The short answer
Restricted earth-fault protection (REF) is a unit protection scheme for earth faults inside a grounded winding zone. The zone is bounded by the three phase CTs at the winding terminals and a neutral CT between the winding neutral and earth or the neutral-grounding impedance. It is called “restricted” because it deliberately ignores faults outside these CT boundaries.
REF is primarily used to close the sensitivity gap left by transformer differential protection (87T). A fault close to the neutral of a grounded-wye winding can produce low terminal phase current even while high local current damages the faulted turns. REF measures the neutral fault-loop current directly and can operate faster and more sensitively than 87T or coordinated neutral overcurrent.
1. What REF protects—and what it does not
| Condition | Expected REF response | Reason |
|---|---|---|
| Earth fault between neutral CT and phase CTs | Operate | Fault is inside the restricted zone. |
| Feeder earth fault beyond the phase CTs | Restrain | Neutral current and phase residual current represent an external fault. |
| Neutral conductor fault between neutral point and neutral CT | Operate if inside the defined CT boundary | Physical CT location determines coverage. |
| Phase-to-phase or turn-to-turn fault without earth | No dependable REF operation | REF needs earth/neutral current; 87T and other functions provide coverage. |
| Fault on a delta winding with no measurable neutral path | Conventional REF not applicable | No accessible neutral-current boundary. |
| Core, tank or lead fault outside CT zone | Not covered by REF | Protection zones are physical, not conceptual. |
REF complements 87T, Buchholz/sudden-pressure protection, 50/51 and neutral backup. It does not replace them.
2. Why 87T can be insensitive near the neutral
Transformer differential must tolerate ratio and phase displacement, tap-changer mismatch, CT errors, magnetizing inrush, overexcitation and external-fault CT saturation. It therefore uses a minimum pickup and percentage restraint. For an earth fault close to the neutral end of a wye winding, only a small fraction of the winding voltage drives current from the terminal into the fault. Terminal current seen by 87T can remain below its operating characteristic.
The problem is especially important for low-impedance-grounded windings: the grounding impedance limits current and can make 87T insensitive to earth faults over a significant part of the winding. REF uses current in the neutral fault loop and is specifically intended to improve this coverage. Winding coverage is not safely represented by a universal “percentage of turns”; it must be calculated using transformer winding and grounding data.
3. Grounding determines whether REF is suitable
| Grounding method | REF application |
|---|---|
| Solidly grounded | Normally effective; particularly valuable for faults close to the neutral. |
| Low-resistance or low-impedance grounded | Strong application; verify pickup below minimum internal fault current and inside NGR thermal duty. |
| High-resistance grounded | Current may be below practical REF sensitivity; compare with 59N, neutral-displacement voltage and insulation monitoring. |
| Isolated or resonant grounded | Conventional neutral-current REF is normally unavailable unless a defined grounding device creates a measurable path. |
| Multiple or switchable neutral paths | Every path and switching state must be modelled; bypassing the neutral CT invalidates the scheme. |
Install the neutral CT so that every intended neutral-to-earth path passes through it. Check NGR bypasses, parallel transformer neutrals, cable screens, tank earths, neutral earthing switches and temporary maintenance grounds. The transformer tank earth normally must not carry winding neutral current around the CT.
4. Basic current relationship
With the project polarity convention, the phase residual current is:
The neutral CT measures IN. A simplified REF differential quantity is:
The sign is illustrative: one reversed CT changes subtraction into addition. Numerical REF may instead use a current-polarized directional comparison, percentage restraint or phase comparison. Use the selected relay’s reference arrows, not a generic diagram.
5. High-impedance REF
In high-impedance REF, the residual connection of the phase CTs and the neutral CT are paralleled with a high-resistance relay branch. For load and external earth faults, secondary current circulates among CT circuits. If one CT saturates, the high relay impedance forces most spill current through the saturated-CT resistance rather than through the relay.
A representative worst-case stability voltage is:
The lead factor k depends on the actual return path. Relay pickup is then selected above Vs using the manufacturer’s security factor. If an external stabilizing resistor is required:
The final design must check CT knee point/accuracy voltage, excitation current, winding resistance, installed lead resistance, relay current, MOV/Metrosil leakage, fault duration and insulation level. Traditional rules such as Vk ≥ 2Vs are valid only when required by the selected application guide.
| Advantages | Disadvantages |
|---|---|
| Simple, proven and fast | Normally requires matched CT ratios and compatible excitation curves. |
| Excellent external-fault stability when engineered correctly | Dedicated CT cores and controlled secondary wiring are strongly preferred. |
| High sensitivity can be achieved | Stabilizing resistor and nonlinear voltage limiter require calculation. |
| Limited dependence on complex algorithms | High secondary voltage creates insulation and personnel-safety requirements. |
| Economical for a fixed zone with suitable CTs | An open or shorted CT circuit can falsely operate, desensitize or disable the scheme. |
6. Worked high-impedance REF screening calculation
Consider a solidly grounded winding with maximum external earth-fault current 12 kA and 1200/1 phase and neutral CTs. Assume the worst CT secondary resistance is 3.5 Ω and round-trip lead resistance is 1.0 Ω.
Vs = 10 × (3.5 + 1.0) = 45 V
With an illustrative K = 1.5: Vpickup ≥ 67.5 V
If four CTs each draw 10 mA excitation current at 70 V, the relay draws 25 mA and the nonlinear resistor draws 5 mA, estimated primary sensitivity is:
This result must be compared with the minimum internal earth-fault current at several winding locations. If the grounding system limits current to 50 A, this scheme cannot meet the required sensitivity without redesign. If 70 V is above the CT knee point, excitation-curve calculation may be unreliable and complete-circuit testing is required.
