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Cop
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Oct
201
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TIA
Slide 1
Shocking Truths About Welding
Welding Electrical Safety UpdateGlen Allan
Manager WTIA OzWeldTechnology Support Centres Network
Cop
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Slide 2
Overview
• Myth busting• Review of welding electrocutions over the
past 16 years and key learning's• The ongoing need for awareness of welding
electrical safety• Developments in Standards• Developments in Equipment• Improvements in Industry Practice• Ongoing Issues
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Slide 3
Myth Busting – True or False
• Electric shock is something welders should expect in a days work
• Welders have to learn to get used to electric shock• Welders are capable of sustaining higher levels of
electric shock than other people• The electric shocks that welders get from welding
power sources are from a low voltage source & are therefore harmless
• All welding power sources have the same electrical hazard – MMAW, GTAW, GMAW, FCAW, SAW
• A current of 1/10th of one amp (100 mA) for 1 second can(and likely will) kill you (even if you are a welder)
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Slide 4
Is The Welding Circuit Hazardous?
ACMEACMEACMEACME
Welding Welding Welding Welding
IncIncIncInc. Applied VoltagePower Source
Work
Electrode
22 fatalities in Australia 1958 to 1996
Cannington Mine 1997
Six fatalities in Australia in the past 16 years
HRD
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Slide 5
Overview of WeldingElectrocutions in Australia
The electrocutions in Australian industry over the past sixteen years from contact with a welding circuit include:
• SA Shipyard 1996• Qld Cannington Mine December 14 1997• WA Shipyard 1999• NSW Kemblawarra October 22 2004• NT Darwin Harbour February 02 2006• Qld Townsville March 16 2011
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Slide 6
Learning's from WeldingElectrocutions in Australia
• All electrocutions were from contact with an a.c. welding circuit
• All electrocutions occurred using the manual metal arc welding (MMAW) process
• All environments should have been classified and managed as Category C in accordance with AS 1674.2 – 2007
• As little as 5% w/w moisture renders clothing and PPE sufficiently conductive for electric shock and electrocution – TestSafe & UoW
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Slide 7
Voltages on Primary and Secondary Circuits of Welding Power Sources
Up to 80 V a.c. or 113 V d.c.240 V a.c. or 415 V a.c.Extra-low voltage
Not exceeding 50 V a.c. or
120 V ripple-free d.c.(Ref: AS/NZS 3000:2007)
a.c. Power sources typically ~ 1.6 x ELV
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Slide 8
Constant Current (CC) Power Sources
• Drooping volt-amp curve • “Constant” current in the operating
range• Voltage changes with arc length –
only a minor change to amperage• Limited short-circuit current for
reduced stubbing and low spatter• Used for MMAW (stick), some wire
feeders (variable speed), air carbon-arc gouging
• May be d.c.+ve, d.c.-ve or a.c.• High no-load voltage necessary for
arc starting and re-ignition especially for a.c. and “low hydrogen” rods
• High no-load voltages on these power sources exceed ELV for a.c. and d.c with ripple
0102030405060708090
100
100
110
120
130
140
Current Amps
Vol
tage Striking Voltage for GP electrodes
Striking Voltage for LH electrodes
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Slide 9
0102030405060708090
100
100
110
120
130
140
Current AmpsV
olta
ge
Constant Voltage (CV) Power Sources
• Relatively flat volt-amp curve• Maintains a relatively stable,
consistent voltage regardless of the amperage output
• Current increases rapidly with decreased voltage
• High short-circuit current helps to maintain a consistent arc length
• Used for wire feeder processes with constant wire feed
• Usually d.c. electrode +ve output for GMAW and FCAW (–ve for some FCAW applications)
• Much lower no-load voltage combined with d.c. output makes these power sources inherently safer
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Slide 10
Highest risk
Lowest risk
Risk vs Type of Current
• Low Frequency a.c.• a.c. 50Hz – causes fibrillation of the heart
• High frequency (HF) a.c.
