Reducing Electrical Hazards in Maintenance Welding
Welding in any workplace introduces many hazards that must be controlled to prevent accidents, injuries, and property damage. Welding in a plant or on-site maintenance environment adds many additional challenges.
A weld that could be readily performed in a controlled space such as a maintenance shop becomes more complex when the location is remote or outside. One of the significant differences in field (or in-situ) welding is the increased likelihood of electrocution or inadvertent damaging of the facility’s electrical system.
A disturbing cluster of preventable electrocution and electrical shock incidents involving maintenance workers using arc welding has occurred in Canada in recent years. Earlier this year, the seventh edition of the national standard CSA W117.2:19, Safety in welding, cutting, and allied processes was published. This edition has many progressive updates, of which many are in direct response to the most recent reviews of injuries and fatalities, including electrical incidents.
A Review of the Basics
Arc welding involves the use of an electrical power source (welding machine) that is isolated from the electrical network through insulation or by remote generation/battery power. Standard power sources are generally limited to a secondary no-load voltage (open-circuit voltage or OCV) of 113 VDC peak or 113 VAC peak (80 V r.m.s.). This is the maximum voltage across the main output terminals that a person can be exposed to if they contact the electrode while in contact with the work.
If the welding machine is electrically powered, it can have a primary input (or main) voltage of up to 600 VAC. A key function of the power source is to lower and regulate this voltage for the secondary or output welding circuit, which is typically a much higher amperage.
The intention is that this secondary circuit be completed as an isolated closed-loop system with two cables:
• Electrode Lead — is the secondary circuit conductor transmitting energy from the power source to the electrode holder, gun, or torch.
• Workpiece Lead — is the secondary circuit conductor that is attached to the workpiece by the return current clamp and completes the welding circuit.
For an electrical shock or electrocution to occur, an electrical current must pass through a person’s tissue, organs, or bodily fluids. Electrical current levels as low as <0.025 A are known to induce an involuntary muscle contraction, preventing the person from letting go of an electrically live object or releasing a switch. Although there are many variables, such as the pathway that the current travels through the body and the duration of the event, it is clear that amperage levels of well below 0.5 A can be lethal.
It is believed that under certain circumstances, such as when the skin is soaked with perspiration or the skin is broken, that the resistance of the human body can be <100 Ohms. Applying Ohm’s Law, it is clear then that an arc welding machine produces more than sufficient voltage to electrocute a person, and, unfortunately, this has been verified by an analysis of accident investigations resulting in welder electrocutions and reported shock incidents.
While, in many ways, the electrical hazards from the primary (or main) connection of a welding machine is not much different than any other electrically powered industrial equipment, there are additional hazards that can be caused when these machines are used. These hazards can be grouped into two categories:
1. Secondary voltage hazards; and;
2. Stray welding current damage.
The Secondary Voltage Hazard
The secondary (or output) voltage on many arc welding machines is high enough to electrocute a person when conditions are unfavourable. Typically, maintenance workers are at the highest risk because of the nature of welding processes they use and the challenging work environments they endure. Many experienced maintenance welders tell tales of electrical shocks they have taken, but they are the lucky ones; other workers have, unfortunately, not lived to tell stories.
The four contributing factors in most reported serious welding shocks or electrocutions are as follows:
1. When work environments are hot, humid, damp, or high in electrolytes (e.g., salts), and the moisture or worker’s perspiration reduces the electrical insulating properties of standard PPE (leather gloves or work clothes);
2. When the welding machine is producing high no-load (or OCV) voltage across the terminals;
3. When the welder is in a restricted or confined space and in contact with the work (or structure) that is being welded, or even a damp concrete floor; and
4. When the worker is using a process such as shielded-metal arc welding where the welding machine voltage is normally ‘ON’ and there is a requirement to routinely touch the electrode to change this consumable.
Along with best practices (using dry insulating mats), for these situations there is an engineering control called a Voltage Reduction Device (VRD) available that can reduce the shock hazard. VRD technology is available on certain models of new arc welding machines or can be purchased as an accessory and retro-fitted to existing machines. Unfortunately, VRD technology is not well known by end-users and is not yet in widespread use.
A VRD is designed to reduce the voltage present between the electrode holder and the work to a safer level, and when the arc is struck, the technology detects a drop in resistance and automatically increases voltage to a suitable level to perform the weld. Users and laboratory testing confirm that these devices do not interfere with the welding operation. The revised CSA W117.2:19 now requires the use of VRD technology under specific circumstances, such as those described above.
Stray Welding Current Damage
Stray Welding Current (SWC) is a fault condition where serious property damage can occur, and the damage to the electrical network can be severe and lead to electrocution of workers. This is a fault where potentially hundreds of amperes of secondary welding current return to the isolated power source through unintended means, usually the building’s protective grounding or bonding conductors. Often this occurs when welders neglect to place the return current clamp at a location close to the point of welding; however, there are scenarios where minor equipment malfunctions can be the root cause.
SWC can be generated by any arc welding machine. In some cases, stray current damages other electrically powered devices in the vicinity of the welding operations. Other equipment can be damaged by this, including machinery bearings, overhead cranes, lifting chains, and slings. There is also the potential arcing in unexpected locations in a facility to cause fires and explosions.
Stray current damage can be a problem economically, since damage may be initially hidden, before it becomes a safety issue. Thousands of dollars of damage can occur in a facility or to machinery and systems from one stray current event, which may not be immediately detected.
Welding machines are bonded to earth potential using a grounding (protective Earth) wire that connects the metallic chassis through the electrical network to an earthing system. The grounding wire is a critical part of the electrical safety system. Its purpose is to prevent electrical shocks in the case that a fault occurs where an energized conductor comes in contact with an exposed metallic component by shorting that conductor and opening the overcurrent protection device.
SWCs through grounding conductors are not interrupted by the normal circuit protection systems. Electrical codes do not allow for or specify fuses or other overcurrent protection devices in these grounding/bonding conductors, as they are intended to be an uninterrupted low-impedance pathway for fault currents. Even circuits protected by ground fault circuit interrupter (GFCI) devices are ineffective at preventing stray current faults, since these devices work from the principle of detecting AC current imbalances in the primary current carrying conductors using a differential transformer, ignoring the grounding conductors.
SWCs are usually avoided through the strict adherence to rules of the applicable welding safety standards such as CSA W117.2:19. These practices include locating the workpiece lead return attachment point as close as practicable to the arc, using well-maintained welding cables of sufficient amperage capacity, and ensuring that the work return current clamp is firmly attached on an intentionally cleaned spot.
It’s All About Safety
The use of welding machines, especially in maintenance environments, can pose significant electrical hazards. They can provide enough voltage output to cause electrocution under certain circumstances, but there is technology readily available called a VRD, which can greatly reduce this hazard. Welding machines can also generate a stray current that will travel through the electrical system grounding or bonding wires, which can severely damage these critical safety devices, leading to significant property damage and even electrocutions.
The proper installation of the machine and routine inspection of plugs, receptacles, and cords are basically the same as a band-saw or other fabrication equipment operating from similar input power. It should also be mentioned, however, that many arc welding machines also provide an auxiliary power outlet for convenience. These outlets are used to operate electric tools that a welder may need at their worksite. Depending upon the situation, these outlets may require GFCI protection as dictated by the applicable standards and manufacturer’s recommendations.
Jim Galloway is a professor at Conestoga College in Cambridge, Ont., where he teaches in the Welding Engineering Technology program. He also volunteers on several CSA technical committees, including CSA W117.2.