7. Numerical low-impedance REF
A low-impedance relay receives the three phase currents and neutral current on separate inputs. It normalizes CT ratios and applies restrained differential or current-polarized directional logic. Different phase and neutral CT ratios are often allowed within relay limits, and external stabilizing resistors or varistors are normally unnecessary.
| Advantages | Disadvantages |
|---|---|
| Different phase and neutral CT ratios can often be compensated | Security depends on the relay algorithm, settings and CT transient performance. |
| Low secondary voltage and simpler external wiring | Requires a suitable neutral-current input and correct channel mapping. |
| Individual currents, oscillography and CT supervision are available | Wrong ratio, phase or polarity settings can create convincing false differential current. |
| Good retrofit flexibility | Firmware/configuration management becomes protection-critical. |
| Adjustable restraint, directional security and optional harmonic logic | Very sensitive settings can conflict with standing spill, neutral harmonics and CT error. |
Do not assume numerical ratio correction rescues inadequate CTs. Verify external-fault CT saturation using the relay-specific CT dimensioning method and, for critical applications, waveform playback.
8. High- versus low-impedance REF selection
| Decision factor | High-impedance REF | Low-impedance REF |
|---|---|---|
| CT ratios | Must normally match | May differ within compensation range. |
| CT cores | Dedicated matched cores preferred | Existing protection CTs may be usable after performance review. |
| External components | Stabilizing resistor and MOV/Metrosil may be needed | Normally none. |
| Secondary voltage | Potentially high | Low. |
| Diagnostics | Limited unless numerical high-Z relay used | Individual current records and supervision. |
| Retrofit | Difficult with mismatched CTs | Usually more flexible. |
| Main risk | CT circuit/resistor/voltage design error | Settings, mapping and algorithm/application error. |
Use high impedance when suitable dedicated CTs already exist and the owner can maintain the circuit. Use low impedance when ratios differ, retrofit flexibility and records are important, or the transformer relay already includes a proven REF function.
9. Settings workflow
- Freeze the transformer vector group, winding connections, grounding impedance and all switching states.
- Draw phase- and neutral-CT locations and the physical REF zone.
- Calculate maximum external earth fault for stability.
- Calculate minimum internal earth fault versus winding position for sensitivity.
- Obtain CT ratio, class, excitation curve, resistance and actual lead data.
- Select high- or low-impedance implementation and use the exact relay manual.
- Set pickup above maximum healthy spill, CT error and standing neutral current, but below the minimum in-zone fault with stated margin.
- Set restraint, directional angle, harmonic/high-security mode and delay only from relay-specific studies.
- Check total clearing time against NGR and transformer withstand.
- Document every assumption and sensitivity/security margin.
10. Trip and re-energization logic
A confirmed REF trip normally indicates an internal transformer winding/lead earth fault. Trip every source that can energize the transformer, initiate breaker failure, operate 86 lockout, block autoreclose and ATS, and capture disturbance records. Include tertiary sources and parallel LV infeed where applicable. Re-energization should require investigation and authorized reset.
Keep time-delayed 51N/51G neutral protection as backup and coordinate it with downstream earth-fault devices and NGR duty. REF itself should not be delayed for external coordination because it is a unit scheme.
11. Commissioning tests
- Verify primary CT ratio, polarity, phase identity and neutral-current direction.
- Confirm all neutral current passes through the neutral CT in every operating state.
- Measure CT and lead resistance; compare with calculations.
- For high-Z, test relay branch, stabilizing resistor, nonlinear resistor, insulation and calculated pickup voltage.
- Apply balanced through current and prove stability.
- Simulate an external earth fault and prove restraint/direction.
- Simulate internal faults at several current levels and angles.
- Test CT saturation/high-security behavior with suitable waveforms.
- Open and short selected CT circuits only through an approved, de-energized or safely shorted test arrangement; verify supervision.
- Trip the actual breakers and verify 86, breaker failure, alarms, records and ATS/autoreclose blocking.
12. Common application errors
- Specifying only “64” or “87N.”
- Applying REF to a winding without a measurable neutral path.
- Placing the neutral CT so an NGR or parallel earth conductor bypasses it.
- Using mismatched CT ratios in a high-Z circuit.
- Ignoring excitation current in the high-Z sensitivity calculation.
- Using room-temperature lead resistance instead of the installed hot-loop value.
- Copying a directional angle from another relay.
- Assuming REF detects phase-to-phase or interturn faults.
- Tripping only one side of a transformer with another source connected.
- Allowing automatic transfer or reclose after an internal REF trip.
Related guides
Engineering limitation
This guide explains a defensible engineering workflow; it is not a project setting calculation. Final protection functions, settings, wiring and trip logic must be based on the approved single-line diagram, short-circuit and coordination studies, equipment data, grid code, relay manual, and verified commissioning results. Changes require formal protection-management control.
References and further reading
- IEEE C37.91-2021 — practical transformer-protection guidance
- IEEE C37.110-2023 — CT application, saturation and distorted secondary current
- IEC 60255-187-1:2021 — differential-protection functional requirements and tests
- IEC 61869-2:2012 — requirements for inductive current transformers
- SEL — A Call to Action: Say Yes to REF Protection — REF purpose, sensitivity, logic and commissioning
- ABB RET615 Product Guide — high- and low-impedance REF implementation
- ABB RET630 Application Manual — numerically stabilized REF application
Standards must be applied using the edition required by the project, utility and local law. Standards summaries on public pages are not substitutes for the controlled documents.