In short. Earth faults are the most common faults and can be small compared with phase faults, so phase overcurrent may not see them. Dedicated earth-fault protection measures current returning through earth, and its design depends on how the system is earthed.
How it is measured
| Scheme | How it senses | Notes |
|---|---|---|
| Residual connection | Sum of three phase CTs | Prone to false residual during high phase current if CTs saturate or mismatch |
| Core-balance CT | One CT around all conductors | Sensitive, good for feeders and motors |
| Neutral CT | CT in the neutral-earth link | Used at source neutral |
| Residual-current devices | Leakage detection at low levels | Personnel and equipment protection |
Earthing affects the current
- Solidly earthed: high earth-fault current, easy detection, high damage and arc energy.
- Resistance earthed: limited fault current, needs a sensitive relay.
- Unearthed or high impedance: very small first-fault current, alarm and locate rather than trip.
See neutral-earth architecture.
Settings
- Set the pickup above normal leakage and imbalance.
- Choose time delay to coordinate with downstream earth-fault devices.
- Confirm the minimum earth-fault current exceeds pickup with margin.
- Consider more than one stage: alarm and trip.
Generator and changeover issues
When a generator and utility share a solid neutral, neutral current can return by an unexpected path, causing false trips. Four-pole switching or a single earthing point avoids this. See ATS engineering. Generator stator earth faults need dedicated protection, often restricted earth fault. See restricted earth-fault.
Illustrative example
A 415 V site has a main breaker with ground-fault protection set at a few hundred amps and 0.3 second delay, and feeder devices set lower and faster. After adding a generator with its own neutral link, the main breaker started tripping on utility changeover because neutral current returned through both links. Opening one link, or switching the neutral at the ATS, restored correct operation.
Why earth-fault protection is separate
Earth-fault currents can be much smaller than phase-fault currents, particularly where neutral earthing is through resistance or on a generator. A phase overcurrent relay set above load current may not respond to an earth fault of modest magnitude, which can still cause fire, shock risk or damage. A dedicated earth-fault element is set with far greater sensitivity.
How the earth-fault current is detected
| Method | Principle |
|---|---|
| Residual connection | Sum of three phase CT currents; zero in a healthy system |
| Core-balance CT | One CT encircling all phase conductors; very sensitive |
| Neutral CT | Measures current in the neutral-to-earth connection |
Core-balance CTs give the best sensitivity but require cables to pass through correctly, with the cable screen earth lead passed back through to avoid false readings.
Relation to the earthing system
- Solid earthing gives high earth-fault current and fast tripping.
- Resistance earthing limits current, requiring sensitive relays.
- Unearthed or high-impedance systems need insulation monitoring and other schemes.
See neutral earth architecture.
Generator considerations
With a neutral switched or bonded differently between utility and generator, the earth-fault path changes. Verify that protection works in each source configuration, and avoid multiple neutral-earth bonds that cause circulating currents or blind spots.
Setting approach
- Choose pickup well below the minimum earth-fault current expected.
- Make it high enough to avoid nuisance trips from leakage and unbalance.
- Use time grading for selectivity between levels.
- Test with injection.
Common faults found
- CT wiring errors in the residual connection.
- Core-balance CT with screen earth not returned through the core.
- Double neutral-earth bond causing false readings.
Common mistakes
- Multiple neutral-earth links on parallel sources.
- Residual CT mismatch.
- No testing of the earth-fault path.
What to send us
Send the earthing arrangement and single-line diagram. Send your application details. See also earthing and neutral grounding.
Questions engineers ask
Why is a separate earth-fault relay needed?
Earth-fault current can be below phase pickup, so phase protection may not see it.
What causes nuisance earth-fault trips?
CT errors, leakage, multiple neutral links and harmonics.
Should earth faults always trip?
On high-impedance systems the first fault is often only alarmed. The earthing design decides.
How can it be tested?
Injection testing of the relay and checks of the CT circuits. See injection testing.
This guide gives general principles. Earth-fault design must suit your earthing system.