HV / LV Earthing Systems: (TN-S, TN-C, TN-C-S, TT & IT)
Electrical power systems rely on a safe and reliable connection to earth. A well-designed earthing arrangement provides a controlled path for fault currents, protecting people and equipment, stabilising voltages during lightning strikes or switching events, and ensuring protective devices operate as intended. Without a well-designed low-impedance path to earth, even a minor insulation fault can present serious risks to your operations.
This article explores the five main types of earthing systems (TN-S, TN-C, TN-C-S, TT, IT) with guidance on their application in both LV and HV environments. It compares each system for safety, cost, and reliability, and outlines key design and testing considerations to support compliance and long-term performance.
Key Observations
- What is earthing and why it matters
- Types of earthing systems
- TN system and its variants (TN‑S, TN‑C and TN‑C‑S)
- TT system
- IT system
- Comparison of earthing systems
- Practical guidance for earthing projects
- Earthing audits and periodic testing
- Design considerations and soil resistivity
- Common mistakes to avoid
- Conclusion and next steps
What is Earthing & Why it Matters
Earthing connects exposed conductive parts of electrical equipment and the neutral point of supply to the general mass of earth. This deliberate connection is fundamental to electrical safety and system performance. Key benefits include:
Protects Personnel
Provides a low-impedance path for fault currents, ensuring protective devices disconnect the supply quickly to prevent electric shock.
Safeguards Infrastructure
Reduces the risk of overheating, arc flash and stray currents. In HV systems, it can also limit earth potential rise (EPR) to prevent dangerous voltage differences.
Maintains Resilience
Stabilises system voltages during lightning strikes, switching surges, and unbalanced faults, helping to maintain continuous operation.
Ensures Compliance
Supports compliance with standards such as BS 7671 and BS 7430, ensuring installations meet safety and performance requirements.
Types of Earthing Systems
In the UK, there are five recognised types of earthing systems – TN-S, TN-C, TN-C-S, TT, and IT. They are used across both low voltage (LV) and high voltage (HV) networks, but HV applications are required to handle much larger fault currents and higher earth potential rise (EPR)
Each arrangement is defined by how the supply neutral and protective conductors connect to earth.
TN Systems (TN-S, TN-C, TN-C-S)
In TN systems, the supply neutral is earthed, and all exposed conductive parts are connected to that point.
- TN-S uses separate protective earth (PE) and neutral (N) conductors throughout, helping to reduce electrical interference and maintain a stable reference point.
- TN-C-S uses a combined PEN conductor in the supply network, which is then split into PE and N within the consumer’s installation.
- TN-C uses a single combined conductor for the entire run (now restricted to older networks due to the safety risks if the PEN is damaged).
For HV applications, TN-S is common in substations and renewable export connections where a dedicated PE is installed. TN-C is rare in new HV designs but can still be found in legacy rural overhead lines. TN-C-S is typical where an 11 kV/0.4 kV transformer supplies a PME network, with additional earthing points to manage earth potential rise (EPR).
TT Systems
TT systems earth the supply neutral at the transformer, but the consumer provides their own protective earthing arrangement via electrodes such as rods, plates, or mats.
- In LV networks, TT is common in rural or temporary sites where a PME supply is not permitted or practical. External loop impedance is typically around 21 Ω, plus the resistance of the installation’s own electrode.
- In HV networks, the earthing system must be able to discharge fault currents of several kilo-amps while keeping step and touch voltages within safe limits.
Soil resistivity testing is a critical step in TT design. It indicates how readily current flows into the surrounding ground and informs the correct sizing, depth, and arrangement of electrodes.
IT Systems
In IT systems, all live conductors are either isolated from earth or connected via a high impedance, while exposed conductive parts are earthed separately.
In HV systems, it appears in applications such as mines, offshore installations, and industrial processes where unplanned shutdowns carry significant risk. Neutral earthing resistors or Petersen coils may be added to control fault current, and all connected equipment must be able to withstand full phase-to-phase voltage.
- The first insulation fault will not disconnect the supply, allowing essential services to keep running. An insulation monitoring device alerts operators so that the fault can be addressed before a second fault occurs.
- In LV systems, IT is often used in environments where continuity is essential, such as medical facilities, laboratories, and precision manufacturing.
- In HV systems, it appears in applications such as mines, offshore installations, and industrial processes where unplanned shutdowns carry significant risk. Neutral earthing resistors or Petersen coils may be added to control fault current, and all connected equipment must be able to withstand full phase-to-phase voltage.
| Earthing System | LV Applications | HV Applications |
|---|---|---|
| TN-S | Specialist LV supplies such as data centres, laboratories, and sites requiring a clean earth with low electrical noise; less common in modern public distribution as DNOs often use TN-C-S. | Industrial HV substations and renewable export substations where the transformer neutral is solidly earthed and a dedicated PE is run. |
| TN-C | Historic overhead distribution networks; rarely used for new LV installations due to safety concerns. | Legacy rural HV feeders using combined PEN conductors; generally phased out in favour of safer arrangements. |
| TN-C-S | Most common public LV supply for residential, commercial, and light industrial premises. | 11 kV/0.4 kV distribution substations supplying PME networks, with PEN conductor split into PE and N on the LV side. |
| TT | Rural properties, construction sites, and locations where PME (TN-C-S) is prohibited. | Pole-mounted HV/LV transformers without a continuous PE conductor; isolated generation sites and temporary HV supplies without grid-provided earthing. |
| IT | Hospitals, medical facilities, clean rooms, and critical manufacturing lines (where supply continuity is essential). | Mines, offshore platforms, and industrial processes requiring uninterrupted operation through a single fault. |
Power Earthing Considerations for High Voltage Applications
Earth Potential Rise (EPR) & Step and Touch Voltages
When a large fault current enters a substation or transmission tower earth grid, the local earth potential rises relative to remote earth. This earth potential rise (EPR) is greatest at the fault location and reduces with distance. The resulting voltage gradient can be hazardous:
- Step voltage – The potential difference between two points on the ground spaced one metre apart, which can drive current through a person’s legs.
