Generator Earthing (Grounding)

Incorrect earthing can cause residual current devices to trip unnecessarily when the supply changes over to the generator. A common example is a neutral–earth link made both in the main board and at the generator; part of the neutral current then returns through the protective conductor, and the device detects it as leakage.

Earthing errors lead either to nuisance tripping or to protection that fails to trip when it should. Correct earthing gives a clear answer to three questions:

1. How are the frame and metalwork earthed?
2. Where is the generator neutral connected to earth?
3. Does the ATS switch the neutral?

These decisions should be made in the electrical design, together with the facility’s earthing system.

Three separate jobs: protective, system, equipotential

• Protective earthing: Base frame, canopy, panel enclosures, metal fuel lines. These parts, which are not normally live, are connected to the protective conductor (PE). In an insulation fault, this limits the touch voltage.
• System (functional) earthing: The generator’s star point (neutral) is connected to earth. Fault current can return to the source, and the fuse, circuit breaker or residual current device trips.
• Equipotential bonding: Metal pipes, cable trays and structural elements are bonded together. No dangerous voltage difference can build up between them.

Earthing systems (IEC 60364)

System	Short definition	What to watch on the generator
TN-S	Neutral and protective conductor separate throughout	Neutral–earth link at a single point
TN-C-S	PEN in one section, then split into N and PE	N and PE must not be rejoined after the split point
TT	Source neutral earthed, exposed parts on a separate electrode	Residual current protection is essential
IT	Source isolated from earth or earthed through high impedance	Special applications, insulation monitoring required

Which system is used varies by country, distribution company and facility; for example, TN systems are common in some countries and TT systems in others (such as France). The generator connection must be designed to suit the existing system.

Neutral–earth link and ATS pole count

The example at the start is the most common mistake: making the neutral–earth link in the wrong place or at more than one point. Neutral current then also circulates through the protective conductor. Residual current devices trip for no apparent reason. Interference and corrosion come on top of that.

• 4-pole ATS (neutral switched): The generator behaves as a separate source. The neutral–earth link is usually made at the generator.
• 3-pole ATS (common neutral): The mains and the generator share the same neutral. The neutral–earth link is usually made at a single point only, in the main distribution board. The link at the generator is removed.

The choice depends on the grid’s earthing system, the residual current protection and local regulations. We cover the ATS side in our article what is an ATS.

Protective conductor cross-section

The simplified table used in IEC 60364-5-54 when the protective conductor is made of the same material as the phase conductor:

Phase conductor cross-section (S)	Minimum protective conductor cross-section
S ≤ 16 mm²	S
16 mm² < S ≤ 35 mm²	16 mm²
S > 35 mm²	S ÷ 2

This is a general approach. The exact cross-section is calculated from the short-circuit current and disconnection time, or determined according to the regulations in force.

Electrode and resistance

The electrode can be a driven rod, a buried strip or a foundation earth electrode. Earth resistance varies with soil type, moisture and season. A value measured in dry summer soil can be noticeably higher than one measured in winter.

The required resistance depends on the system. For example, in TT systems, according to IEC 60364-4-41:

RA × IΔn ≤ 50 V

RA is the sum of the electrode and protective conductor resistances; IΔn is the rated residual operating current of the residual current device. For a 300 mA device, RA ≤ 50 ÷ 0.3 ≈ 167 Ω. In practice, design values are often kept well below this. In TN systems, the fault loop impedance must be low enough to trip the protective device within the required time.

Special cases

• Construction sites and trailer-mounted sets: At every new location the electrode is driven again and measured. “We measured it on the last site” does not count. See construction site generator.
• Portable generators: Whether earthing is required depends on how the set is used; the manufacturer’s manual is the reference.
• Sets running in parallel: Circulating currents can occur between neutrals. Neutral earthing must be designed at system level.
• Generator room: An equipotential busbar is connected to all the metalwork in the room. See generator room and installation.

Measurement and periodic inspection

Earthing resistance is measured by authorised persons with a calibrated instrument, using standard methods such as the three-pole (fall-of-potential) method. Continuity and tightness of connections are also checked. Over time, vibration can loosen the earthing connection on the generator base frame. In many countries, including Türkiye, periodic inspection of workplace earthing installations is required by law; the interval and method are set by the relevant regulations. Earthing connections should also be checked during maintenance; see scheduled maintenance.

Checklist

• Is the facility’s earthing system (TN-S, TN-C-S, TT) known?
• Are the location of the neutral–earth link and the ATS pole count written into the design?
• Is the neutral–earth link made at one point only?
• Are the base frame, canopy and panel enclosures connected to the protective conductor?
• Has the protective conductor cross-section been calculated?
• Has the earthing resistance been measured and recorded?
• Does the residual current protection suit the system type?
• On portable sets, is a new measurement taken at every location?

This article is for general information. The earthing design and measurements must be carried out by authorised electrical engineers in accordance with the regulations in force.

Next step

The Terex Diesel team can work with your electrical designer on the generator’s neutral connection and the ATS arrangement. For product and connection information, contact us.