Testing of the earthing system
Commissioning of new earthing system is essential as a validation step for the design and installation process and for the design inputs. In most cases commissioning should measure the outputs of the earthing system in terms of produced voltages and current distributions rather than solely resistance.
The commissioning should consider closely the key performance criteria identified in the hazard identification and treatment analysis phases.
Commissioning will determine the earthing system initial compliance and set a benchmark or baseline for ongoing supervision.
As it is not always possible to foresee all hazard mechanisms at the design stage commissioning testing should also determine the need for any localised secondary mitigation and any additional requirements for telecommunication coordination and pipeline interference coordination or mitigation.
- Visual inspection
- Continuity testing
- Earth resistivity testing
- Injection testing
- Transfer, touch and step voltage testing
- To be aware of
The visual inspection typically involves checks of //
- Design compliance and as-built drawing accuracy
- Condition of earthing conductors and connections
- Condition of earthing electrodes
- Presence and condition of earthing bonds to equipment
- Condition of surface layer materials if required
- Condition of access fences if required
- Presence of transfer hazards.
Continuity testing is used to measure the resistance between items of plant within the main earth grid and to components that should be effectively bonded to the grid. This test is especially important in large earthing systems where visual inspection of all conductors and connections is more difficult.
Adequate bonding is essential to ensure that personnel are working only on equipment that is effectively connected to the earthing system.
It is often necessary to carry out earth resistivity tests in conjunction with performance assessments to allow accurate error corrections and safety criteria determination.
Even where resistivity testing was undertaken at the design stage, additional testing (however brief ) may help to define measurement errors and periodic variations.
The remaining tests require the presence of a simulated power system line to ground fault. To achieve this, a circuit is established between the earthing system under test and a remote injection point.
The simulated fault is typically made sustainable by injecting a small current, commonly between 2 and 20 amps. The effects are made measurable, even on live systems, by injecting at a frequency away from power system frequency and using frequency tuneable measuring equipment.
The test is referred to as a Low Current, Off Power Frequency Injection Test.
With test current flowing through the simulated fault circuit, voltages will be present in the same locations and in proportion to those generated during a real earth fault. The earthing system’s EPR is measured by performing a fall of potential test.
This test requires a test lead to be run out from the earthing system to allow a series of voltage measurements to be made between the earthing system under test and the ground. The route and distance is chosen to minimise measurement errors.
The measurements taken from the fall of potential test must be processed for the difference between test and power system frequency and for distance to remote earth. They can then be used to determine the earth system impedance and the EPR under actual fault conditions. Adjustments should also be made for mutual earth resistance and for mutual inductance as required.
Direct remote earth measurements, such as voltage measurements to remotely earthed communications or pilot wires, can also supply supplementary test data. However, with single point measurement alone it is very difficult to correctly assess and correct the many error sources that can be part of any measurement taken.
In the situation where fault current may leave the earthing system through paths alternate to the earth grid (such as cable sheaths or overhead earth wires), the current through those alternate paths should also be measured.
This allows analysis of how fault energy is dissipated, its effect on the alternate paths (for example, cable sheath capacity) and calculation of the earth grid impedance from the total system impedance.
In complex systems the results are particularly important in modelling alternate fault scenarios and in-feeds not simulated during testing.
While test current is flowing in the fault circuit, measurements are made of actual transfer, touch and step voltages. The purpose of such measurements is to directly measure the earthing system’s outputs and the compliance with the determined safety criteria.
When measuring touch and step voltages it is important to measure the prospective touch and step voltages using a high impedance voltmeter and to measure the effective or loaded touch and step voltages appearing across an appropriate resistance that represents the human body.
Care should be taken not to confuse the prospective step and touch voltages (i.e. open circuit case) with the effective step and touch voltages criteria. The loaded touch and step voltage cases are more variable due to variations in contact resistance.
Therefore, the loaded case is only used when necessary and precautions taken (for example, take multiple measurements, use electrode contact initially, and only use a weighted plate on moistened soil if necessary).
Where telecommunications equipment is installed within the area of influence of a high voltage earthing system consideration is required of the hazards that may be created. In such cases notification must be given to the appropriate telecommunications group.
Pipeline interference / coordination
Where pipelines are installed within the area of influence of a high voltage earthing system consideration is required of the hazards that may be created. These hazards must be reviewed during commissioning. In such cases notification must be given to owner / operator of the pipeline.
Reference // Power System Earthing Guide—part 1: management principles by ENA