The equipment keeps tripping on earth fault. The sensitive instruments in the laboratory are producing noisy readings that nobody can explain. The server room UPS keeps throwing nuisance alarms. And every time a specialist is called in, they ask the same first question: when was the earthing system last tested?
Earthing is one of those fundamental electrical topics that gets taken for granted in most buildings. It gets installed during construction, tested once during commissioning, and then largely forgotten until something goes wrong. But earthing systems deteriorate. Connections corrode. Earth electrodes age. Supplementary bonding gets damaged during refurbishment works. And unlike a failed light fitting, a degraded earthing system gives you almost no warning before it causes a serious safety event.
This guide covers earthing system in buildings completely — what earthing is, how the different system types work, what the maintenance requirements are, and how to test and verify an earthing system is performing correctly. Written for MEP engineers, electrical engineers, and FM professionals who are responsible for the installation but don’t always have the background to assess its condition.
What Is Earthing and Why Does Every Building Need It?
Earthing — called grounding in North American terminology — is the deliberate connection of electrical equipment and metalwork to the general mass of earth. It serves two fundamental purposes: safety and functional performance.
For safety: if a live conductor touches metalwork — a panel enclosure, a motor casing, a light fitting — without an earth connection, that metalwork becomes live at line voltage. Anyone who then touches it receives an electric shock. With a proper earth connection, the fault current flows to earth instead of through a person, the protective device (MCB, RCD, or MCCB) detects the fault current and trips, and the equipment is made safe.
For performance: earthing provides a stable reference voltage for electrical systems. In buildings with sensitive equipment — laboratories, data centres, medical facilities, communications rooms — a poor or noisy earth causes equipment malfunction, data errors, and instrument inaccuracy that is often misdiagnosed as an equipment fault for months before anyone checks the earth system.
The key distinction to understand: earthing is not the same as neutral. The neutral conductor carries return current back to the source. The earth conductor carries fault current to earth and provides a reference voltage — it should carry no current during normal operation. If your earth conductor is carrying current continuously, something is wrong with the installation.
Types of Earthing Systems Used in Buildings
The IEC 60364 standard (and its UK equivalent BS 7671) defines earthing systems using a two-letter code. Understanding which system your building uses is essential before testing or modifying the earthing arrangement. The first letter describes how the source (transformer or generator) neutral is earthed. The second letter describes how the building’s exposed metalwork is earthed.
| System | Description | Where Commonly Used |
| TN-S | Source earthed at transformer. Separate neutral and protective earth (PE) conductors throughout. PE connected to source earth. | New commercial buildings, modern industrial installations. Cleanest earth — recommended for sensitive equipment. |
| TN-C-S (PME) | Combined neutral and earth (PEN) conductor from supply to building, then split into separate N and PE at the building’s main earthing terminal. | Most common in UK and European commercial buildings supplied from the public LV network. Simple and cost-effective. |
| TN-C | Combined neutral and earth throughout — PEN conductor. No separate earth conductor. | Older industrial installations. Not permitted for new installations in most regions. |
| TT | Source neutral earthed at transformer. Building equipment earthed via its own independent earth electrode — no connection to source earth. | Rural areas or where PME is not available. RCDs essential for personal protection. |
| IT | Source neutral not earthed, or earthed through high impedance. Equipment earthed via independent electrode. | Hospitals (TN-S within hospital), some industrial. Allows operation with single earth fault without tripping. |
In most commercial buildings in the UK and internationally, you will encounter TN-C-S (PME) or TN-S systems. The earthing system determines what testing methods are appropriate, what electrode resistance is acceptable, and what fault current levels the protective devices need to handle.
Components of a Building Earthing System
A building earthing system is not just a wire and a rod in the ground. It’s a complete network of conductors, connections, and electrodes that works together. Understanding each component helps you maintain the system correctly:
Earth Electrode
The earth electrode is the physical connection to ground — a metal rod, plate, tape, or foundation reinforcement that establishes electrical contact with the soil. The most common type in commercial buildings is a driven rod electrode — a copper-bonded steel rod typically 1.2–2.4 metres long, driven vertically into the ground. Multiple rods may be used in parallel to achieve a lower earth resistance, spaced at least twice their length apart to avoid mutual interference.
Earth electrode resistance depends primarily on soil resistivity — which varies significantly with soil type, moisture content, and temperature. Sandy, dry soils have high resistivity and require more electrodes. Clay soils with moisture content have low resistivity and achieve good earth resistance with a single rod. Always test the electrode resistance after installation and annually thereafter.
Main Earthing Terminal (MET)
The Main Earthing Terminal is the central connection point in the building where all earthing conductors meet — the earth electrode conductor, the main protective bonding conductors, and the main earth bar in the LV switchboard. In a TN-C-S system, this is also where the PEN conductor is split into separate neutral (N) and protective earth (PE) conductors.
The MET must be accessible for testing, clearly identified, and the connections must be secure and corrosion-free. Any work on the MET requires isolation of the supply — the MET is live to earth fault potential during a fault condition.
