Transcript Mines Safety Roadshow 2008 - Department of Mines and Petroleum

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:
[email protected]
or visit
www.docep.wa.gov.au/ResourcesSafety
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Toolbox presentation: Arc welding hazards and
safeguards – electrical
Electric arc welding
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Welding hazards
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Electric shock
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Radiation
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Burns
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Heat stress
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Fire
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Explosions
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Asphyxiation
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Fumes and gases
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Electric shock
Third highest category of all reportable occurrences
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
12% of all shocks
involved welding
0
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1000
2000
3000
4000
5000
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40
30
Fibrillation
Cramps
Breathing
Problems
50
Muscular
Paralysis
Milliamps
60
Involantary Reflexes
70
Perceptable Current
80
Ventricular
Physiological effect of current
20
10
0
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Factors affecting electric shock severity
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Body impedance
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Current path through body
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Shock current magnitude
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Duration of shock
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AC or DC supply
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Body impedance
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Determines the current that flows
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Can be relatively high ( > 1000 ohms) when the skin is
cool and dry
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Reduces significantly (200-500 ohms) if the skin becomes
hot, wet or moist
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Lowers with increased contact area or pressure
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Current path through body
Body impedance variations
Hand to hand
Hand to foot
One hand to neck
Both hands to neck
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= 100%
= 100%
= 40%
= 20%
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Shock current magnitude
Assume worst conditions:
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80 volts AC and
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260 ohms resistance
Ohm’s Law
I = V/R = 80/260 = 300 mA
AC current above 30 mA can prove fatal
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Duration of shock
If not rapidly isolated, electric current through the body
will cause:
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muscular contraction (no “let go”) and then
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fibrillation (and likely death)
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AC or DC supply
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AC current is 3-4 times more harmful than DC
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Although thresholds of physiological effects are higher for
DC, electrocution is still possible
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Incidents
98% of people walk away unharmed from an electric shock
INJURIES
2%
SHOCKS
98%
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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
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Welding incidents (continued)
Welder made contact through
chest to boiler wall with faulty
electrode cable
I = V/R = 70/200 = 350 mA
Result = Death
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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
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Recognised safety devices
Sound welding practice with
manual trigger switches
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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)
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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
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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
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Gazetted on 7 July 2006 to apply to all mining and nonmining workplaces
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Sets maximum allowable welding voltages that may be
used in certain conditions
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Category A environment
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Area where there is low risk of electric shock
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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)
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Category B environments
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Environment where there is significant risk of welder
being in contact with parts of work piece, or freedom
of movement is restricted
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Includes steel building structures, vessels, tanks and
confined spaces
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Ambient temperature generally less than 32°C
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Allowable voltages 48 V AC or 113 V DC
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Category C environments
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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)
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Ambient temperature above 32°C

Allowable voltages 25 V AC or 35 V DC
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Summary of electric shock hazard
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Arc welding can and has killed people in high risk environments
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Everyone involved needs to better understand the hazards, adopt
sound practice and use appropriate safety devices
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Study and adhere to the Code of Practice: WTIA Tech Note 7-04
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Under adverse conditions, strive to improve control measures
….. and don’t be an idiot …..
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Idiot of the week!
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