• d.c. with ripple (usually 50 Hz)
• d.c. pulsed• Pulsed at 25 – 500 Hz
• d.c. smoothed
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Slide 11
Combined safety factor relative to 50Hz
0
0.5
1
1.5
2
2.5
3
3.5
4
4.5
5
1 10 100 1000 10000
Frequency (Hertz)
Saf
ety
Fac
tor
Effect of a.c. Frequency
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Slide 12
Amperage effects – a.c.Short exposure – @ 50 Hz
Threshold of reaction – 0.5 mA
Threshold of “let-go” – 10 mAPain, cramps, muscle control increasingly difficult – tending
towards immobilisation – 0.5-10 mA
Intense pain, severe cramps, loss of voluntary muscle
control, involuntary movements – 10-30 mA
Loss of consciousness – 60 mA
Heart cramps increasing risk of ventricular fibrillation – 50 mA
Ventricular fibrillation – 70 mA
Probability of death very high > 70 mA
Threshold of ventricular fibrillation – 30 mA
Refer AS/NZS 60479.1 2010 Effects of current on human
beings and livestock Part 1: General Aspects – Section 5
Body Current
mA
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Slide 13
Amperage effects – d.c.Short exposure – ripple free
Threshold of reaction – 2 mA
Sensation only at making and breaking of the current with the possibility of involuntary movements and cramp like
sensation, no loss of voluntary muscle control –
10-30 mA
Loss of consciousness > 300 mA
For longitudinal rising current(feet positive), duration shorter
than 0.2 sec – threshold of ventricular fibrillation– 30 mA – (as for a.c.)
Sensation of warmthin the extremities and painful
sensations on the skin – 100 mA
Transverse currents cause reversible cardiac dysrhythmias, current marks, burns, dizziness and sometimes unconsciousness – up to 300
mA
Body Current
mA
For longitudinal downward current (feet negative),
duration shorter than 0.2 sec – threshold of ventricular fibrillation
– 60 mA – (2 x a.c.)
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Slide 14
Refer AS 60479.1 – 2010 Effects of current on human beings and livestock Pa rt 1: General aspects
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Slide 15
Refer AS 60479.1 – 2010 Effects of current on human beings and livestock Pa rt 1: General aspects
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Slide 16
a.c. vs d.c.
• An a.c. (or d.c. with ripple) welding current is considered to be 2 to 4 times more hazardous than ripple free d.c.• With a.c. there is a higher risk of heart fibrillation• Difficulty “letting go” at 10 mA or greater• Good impedance match at 50Hz
• Welders have died from d.c. welding current• Very short duration d.c. shocks are likely to cause
fibrillation if they coincide with the vulnerable period of the heartbeat – approx. 1/5 of the cycle
• Timing is all important – Russian Roulette • No welding electrocutions on d.c. in Australia, so far
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Slide 17
Welding Environments
• Australian Standard 1674.2 – 2007 classifies welding environments into three categories
Category “A”
Category “B”
Category “C”
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Slide 18
AS 1674.2 – 2007 Welding Environments
• Definition• 1.3.6.1 Category A environment• 1.3.6.2 Category B environment • 1.3.6.3 Category C environment
• Classification• 2.2 (a) Category A environment• 2.2 (b) Category B environment• 2.2 (c) Category C environment
• Control measures• 2.3.1 Category A environment• 2.3.2 Category B environment• 2.3.3 Category C environment
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Slide 19
Allowable Voltages – WeldingAS 1674.2 – 2007, WTIA TN 7 – 2004, WTIA TN 22 – 2003
Working environment categories Maximum permitted open circuit voltage (OCV)
Category A environment (AS 1674.2 Clauses 1.3.6.1, 2.2 (a) and 2.3.1) is where the risk of electric shock or electrocution is low due to controls to prevent the possibility of the welder being in contact with the workpiece in the event of being in contact with a live part of the welding circuit.
d.c. 113 V peak, ora.c. 113 V Peak, 80 V r.m.s.
without an observer
Category B environment (AS 1674.2 Clauses 1.3.6.2, 2.2 (b) and 2.3.2) is where there is a high probability of the welder being in contact with the workpiece. Freedom of movement may be restricted.
d.c. up to 113 V peak,
with an observerd.c. 35 V peak,
without an observer
a.c. up to 68 V peak, 48 V r.m.s., with an observer
a.c. 35 V peak, 25 V r.m.s.,
without an observer
Category C environment (AS 1674.2 Clauses 1.3.6.3, 2.2 (c) and 2.3.3) is where there is a high probability of the welder being in contact with the workpiece and the risk of an electric shock or electrocution is greatly increased due to the presence of moisture, e.g. from high humidity, high ambient temperature, perspiration or water.
d.c. 35 V peak, always with an observer, or
a.c. 35 V peak and 25 V r.m.s., always with an observer
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Slide 20
Allowable Voltages – WeldingAS 1674.2 – 2007, WTIA TN 7 – 2004, WTIA TN 22 – 2003
Not Permitted35 V peak or25 V r.m.s.
