Item Measurement of the ground loop resistance of the ungrounded rod

All mining sites have grounded electrical systems to safely direct current to earth or transformer neutral in the event of a lightning strike, surge, or equipment ground fault. It is common for electrical codes and national or technical standards to specify the minimum resistance of ground connections to ensure connection reliability. This application note shows how one of the world’s leading mining and exploration companies uses Fluke ground testers to maintain safety in the field.

Rio Tinto is one of the world’s leading exploration and mining companies. The company seeks out, extracts and processes the earth’s mineral resources – the metals and minerals essential for the manufacture of thousands of everyday products that meet society’s needs and help improve the quality of life.

These products include aluminum, diamonds, energy products, gold, industrial minerals and iron ore.

The company extends its business worldwide and is particularly present in Australia and North America, with significant operations also in South America, Asia, Europe and South Africa.

The Pilbara region

For over forty years Rio Tinto has been innovating and driving growth in the Pilbara region.

Rio Tinto’s iron ore operations in the Pilbara have grown to an annual capacity of 220 million tons, and the company plans to expand further. With its network of 12 mines, three shipping terminals and Australia’s largest privately owned freight train network, the company’s Pilbara operations make up a large part of the world’s iron ore business. These operations began in 1966 and have grown to meet the growing needs of the steel industry around the world.

Rio Tinto is a world-renowned asset manager who manages and maintains mining, rail, power and port facilities in the Pilbara on behalf of Hamersley Iron and Robe River property owners.

Grounding systems

Regular grounding testing is a very important practice for mining companies like Rio Tinto. In addition to protecting people and equipment, the purpose of a ground is to provide a safe path for the dissipation of fault currents, lightning strikes, transient voltage surges, static electricity discharges, and EMI and RFI signals and interference.

A ground is a conductive connection, intentional or accidental, between an electrical circuit or equipment and earth or a conductive body acting as ground. It helps stabilize voltage to ground under normal operating conditions and limit voltage surges caused by lightning strikes, in-circuit surges, or accidental contact with high-voltage lines.

In the event of a short circuit or lightning strike near the mining area, a low impedance ground connection will facilitate the routing of the energy to earth. Keeping the potential gradient low helps minimize damage.

Without an effective grounding system, employees would be at risk of electric shock, not to mention instrumentation errors, harmonic distortion issues, power factor issues, and a variety of possible intermittent problems. If fault currents cannot escape to earth through a properly designed and maintained grounding system, they will find other means, which may include people.

Incorrect grounding leads to unnecessary downtime. It represents a serious hazard and increases the risk of equipment failure. An effective grounding system increases equipment reliability and reduces the possibility of damage from lightning strikes or fault currents.

Ground test required

Over time, corrosive soils with high moisture content, high salt content, and high temperatures can damage ground rods and their connections. Previous shorts may have fused connections that are not easily visible. For example, the resistance of a grounding system that had low ground resistance values ​​when it was first installed can increase if the ground rods are degraded by corrosion or by cuts in the interconnecting wires and changes in the water table.

Earth resistance is the earth’s resistance to the flow of electric current. The test results indicate the resistance of the earth rods to the connecting wires, which should be less than 1 ohm in the case of Rio Tinto installations. Resistivity refers to a material’s ability to conduct current, a complex property to measure in earth as it is affected by many factors, including soil composition, mineral content, temperature and depth.

Intermittent electrical problems at a mining site can be related to improper grounding and poor quality electrical power.

For this reason, it is strongly recommended that all ground connections and earth connections be checked during installation and at least once a year. During these periodic checks, if more than a 20% increase in resistance is found, the technician should identify the source of the problem and reduce the resistance by replacing or adding ground rods to the grounding system.

Rio Tinto conducts grounding tests on a regular basis to ensure the safe operation of the mine. This is the first line of defense. The ability to detect and monitor ground junctions can provide critical data to enable proper risk assessments for mining operations.

Anode soils provide a safety ground for mining electrical equipment. The goal is to achieve the lowest earth resistance value economically and physically achievable. Ideally, ground should have a resistance of zero ohms.

David Oxley, electrical manager in the reliability assurance department at Rio Tinto’s Cape Lambert site in the Pilbara region, oversees these inspections.

He explains: “We have to make sure that the grounding is optimal on all substations and that we comply with local regulations”.

David Oxley and his team carry out regular checks on their substations.

Procedure

Specialized tools make it easier for maintenance and safety teams to measure earth resistance and troubleshoot.

At Rio Tinto, the Fluke 1625 is used to measure earthing system capacity in substations.

Measurement of earth resistance: measurement of potential drop

The classic potential drop test method is used at Rio Tinto’s Cape Lambert facility to measure the ability of a grounding system or an individual outlet to dissipate energy from a site.

Generally, the affected earth electrode must first be disconnected from the site. Then the tester is grounded. So, to perform the 3-pole potential drop test, two earth rods should be placed in the ground in a straight line away from the earth electrode, usually with a gap of 20 meters. Using the Fluke 1625 to source a current between the two outer earth rods (the auxiliary earth rod and the earth electrode), the voltage potential drop between the two inner earth rods is measured. The Fluke Tester automatically calculates earth resistance using Ohm’s Law (V = IR).

Ground resistance measurements are often corrupted by the existence of ground currents and their harmonics. To avoid this phenomenon, the Fluke 1625 uses an automatic frequency control system. Automatically selects the test frequency with the lowest noise level, so you get an accurate terrain value.

Save time with selective testing

The Fluke 1625 is a special ground tester. It not only allows you to measure earth resistance using the standard potential drop test, but also saves time during these measurements, thanks to the selective method. With the selective method, the electrode to be tested does not have to be disconnected during the measurement, which ensures greater safety.

The selective method makes it possible to measure the resistance of a specific earth electrode without the need to dissociate it from the network or from the structure’s distribution system. This means that risks are minimized as there is no risk of disconnection of the earth connection of a live system.

With selective counting, two earth rods must be buried in the ground in a straight line, at a distance of 20 meters from the earth rod. The Fluke 1625 is then connected to the ground in question, which has the great advantage of not having to interrupt the connection to the site.

Instead, a special clamp is placed around the grounded socket which removes the effects of parallel resistances in the grounded system and allows only the socket under test to be measured. A known current is generated by the Fluke 1625 between the outer rod and earth, while the voltage potential drop is measured between the inner rod and earth. Only the current flowing in the earth connection is measured by the clamp. The generated current also flows through other parallel resistors, but only the current through the clamp is used to calculate the resistance.

“The Fluke 1625 also allowed us to run tests without having to shut down any of our 20 substations. This feature minimized downtime and gave us the freedom to test all substations at any time. »

Steve Hood, Managing Director of Fluke Australia explains: “The Fluke 1625 is intended for electrical installations or other high energy environments, offering more flexibility for more demanding applications. »

“Under these circumstances, for Rio Tinto, the selective method is much safer and easier to test earthing as it is not necessary to disconnect or de-energize any of the substations to perform this test. Grounding tests performed ensure a safe working environment for employees and maximize returns for investors by operating responsibly and without downtime. »

David says, “The team learned to use the Fluke 1625 tool almost instantly. Fluke made sure to meet our requirements and needs by providing extensive training and support. This tool is now indispensable for our work in the mine”.