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Earthing -- called grounding in North American terminology -- is the intentional electrical connection between the generator's metallic enclosure, the electrical system's neutral point, and the general mass of earth. It is one of the least visible aspects of a generator installation and one of the most important for human safety.
A generator without correct earthing is a silent hazard. Under normal operation, no current flows through the earth conductor and nothing appears wrong. But in a fault condition -- an insulation breakdown, a wiring error, or a loose connection that allows live voltage to appear on a metal enclosure -- the absence of correct earthing means there is no safe path for fault current to flow. The fault current finds its own path instead: through the person who touches the enclosure.
This guide explains the earthing requirements for commercial and industrial diesel generator installations in plain language: why each connection is needed, how to design the electrode system, how to size the conductors, and how to verify the installation is correct before commissioning.
To understand earthing, consider what happens when a live conductor contacts a metal enclosure inside the generator -- a scenario that can occur through insulation breakdown, mechanical damage to wiring, or vibration-induced chafing of cable insulation against the generator frame.
⚡ Without earthing: the enclosure becomes live
If the generator frame is not connected to earth, the fault voltage (400V or 230V) appears on the generator canopy, base frame, and any metal connected to it. There is no path for fault current to flow -- no protective fuse or circuit breaker trips. The fault is invisible. Anyone who touches the generator enclosure while standing on the ground completes the circuit through their body. At 400V, contact current through a human body is typically fatal within 100-300 milliseconds.
⚡ With correct earthing: the fault trips protection
If the generator frame is correctly connected to earth, a fault from live conductor to frame creates a low-impedance fault current path back to the source through the earth conductor and neutral. This fault current is large -- tens or hundreds of amps -- and causes the overcurrent protection (fuse or circuit breaker) to trip within milliseconds. The fault is cleared before it can harm anyone who might touch the enclosure. Earthing does not prevent faults; it ensures faults are detected and cleared automatically.
The earthing principle in one sentence: a correct earth connection ensures that any fault current has a safe, low-impedance return path that causes protection to operate -- rather than finding its return path through a person who touches the faulted enclosure.
A complete generator earthing system involves three distinct connections, each serving a different purpose. Confusing them is the most common earthing design error.
1 Equipment Earth (Protective Earth -- PE)
The equipment earth connects all exposed metallic parts of the generator -- the canopy, base frame, alternator frame, control panel enclosure, and engine block -- to the earth electrode. This is the connection that makes fault current flow safely if a live conductor contacts any metal surface. The equipment earth conductor is always green/yellow in European convention and green in North American convention. It must be continuous from the generator frame to the earth electrode, with no breaks, switches, or fuses in the earth conductor path.
2 Neutral Earth (System Neutral Earthing)
In a three-phase generator, the star-point (neutral point) of the alternator windings can be earthed or left floating (unearthed). Earthing the neutral provides a reference voltage for the system -- it defines what zero volts means in relation to earth. Without a neutral earth, phase voltages are not defined relative to earth and can float to unexpected values. Earthing the neutral also allows single-phase faults (live to earth) to produce the large fault current needed to operate protection. The neutral earth connection is typically made through a solid link (direct earthing) or through a neutral earthing resistor (NER) in high-impedance earthed systems.
3 System Earth (Main Earth Electrode)
The system earth is the physical connection to the general mass of earth -- the buried electrode (rod, plate, or ring) that provides the ultimate return path for fault current. All other earth conductors in the system must ultimately connect to this electrode or to a main earthing terminal connected to it. The quality of the system earth -- measured as its resistance to the general mass of earth -- determines the effectiveness of the entire earthing system.
The earth electrode is the physical component buried in the soil that provides electrical contact with the mass of earth. Electrode design determines whether the target earth resistance can be achieved in the soil conditions at your site.
Earth resistance target: For most commercial and industrial generator installations, the target earth resistance is 1 ohm or less. Some standards allow up to 10 ohms for general installations, but 1 ohm provides significantly better fault protection and should be the design target. High-value installations (hospitals, data centres, hazardous areas) should target 0.5 ohm or less.
Soil resistivity -- the key variable: Earth resistance is primarily determined by soil resistivity -- the electrical resistance of the soil, measured in ohm-metres. Clay soils have low resistivity (10-100 ohm-m); sandy or rocky soils have high resistivity (1,000-10,000 ohm-m). High soil resistivity requires longer or more numerous earth rods, or enhanced electrode designs, to achieve the target resistance. Soil resistivity can be measured with a four-electrode test (Wenner method) before electrode installation.
