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Confined Vs. Enclosed Spaces

“confined space” A space which, because of its construction, location, contents, or work activity therein provides limited means of entry or exit, has poor natural ventilation, contains or may develop a dangerous atmosphere, and may require the use of protective equipment including the capability of immediate rescue of a worker who enters.
Confined spaces are not meant for continuous occupancy, and are likely to be entered only for purposes such as inspection, maintenance or construction. Limited entry and exit points not only make escape or rescue difficult, but also can restrict natural ventilation.

“enclosed space”  A confined space is different from an enclosed space, which is any area enclosed on all sides and overhead, such as a machinery room, storage facility or boiler room, where workers may find ordinary job hazards intensified. Enclosed spaces may be meant for frequent or lengthy human occupancy, and often have accessible points of entry and exit. Requirements for safe entry to enclosed areas are usually less strict than for confined spaces.

"high hazard atmosphere" means an atmosphere that may expose a worker to risk of death, incapacitation, injury, acute illness or otherwise impair the ability of the worker to escape unaided from a confined space, in the event of a failure of the ventilation system or respirator;

"low hazard atmosphere" means an atmosphere which is shown by pre-entry testing or otherwise known to contain clean respirable air immediately prior to entry to a confined space and which is not likely to change during the work activity, as determined by a qualified person after consideration of the design, construction and use of the confined space, the work activities to be performed, and all engineering controls required by this Regulation;

"moderate hazard atmosphere" means an atmosphere that is not clean respirable air but is not likely to impair the ability of the worker to escape unaided from a confined space, in the event of a failure of the ventilation system or respirator.

"clean respirable air" when used to describe the atmosphere inside a confined space, means an atmosphere which is equivalent to clean, outdoor air and which contains
(a) about 20.9% oxygen by volume,
(b) no measurable flammable gas or vapour as determined using a combustible gas measuring instrument, and
(c) no air contaminant in concentrations exceeding either 10% of its applicable exposure limit in Part 5 (Chemical and Biological Substances of the OHSR or an acceptable ambient air quality standard established by an authority having jurisdiction over environmental air standards, whichever is greater;

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