The NSW Resources Regulator recently issued a safety alert highlighting safety risks associated with certain materials – such as rubber, polyurethane, PVC, polyethylene, polypropylene and polyester – which have the ability to accumulate and store an electrical charge.
This charge can cause sparking, with enough energy to ignite methane gas. The sparking may come from the material or from metallic fittings attached to the material that forms part of a manufactured product.
Resources Safety and Health Queensland released safety alert No.381, on 29 September 2020, advising of issues associated with the use of products that have been assessed as anti-static, in accordance with Mining Design Guideline MDG 3608 for Fire Resistant Anti-Static (FRAS) materials.
The NSW Work Health and Safety (Mines and Petroleum Sites) Regulation 2014 (the Regulations) requires that any component of the ventilation system of an underground coal mine and any conveyor belting and conveyor accessories used at an underground coal mine or in a reclaim tunnel at a coal mine must be FRAS (clause 87).
Materials such as rubber, polyurethane, PVC, polyethylene, polypropylene and polyester are often used to make base materials that are then manufactured into products for use in underground coal mining.
Fibre reinforced resin materials such as fibre glass and carbon fibre composite materials are also used.
The products manufactured include ventilation sheeting (brattice) and ventilation stoppings, ventilation ducting (rigid and flexible), dust curtains, venturi blowers, air fans, pipes, conveyor belting and conveyor accessories.
These products, when subjected to friction, may accumulate a static charge that can cause spark ignition of methane gas. The friction may be generated by air movement, especially with higher air velocities from compressed air lines or from auxiliary ventilation fans or contact between a moving conveyor belt and associated accessories such as non-metallic conveyor idlers, scraper blades and skirting rubbers.
The presence of dust in the air may exacerbate the charging effect. It has also been identified that in some circumstances, sparking energies increase with increases in humidity and moisture.
Many of these products are supplied in different colours and sizes, and with different fixtures to allow for varying methods of suspension or attachment. The methods of mounting include the use of non-conductive materials, such as nylon rope, nylon straps, plastic cable ties and magnets, and conductive materials such as chains.
Controls to prevent the build-up of a static charge include minimising surface area and bonding the materials to earth.
It is important to note that while chains are conductive, they may not provide reliable earthing and static discharge in all situations.
SIMTARS undertook surface resistance and incendivity testing in accordance with MDG 3608 on samples of different materials and products, and with different colours of the same FRAS materials. Testing included samples of bare FRAS material and samples fitted with metallic components such as eyelets, rivets and channel.
The tests revealed the following:
It should be noted that metallic objects such as pipes and metallic vent ducting, while not tested by SIMTARS, are considered as potential sources of static discharge when not provided with effective earthing.
The safety alert said mine operators of underground coal mines must ensure that control measures are implemented for the use of products that may be subject to accumulation of static charge. The control measures should include:
Manufacturers of FRAS rated materials must also ensure that testing is undertaken in accordance with the relevant requirements of MDG 3608 by an independent testing facility for:
Manufacturers and suppliers of products incorporating FRAS materials as components must ensure that: