Hot work: fire safety rules for welding and cutting
Welding sparks reach around 1500 °C and travel ten metres. Most of the fires they start begin hours later, after the welder has packed up — which is why the rules are about what happens afterwards as much as during.
The hot work permit
Outside a designated welding bay, hot work in most industrial and construction settings needs a written permit. It is not bureaucracy for its own sake: the permit forces someone to walk the area and answer specific questions before the work starts.
A permit records what work is being done and where, what combustible material is within the danger zone and what has been done about it, what fire-fighting equipment is present, who is keeping the fire watch and for how long afterwards, and who signs the area off at the end.
Preparing the area
Ten metres in every direction is the working assumption. Within that radius: remove what can be moved, cover what cannot with fire blankets or metal sheet, and pay particular attention to the things nobody thinks of.
- Openings in the floor. Cable penetrations, drains, gaps under doors. A spark that goes through a hole lands somewhere you are not watching.
- Dust and lint. Accumulated dust on beams and ledges ignites readily and smoulders quietly.
- Insulation in cavities. Especially older mineral fibre with a paper facing, and any foam.
- The other side of the wall. Steel conducts heat: welding one face of a partition can ignite what is stacked against the other.
- Hydraulic hoses and cable trays above the work.
The fire watch
The single most important rule on this page: stay for at least an hour after the work is finished. A spark that lodges in dust or insulation smoulders before it flames, and the delay is typically between thirty minutes and several hours.
The fire watch is a person with nothing else to do and an extinguisher within reach, who checks not only the work area but the space below it and the far side of any wall that was welded.
Extinguishers
| Type | For | Not for |
|---|---|---|
| Water | Wood, paper, textiles | Live electrical equipment, liquids |
| Powder | Most situations, including electrical | Confined spaces — it blinds you instantly |
| CO₂ | Electrical equipment, the welding set itself | Outdoors in wind; asphyxiation risk in confined spaces |
| Foam | Flammable liquids | Live electrical equipment |
Gas cylinders
Upright, secured with a chain, valves closed when not in use, and away from the heat and the spatter. An acetylene cylinder that has been knocked over needs standing upright for a considerable time before use, and one that has been in a fire is dangerous for hours afterwards because acetylene decomposes exothermically once it starts.
Never test for gas leaks with a flame. Soapy water, every time.
Frequently asked questions
How far do sparks actually travel?
Up to about 10 metres horizontally from grinding and cutting, and considerably further downwards from work at height. The standard clearance in a hot work permit is 10 m in every direction, with anything combustible inside that radius removed or covered.
What is a fire watch and how long does it last?
Somebody whose only job is to watch for fire, with an extinguisher, during the work and for at least an hour afterwards — some regimes require two. Most hot work fires start after everyone has gone home, from a spark that smouldered in dust or insulation for an hour.
Can I weld a container that held fuel if it is empty?
Empty is exactly the dangerous state: liquid fuel does not explode, its vapour does, and an "empty" drum is full of vapour in the right mixture. Cleaning, steaming and gas testing are mandatory, and even then this is specialist work. Every year somebody is killed by a drum that was empty.
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