In short. Generator earthing answers two separate questions: how the exposed metalwork is bonded so a touch is safe, and how the alternator neutral is referenced so that an earth fault produces a current the protection can clear. The right arrangement depends on the utility neutral, the transformer, whether the transfer switch switches the neutral, and how many sets run in parallel. An earth pit with low resistance proves none of this on its own.
Two questions that are often confused
Equipment earthing, often called body earthing in India, bonds the generator frame, the canopy, the switchboard enclosure and every exposed conductive part to the protective earth so that no dangerous voltage can appear on them. System earthing, or neutral earthing, decides how the alternator star point is connected to earth and therefore how much current flows when a phase conductor touches earth, and along which path it returns. A site can have a textbook body earth and a neutral arrangement that leaves earth faults undetected, or a sound neutral and a canopy that is not bonded at all. IS 3043 covers both, and the broader principles are set out in earthing and neutral grounding.
Why the neutral path decides protection
When an insulation failure connects a phase to the frame of a motor, the fault current flows through the protective conductor back to the source neutral. Only if that loop has low enough impedance does the current reach the pickup of the breaker or earth-fault relay. The relationship is simple.
Ief = U0 / Zs
Ief is the earth-fault current in amperes, U0 the phase-to-neutral voltage, 230 V on a 415 V system, and Zs the total earth-fault loop impedance in ohms, including the alternator winding, the phase conductor, the protective conductor and every joint between them. Worked example (illustrative): a feeder to a remote pump has a loop impedance of 0.5 Ω, giving an earth-fault current of 460 A. If the feeder breaker is a 100 A unit with an instantaneous pickup at ten times, 1,000 A, the fault is not cleared instantaneously; it sits on the thermal curve while the pump casing is live. A sensitive earth-fault element, a larger protective conductor or a lower pickup fixes it. On generator supply this check matters more than on the grid because the alternator adds its own impedance to the loop, as protection coordination explains.
How the generator neutral is treated in practice
| Situation | Usual neutral arrangement | What it depends on |
|---|---|---|
| Single LV set as the only source | Star point solidly earthed at the set, separate from body earth | Two neutral pits and two body pits is common Indian practice |
| LV set behind a 3-pole ATS, utility neutral shared | Neutral continuous to the utility earth; generator star point not separately earthed, or earthed with care | Avoiding two earthed neutrals on one system |
| LV set behind a 4-pole ATS | Generator is a separately derived source with its own solidly earthed neutral | ATS must switch neutral overlapping or with proper sequencing |
| Several LV sets in parallel | One neutral earthed, or each through a neutral contactor controlled by the master, or a common earthing point on the bus | Preventing third-harmonic circulating current |
| MV generator plant | Low- or high-resistance earthing through a neutral grounding resistor | Stator damage limits and relay sensitivity |
Three-pole or four-pole transfer: the central decision
If the automatic transfer switch switches only the three phases, the neutral runs unbroken from the utility transformer star point, through the load, to the generator star point. The generator neutral then shares the utility’s earth. Earthing the generator star point as well creates two earthed neutrals, and part of the normal neutral current returns through the earth and protective conductors instead of the neutral. Earth-fault relays at either source see this as a fault and trip without cause, and the current flowing in bonding conductors creates a heating and corrosion problem.
If the ATS switches the neutral as well, the generator becomes a separately derived source. Its star point is solidly earthed, its earth-fault protection sees only its own system, and the two neutrals never meet. The price is a four-pole switch whose neutral contact never leaves the load neutral floating during transfer, and a generator earth electrode system still bonded to the site main earth. In our experience the four-pole arrangement is the cleaner answer wherever earth-fault protection exists at the generator; the three-pole arrangement is acceptable for small sets where the only earth-fault protection is downstream of the transfer point. The question is treated in depth under neutral protection and neutral earth architecture.
Paralleled generators and the circulating current problem
Alternator voltage waveforms are not perfect sine waves. They contain a third harmonic, and when two machines whose neutrals are both earthed run in parallel, that third-harmonic voltage difference drives a current around the loop formed by the two star points and the earth. It heats the windings and appears on the neutral CTs as a standing earth-fault current. The solutions are to earth only one neutral at a time through contactors controlled by the master controls, to earth all neutrals through a common point with the earth-fault CT upstream of the junction, or on MV plants to use a single earthing transformer on the bus. Which to choose depends on whether sets must be able to run alone, which they almost always must.
Electrodes, bonding and what resistance proves
The earth electrode is the last link, not the first. In Telangana’s dry laterite and rocky ground, a single pipe or plate electrode can read well above the few ohms wanted, and chemical or maintenance-free electrodes with backfill are commonly used to bring the value down. An earth tester reading proves the electrode resistance to remote earth; it does not prove that the generator body is bonded, the protective conductor continuous, or the fault loop low enough for protection to operate. Those need separate tests: bond continuity, loop impedance at the far end of each feeder, and a trip test of the earth-fault relay. Soil and electrode design is covered under earth electrode and soil resistivity engineering. Testing in the dry season gives the conservative figure; monsoon readings flatter the electrode.
Related photographs and videos
Common mistakes
- Earthing the generator star point behind a three-pole ATS on a system whose utility neutral is already earthed. Neutral current divides into the earth path and the earth-fault relays trip on load.
- Earthing every neutral in a paralleled plant. Third-harmonic current circulates between machines and the earth-fault protection is blinded.
- Accepting a low earth-pit reading as proof of safety. The loop impedance at the far end of the feeder is never measured and a fault there is never cleared.
- Running the generator body earth and neutral earth to the same electrode with no bonding to the main site earth. A fault raises the canopy to a dangerous potential relative to the building.
What to send us
Send the single-line diagram showing the utility transformer, the ATS or changeover and whether it is three-pole or four-pole, the generator rating and the number of sets, photographs of the existing earth pits and the neutral link at the generator, and the latest earth resistance readings with their dates. Tell us whether any earth-fault relay has tripped unexpectedly. Use the application engineering enquiry page.
Questions engineers ask
How many earth pits does a diesel generator need?
Indian practice, following IS 3043 and the CEA regulations, is two independent electrodes for the neutral and two for the body, with the two systems interconnected to the site main earth. The number is a convention for redundancy; what the installation must achieve is a loop impedance low enough for the protection to operate and a body bond that keeps touch voltage safe.
Should my ATS be three-pole or four-pole?
Four-pole where the generator has its own earth-fault protection, where the generator is far from the utility earth, or where the system includes UPS or sensitive loads that need a clean neutral reference. Three-pole is acceptable for a small set near the utility transformer where earth-fault protection exists only downstream of the transfer point and the generator neutral is not separately earthed.
Why does my earth-fault relay trip when two generators run together?
Almost certainly third-harmonic circulating current between the two earthed star points. It flows continuously when both neutrals are earthed and the relay reads it as a fault. The remedy is to earth one neutral at a time through controlled contactors or to move the earth-fault measurement upstream of a common neutral earthing point.
What earth resistance should a generator installation achieve?
Values of 1 Ω or lower are commonly specified for generator neutral earths in India, and below 5 Ω for small installations, but no single figure proves protection performance. The design value comes from the earthing study and the protection settings, and loop impedance at the loads is the test that confirms it.
Technical content reviewed by Srinivas Reddy, Managing Partner, Reddy Generators. Last updated 19 October 2026.