Electrical safety in welding
A welding set is a high-current source with an exposed electrode, operated by someone holding metal while standing on metal. The voltages are low by industrial standards and they kill welders every year.
The voltage that matters
Two figures. Welding voltage is 15–35 V while the arc is burning — genuinely low. Open-circuit voltage is what sits on the electrode before the arc strikes: 50–90 V on a DC inverter, up to 113 V on some AC machines.
The dangerous threshold is around 50 V, and it falls with wet skin. Dry skin has a resistance of some tens of kilohms; wet skin has a few hundred ohms. The same voltage that is a mild shock in a dry workshop in March is a lethal one in a humid tank in August.
Situations with increased risk
EN 60974-1 defines them, and machines suitable for them carry the S marking with reduced open-circuit voltage:
- Confined conductive spaces — tanks, vessels, ducts, pipes.
- Restricted spaces where the body is in contact with conductive parts on several sides.
- Wet, damp or hot environments where clothing is soaked with sweat.
In these conditions: an S-marked machine, dry insulating mats, insulated gloves in good condition, and someone outside who can cut the power. Never work alone.
The return circuit
The return path is the half of the circuit nobody looks at, and it causes three separate problems when it is wrong.
A poor connection wastes voltage, so the arc gets less than the machine sent — the symptom is a weak arc and a sticking electrode.
Stray current finds another path. If the clamp is on a bench rather than the work, the return current will travel through whatever conducts: a bearing, a hinge, a wire rope, another machine's earth. Bearings destroyed by welding current are a well-known failure and an expensive one.
A hot clamp is a fire risk. A resistive connection at 200 A dissipates real power at exactly the point where nobody is looking.
So: clamp directly to the workpiece, on ground-bright metal, as near to the joint as you can get.
The equipment
- Leads. Inspect before use. Cuts in the insulation, exposed copper at the connectors, taped repairs — all of them need fixing rather than noting.
- The electrode holder. Insulated, undamaged, and never tucked under an arm or left on the bench with an electrode in it while the machine is live.
- Cable size. Undersized cable overheats along its length — see the cable calculator.
- The supply. RCD protection on the mains side. An RCD that trips is reporting a real fault; find it rather than resetting it — the causes are listed in why your welder trips the breaker.
The habits that matter
Switch off before changing an electrode in a risky environment. Do not weld in wet clothing or standing in water. Keep dry insulating material between you and the workpiece where you can. And treat the electrode as live at all times, because between arcs it is.
Frequently asked questions
Welding voltage is low. Is it actually dangerous?
Yes. Open-circuit voltage is typically 50–90 V DC and can reach 113 V on AC machines, and the danger threshold is about 50 V — lower when the skin is wet. Sweat drops skin resistance by an order of magnitude, and a welder in summer is a welder with wet skin.
What does the S marking on a machine mean?
That the machine has reduced open-circuit voltage and may be used in environments with an increased risk of electric shock — confined spaces, damp locations, work where the body is in contact with the structure. Without that marking, a machine should not be used in those conditions.
Where should the earth clamp go?
Directly on the workpiece, as close to the joint as practical, on bare metal. Not on the bench that the work is resting on, and never anywhere that lets the return current find its way through a bearing, a hinge or another machine — that path damages what it passes through.
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