Not Permitted35 VAS 1674.2
Category CElectrically
Hazardous (wet)
35 V peak or25 V r.m.s.
68 V peak or48 V r.m.s.
35 V113 VAS 1674.2
Category BElectrically
Hazardous (dry)
113 V peak or 80 V r.m.s.113 VAS 1674.2 Category A
Non electrically Hazardous
Working Without an Observer
Working With an Observer
Working Without an Observer
Working With an Observer
Maximum OCV – a.c. or d.c. @ > 10% ripple
Maximum OCV – d.c. @ < 10% ripple (ripple free)
Environment
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Slide 21
MMAW in a Category C Environment
• Requirements for use of a MMAW welding power source in a Cat. C environment:• A hazard reducing device (HRD), i.e. a switch or
voltage reducing device (VRD), and• An observer providing constant surveillance of the
welder (person welding)
• The observer:• Maintains visual contact with the welder• Has the capacity to immediately isolate the output
circuit of the welding power source in the event of an incident
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Slide 22
Personal Protective Equipment (PPE)
• PPE is the least effective control but is essential for cutting and welding
• All PPE should be:• Kept dry – changed if necessary• Used correctly – often PPE is the only protection
against electrical, heat and radiation hazards• Maintained in good condition and replaced as
necessary• Supplied / purchased in accordance with relevant
Standards
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Slide 23
Welding Circuit Connections
• Welding circuits carry large currents and rely on very good connections to carry that current• Connections to the power source• Line connectors• Connection to work
• Work return clamps must make a good connection on clean metal and should be placed as close as practicable to the welding point
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Slide 24
Welding Circuit Connections
• The current path in the work piece between the return clamp and the arc must be low resistance and ideally contain no joints
• Any unavoidable joints in the work piece must be either sound welds or clean tightly bolted connections with clean metal to metal contact
• The use of dual return clamps (equipotential bond) connected to a common return cable to ensure good connection to both parts being joined is recommended
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Slide 25
25
Preventing Electrical Shock
VRDs do not provide protection in all situations!
• When changing an electrode (stick) with the power on, the welder is relying on the VRD and the insulation of their gloves to prevent electrical shock
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Slide 26
26
VRDs Do Not Provide ProtectionIn All Situations!
• Electric shocks to people in contact with the welding circuit while holding a work piece in place and are quite common
• An equipotential bond (current carrying cable) attached to the piece being held and connected to the return circuit or main job provides a low resistance path
• Alternatives such as using clamps or insulated tools to secure the unattached piece should be considered
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Slide 27
27
Power Off for Electrode Changes
• It is a requirement of a number of organisations that power to the electrode holder is isolated when performing an electrode change either by:• Using an output circuit safety switch, or• Turning off power to the welding power source
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Slide 28
Industry Changes 1996 to Present
• Improved awareness of the hazards inherent in electric arc welding
• Widespread elimination of a.c. power sources for MMAW, unfortunately not yet universal
• Many organisations now permit only d.c. output welding power sources for MMAW
• Greater awareness of conditions that constitute a Category C environment, and
• Implementation of requirements of AS 1674.2 for management of Category C environments
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Slide 29
Industry Changes 1996 to Present
• Most suppliers of welding equipment offering hazard reduction by way of switches and/or voltage reduction devices (VRDs)
• Equipment manufactured with integral VRDs providing superior performance to retro-fitted VRDs
• Widespread use of HRDs in Australian industry, by way of:• A switch, or • A VRD, and• In some cases, both a VRD and a switch
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Slide 30
Industry Changes 1996 to Present
• Hazard identification, risk assessment and hazard management processes implemented
• Better understanding of conditions that exacerbate electrical hazards and the tendency to deterioration of conditions during a days work, i.e. Cat. B deteriorating to Cat. C
• Better understanding and implementation of simple precautions such as use of insulation mats, using correctly selected and maintained PPE, and minimising moisture
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Slide 31
Ongoing IssuesInspection and Testing
• Inadequate understanding of welding circuits• Tag and test personnel unaware of the link
between AS 3760 and AS 1674.2• Poor understanding and implementation of
requirements of AS 1674.2 – 2007 for periodic inspection and testing of output circuits of welding power sources
• Poor understanding and implementation of requirements of AS 1674.2 – 2007 for periodic inspection and testing of accessories
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Slide 32
Ongoing IssuesEquipment Use
• Lack of diligence in implementation of pre-start safety checks and simple maintenance
• Inadequate maintenance of welding equipment particularly electrode holders, electrode and return cables and work clamps
• Incorrect placement of return clamps and poor preparation of surfaces for high currents
• Lack of appreciation of the very low moisture levels required to change an environment from Category B to Category C classification
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Slide 33
