Introduction
In earthing studies, one of the most important steps to undertake is to calculated the fault current actually injected into the ground grid vs fault current that will return via metallic paths such as cable sheaths, as it is only the ground grid injection that causes an EPR and associated touch and step voltages.
What does this mean? Going back to basics, when a phase earth fault occurs, to complete the circuit the fault current has to return to the upstream neutral point. Some fault returns through metallic paths (cable sheaths, conductors, steel work etc.) and some returns though the mass of the ground, the current flowing through the earthing grid, will cause a voltage drop across the earthing system, which is known, in earthing terms, as the Earth Potential Rise (EPR) – if you follow North American practice – it is known as GPR. The EPR will persist until the upstream protection operates.
Fault current analysis is essential for designing safe and effective power systems, especially in the context of earth faults where the return current flows through the ground or neutral path. Accurately determining earth fault currents is critical for ensuring that protective devices operate correctly and for assessing safety risks such as touch and step voltages. If this is over-estimated, the EPR will be higher than expected, and the system may be unsafe, if it is under-estimated then the system will be safe, but more expensive and complex than necessary.
In UK distribution networks, two main approaches are commonly used to calculate earth fault currents:
- The ENA S34 methodology, a standardised and conservative method.
- Detailed modelling, using specialised simulation tools that account for system-specific parameters.
We compare these two methods with a focus on how each handles earth return current paths, highlighting the compromise in accuracy, complexity, and practicality.
ENA S34 Methodology: Simplified Earth Return Fault Estimation
ENA Engineering Recommendation S34 provides formulas for calculating earth fault current contributions in HV distribution systems. It is particularly focused on 11kV and 33kV single-phase-to-earth faults, which are the most common fault type in many distribution networks. There are a few models for 132kV cables, but these are somewhat limited. It also has some simplified formula for 132, 275 and 400kV OHL tower lines, which are useful approximations.
- S34 simplifies the earth return path by using standard impedance values for typical network arrangements, to consider the self and mutual impedance between cables.
- The method is fast, spreadsheet-based and low complexity, and it is quick and easy to calculate.
- The earth return path is modelled by using the ‘C-factor method’, typically based on published values or worst-case assumptions, based on typical cable properties.
Pros:
- Quick calculations with minimal data: ideal for high-level planning and basic earthing studies.
- Ensures conservative estimates for Earth Potential Rise (EPR) calculations.
- Standardisation across DNOs ensures consistency.
Cons:
- Lack of spatial detail: S34 cannot account for variation in soil resistivity, conductor geometry, or return path segmentation.
- Over-simplification: Earth return path impedance is often estimated too conservatively, potentially leading to mitigation requirements (e.g., larger earthing systems).
- Ignores dynamic effects: No provision for current splitting through multiple ground paths.
- Limited model types at 33kV and 132kV
Detailed Modelling: Accurate Representation of Earth Return Paths
Detailed modelling of earth fault currents involves creating an electrical model of the earth return path using tools such as PSCAD or CDEGS for earthing system analysis. CDEGS has two specific modules for this known as TRALIN and SPLITS, these are powerful tools – but not the easiest to use. In many times it is easier and simpler to create a model in PSCAD.
- Incorporates site-specific soil resistivity measurements (e.g., Wenner or Schlumberger tests).
- Models the geometry of return paths, including cable sheath impedance, metallic return conductors, and earth electrode configurations.
- Can represent multiple return paths, mutual coupling, and current distribution through the ground, neutral, and adjacent installations.
Pros:
- High accuracy: Reflects a more realistic behaviour of the fault return current, reducing over or under estimation of fault currents.
- Customisable: Suitable for more complex configurations such as urban substations, rural networks, or networks with multiple earthing points.
- Supports safety assessments: Enables accurate calculation of touch and step voltages, hot zone extents, and transferred potentials.
Cons:
- Data-intensive: Requires detailed network information and conductor data.
- Time-consuming: Model creation, validation, and interpretation may require specialist expertise and tools.
- Complexity: Outputs are more difficult to interpret without earthing system experience.
Summary
Ground return fault current calculations are crucial for designing safe and compliant earthing systems. The ENA S34 method provides a quick and conservative approach that is suitable for many typical applications. However, for more complex systems, especially those with multiple return paths, parallel earth conductors, or critical safety requirements detailed modelling offers the precision and flexibility needed to capture more realistic behaviour. For most projects, a common approach is recommended:
- Look at the site configuration and earth return path possibilities to determine if the more simplified S34 approach will work.
- Start with ENA S34 to screen worst-case conditions and guide early decisions. If the calculations result in good outcomes, there is no need to go into more detailed analysis.
- If necessary or if the site configuration is complex, follow up with detailed modelling where higher accuracy is needed for earthing design and safety assessments.
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