An earth fault close to the neutral end of a star winding draws so little line current that ordinary differential protection may never see it. A small, sensitive and fast zone scheme around that winding catches it before the damage spreads through the core and tank.
Winding earth faults near the neutral point are weak on the line side but very destructive inside the tank. REF protection watches only the zone between the line CTs and the neutral CT, so it can be set sensitive and still stay stable for outside faults.
What Is Restricted Earth Fault Protection?
Restricted earth fault protection, ANSI code 64REF, is a unit protection that operates only for earth faults inside a defined zone, usually one star connected winding of a transformer or generator. It compares the residual current of the phase CTs with the current in the neutral CT, as a special case of the protective relays family called differential protection.
For any fault outside the zone, the two currents are equal and the relay sees nothing. For a fault on the winding, the currents no longer balance and the relay trips within a few cycles, independent of the IDMT overcurrent grading of the rest of the network.
The word restricted means the protected zone is bounded strictly by the CT locations. Anything beyond the CTs, such as a feeder cable fault, is simply a through fault for this scheme.
Why Differential Protection Misses Faults Near the Neutral
On a solidly earthed star winding, the voltage driving an earth fault is proportional to its distance from the neutral. A fault at 10 percent of the winding from the star point has only 10 percent of phase voltage behind it, and the transferred line current is small, which is the gap discussed in transformer differential protection 87T.
When the neutral is earthed through a resistor, fault current is limited to the resistor rating, sometimes only a few hundred amperes. The sizing logic in neutral grounding resistor sizing explains why such low currents are chosen and why sensitive earth fault detection becomes essential.
Electrical Engineering Materials notes that transformer differential protection usually starts with a setting of around 20 percent, while restricted earth fault protection uses a lower setting of about 5 to 10 percent. That extra sensitivity covers a much larger part of the winding.
Restricted earth fault protection does not see phase to phase faults at all, because such faults produce no residual current. It is always used together with differential or overcurrent protection, never alone.
How the 64REF Scheme Works Step by Step
The three line CTs are connected in parallel to produce residual current, and the neutral CT is connected across them with opposite polarity. Correct polarity is vital, so every CT must follow the rules in current transformer working principle.
On a delta winding, a zero sequence path does not exist, so the three line CTs alone form a balanced group. The winding connection is read from the transformer vector group before designing the scheme.
High Impedance vs Low Impedance Restricted Earth Fault
A voltage operated relay with a series stabilising resistor sits across the paralleled CTs. A saturated CT behaves like a low resistance, so spill current bypasses the relay.
A numerical relay reads each CT separately, calculates residual and neutral currents and applies a biased characteristic.
| Feature | High Impedance | Low Impedance |
|---|---|---|
| CT requirement | Identical ratio, Class PS, low Rct | Can differ, ratio matched in software |
| Extra hardware | Stabilising resistor and Metrosil | None |
| CT sharing | Needs dedicated cores | Can share with other functions |
| Stability method | High relay branch impedance | Bias or directional check |
Low impedance schemes are standard in modern numerical transformer relays and fit neatly into IEC 61850 substation automation projects. High impedance schemes remain popular in India for generators and large power transformers because they are simple and extremely stable.
Stabilising Voltage and Resistor Formula
The design assumes the worst case, where one line CT saturates fully during the maximum external earth fault. A saturated CT has no magnetising impedance, so the healthy CTs drive current through its winding resistance and the lead resistance.
Stabilising resistor: Rs = Vs ÷ Is
CT knee point: Vk ≥ 2 × Vs
If = maximum through fault current, CT secondary amperes
Rct = CT secondary winding resistance, RL = one way lead resistance
Example:
Through fault 6600 A, CT 600/1, so If = 11 A
Rct = 3 Ω, RL = 1.5 Ω, loop = 3 + 3 = 6 Ω
Vs = 11 × 6 = 66 V
Rs = 66 ÷ 0.1 = 660 Ω
Vk ≥ 2 × 66 = 132 V
Is is the relay current setting, often 0.1 A for a 1 A relay, and the small relay burden is ignored here. Real designs add a margin factor to Vs, as the Protecta guide recommends, and then pick the nearest higher resistor tap.
REF Stabilising Resistor Calculator
Second Worked Example: Primary Operating Current
A high impedance scheme draws magnetising current from every paralleled CT at the setting voltage, so the actual primary sensitivity is poorer than the relay setting alone suggests. The primary operating current is POC = CT ratio × (Is + n × Ie), where n is the number of CTs and Ie is the magnetising current of one CT at Vs.
With four CTs of 600/1, Is = 0.1 A and Ie = 0.02 A at 66 V, POC = 600 × (0.1 + 4 × 0.02) = 600 × 0.18 = 108 A. If the earthing resistor limits the fault to 300 A, the scheme covers faults down to about 36 percent of that value, which is a healthy margin.
Ask the CT maker for the magnetising curve and the Ie value at your calculated Vs, not only the knee point. A CT with high magnetising current can quietly spoil the sensitivity of a well designed scheme.
CT Requirements for Restricted Earth Fault Schemes
High impedance restricted earth fault protection needs all CTs of identical ratio, low secondary resistance and a knee point well above the stabilising voltage. In India these are specified as Class PS to IS 2705, and Electrical Engineering Materials also states that the neutral CT must be Class PS.
- All line and neutral CTs have the same ratio and Class PS rating.
- Knee point voltage is at least twice the calculated Vs.
