OHS Code Explanation Guide

Published Date: July 01, 2009
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Part 36 Mining

Section 743 Incombustible dust

When coal dust is airborne it is potentially explosive. A coal dust explosion is often initiated by a methane explosion whereby its pressure wave raises any coal dust into the air and its flame front then ignites the cloud of coal dust, with devastating effect. The most recent example in Canada was at the Westray Mine in Nova Scotia in 1992. The coal dust hazard is typically mitigated by using inerting agents, usually water and/or incombustible dust consisting of finely ground limestone rock. The latter is usually referred to as stone dust or rock dust.

Subsection 743(1)

The mining area that falls within 10 metres of the active working face is generally considered to be constantly changing as mining excavation occurs. This section therefore does not apply to this area while coal cutting is in progress.

Subsection 743.1(1)

This subsection requires the employer to prevent accumulation of combustible dusts, typically coal dust. The Westray Inquiry into the coal dust explosion at the Westray Mine, Nova Scotia in 1992, highlighted, among other things, that an important first step in mitigating the hazard of combustible dust is to ensure that accumulations of coal dust do not build up anywhere. The exception to this is the immediate mining area (see subsection 743(1)). Particular attention must be paid to conveyor systems which are well known for creating accumulations of coal dust. If unattended, these accumulations can build up around conveyor rollers. Should a roller fail and jam, the moving belt could generate enough heat to ignite the accumulation and cause a fire.

Subsection 743.1(2)

The employer must prepare a formal stone-dusting plan (which is in effect a coal dust inerting program) for filing with the Director. This plan demonstrates how the employer will achieve the minimum incombustible content of 80 percent (see section 743(3)). The plan must include the approach, methods to be used, and testing procedure and frequency.

Subsection 743(2)

Incombustible dust is liberally applied in order to inert combustible dusts and minimize the potential for a coal dust explosion. All areas that are accessible to workers must be treated with the exception of the areas noted in subsection 743(1). Other methods or materials may be allowed in place of the specified incombustible dust but must first be approved by the Director.

Subsection 743(3)

The quantity of incombustible dust applied to coal dust has been defined after extensive research. To effectively inert coal dust, post-dusting samples must consist of at least 80 percent incombustible dust.

Subsection 743(4)

The requirement for a minimum of 80 percent by weight of incombustible dust to coal dust is waived if the area under consideration contains at least 30 percent moisture. Such a high moisture content alone suppresses coal dust and its potential for explosion. 

Subsection 743(5)

The requirement for 80 percent incombustible material must be increased by one percentage for every 0.1 percent of flammable gas in the ventilation current.

Subsection 743(6)

Repealed.

Subsection 743(7)

The purpose of cleaning the roadway area is to remove coal dust or any other combustible matter so that stone dust or rock dust forms the base. The effectiveness of the stone dust is reduced when applied on top of thick layers of coal dust.