Electrode Type | Typical Application | Advantage | Limitation | Typical Resistance |
Single earth rod | Small installations; | Simple to install; | May not achieve 1 ohm | 5-50 ohm (variable; |
Multiple earth rods | Medium installations; | Reduces resistance; | Higher installation cost; | 1-10 ohm achievable |
Deep-driven earth rod | Sites where surface soil | Accesses lower-resistivity | Requires specialist | Often achieves <1 ohm |
Earth plate | Sites where rod driving | Broad contact area; | Excavation required; | 2-20 ohm depending |
Earth ring | Large installations; | Very effective; | Requires extensive trenching; | <1 ohm achievable; |
Chemical earth rod | High-resistivity soil; | Backfill compound lowers | Higher cost; compound | Achieves <1 ohm in |
The most common earthing failure in developing markets: a single 1.2m earth rod driven into dry, sandy, or rocky soil and assumed to provide adequate earthing. In high-resistivity soils, a single short rod may achieve 50-200 ohms -- far above the 1-ohm target. This makes the protection system effectively non-functional for earth fault clearance. Always test earth resistance after installation; never assume a rod meets the target without measurement.
The protective earth (PE) conductor connects the generator frame to the earth electrode. It must be large enough to carry the prospective fault current without overheating or breaking, long enough to maintain continuity, and adequately protected against mechanical damage.
Sizing rule -- IEC 60364 method: The minimum cross-sectional area of the PE conductor is determined by the phase conductor cross-section: if the phase conductor is 16mm2 or smaller, the PE conductor must be the same size; if the phase conductor is 16-35mm2, the PE conductor must be minimum 16mm2; if the phase conductor is above 35mm2, the PE conductor must be minimum 50% of the phase conductor area, with 25mm2 minimum. Never size the PE conductor smaller than these minimums -- the conductor must survive the fault current for the protection clearing time.
Phase Conductor Size | Minimum PE Conductor Size | Typical Generator Size | PE Conductor Material |
Up to 16 mm2 | Same as phase (up to 16 mm2) | 20-30 kW | Copper -- green/yellow sheath |
16-35 mm2 | 16 mm2 | 30-60 kW | Copper -- green/yellow sheath |
35-70 mm2 | 25 mm2 minimum | 60-150 kW | Copper -- green/yellow sheath |
70-150 mm2 | 50% of phase (35-75 mm2) | 150-400 kW | Copper -- can use bare |
150-300 mm2 | 50% of phase (75-150 mm2) | 400-1,000 kW | Copper -- bare for |
Above 300 mm2 | 50% of phase (>150 mm2) | >1,000 kW | Copper -- structural |
Connection to earth electrode: The PE conductor connects to the earth electrode via a clamp or brazed/welded joint. Bolted clamps are acceptable if they are weatherproof (stainless steel or copper alloy), properly torqued, and accessible for inspection. Brazed joints are preferred for buried connections -- they are more corrosion-resistant and mechanically secure than clamp connections in soil.
Protection from mechanical damage: The PE conductor must be protected from mechanical damage from the generator frame to the point it enters the ground. Protect with steel conduit or armoured cable where the conductor is exposed to vehicle traffic, foot traffic, or generator vibration. Underground sections should be at minimum 600mm burial depth to protect from surface damage.
Whether to earth the generator neutral at the generator is one of the most consequential earthing decisions -- and one that depends on how the generator connects to the existing building electrical system. Getting this wrong creates either a dangerous floating neutral or a double-neutral earth that interferes with earth fault protection.
⚡ Scenario A: Generator as standalone power source (no grid connection, no ATS)
Earth the neutral at the generator. The generator is the only power source; its neutral defines the voltage reference for the system. Connect the neutral point to the main earth terminal (MET) of the distribution board, and from there to the earth electrode. This is a TN-S or TN-C-S system depending on your distribution design.
⚡ Scenario B: Generator connected via ATS to an existing building with grid earthing
Do NOT earth the generator neutral at the generator if the building already has an earthed neutral from the grid supply. The ATS must either: (a) switch the neutral connection along with the phase conductors (4-pole ATS -- always preferred); or (b) maintain the existing neutral earth at the building MET for both grid and generator supplies (3-pole ATS with separately maintained earth). Double-earthing the neutral through both the grid and the generator simultaneously creates circulating currents that interfere with earth fault protection and residual current devices.
⚡ Scenario C: Portable or temporary generator connection
A portable generator used for temporary power must be earthed independently of any building earth system it connects to. Drive a temporary earth rod at the generator location and connect the generator frame and neutral to this rod. Do not rely on the building earth system for a portable generator -- the building earth may be disconnected from the generator if it is not directly wired into the building system.
⚠ 3-pole vs 4-pole ATS: the neutral earthing implication
A 3-pole ATS switches only the three phase conductors between grid and generator. The neutral conductor remains connected throughout. In this configuration, the neutral earth must be maintained at the building MET regardless of which source is supplying power -- the generator neutral must NOT be separately earthed at the generator. A 4-pole ATS switches all four conductors including the neutral. In this configuration, the generator neutral can be earthed at the generator (forming an independent earthing system when on generator) and the grid neutral earth at the building MET is valid when on grid. For new installations, 4-pole ATS is strongly recommended -- it provides cleaner isolation between grid and generator earthing systems and eliminates the neutral conflict problem.
