Articles
Distribution Earthing – Use the Biggest Level First
Are you wasting money on installation with your design strategy?
In this article we explore the effectiveness of different earthing strategies across a range of design scenarios. Three fictional case studies are used to demonstrate how alternative approaches can yield varying efficiencies depending on the specific power system context.
Each case study considers a padmount substation located in different zones of the power system as follows:
- Close to the supplying Zone Substation, with a high fault level, entirely cable fed, and supplying a ‘strong MEN area’.
- Located a ‘medium’ distance from the supplying Zone Substation, so a slightly lower fault level, dominantly cable fed. The HV supply includes a short OH line section with a LV neutral but no direct HV earthing connection. This case also supplies a ‘strong MEN area’.
- This case is located in a semi-rural setting. It has the lowest fault level, a section of OH HV feed > 1km, no earthing continuity to the Zone Substation, and supplies a ‘smaller, isolated MEN’.
The outcomes of the three case studies and variations are presented in a table at the end of the article. The results presented have been limited to asset touch voltage and the LV transfer voltage (either direct EPR transfer in common arrangement or soil voltage transfer onto the LV earthing in separate arrangement).
Some inputs are consistent between the cases to better illustrate key differences between the cases. The analysis has also ignored third-party asset risk and is calculated based on prospective fault currents and clearing times to simplify the comparison of design outcomes.
The specific details of the earthing designs will be detailed in each case study. The common details are:
Case Study 1
The key parameters used in the calculation of Case 1 are presented in the table below.
| Parameter | Value |
| Fault Level (Prospective) | 7kA |
| Clearing Time | 0.2s |
| MEN Resistance | 0.2Ω |
| DU Voltage Criteria | 4346V |
| TDMEN Voltage Criteria | 535V |
This case is the most straightforward case study of the three. The low impedance return path associated with the HV supply provided by the bonded cable screen and the low impedance MEN reduces the EPR to 301V. This is 56% of TDMEN criteria so the installation easily achieves compliance.
However, without a bonded cable screen connection to the ZS the earthing design becomes significantly harder, as despite a low impedance MEN connection, the EPR without screen bonding rises to almost 1300V.
A compliant outcome may be possible through significant design modifications, such as an extended local earth grid or an alternative earthing strategy, which is beyond the scope of this article. For the scenario considered then, a metallic screen return is likely to provide the most effective earthing design.
Case Study 2
The key parameters used in the calculation of Case Study 2 are presented in the table below.
| Parameter | Value |
| Fault Level (Prospective) | 3.5kA |
| Clearing Time | 0.3s |
| MEN Resistance | 0.2Ω |
| DU Voltage Criteria | 4160V |
| TDMEN Voltage Criteria | 446V |
| Power Supply with three sections of line | 1) 800m UG Cable
2) 500m OH Line with LV Neutral (7/3.75 ACC) 3) 200m UG into PM under design |
Case 2 is a tricker design from the outset. While the design retains a low impedance MEN connection, with the given fault level compliance is not immediately achievable with the base earthing layout with a common design strategy.
A compliant separate earthing design is relatively straightforward by increasing the separation to the LV earthing to approximately 13m (15m from the centre of the asset). If this separation is achievable then experience suggests this would likely be the most effective solution. A separated design strategy would also benefit from the reduction in fault level due to the corresponding increase in the local earth grid impedance which is not considered here.
If the substation land allocation does not allow space for 13m of separation, or if common earthing is required or preferred for the installation, then achieving a compliant design is far more difficult. With an EPR at approximately 135% of TDMEN criteria and an already low MEN impedance, installation of a lower impedance local earth grid is unlikely to be reasonably practicable even with a relatively low soil resistivity.
One option to achieve a compliant common earthing system here would be to utilise the LV neutral conductor on the overhead line to enable a metallic connection back to the zone substation. With the inductive effect of a metallic earth return, the EPR and earthing hazards are now significantly reduced with an EPR of approximately 250V which is 55% of the TDMEN criteria.
An additional benefit of this strategy in some design scenarios is the mitigation of voltage hazards arising at upstream locations. The nature of the inductive return of fault current is such that it can create voltage hazards at upstream open points which can be larger than the faulted substations EPR. This voltage hazard is beyond the scope of this article but certainly worth considering in any distribution design. If the LV conductor was not available, then installation of some form of overhead earth wire may still be a viable option to achieve a common earthing system when considering the balance of cost/risk in an earthing design.
Case Study 3
The key parameters used in the calculation of Case Study 3 are presented in the table below.
| Parameter | Value |
| Fault Level (Prospective) | 1.5kA |
| Clearing Time | 0.5s |
| MEN Resistance | 20Ω |
| DU Voltage Criteria | 4065V |
| TDMEN Voltage Criteria | 270V |
Case 3 requires a different strategy as without a low impedance MEN to connect to, and unlike Case 2, the distance back to the zone substation is unlikely to be bridged with an LV connection or an underslung earth wire. As such, the base common earthing system is far from compliance. As with Case 1 a compliant common earthing design may be possible, but the installation is not likely to be reasonably practicable with an EPR of 643% of TDMEN criteria for the base common layout.
A separate earthing design is straightforward, like in Case 2 we only require a small increase in the separation from the base earthing layout with a compliant design with at least 6m separation (8m from the centre of the asset) achieving an LV transfer of 90% of TDMEN. Further increasing the separation to 13m (15m from the centre of the asset) further reduces the LV transfer risk to 59% of TDMEN.
Results and Conclusion
The following table presents a summary of the results for each of the considered cases and the discussed alternative design options.
| Scenario | Asset Touch Voltage | LV Transfer Voltage | ||
| Voltage | % Criteria | Voltage | % Criteria | |
| Case 1 Bonded Screens |
100V | 2% | 301V | 19% |
| Case 1 Unbonded Screens | 412V | 10% | 1,247V | 233% |
| Case 2 Base Common earthing 0.2Ω MEN |
199V | 5% | 601V | 135% |
| Case 2 Separate Earthing 15m Separation |
1562V | 38% | 368V | 83% |
| Case 2 Common – Bonded to ZS |
82V | 2% | 248V | 55% |
| Case 3 Common Earthing |
575V | 14% | 1,737V | 643% |
| Case 3 Separate Earthing 8m Separation |
671V | 17% | 243V | 90% |
| Case 3 Separate Earthing 15m Separation |
669V | 17% | 158V | 59% |
These results show that each earthing design has its unique characteristics and the strategy which works best for one design will not necessarily work in another. The lowest system impedance or grid resistance and lowest possible EPR isn’t always the best solution to get the lowest voltage hazard risks.
When starting an earthing design, it is important to consider the design choices available and to assess which earthing strategy may be most effective and try that first.
By Myles Wellington, Consulting Engineer, Safearth
For more information contact Safearth on 1800 327 844
streets.safearth.com | streets@safearth.com