- Touch voltage – The potential difference between the ground a person is standing on and a conductive structure they touch, such as a tower leg or substation frame.
HV earthing design must ensure step and touch voltages remain within permissible safety limits. Mitigation methods include:
- Bonding or isolating metallic items such as rails, fences, pipes, and telecom cables to avoid dangerous transferred potentials.
- Expanding the earthing system to lower EPR.
- Installing additional conductors to equalise surface potentials.
- Applying high-resistivity surface layers, such as crushed stone, to increase foot resistance.
- Reducing fault clearance times through faster protective device operation.
- Erecting barriers or using personal protective equipment.
Standards and Compliance
BS EN 50522 covers earthing for power installations above 1 kV AC. It addresses earthing arrangements, soil resistivity testing, and bonding of conductive parts, with the primary aim of reducing electric shock and fire risks. The standard also requires periodic testing and maintenance to ensure continued performance. HV designers may also reference ENA TS 41-24 and IEEE Std 80 for specific step and touch voltage limits and permissible body currents.
Earthing Grids and Soil Resistivity
Unlike LV installations, which may use a single rod or small electrode, HV substations employ large earth grids bonded to all metallic structures. Soil resistivity measurements are critical to design, as variations in soil layers affect current dissipation. Engineers model the grid using specialist software to:
- Calculate fault current distribution.
- Predict step and touch voltages.
- Size conductors and determine mesh spacing.
A typical HV grid consists of buried copper or galvanised steel tapes arranged in a mesh, with earth rods at the corners and along the perimeter. In high-resistivity areas, designers may add more rods, chemical electrodes, or soil conditioning materials to achieve target values. For HV cable circuits, sheath bonding and cross-bonding techniques help reduce circulating currents and maintain a low-impedance path to earth.
Practical Guidance for Earthing Projects (HV and LV)
Soil Resistivity Testing – Always carry out on-site resistivity surveys before designing HV earthing systems and strongly consider them for LV projects. Accurate data informs electrode design, grid layout, and the ability to meet step/touch voltage or disconnection time requirements.
Avoid: Skipping soil resistivity testing or relying on assumed values, which often leads to undersized or ineffective systems.
Design to Limit Loop Impedance and EPR
- LV: Use the supply authority’s declared Ze values as a starting point and ensure the total Zs allows protective devices to disconnect within the required time.
- HV: Size and configure the earth grid to control step and touch voltages within safe limits, accounting for worst-case fault currents.
Avoid: Applying generic earthing designs without considering soil conditions, local fault levels, and the network configuration.
Earthing Audits and Periodic Testing – Inspect electrodes, bonds, and connections regularly.
- LV: Verify loop impedance and continuity of protective conductors.
- HV: Use methods such as fall-of-potential or transfer impedance to check grid performance.
Avoid: Neglecting verification after installation or modifications – performance must be tested before energising.
Maintenance After Modifications – Any system changes, whether LV or HV, require retesting to confirm compliance. HV updates must also account for altered fault current contributions and EPR conditions.
Avoid: Failing to re-bond conductors after maintenance. Temporary disconnections that aren’t restored correctly can compromise protective paths.
Documentation and Compliance – Maintain detailed records, including layout drawings, soil resistivity results, test data, and bonding schedules. Accurate documentation supports audits, eases troubleshooting, and simplifies future upgrades.
Avoid: Leaving undocumented changes or poor record-keeping, which makes audits, fault-finding, and upgrades difficult and costly.
Installation Practices – Ensure buried conductors are laid straight and correctly tensioned.
Avoid: Leaving excessive slack or loops underground, which increases inductive impedance and raises fault path resistance.
How EPS Delivers Safe, Compliant Earthing Systems
Protection isn’t proven by design alone – earthing systems need rigorous testing to keep people safe and protect site infrastructure. EPS offer a comprehensive suite of site testing services to prove systems are safe, compliant, and ready for all eventualities. Our highly experienced specialist earthing coordinators carry out Soil Resistivity Testing, Fall of Potential, and Step & Touch Voltage Assessments, as well as Lightning Protection Checks and full Earthing Audits.
How We Can Help:
- Annual & Periodic Testing
- Soil Resistivity Testing
- Fall of Potential (FoP) Testing
- Lightning Protection System (LPS) Inspections & Strike Risk Assessments
- Visual Surveys & Earthing Continuity Testing
- Detailed Reporting with Actionable Recommendations
- Gap Analysis Reporting for Power Earthing & Lightning Protection
With decades of combined experience, EPS don’t just hand over readings or overly lengthy reports; we provide clear recommendations, supported by the relevant data, that keep your operations safe and compliant. If you’d like to see how we approach earthing site testing in full, explore our range of power earthing inspection and testing services here.
View Our Work
Whether you’re starting a new project, upgrading an existing site, or addressing compliance issues, EPS provides the technical expertise and hands-on delivery to ensure your earthing system performs when it matters most.