Protective Conductors (PE) and Equipotential Bonding
From the MET, protective earth conductors run to every piece of electrical equipment and panel in the building — these are the green-and-yellow conductors inside cables and conduit. Their purpose is to provide a low-impedance fault current path back to the source so protective devices operate quickly.
Separate from the PE conductors, equipotential bonding conductors connect all metalwork that could become live — gas pipes, water pipes, structural steelwork, HVAC ductwork, and any other metallic services — to the MET. The purpose is to ensure that in a fault condition, all metalwork rises to the same potential simultaneously, so there is no dangerous voltage difference between items a person might simultaneously touch.
In wet areas — bathrooms, swimming pools, kitchens, plant rooms — supplementary equipotential bonding provides an additional connection between exposed metalwork within the zone. This is the bonding most often damaged during refurbishment and most often missed during maintenance inspections.
| 📌 For the full electrical PPM programme that includes earthing system testing, see the Electrical Preventive Maintenance Checklist. |
Earthing System Testing: What to Measure and How
An earthing system that looks intact visually can still be completely ineffective. Corroded connections, high-resistance joints, and deteriorated electrodes are invisible without testing. Here are the tests that verify your earthing system is actually working:
Earth Electrode Resistance Test
This test measures the resistance from the earth electrode to the general mass of earth. The fall of potential method is the standard approach — a test current is injected through the electrode, and voltage measurements are taken at intermediate points to derive the resistance. You need a dedicated earth electrode tester (not a standard Megger) and access to drive temporary test spikes into the ground at measured distances from the electrode.
Acceptable earth electrode resistance depends on the earthing system type. For a TT system where the electrode is the primary safety protection: maximum 200 Ω (with RCDs providing the primary fault protection). For TN systems where the electrode supplements the source earth: as low as possible — typically less than 10 Ω is targeted for commercial installations. Always compare against your commissioning test result — an increase in resistance over time indicates electrode deterioration.
Earth Fault Loop Impedance Test
This test measures the complete impedance of the earth fault current path — from the supply transformer through the live conductor, through the fault, and back through the protective conductor and neutral to the transformer. A low Zs (earth fault loop impedance) value means protective devices will operate quickly in a fault condition. A high Zs means the fault current is insufficient to trip the protective device within the required time — the equipment remains live and dangerous.
Test at the furthest point from the supply on each circuit — this is where impedance is highest. Acceptable Zs values depend on the type of protective device: for a 32A Type B MCB, maximum Zs is 1.37 Ω. For a 16A Type B MCB: 2.87 Ω. Values above these limits mean the MCB will not trip within 0.4 seconds for a socket outlet circuit or 5 seconds for a fixed equipment circuit — both are serious safety failures.
Continuity of Protective Conductors
This test verifies that the protective earth conductor runs continuously from the distribution board to every outlet, connection point, and piece of equipment. Use a low-resistance ohmmeter (not a standard multimeter — the resistance values are too low for accurate measurement). For a 4mm² copper conductor run of 20 metres, expected resistance is approximately 0.09 Ω. Values significantly above this indicate a break or high-resistance joint in the protective conductor.
Insulation Resistance Test (Megger)
Although not strictly an earthing test, insulation resistance testing reveals whether current is leaking from live conductors to earth — which affects earthing system performance and indicates cable deterioration. A healthy installation should show insulation resistance of minimum 1 MΩ between live conductors and earth at 500V DC. Values below 0.5 MΩ need investigation. Values below 0.1 MΩ indicate a serious fault requiring immediate attention.