Earth resistance testing is a mandatory step before commissioning any generator installation. It is the only way to confirm that the electrode installation achieves the target resistance -- visual inspection cannot tell you anything about earth resistance quality.
Step 1: Equipment Required
An earth resistance tester (also called an earth tester or Megger earth tester). The most common method is the three-terminal (fall-of-potential) method, which requires the tester and two auxiliary electrodes (current and potential spikes) driven into the ground at specific distances from the main electrode. The tester injects a known alternating current into the earth and measures the voltage drop to calculate resistance.
Step 2: Measurement Procedure
Disconnect the earth conductor from the electrode being tested (testing with the conductor connected measures the parallel combination of all earth paths, not the single electrode). Drive the current auxiliary electrode (C2) at a distance of 30-40m from the main electrode. Drive the potential auxiliary electrode (P2) at a distance of 15-20m from the main electrode, in line between the two. Connect tester leads: C1 to main electrode; C2 to current spike; P1 to main electrode (or direct to MET); P2 to potential spike. Test and read resistance.
Step 3: Interpreting the Result
Target: 1 ohm or less for commercial/industrial generator installations. If result is above 1 ohm: add a second earth rod in parallel (minimum 2x rod length spacing from the first); retest. If result remains above 5 ohms after multiple rods: soil has high resistivity; use chemical backfill, deeper rods, or earth ring design. Document the test result: date, tester model, electrode configuration, and measured resistance. Include in commissioning certificate.
✔ Simplified test method for sites where full fall-of-potential is impractical
Where space for auxiliary spikes is unavailable (confined urban sites, rooftop installations), a two-electrode test using a known-resistance reference electrode gives an approximate result. Alternatively, a clamp-on earth resistance tester measures earth resistance without disconnecting the electrode -- useful for testing in parallel earth systems. These simplified methods give indicative results; they are not a substitute for full fall-of-potential testing where safety-critical installations are involved.
Mistake | Consequence | Correct Practice |
Single short earth rod | Earth resistance 20-200 ohm; | Test soil resistivity first; |
Earth conductor with | Earth conductor can be opened; | Earth conductor must be continuous |
Undersized PE conductor | PE conductor overheats and | Size per IEC 60364 or local |
Neutral earthed at both | Circulating currents; RCDs | Use 4-pole ATS to isolate |
Earth rod painted or | Paint coating prevents soil | Never coat or paint |
Earth connection on top | High-resistance connection; | Clean contact surfaces to |
No earth resistance test | Installation assumed safe | Always test with calibrated |
Requirement | Specification | Standard Reference |
Equipment earth (PE) conductor | Green/yellow; sized per phase conductor; | IEC 60364-5-54; |
Earth electrode resistance | 1 ohm or less for commercial/industrial; | IEC 60364-5-54; |
Neutral earthing decision | Standalone generator: earth at generator MET; | IEC 60364-3; |
Earth resistance test | Fall-of-potential method with | IEC 61557-5; |
Earth conductor protection | Mechanical protection where | IEC 60364-5-54 |
Labelling | All earth connections labelled | IEC 60364-5-54; |
All Leading Power generators are supplied with a dedicated main earthing terminal (MET) on the generator frame, clearly labelled with the earth symbol, sized and positioned for connection to the site earth electrode. The MET is bonded internally to the alternator frame, engine block, and all metallic enclosure components.
· Main earthing terminal (MET): M10 or M12 stainless steel bolt on generator base frame; clearly labelled; accessible without removing any panels
· Internal bonding: all metallic components are bonded to the MET at the factory -- alternator frame, engine mounting points, canopy structure, control panel enclosure, fuel tank
· Neutral earthing link: removable neutral-to-earth link in the generator terminal box -- can be installed (standalone operation) or removed (ATS-connected operation with separate neutral earth)
· Documentation: earthing connection diagram supplied in installation manual; MET location and specification shown on generator drawing
· Technical support: earthing design questions (electrode selection, neutral earthing decision for ATS applications) answered by Leading Power engineering team within 24 hours
· Earth resistance testing: we recommend customers arrange third-party earth resistance testing after electrode installation; we can provide referrals to qualified testing contractors in major markets
Leading Power is a CE-certified diesel generator manufacturer based in Fu'an, Fujian, China. Established in 2008. 5kW-3,000kW generator sets. All units supplied with main earthing terminal, internal bonding, and removable neutral-earth link. Complete earthing documentation in installation manual. 24-hour technical support.