- Rct and magnetising current at Vs are stated on the test report.
- Lead resistance is measured, not estimated, on the actual route.
- CT polarities are marked and checked with a battery flick test.
- CT star point is earthed at one place only, at the relay panel.
- Metrosil and resistor ratings match the calculated fault voltage.
An open CT secondary in this circuit can create dangerous voltage, especially on the neutral CT during a fault. Review CT secondary open circuit risks before any work on live panels.
Metrosil Voltage Limiter and Its Purpose
During an internal fault, the high impedance relay branch forces the CTs toward the full fault voltage, which can reach several kilovolts in peak. A Metrosil, a non linear resistor, is connected across the relay branch to clamp this voltage and protect the CT wiring and relay.
The Protecta guide applies a voltage limiter when the expected peak exceeds about 2 kV, and many relay makers use a similar threshold between 2 and 3 kV. It works much like a surge arrester, drawing little current at setting voltage and large current at high voltage.
5 Essential Steps to Set Restricted Earth Fault Protection
The first step relies on a proper fault study, as shown in short circuit fault current calculation. Use the transformer percentage impedance and source fault level together, not the transformer alone.
Restricted Earth Fault on Generators
On a generator, restricted earth fault protection covers the stator winding between the terminal CTs and the neutral CT, and works alongside stator earth fault and differential relays. The same careful CT work applies during generator synchronizing and commissioning checks.
Testing a Restricted Earth Fault Relay
Commissioning begins with CT ratio, polarity, magnetisation curve and loop resistance tests, followed by secondary injection of the relay and resistor. A primary injection through one line CT and the neutral CT then proves stability, while injection through the neutral CT alone proves operation.
Transformer energisation is the final practical test, because heavy transformer inrush current can saturate CTs unevenly. A stable scheme should also pass routine checks listed under transformer routine and type tests.
If a REF relay trips on energisation or through faults, measure lead resistance again and compare CT magnetising curves. Unequal CT cores or a wrongly earthed star point cause most false trips.
A healthy high impedance scheme reads almost zero spill voltage during normal load. A steady voltage of a few volts at full load often points to a reversed CT or a ratio mismatch.
- Detects earth faults very close to the neutral point.
- Very fast and fully stable for external faults.
- Simple and low cost high impedance hardware.
- Works alongside differential and backup overcurrent relays.
- Sees only earth faults, never phase faults.
- High impedance version needs dedicated matched CTs.
- Needs careful lead resistance and knee point data.
- Metrosil and resistor add panel space and heat.
Applications in Plants and Substations
High Impedance Differential Technical Guide
REF Protection Explained Video
Restricted Earth Fault FAQ
It is a unit protection that trips only for earth faults inside the zone bounded by the line CTs and the neutral current transformer. It compares residual current with neutral current and stays silent for outside faults.
It is used mainly on star windings of transformers and generators. Its low setting lets restricted earth fault protection detect faults close to the neutral point.
A fault near the neutral of a star winding produces very little line current. A differential relay with a setting of around 20 percent may not operate for such a fault.
REF compares currents on the same winding only, so it can be set around 5 to 10 percent. That higher sensitivity protects a much larger portion of the winding.
A high impedance scheme uses a voltage operated relay with a stabilising resistor across the paralleled current transformers. It needs identical Class PS cores but is extremely stable for external faults.
A low impedance scheme is a numerical function that reads each CT input separately. It allows different ratios and shared cores, with stability coming from a biased characteristic.
First find the stabilising voltage using the maximum through fault current times the CT resistance plus twice the one way lead resistance. Then divide that voltage by the relay current setting.
For example, 11 amperes secondary through a 6 ohm loop gives 66 volts. With a relay setting of 0.1 amperes, the stabilising resistor becomes 660 ohms.
A common rule is a knee point of at least twice the stabilising voltage. This ensures the CTs can drive enough voltage to operate the relay quickly for internal faults.
For a stabilising voltage of 66 volts, the minimum knee point is 132 volts. The magnetising current at that voltage must also be known to calculate the real sensitivity.
During an internal fault, the high impedance branch can push the CT voltage to several kilovolts in peak. That voltage can damage wiring insulation, terminals and the relay itself.
The Metrosil is a non linear resistor connected across the relay branch to clamp this voltage. It draws very little current at the setting voltage, so sensitivity is hardly affected.
Inrush current flows in the line terminals but produces no current in the neutral CT circuit. A correctly designed and connected scheme therefore remains stable during every energisation.
False trips on inrush usually point to unequal CT saturation or wrong polarity in the wiring. Check the lead resistance, the CT magnetising curves and the single earthing point of the star.
Related Articles
- Transformer Differential Protection 87T
- Current Transformer Working Principle
- Neutral Grounding Resistor Sizing
- CT Secondary Open Circuit Risks
- Protective Relays Explained
External References
- Technical Guide to the High Impedance Differential Protection, Protecta
- High Impedance Restricted Earth Fault Protection 64H, Electrical Engineering Materials
- Protective Relay, Wikipedia
What We Learn Today
- Restricted earth fault protection compares residual line current with neutral current, so it trips only for earth faults inside the protected winding zone.
- High impedance schemes need matched Class PS CTs, a stabilising resistor and often a Metrosil, while low impedance schemes run inside numerical relays.
- Stabilising voltage equals If × (Rct + 2RL), the resistor equals Vs ÷ Is, and each CT knee point should be at least twice Vs.