An earthing system, also called a grounding system, provides a safe path for fault current to flow into the earth, protects people from electric shock, and stabilizes voltage during normal operation. International standards recognize several earthing arrangements, each with different characteristics and applications. This guide explains the main types, their components, and how to choose the right one for your installation.
What Is an Earthing System?
An earthing system is the set of conductors, electrodes, and connections that link the exposed conductive parts of an electrical installation to the general mass of earth. Its purpose is threefold: protect people from electric shock, allow protective devices to detect and clear faults, and limit overvoltages caused by lightning or switching.
Every low-voltage installation has an earthing system, whether or not the local code calls it “grounding” or “earthing”. The choice of system affects equipment safety, fault clearance, and the selection of residual current devices (RCDs).
Main Types of Earthing Systems
The international standard IEC 60364 classifies low-voltage earthing systems using a two- or three-letter code. The first letter indicates the relationship between the power system and earth; the second letter indicates the relationship between the exposed conductive parts of the installation and earth. Additional letters indicate the function of the neutral and protective conductors.
TN System
In a TN system, the power supply has one point directly earthed (usually the neutral), and the exposed conductive parts of the installation are connected to that point through a protective conductor. The T stands for “terre” (French for earth).
TN-C: Neutral and protective functions combined in a single conductor (PEN) throughout the system. Simple and economical, but any break in the PEN conductor exposes all downstream equipment to full line voltage.
TN-S: Neutral and protective conductors are separate throughout the system. Safer than TN-C, common in modern installations and required for many sensitive electronic loads.
TN-C-S: Combined PEN in the supply and separate N and PE in the installation. The most common arrangement for residential and small commercial installations, also known as PME (protective multiple earthing) in the UK.
TT System
In a TT system, the power supply has one point directly earthed, and the exposed conductive parts of the installation are connected to a local earth electrode independent of the supply electrode. RCDs are typically required for fault protection because the earth fault loop impedance is high.
TT is common in rural areas, mobile installations, and sites where the supply earth cannot be relied upon.
IT System
In an IT system, the power supply is isolated from earth or connected through a high impedance, and the exposed conductive parts of the installation are earthed locally. The first fault does not cause a trip; an insulation monitoring device signals the fault so it can be located and fixed before a second fault occurs.
IT is used in hospitals (for medical IT systems in operating rooms), control systems, and any installation where continuity of supply is critical.
Comparison of Earthing Systems
| System | Source Earth | Exposed Parts Earth | Fault Loop | RCD Required | Typical Use |
|---|---|---|---|---|---|
| TN-C | Yes (PEN) | Connected to PEN | Low | Optional | Old, simple industrial |
| TN-S | Yes (N, PE) | Connected to PE | Low | Optional | Modern commercial, data center |
| TN-C-S | Yes (PEN → N, PE) | Connected to PE | Low | Recommended | Residential, small commercial |
| TT | Yes | Local electrode | High | Mandatory | Rural, mobile |
| IT | Isolated / impedance | Local electrode | Very high | Insulation monitor | Hospitals, critical processes |
Key Components of an Earthing System
- Earth electrode — Rod, plate, or foundation concrete-embedded conductor that makes contact with the soil.
- Earth conductor — Connects the earth electrode to the main earthing terminal or busbar.
- Main earthing terminal — Common connection point for the earth electrode, incoming supply earth, and installation protective conductors.
- Protective conductor (PE) — Connects exposed conductive parts of equipment to the main earthing terminal.
- Equipotential bonding — Connects metal services (water, gas, structural steel) to the main earthing terminal to reduce touch voltage.
Standards to Follow
The most important international standards for earthing are:
- IEC 60364 — International standard for low-voltage electrical installations, including earthing arrangements.
- IEC 61936-1 — Power installations exceeding 1 kV AC.
- IEEE 80 — Guide for safety in AC substation grounding (commonly used in North America).
- NFPA 70 (NEC) — US National Electrical Code, with detailed grounding and bonding requirements.
- BS 7671 — UK wiring regulations, with detailed earthing arrangements.
Requirements may vary by application, market, and applicable standard. Always follow the local code and the consultant’s design.
How to Choose the Right Earthing System
The earthing system is usually determined by the supply authority and the local code. Within those constraints, the designer may choose among the available options. Consider the following factors:
- Continuity of supply — If continuity is critical (e.g., hospitals, control systems), IT or TN-S with high-quality components may be preferred.
- Length of feeders — Long feeders increase earth fault loop impedance. TN systems keep the loop low; TT systems may need lower-resistance electrodes or RCDs.
- Soil resistivity — High soil resistivity makes local electrodes less effective, which can rule out TT or IT in some sites.
- Need for RCDs — TN-C cannot use RCDs on the PEN conductor; TN-S, TN-C-S, and TT can.
- Existing infrastructure — Upgrading an existing installation to a different earthing system can be expensive.
How to Inspect Earthing System Quality
- Earth electrode resistance — Measure with an earth tester; compare with the design value.
- Continuity — Verify PE continuity from each exposed part to the main earthing terminal.
- Equipotential bonding — Check connections to water, gas, and structural steel.
- Conductor sizing — Verify PE and bonding conductor cross-section against the design.
- Corrosion — Inspect earth electrodes and connection points for corrosion.
- Mechanical protection — Check that exposed earth conductors are protected against damage.
Frequently Asked Questions
What is the safest earthing system?
There is no single “safest” system. TN-S and IT are widely considered among the safest for their respective applications. Safety depends on correct design, installation, and maintenance, regardless of the system type.
Can I use an RCD in a TN-C system?
No. RCDs require separate neutral and protective conductors. In a TN-C system, they are combined in the PEN conductor, so an RCD cannot be installed upstream of the PEN split. Convert to TN-C-S or TN-S first.
Why are RCDs mandatory in TT systems?
In a TT system, the earth fault loop impedance is high because the fault current flows through the soil. This high impedance limits the fault current to a level that may not trip a standard breaker, so an RCD is required to detect the small residual current and disconnect quickly.
How often should the earthing system be tested?
Earth electrode resistance should be tested at least once a year, and after any major excavation, lightning strike, or modification. Continuity should be tested as part of the periodic installation inspection.
What is the difference between earthing and grounding?
“Earthing” is the term used in British, European, and IEC standards. “Grounding” is the term used in North American standards. The two terms refer to the same function.
Can I install my own earth electrode?
For safety and code compliance, earth electrode installation should be carried out by a qualified electrician and tested with proper instruments. Always follow the local code and have the installation inspected before energizing.
Source Earthing Components from BANGE Electric
BANGE Electric provides integrated electrical distribution and cable system solutions, including distribution boxes, switchgear, and complementary components for earthing systems. We support specification customization, consistent quality control, export packaging, and reliable international supply.
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