Lightning protection for mines in Australia: AS 1768 Appendix M and the regulators

A mine lightning management plan (LMP) should identify the lightning hazards across the whole site, describe the controls in place for each, and set out what happens in every phase of a storm, from the first warning to the all clear. That risk based plan is the central recommendation of AS 1768:2021 Appendix M, set out in Clause M.2. Clause M.4 extends the plan to surface structures, from processing plant to offices and accommodation.

Detection and warning belong inside the plan. Clause 6.5 separates detectors, which sense the radio burst of a flash already made, from warning systems, which watch the electric field and can alert before the first flash. It grades them in classes A to D, refers to IEC 62793, and warns that none is fully reliable, hand-held units least of all. Its NOTE adds that these systems lower the overall risk but never stand in for the protection system itself. Clause M.6 points mines to the same clause.

Clause 6.4.1 sets a baseline: people outdoors are most at risk once a storm is within 15 km, and a retreating storm stops counting as a threat 30 minutes after the last thunder. Mine trigger plans often use several distance bands instead, as the Queensland alert below shows.

AS 1768:2021 Clause M.5.1 asks planners to consider gas layers, gas in sealed goaf, unsealed goaf fringes, surface gas drainage plant linked to the workings, and unintended operation of equipment or explosives. Workers below may not know a storm is overhead, so surface controls should be designed to eliminate, or at least reduce, the lightning energy that reaches the workings.

Energy travels down four ways: through boreholes; along conveyors, pipes, cables, vent shafts, mesh and reinforced portals; through the strata from a ground flash; and by coupling into long conductors, live or not.

Know every connection (M.5.2). Keep survey records of each entry point: lined or instrumented boreholes, conveyors and steel cord belts, cables, rails, pipelines, lined vent shafts and winders.

Break the path (M.5.3). Insulated sections in pipes and conveyors, fibre optic links and separated cable screens interrupt the route to the seam. Line the breaks up rather than staggering them, so differently earthed items cannot put a hand to hand voltage across a worker, and signpost each zone.

Design the earthing early (M.5.4).

• Design lightning protection and mine earthing together at the civil design stage, taking in AS 2067, AS/NZS 3007 and AS/NZS 3000 as the clause directs, from a three dimensional soil resistivity model, aiming for risk as low as reasonably practicable.
• Past practice kept the lightning earth at least 15 m below ground and 3 m through the air from the mine underground earthing system. The clause asks for a distance calculated for the site instead.
• Where isolation fails, as at winders, shunt the energy into the ground away from the workings. Earth every non-power conductor entering the workings deliberately, never through chains.
• Overhead supply lines within 1.5 km should carry overhead earth wires, with surge arresters at their ends.

Boreholes (M.5.5, M.5.6).

• A study should decide whether a borehole needs an LPS or its own earth.
• The same study decides whether the casing is insulated, bonded or kept apart from the mesh.
• Decommissioning plans for a retired borehole should consider removing or ending its conductive casing at least 6 m below ground, and taking out its cables and pipes.

In the workings (M.5.7).

• Coal mines should consider gaps or non-conducting mesh at chosen points, knowing current can bypass a gap through the strata.
• Remove conductors from sealed areas where possible, and break anything crossing a goaf seal.
• Stop underground blasting when storms are forecast, and use detonators designed to resist electrical discharge.

Yes, and here the standard stops advising and starts requiring. AS 1768:2021 Clause J.5.1 says any building that keeps explosive substances or articles must be protected, using one of three methods: roof protection to Section 3, several masts or a suspended air terminal network, or a single vertical air terminal for a small structure or magazine. LPL I applies in all cases, which the clause ties to a 20 m rolling sphere. Masts and air terminals must stand at least 2 m clear of the store, and the store's own fabric may not form part of the LPS unless that cannot ignite its contents.

J.5.1 reaches beyond the magazine: buildings within 50 m with their own fire or explosion hazard, buildings within 30 m that could throw masonry onto the store or start a fire if struck, and any open area designated for storing explosives all need protection too.

Surge protection (J.5.2). SPDs are required as part of the LPS wherever explosive material may be present, installed to Appendix F and placed outside the area where the material is held. Where power comes from an overhead line, J.5.6.5 requires a buried cable section before the point of entry and SPDs at the end of the overhead line, as well as SPDs on every incoming power, data and control service.

Buried and mounded stores (J.5.3, J.5.4). A buried store with at least 500 mm of cover and no services from above may use a surface mesh of 3 m to 8 m. A mounded store with less than 500 mm of cover is protected to Section 3.

Earthing and bonding (J.5.6). A ring conductor should link the earth terminations, and it must be buried at least 500 mm deep and at least 1 m from the walls unless bonding, testing or corrosion reasons justify leaving it exposed. The network should be 10 ohms or less, a recommendation inside a normative appendix. Racking, incoming services, and fences at no more than 10 m spacing are bonded, and no unprotected mast, antenna or tree may stand within 15 m.

Appendix J also covers gas and dust hazardous areas in J.3, relevant to coal handling and gas drainage plant; the hazardous area guide explains those zone rules.