Electricity and Control April 2023

MEASUREMENT + INSTRUMENTATION

Ground systems

Ground systems can range from simple to more complex, comprising: a single ground electrode, multiple ground electrodes connected, a mesh network, and ground plate. and in such situations alternative methods, including grounding cement, can be used.

Ideally, the ground should have a resistance value of zero ohms. There is no single standard ground resistance threshold recognised by all agencies, but a ground resistance value of 5.0 ohms or less is recommended. It is important to ensure that the system impedance to the ground is less than 25 ohms. In facilities with sensitive equipment, it should be 5.0 ohms or less. This is the resistance value adopted by the telecommunications industry, for example, as the resistance value limit for grounding and bonding. The aim should be to achieve the lowest ground resistance value that makes sense economically and practically. Testing methods Several methods of earth ground testing are available. Soil resistivity testing, which uses stakes, is most needed when determining the design of the grounding system for new installations (green field applications) to meet ground resistance requirements. The Fall-of-Potential test method is used to measure the ability of an earth ground system or an individual electrode to dissipate energy from a site. 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. Selective testing is similar to the Fall-of-Potential method, pro viding all the same measurements, but in a much safer and eas ier way. With selective testing, the earth electrode in focus does not need to be disconnected from its connection to the site. Stakeless measurement can be done by measuring earth ground loop resistances for multi-grounded systems using only current clamps. This test technique eliminates the dangerous and time-consuming activity of disconnecting parallel grounds and finding suitable locations for auxiliary ground stakes. It also allows for earth ground tests to be performed in places not con sidered before: inside buildings, on power pylons, or wherever there is no soil. In situations where driving ground stakes is neither practical nor possible, a two-pole ground resistance/continuity measure ment can be done. To perform this test the technician must have access to a good, known ground such as an all-metal water pipe. Comtest supplies safe testing tools for earthing systems and offers CPD-accredited seminars on earth, ground and installation testing, for professional electricians. □

Diameter of the ground electrode Increasing the diameter of the ground electrode has little effect in lowering the resistance. For example, even doubling the diame ter of a ground electrode will reduce the resistance by just 10%. Number of ground electrodes Another way to lower ground resistance is to use multiple ground electrodes. In such a grounding system, more than one electrode is driven into the ground and connected to others, in parallel, to lower the resistance. For additional electrodes to be effective, the spacing of additional rods needs to be at least equal to the depth of the driven rod. Without proper spacing of the ground electrodes, their spheres of influence will intersect and the resist ance will not be lowered. When determining the ground rods/grounding systems to meet specific resistance requirements, it must be noted that the soil differs in different areas, it is layered and rarely homogene ous, and the resistance values will vary substantially. Design of the grounding system As noted above, a simple grounding system using a single ground electrode driven into the ground is usually adequate for residential and commercial installations. More complex ground ing systems consisting of multiple ground rods, connected, mesh or grid networks, ground plates, and ground loops, increase the extent of contact with the surrounding earth and lower ground resistances significantly. Why test? Over time, corrosive soils with high moisture content, high salt content, and high temperatures can degrade ground rods and their connections. So, although the grounding system when in itially installed had low earth ground resistance values, the re sistance of the system can increase if the ground rods are eaten away. It is recommended that all ground rods and ground con nections be checked annually as a part of a standard predictive maintenance plan. If an increase in resistance of more than 20% is found, the source of the problem needs to be investigated and the grounding system corrected to lower the resistance. What is a good ground resistance value? Some confusion exists around what constitutes a good ground resistance value and what the resistance value needs to be.

For more information contact visit: www.comtest.co.za

APRIL 2023 Electricity + Control

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