In short. The neutral-earth design decides fault current, protection type, safety and whether sources interfere. The rule of thumb on a multi-source system is one neutral-earth connection at a time in the energised circuit, with the neutral switched or arranged so that earth-fault protection sees the correct current.
Earthing methods
| Method | Earth-fault current | Notes |
|---|---|---|
| Solid earth | High | Easy detection, high damage and arc energy |
| Resistance earth | Limited | Reduces damage, needs sensitive relays |
| Reactance earth | Limited | Less common at LV |
| Unearthed / high impedance | Very low | Continues running on first fault, needs fault location |
The multiple-source problem
If the utility transformer and the generator are both solidly earthed at their neutrals, and a three-pole ATS switches only the phases, the neutral conductors are tied together. Neutral current can then split between two earth paths. Effects include false earth-fault trips, stray currents in structures and unsafe touch voltages.
Solutions
- Four-pole switching: the ATS switches the neutral so only one source’s neutral-earth connection is live.
- Three-pole with solid neutral: the generator neutral is not earthed locally but uses the system earth point, with careful design. This suits some systems only.
- Parallel generators: earth one generator neutral at a time, or use neutral earthing through impedance, with a switching scheme.
See ATS engineering.
Neutral protection
- Neutral conductors can carry large currents from unbalanced or harmonic (triplen) loads, so they may need to be full-size, with protection.
- Neutral overcurrent protects against overheating in neutral conductors and generator neutrals.
- Harmonics increase neutral current and overheating. See harmonics.
Design steps
- Choose the earthing method from safety, availability and fault-current goals.
- Define the neutral-earth connection for each operating mode.
- Choose the pole count of transfer devices to match.
- Set earth-fault protection for the resulting current. See earth-fault protection.
- Document and label the arrangement.
Illustrative example
A site with a three-pole ATS and two neutral-earth links sees nuisance earth-fault trips whenever the load is unbalanced, because neutral current returns through both paths. Changing to a four-pole ATS, so that only one neutral link is in circuit at a time, removes the trips and restores correct earth-fault readings.
Common neutral arrangements
| Arrangement | Notes |
|---|---|
| Solidly earthed neutral | High earth-fault current, simple detection |
| Resistance earthed | Limits fault current, reduces damage, needs sensitive detection |
| Unearthed or high impedance | Low fault current, continuity of service, needs fault locating |
Generator neutral choices with an ATS
- Three-pole transfer switch, one common neutral-earth link: the neutral is solidly connected to the supply neutral; only one source-to-earth link exists.
- Four-pole transfer switch, switched neutral: each source has its own neutral-earth link; the switch isolates neutrals, preventing parallel paths.
Wrong choice causes circulating neutral current or unwanted earth-fault trips when the generator runs.
Symptoms of a wrong neutral design
- Earth-fault relays trip when the generator starts.
- Current flows in the earth conductor in normal operation.
- Voltage differences between neutral and earth.
Third-harmonic and neutral current
Triplen harmonics add in the neutral, which may need a full-size or oversize conductor. See harmonics.
Design documentation
Draw the neutral and earth path for every operating state: mains, generator, transfer and parallel. See neutral grounding and earth fault protection.
Common mistakes
- Two neutral-earth links in circuit.
- Undersized neutral for harmonic loads.
- Earthing arrangements not documented.
What to send us
Send the earthing diagram and sources. Send your application details. See also earthing and neutral grounding.
Questions engineers ask
Do I always need four-pole switching?
Not always, but it is common when both sources are earthed at their own neutrals.
What causes neutral overheating?
Unbalanced loads and triplen harmonics.
Why use resistance earthing?
To limit fault current and damage while still allowing detection.
Is an unearthed system safer?
It keeps supply on a first fault, but needs monitoring and fault location, and has its own risks.
This guide gives general principles. Earthing design must follow local regulations.