Marlon Fonseka made a presentation at the SirfRT on 15 April 2016 on his experiences in conducting earthing system testing. Marlon is an experienced electrical engineer in the field of earthing system design and testing. He has also attained a CDEGS Level I Certification from SES Technologies – the leading software for earthing and electromagnetic interference. The following points were raised in his presentation:
Purpose of an Earthing System
The fundamental purpose of an earthing system is to ensure:
- The safety of personnel and the general public in regards to the prevention of electric shock
- Proper operation of safety devices
- The protection of electrical assets from dangerous voltages and currents
Routine earth testing and integrity checks are required to ensure that the earthing system adequately achieves the above goals.
Interference due to underground or above-ground assets
Care must be taken on the selection of test plans and set ups to avoid erroneous and inconclusive test results. Assets such as underground power cables, pipes or fences can in some cases, greatly affect the test measurements. Routes where assets such as electrical circuits with overhead earth wires, bonded back to the site under test, should also be avoided.
Soil resistivity is the biggest factor governing the performance of the earthing system. The top soil layer can be replaced to a certain depth and ground enhancing material can be added to the immediate surrounds of earth grid conductor. However, the total earth that is affects the earthing system, which can be several meters below ground level, cannot be economically dug out or replaced, so it is a fixed variable that we as engineers need to work with.
All fault scenarios need to be assessed in combination with clearing times to ensure that the worst case earth return fault level is used to determine the safety of the site
Changes affecting the performance of the earth grid
There are several factors that can affect the performance of an earth grid over time. These include any changes in the infrastructure immediately surrounding the high voltage asset, changes to the site layout and deterioration of earth grid conductors.
Recommended test during maintenance
The following is a list of recommendations to include in a routine maintenance program:
- Off frequency current injection testing to determine:
- Touch, step and transferred voltages
- Earth potential rise contours
- Earth grid impedance
- Soil resistivity testing to determine tolerable touch and step voltage limits
- Assessment of the physical integrity of the earthing using the following methods:
- Continuity testing
- Visual inspections
- Assessment of the earth grid conductor sizes to determine the withstand capability
- Assessment of the switchyard surfacing to determine its suitability as a protective layer