Essential Things to Know For Earthing Design and Testing
Why Is Earthing Important?
The fundamental purpose of an earthing system is to provide a safe return path for fault currents. When an earth fault strikes on a high voltage system, the fault current generates a voltage at the source of the fault. This voltage can reach dangerous levels, which can pose a hazard to personnel as well as to members of the public in the vicinity of the asset.
Earthing System Design
Earthing system designs require careful consideration and a thorough understanding of factors that can influence the performance of an earthing system under fault conditions.
Considerations for a performance based design requires understanding of project geographical constraints and restrictions, as well as local and international safety standards (IEEE 80 and AS 2067).
Initial requirements are:
- Conduct soil resistivity testing, modelling and analysis
- Review fault data and protection documentation
- Review civil and site layout
- Analyse multilayer soil structure
- Review client specific design specifications
- Review equipment, system configuration and documentation
Earthing System Testing
The earthing system is designed and engineered to perform safely during earth fault conditions. However, it is also critical that commissioning and routine earthing testing is undertaken to validate and ensure that the earthing system performs safely.
The following activities should be conducted as part of an earthing and lightning protection review:
- Touch, step and transferred voltage hazard measurements
- Determination of individual and overall earthing system impedance
- Determination of lightning protection earthing impedance and physical integrity
- Determination of earth potential rise (EPR) contours
- Soil resistivity testing
- Continuity of equipment earth connections
- Visual inspections
- Current distribution measurements
- Determination of earth grid conductor sizing suitability Assessment of switchyard surfacing
- Testing Methods Injection Testing
The concept of injection testing is to simulate a system earth fault at a much smaller scale. This is performed by injecting an off-frequency current into the earth grid which in turn causes the earthing system to rise in voltage. The purpose of injecting an off-frequency current (typically 58 Hz), is to ensure that the earthing system is isolated from any interference, including the 50 Hz system frequency and harmonics.
The following parameters are able to be measured during an injection test:
- Earthing system impedance
- Touch, step and transferred voltages
- Location of Earth Potential Rise (EPR) contours d) Earth Grid Voltage Rise (EGVR)
Soil Resistivity Testing
The soil resistivity structure is the biggest contributing factor in the performance of the earth grid. The soil structure governs the way the voltage dissipates from the earthing system during an earth fault event and the susceptibility of the change in earth resistance in relation to the amount of earth grid conductor added.
The most accepted and accurate method in measuring the average resistivity of large volumes of undisturbed earth is the Wenner Array method. The Wenner Array method uses four electrodes spaced at equal distances apart to create the test circuit. The two outer electrodes create the current circuit and the two inner electrodes measure the voltage on the earth’s surface in relation to the current electrodes.
Fall of Potential (FOP) Testing
The fall of potential (FOP) method is used to determine the resistance of a small earthing system. This test is typically used to measure the earthing resistance of lightning protection systems and small kiosk/pole mounted substations. This test only gives a resistance value and does not confirm safety with respect to touch and step voltages.
Continuity tests are conducted to check the connections of primary pieces of plant are adequately bonded to the earth grid. The resistance between a primary piece of plant and reference point is measured using a micro-ohmmeter. This method allows the user to determine the resistance between a path and to identify defects which otherwise cannot be visually inspected.
Design and Management
Electrical systems comprise of various individual components that need to work effectively with each other in order to achieve the system objective. A design process involves selecting such components according to technical, regulatory and safety requirements. A proper design methodology ensures that the design outcome achieves the desired technical objectives as well as regulatory and safety compliance.
It is also crucial to correctly configure the individual components such as protection relays according to the design outcome as well as assess and manage the system to ensure that objectives are met. Design solutions include:
- Earthing System Design
- Lightning Protection Design
- System Configuration and Assessment
- Commissioning Management
- Protection, Control and Metering
- Testing and Commissioning
Electrical systems are designed to fulfil specified functions as well as comply with technical and regulatory standards. Before any electrical installation is put into service, it is vital to verify that the specified functions are being fulfilled, that the installation complies with relevant technical & regulatory standards and that the installation is safe for use.
Effective verification can only be achieved through a well planned and executed testing and commissioning regime.
For more information on West Australian Power Protections Earthing and Lightning Protection Design, Audit and Maintenance Services please call us on (08) 9451 2199 or email email@example.com