The relay may show an earth-fault current, fail to pick up on a test, or trip with no downstream insulation fault. With four parallel 3-core cables, those symptoms can come from where the current is measured rather than from the relay setting. A core-balance current transformer (CBCT) around only one cable does not measure the zero-sequence current of the complete feeder.
What is the earth-fault indication telling you?
An earth-fault element operates from residual current: the vector sum of the currents in every current-carrying conductor supplying the protected load. For a three-phase circuit without a neutral, the measured quantity is IA + IB + IC. With no earth leakage or earth fault, that sum approaches zero.
A CBCT performs this summation magnetically. If 100 A flows toward a load and 97 A returns through the conductors inside its aperture, the CT measures a 3 A residual. Any protective conductor or intentional earth path must bypass the aperture so earth-return current cannot cancel the residual being detected.
Four parallel 3-core cables form one feeder, but current does not necessarily divide identically among the cables. Small impedance and routing differences can make the phase currents within one cable sum to a nonzero value even while the complete feeder sum remains near zero. During a fault, the contribution carried by that single cable is also not a dependable representation of total earth-fault current.
Check: Identify whether the displayed earth-fault value represents all four parallel cables or only one cable before changing pickup or delay settings.
Which conductors must pass through the CBCT?
For one CBCT to protect the complete feeder, pass all four 3-core cables through the same aperture. Every phase conductor must pass through in the same direction. If the circuit includes a current-carrying neutral, pass that neutral through the aperture as well.
| Conductor or connection | Required location | Effect |
|---|---|---|
| All phase conductors in all four cables | Inside one CBCT aperture | Measures the feeder's total zero-sequence current |
| Neutral, if present | Inside the same aperture | Allows normal neutral current to cancel correctly |
| Equipment grounding conductor | Outside the aperture | Prevents fault-return current from cancelling the fault indication |
| Cable sheath or screen earth connection | Arrange so the earth-return path does not pass back through the CBCT | Avoids cancelling part of the measured residual |
| CBCT secondary wiring | Short route to the relay; screened cable where noise is a concern | Reduces induced noise; ground the screen at the protector end |
A CBCT around only one of the four cables is not a complete zero-sequence protection scheme. Do not compensate for incomplete coverage by lowering the relay pickup; that trades an uncertain fault response for nuisance trips.
Check: Trace every current-carrying conductor and every sheath or grounding path through the CT area, then confirm that normal load current has a complete return path inside the aperture.
When should residual phase-CT connection replace the CBCT?
If one CBCT cannot physically accommodate the four large cables, connect the three feeder phase CTs in residual form and feed their summed secondary current to the earth-fault element. Each phase CT must measure the total current for its phase across the complete feeder. A phase CT that covers only one parallel conductor leaves the same coverage problem as a CBCT installed around one cable unless a protection design explicitly sums all matched CT contributions.
| Method | Primary advantage | Recurring limitation |
|---|---|---|
| One CBCT around every phase and neutral conductor | Only one CT is magnetized, giving better sensitivity to low residual current | The aperture may not accommodate four 3-core 300 mm² cables; sheath routing requires care |
| Residual connection of three phase CTs | Uses phase overcurrent CTs and avoids a large ring around the cable group | CT ratio, polarity, excitation, burden, and saturation errors can produce spill current or reduce sensitivity |
Residual connection is the practical configuration when the cable bundle cannot pass through a suitable CBCT and the phase CTs already measure the whole feeder. CBCT measurement remains preferable where low-current sensitivity is required and the installation geometry permits correct routing.
Check: Confirm from the primary conductor layout—not just the relay configuration—that each selected CT measures the entire protected phase.
How should the residual CT circuit be connected?
Wire the three phase-CT secondaries with correct polarity so their balanced-load currents cancel at the earth-fault relay input. A reversed CT or crossed secondary pair creates residual current proportional to load and commonly appears as an earth-fault indication that rises when feeder current rises.
The residual relay burden requires the phase CTs to develop secondary voltage. Some fault current is then used as CT excitation current rather than delivered to the relay, so measured residual current can be lower than the corresponding primary residual. Higher relay and lead burden increases this desensitizing effect. Electromechanical earth-fault relays can present higher impedance at low settings, making burden particularly relevant.
Unequal phase-CT saturation during a large through-current can also create false residual current even without a primary earth fault. Use CT ratio, polarity, burden capability, and excitation characteristics from the installed CT data rather than assuming three nominally similar CTs will behave identically.
- Confirm the polarity marks and secondary terminal assignment for all three CTs.
- Verify that the secondary residual connection matches the relay input diagram.
- Calculate the connected burden from relay input burden, lead resistance, and intermediate devices.
- Inspect secondary earthing and remove unintended parallel current paths.
- Compare the relay's residual indication at balanced load with the individual phase-current indications.
Check: Increase balanced load within normal operating limits and verify that residual current stays low rather than tracking phase current.
How should pickup and delay be commissioned?
Set pickup from the minimum earth-fault current that must be cleared, then check it against maximum standing residual current, CT error, harmonics, and coordination requirements. Conventional residual earth-fault settings are commonly in the range of 10% to 40% of full-load current. Sensitive earth-fault protection may reach 1% or lower when the neutral earthing arrangement limits fault current, but it normally favors a correctly installed CBCT.
Third-harmonic current is zero sequence and can appear as standing residual current. A pickup below that level can produce nuisance trips even when the CT wiring is correct. Record the residual magnitude and harmonic content during representative load conditions before selecting a very low threshold.
A two-stage arrangement may combine normal earth fault at about 10% pickup with an inverse-time curve and a much lower sensitive earth-fault element using a 6 s definite delay. These are application examples, not feeder settings; calculate actual values from the earthing system, minimum fault current, relay characteristics, and upstream/downstream coordination.
Check: Verify that maximum measured standing residual remains below pickup with operating margin and that calculated minimum earth-fault current remains above pickup.
What test proves the complete protection path?
- Use secondary injection to confirm relay pickup, timing, output contact operation, and the configured earth-fault element.
- Apply a primary residual-current test through the installed CT arrangement so the test covers conductor routing, CT polarity, ratio, secondary wiring, and relay measurement.
- Test each phase contribution and confirm that equal simulated phase currents cancel while an intentional residual produces the expected indication.
- Where operationally permitted, execute a functional trip test through the assigned output and confirm breaker opening, indication, and event recording.
- Restore the normal circuit, apply load, and confirm stable phase-current readings with residual current below pickup.
Check: Record primary test current, relay-measured residual, pickup, operating time, and breaker result; acceptance requires the end-to-end values to match the protection design.
Frequently Asked Questions
Why does a CBCT around one parallel cable nuisance-trip?
Unequal current sharing can make the three phase currents in that cable sum to a residual even when the total feeder current is balanced. Measure all four 3-core cables through one CBCT or use phase CTs that cover the complete feeder.
Why does a residual earth-fault connection have lower sensitivity?
Three phase CTs require excitation current, and relay plus lead burden raises the voltage they must produce. CT mismatch or saturation can further reduce genuine residual current or create false spill current.
Why does secondary injection not prove the complete scheme?
Secondary injection tests the relay and downstream trip logic but bypasses the primary conductors and CTs. Finish with primary residual-current injection through the installed measurement path and confirm breaker operation.