Earthing System Maintenance: What Your FM Team Must Do
Most building earthing systems receive no routine maintenance between fixed wire inspections — which in commercial buildings is typically every 5 years. That is too infrequent for a safety-critical system. Here is what should happen more regularly:
Annual earthing checks (visual and physical)
• Inspect the Main Earthing Terminal — all connections secure, no corrosion, label intact and legible, accessible
• Check earth electrode connection — the connection between the electrode conductor and the electrode itself is a common point of corrosion, especially in damp soil. Clean and tighten if necessary
• Inspect exposed equipotential bonding connections — particularly in plant rooms, kitchens, and bathrooms. These get damaged during maintenance works and are rarely reinstated correctly
• Check bonding on all incoming metallic services — gas meter bonding, water service bonding, and any new metallic services installed since the last inspection
• Verify earth conductor colour coding — any conductor that has been replaced or extended should use green-and-yellow insulation. Any deviation needs correction
Every 5 years — or after any significant works
• Full EICR (Electrical Installation Condition Report) — covering all circuits, all protective conductors, and all bonding. Any C1 or C2 observations relating to earthing need immediate action
• Earth electrode resistance test — compare against original commissioning value. More than 20% increase warrants investigation
• Earth fault loop impedance test — verify that all circuits still achieve the required Zs for their protective device type and rating
• After any refurbishment works — always inspect and test the earthing system in the affected areas. Refurbishment is the most common cause of earthing system damage
| ⚠️ Important: If the earth electrode resistance has increased significantly since commissioning, do not simply add more electrodes without investigating why. High resistance can indicate corroded electrode, changed soil conditions, or the electrode being in disturbed ground from construction works nearby. Understand the cause before specifying the remedy. |
Common Earthing Problems in Commercial Buildings
These are the earthing issues that come up most frequently during inspections and fault investigations:
| Problem | What It Means and What to Do |
| High earth electrode resistance | Electrode deteriorated, soil has dried out, or electrode is in disturbed ground. Test using fall-of-potential method. May need additional electrodes or enhanced electrode type (horizontal tape, ground plate). |
| High earth fault loop impedance (Zs) | Protective conductor has a break or high-resistance joint. Test continuity from board to furthest point. Common after refurbishment works where conductors are joined or re-routed. |
| Earth conductor carrying current | Something is wrong — current should not flow in earth conductors during normal operation. Check for neutral-earth interconnection, faulty equipment, or RCD nuisance tripping caused by leakage current. |
| Missing or damaged supplementary bonding | Common in bathrooms and kitchens after refurbishment. Plumbers, tilers, and decorators routinely disconnect bonding and don’t replace it. Visual inspection identifies it — test for continuity to confirm. |
| Corroded bonding connections on pipework | Particularly where dissimilar metals are in contact — copper pipe with steel clamp. Clean the connection point back to bare metal, apply conductive jointing compound, and use correct clamp type. |
| Earth rod not accessible for testing | Earth rod inspection pits get buried or built over during landscaping works. Earthing systems must remain accessible for inspection. Raise issue with facilities/property management. |
| Nuisance RCD tripping related to earthing | Can be caused by high leakage current from equipment, damaged cable insulation, or earth loop problems. Isolate circuits one by one. Megger test suspect cables. Check for multiple neutral-earth connections. |
Frequently Asked Questions
What is the difference between earthing and bonding?
Earthing connects the neutral point of the electrical source (transformer or generator) to earth, and provides the return path for fault current through protective earth conductors connected to equipment. Bonding connects metalwork that is not part of the electrical system — pipework, structural steelwork, HVAC ductwork — to the earthing system so that everything is at the same potential in a fault condition. You cannot have effective personal protection without both: earthing to provide the fault current path, and bonding to prevent dangerous potential differences between things a person might simultaneously touch.
What is an acceptable earth electrode resistance for a commercial building?
For a TN-C-S or TN-S system (the most common commercial installations), the earth electrode resistance should be as low as practically achievable — typically less than 10 Ω is targeted. The electrode supplements the source earth rather than being the primary safety protection, so the specific value is less critical than in a TT system. For a TT system, where the electrode is the primary fault protection, the maximum depends on the RCD sensitivity: for a 30mA RCD, the electrode must be less than 1667 Ω (but in practice less than 200 Ω is recommended). Always compare against the original commissioning test result — the absolute value matters less than whether it has changed.
How often should the earthing system in a commercial building be tested?
The earthing system should be included in the full Electrical Installation Condition Report (EICR), which is typically carried out every 5 years for commercial buildings. However, visual inspection of accessible bonding connections, the Main Earthing Terminal, and earth electrode connections should be part of the annual electrical maintenance rounds. After any significant building works — refurbishment, new services installed, drainage works near electrode locations — the earthing system in the affected area should be tested regardless of when the last EICR was done.
Why does my building have RCD nuisance tripping related to earthing?
RCD nuisance tripping is usually caused by leakage current — small amounts of current flowing from live conductors to earth through damaged cable insulation, moisture in wiring accessories, or certain types of electronic equipment that have deliberate earth leakage for EMC filtering. It can also be caused by multiple neutral-earth interconnections creating circulating currents. To diagnose: isolate circuits one by one until the tripping stops — the last isolated circuit is the source. Megger test the cable insulation on that circuit. If the cable is sound, the leakage is coming from connected equipment.
What happens if a building has no earthing or defective earthing?
Without a functional earthing system, a fault current cannot flow to earth and the protective device cannot detect and clear the fault. Equipment that develops an insulation fault becomes live at line voltage — anyone who touches it while in contact with earth (concrete floor, metal pipework, another grounded surface) receives the full fault current through their body. At 230V, fault currents through the human body are easily fatal. Defective earthing — high resistance connections, broken conductors, corroded electrodes — provides partial protection that may not operate within the required disconnection time, meaning the protective device trips too slowly to prevent injury.
Conclusion
Earthing is not a set-and-forget system. Earth electrodes corrode. Bonding connections get damaged during refurbishment. High-resistance joints develop in conductors. The only way to know your earthing system is performing correctly is to test it — and to test it more frequently than the 5-year EICR cycle for a building with any degree of complexity or risk.
For the complete electrical maintenance programme that includes annual earthing inspections, see the Electrical Preventive Maintenance Checklist. For the LV panel testing that goes hand-in-hand with earthing verification, see the LV Panel Maintenance Checklist.
