Protecting combustibles and the hot work zone

Once the decision is made to do hot work in place, the main job is to make sure there is nothing for the sparks, slag and heat to ignite. That means defining a protective zone around the work, removing everything that can burn from it, protecting what cannot be moved, closing off the paths that sparks use to escape, and having fire extinguishing equipment ready. This lesson explains how to prepare the area so that the fire watch has nothing to do but watch.

The protective zone

Codes and standards set a radius around the hot work within which combustibles must be removed or protected. The National Fire Code of Canada uses 15 m. CSA W117.2, like NFPA 51B, uses 11 m (35 ft), and many Canadian employers and insurers use the larger of the two. These distances are minimums for typical work. Cutting, work at height, windy conditions and very fine fuels such as dust or lint can throw ignition farther, and the authorizing individual should extend the zone when the job calls for it.

The zone is three-dimensional. It extends above the work, where sparks can land on beams, ledges, cable trays and roof structures, and below it, where slag falls through gratings, openings and gaps to lower floors. It also extends through walls, floors and roofs wherever heat can conduct or sparks can pass.

Remove, then protect

The first choice is always to remove combustibles from the zone. Moving pallets, packaging, stored goods, waste bins, flammable liquids, gas cylinders and debris out of the zone is cheap and completely effective. Removal also includes housekeeping: sweeping floors clean of dust, sawdust, paper and oily residue, and cleaning dust from overhead beams, ledges and equipment.

What cannot be removed must be protected. The usual methods are:

• Welding blankets and curtains listed or tested for the purpose, made of materials such as fibreglass, silica or treated fabrics. They come in light, medium and heavy grades, and the grade must suit the process; light blankets are for spatter from welding, heavier ones for slag from cutting. Ordinary tarpaulins are not fire-resistant and can make things worse.
• Noncombustible barriers such as sheet metal or cement board, sealed at the edges so sparks cannot get behind them.
• Wetting down combustible floors or materials that cannot be moved or covered, where water will not create an electrical or chemical hazard. Wet materials can dry during long jobs and need rewetting.
• Covering wooden floors with damp sand, metal or noncombustible sheets.

Blankets must cover completely and overlap, be weighted or secured so they do not lift or slide, and be inspected for holes, burns and oil contamination. A blanket with a hole or a gap at the floor gives little protection.

Openings, penetrations and adjacent areas

Every opening in floors, walls and ceilings within the zone is a path for sparks to reach places the fire watch cannot see. Cable trays and pipes that pass through walls often have gaps around them. Doors may not seal at the bottom. Drains and trench covers lead to spaces that can hold vapour or debris. Ductwork openings lead into ceiling spaces. Each opening should be covered tightly with noncombustible material, or a fire watch posted where the sparks would land.

Adjacent areas need checking too. If combustibles lie against the far side of a metal wall, floor, ceiling or roof being heated, they must be moved or protected, and a second fire watch posted on that side if the first cannot see it. The same applies to the floor below when working on an upper floor or roof.

Combustible construction and building elements

Some buildings are themselves combustible: wood-frame construction, heavy timber, combustible roof assemblies, foam-insulated panels and older buildings with hidden wood or paper linings. In these buildings, the structure cannot be removed from the zone, so hot work needs extra care: cold methods wherever possible, noncombustible shields, wetting, a fire watch with access to concealed spaces, and longer post-work monitoring. Heritage and religious buildings are particularly vulnerable because of large concealed roof spaces, old dry timber and the irreplaceable value of what may be lost.

Insulated metal panels deserve a special mention. Panels with expanded polystyrene or polyurethane foam cores look like metal from the outside, but the core can melt, ignite and burn inside the panel, spreading fire quickly and out of sight. Hot work on or near such panels should be avoided wherever possible, and done only with careful protection when unavoidable.

Fire protection equipment at the work

At least one portable fire extinguisher suited to the hazard must be available at the hot work location, and the fire watch must be trained to use it. Many sites require two, or a charged hose line where one is available. Extinguishers should be at least a 2-A:10-B:C or larger multipurpose dry chemical unit for general hot work, or a water-based unit for Class A fires in areas where dry chemical would damage equipment, as the site's fire safety plan specifies. The fire safety course, HSE-105, explains extinguisher ratings in more detail.

Sprinklers must be in service. Hot work in a sprinklered area with impaired sprinklers is generally prohibited unless the impairment procedure allows it with additional precautions, such as continuous fire watches and charged hose lines. Fire alarm systems should remain active; if detectors near the work must be temporarily protected to prevent false alarms, the impairment should be documented and the devices restored as soon as the work stops.

Housekeeping during and after the work

Protection is not set-and-forget. During the work, spatter and slag accumulate; hot slag can burn through a blanket or roll off it. The operator and fire watch should check blankets and covers regularly, brush off slag into a metal container, and keep the floor clear. Electrode stubs, which stay hot, should go into a metal container rather than onto the floor. When the work ends, hot pieces should be cooled or marked, and debris removed before the area returns to normal use.

Choosing and caring for welding blankets

Not all fire-resistant fabrics are alike. Welding blankets and curtains are tested for how well they resist burn-through from spatter, slag and heat, and manufacturers classify them for light, medium or heavy duty. Light-duty blankets suit spatter from arc welding and light grinding sparks. Medium and heavy-duty blankets, often thicker silica or coated fibreglass, are needed under oxy-fuel and plasma cutting, where large globules of molten slag fall on them. Using a light blanket under heavy cutting can let slag burn straight through to whatever is underneath.

Blankets need care. Oil, grease and solvents soaked into a blanket can burn. Holes, frayed edges and burned patches let sparks through. Blankets stored wet or crumpled can degrade. Inspect blankets before each use, keep them clean and dry, and replace damaged ones rather than patching them. Mark blankets by grade so workers do not grab the wrong type.

The walk-around before lighting up

A short structured walk-around just before work starts catches what the morning inspection missed. The operator and fire watch walk the zone together: look up for dust and combustibles on beams and cable trays, look down for gaps, drains and debris, check behind and under blankets, look through doors and into adjacent rooms, confirm the extinguisher gauge is in the green and the pin and seal are intact, and agree where the fire watch will stand. It takes five minutes and often finds something that changed since the permit was signed, such as a delivery of packaging or a door propped open.