A confined space is any area with limited entry and exit containing known or potential hazards and which is not intended for continuous human occupancy. The most common confined spaces in the utility industries are maintenance holes (commonly known as Manholes).
At times, maintenance holes and unventilated vaults must be entered for work activities. Confined-space operations in maintenance holes pose hidden hazards. The most common of these hazards are exposure:
• Combustible or flammable vapors and gases from underground storage or piping facilities.
• Oxygen deficiency by displacement with other gases and the introduction or nitrogen from cable pressurization.
• Toxic gases from decomposing soil, chemical spills, and engine combustion exhaust (from vehicles and equipment).
Maintenance Hole Entry Equipment
Several pieces of equipment are required for safe entry into a confined space:
• Work-area protection devices, such as traffic (reflective) vests, traffic cones, maintenance hole guards, work-area protection signs.
• Maintenance hole cover lifter and maintenance hole hook.
• Atmospheric tester for combustible gas, oxygen deficiency, and other toxics.
• Power ventilator (air blower).
• Maintenance hole pump to remove water.
• First aid kit.
• Portable fire extinguisher (dry chemical).
• Extraction equipment.
Atmospheric Conditions
Atmospheric conditions are considered unacceptable if oxygen levels are less than 19.5 percent or greater than 21.5 percent. The following levels of other hazards are unacceptable:
• A flammable gas, vapor, or mist greater than 10 percent of its lower explosive limit (LEL). LEL means the minimum concentration of the flammable or explosive material which will ignite if an ignition source is present.
• An airborne combustible dust at a concentration that obscures vision at a distance of 1.5 meters or less.
• An atmospheric concentration of a substance greater than the allowed limit in the workplace exposure table 18 of the Saskatchewan OHS Regulations or the Material Safety Data Sheet for that substance.
Testing Procedures
After all work-area protection devices are in place and the atmospheric test equipment has been tested as operational, proceed as follows:
• Lower the sampling hose approximately 5cm through the venting hole in the maintenance hole cover to make the first pre-entry test. If no combustible gas is found, remove the cover and proceed. If the flammable gas concentration is above 60 percent LEL (Lower Explosive Limit), suspend operations and follow company procedures.
• After the maintenance hole cover is removed, continue to test for combustible gas by lowering the sampling tube within 60cm of the maintenance hole or vault floor, generally referred to as stratospheric testing (determine the depth, and divide by 3). If no flammable gas is found, purge the maintenance hole with the power blower for the period of time based on the blower capacity and maintenance hole configuration, following company or manufacturer procedures.
Ventilation
If flammable gas is detected and is above 10 percent but below 60 percent LEL, purge and ventilate the maintenance hole for a minimum of 10 minutes, depending on the maintenance hole or vault size. If combustible gas was detected but subsequently cleared, continue to monitor and ventilate to maintain the flammable gas level below 10 percent LEL.