Ground testing for mine sites

All mine sites have grounded electrical systems designed to protect people and equipment by providing a safe path for the dissipation of fault currents, lightning strikes, transient over-voltages, static discharges, EMI and RFI signals and interference.

To help ensure a reliable connection to earth, electrical codes, engineering standards, and local standards often specify a minimum resistance for the ground electrodes. Given the myriad of requirements, it stands to reason that earth testing is an extremely important practice for mining companies.

By way of explanation, a ground is an intentional or accidental conducting connection between an electrical circuit or equipment and the earth; or to some conducting body that serves in place of the earth. It helps to stabilize the voltage to earth during normal operation, and limits the voltage rise created by lightning, line surges or unintentional contact with higher-voltage lines.

Without an effective grounding system, equipment can be damaged or workers exposed to the risk of electric shock. In addition, it can lead to instrumentation errors, harmonic distortion issues, power factor problems and a host of possible intermittent dilemmas.

Why earth testing?

Over time, corrosive soils with high moisture content, high salt content, and high temperatures can degrade ground rods and their connections. Previous faults may have melted connections that aren’t easily visible. So, although a newly installed ground system will have low earth ground resistance values, the resistance of the grounding system can increase in the event of corrosion to ground rods, breaks in interconnecting wires and/or water table changes.

Earth resistance is the resistance of the earth to the passage of electric current. Resistivity defines a material’s ability to conduct current. This is a complex property to measure within the earth as it is affected by several factors, including soil composition, mineral content, temperature and depth.

The goal in ground resistance is to achieve the lowest ground resistance value possible that makes sense economically and physically. Ideally a ground should be of zero ohms resistance.

It is highly recommended that all grounds and ground connections be checked with the appropriate test instrument upon installation and on an annual basis. During these periodic checks, if an increase in resistance of more than 20 per cent is detected, the technician should investigate the source of the problem, and lower the resistance by replacing or adding ground rods to the ground system.

The most commonly applied tests are:

Fall-of-Potential Test – This test method measures the ability of an earth ground system or an individual electrode to dissipate energy from a site. To conduct this test, the first step is to disconnect the site connection of the electrode of interest; then connect the tester to the earth electrode. For the 3-pole Fall-of-Potential test, two earth stakes are placed in the soil in a direct line away from the earth electrode (normal spacing is around 20 meters). The instrument can then be used to generate a current through the two outer ground stakes (the auxiliary earth stake and the earth electrode) and measure the drop in voltage potential between the two inner ground stakes.

Selective Testing – Considered by some operations to be a safer test method, it allows users to measure the ground resistance of a specific ground electrode without disconnecting it from an array or from a structure’s distribution system. With selective measurement, two earth stakes are placed in the soil in a direct line, away from the earth electrode (again, around 20 meters) and the tester is then connected. A special clamp is placed around the earth electrode, which eliminates the effects of parallel resistances in the grounded system, so only the earth electrode of interest is measured. A known current is generated by the instrument between the outer stake and the earth electrode, while the drop in voltage potential is measured between the inner earth stake and the earth electrode. Only the current through the clamp is used to calculate resistance.

Earth ground tests are an important first line of defense for mining operations, because they provide valuable data for undertaking appropriate risk assessments for mining operations. They also play an important role in ensuring the safety of equipment and workers. With the right test tools and a regularly scheduled test program, organizations can avoid interruptions in business functions while avoiding downtimes unnecessary costs. 

*Colin Plastow is an Industrial Product Manager with Fluke Canada Limited. He may be reached at colinplastow@fluke.com