welding circuit and could receive a shock

  18. Where should these safety devices be used ? Refer to environment category in code of practice WTIA Tech Note 7-04 Health and Safety in Welding Gazetted on 7 July 2006 to apply to all mining and non-mining workplaces Sets maximum allowable welding voltages that may be used in certain conditions 18

  19. Category A environment Area where there is low risk of electric shock Includes bench-top welding, where: work piece is small and there is low risk of electric shock, or work piece is large, but welder and assistant are insulated from conductive parts (ideal conditions) Allowable voltages 80 V AC or 113 V DC(normal safe practice can be used) 19

  20. Category B environments Environment where there is significant risk of welder being in contact with parts of work piece, or freedom of movement is restricted Includes steel building structures, vessels, tanks and confined spaces Ambient temperature generally less than 32°C Allowable voltages 48 V AC or 113 V DC 20

  21. Category C environments Environment where welder is in contact with work piece and risk of electric shock or electrocution is greatly increased due to presence of moisture (sweat or water) Ambient temperature above 32°C Allowable voltages 25 V AC or 35 V DC 21

  22. Summary of electric shock hazard Arc welding can and has killed people in high risk environments Everyone involved needs to better understand the hazards, adopt sound practice and use appropriate safety devices Study and adhere to the Code of Practice: WTIA Tech Note 7-04 Under adverse conditions, strive to improve control measures ….. and don’t be an idiot ….. 22

  23. Idiot of the week! 23

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