In the world of underground coal mining, understanding earthing is paramount. This article delves into the practical aspects of earthing and its significance in underground coal mines, with a particular focus on separation requirements.
Whether seeking testing methodologies or exploring this subject further, this article is intended to provide a brief overview of earthing in underground coal mining.
Earthing on Mine Sites
Earthing on a mine site can be broken down into the following three areas:
- Network earth: the earthing system utilised by the supplying utility network, encompassing elements such as overhead earth wires and potentially the primary high-voltage substation earth grid
- Surface earth: pertains to the HV, MV and LV cable screens within the surface plant, as well as motors, structures and all metalwork throughout the site
- Underground earth: the earthing system connected with the underground operations, electrical distribution, services and metalwork below the surface.
Standards and Guidelines
- Australian standards AS3007, AS1768, AS2067 and the NSW Department of Industry mandate that surface and underground earthing systems must prevent the transmission of electrical energy from surface faults and lightning strikes to underground areas with potential hazardous atmospheres.
- While separation of surface and underground earthing systems and fault limitation for underground workings is required, the need for separating network and surface earthing systems depends on site-specific factors and is not universally essential.
Earthing Hazards on Site
In considering mine site earthing in the three areas of network earth, surface earth and underground the following fault scenarios must be considering.
Network
In electrical systems, potential faults can propagate through various pathways, including overhead earth wires (OHEW) and main substations, affecting surface and underground earthing systems. Decisions regarding connecting or separating the main substation from the network’s earth and linking it to the surface earth depend on factors like fault levels, clearing times, proximity to mine infrastructure and utility configurations. Zero Sequence Earthing recommends connecting these components, as the benefits often outweigh the risks, provided surge protection and lightning schemes permit it. Nevertheless, this choice remains site-specific, balancing current return paths with voltage transfer hazards.
Surface
Faults occurring at surface substations can create high voltage risks for interconnected equipment. Without proper separation or inadvertent bridging, surface faults may transfer to underground locations, as can lightning strikes if not adequately isolated. Conducting an insulation coordination study is crucial to size, position and verify the suitability of surge arrestors and lightning protection, reducing the risk of transmitting lightning energy underground. The surface earthing arrangement hinges on site layout, substation placement, supply methods, fault levels and clearing times. Incorporating fault limitation measures, like Neutral Earthing Resistors (NER) on supplying transformers’ neutrals, is common practice to safeguard both surface and underground feeders. This approach is particularly valuable for remote substations connected via overhead feeders, such as vent fans or bore pumps.
Underground
Faults within underground systems must have a dedicated return path to the surface supplying substation. Otherwise, high local earth resistances can lead to hazardous high voltage situations, particularly when fault levels are elevated. Underground areas with separate earthing systems necessitate fault-limited supplies. Coal seams and rock formations inherently offer high resistance, while portable substations lack extensive earthing systems. In cases where the natural underground earth grid isn’t present, installation is required, either underground or near the portal on the surface. Proper isolation from surface earth and lightning protection is crucial for surface-installed grids.
