Restricted earth fault protection, known by device numbers 64REF or 87N, is a scheme that detects a fault between a transformer winding and earth, especially near the neutral or star point where other protection struggles. It works by comparing the current flowing in the neutral connection against the residual sum of the three phase currents on the same winding. When the winding is healthy those two agree; when insulation breaks down to earth inside the protected zone they diverge, and the relay trips. The word restricted means the protection is confined to the zone between the phase CTs and the neutral CT, so it ignores faults elsewhere on the network.
Restricted Earth Fault (64REF) in one line: Restricted earth fault protection (64REF, also called 87N) is a differential scheme that compares the current in a transformer's neutral connection with the residual sum of its phase currents. If they do not balance, an earth fault exists inside the protected winding zone, so the relay trips - even for faults close to the neutral that percentage differential protection would miss.
Standard transformer differential protection, device 87T, is excellent at internal faults but loses sensitivity as a winding fault moves toward the neutral of a star-connected winding. Near the star point the fraction of the winding between the fault and neutral is small, so the fault drives only a modest voltage and the resulting differential current can be too low to reach the 87T pickup, especially when the neutral is earthed through an impedance that further limits fault current. That leaves a stretch of winding near the star point where an earth fault may not be cleared quickly by differential alone.
Restricted earth fault closes that gap because it looks specifically at earth current. It measures the current returning through the neutral-to-earth connection with a dedicated neutral CT and compares it to the residual current, the vector sum of the three phase CTs. During normal load and during faults not involving earth, no net current flows to earth through the winding, so the neutral current and the phase residual match and the scheme stays stable. The moment a phase conductor faults to the transformer's earthed frame or tank inside the zone, current returns through the neutral that has no matching contribution in the phase CTs, and the imbalance appears.
Because REF responds to earth current rather than the full through-fault current, it remains sensitive right up to the neutral, catching faults that leave the winding-to-earth insulation compromised even where 87T is effectively blind. It is normally applied as a complement to differential protection, not a replacement, giving a transformer both broad differential coverage and sharp earth-fault sensitivity near the star point.
The intelligence of REF is that it distinguishes a fault inside its protected zone from one outside using the direction of earth current, defined entirely by the CT connections. For an earth fault beyond the phase CTs - out on the feeder, downstream of the transformer - the fault current flows through the phase CTs and returns through the neutral CT in a way that keeps the two in balance from the relay's point of view, so REF correctly restrains and lets the appropriate feeder protection handle it. This is the through-fault stability that keeps the scheme from over-tripping.
For an earth fault inside the zone, between the phase CTs and the neutral CT, the returning earth current appears at the neutral CT with no matching current at the phase CTs, so the two no longer balance and the relay operates. The zone is thus precisely bounded by the CTs themselves, which is exactly why it is called restricted: it protects only what lies between those transformers and nothing beyond.
This boundary discipline gives REF the same virtue as any unit protection. It does not need to coordinate with downstream relays and can operate fast, because it never responds to anything outside its zone. Correct CT polarity and placement are therefore critical - a reversed neutral CT can turn an out-of-zone fault into an apparent in-zone one, causing the scheme to misjudge direction and either trip on external faults or fail to trip on internal ones.
REF is implemented in two classic ways. A high-impedance scheme parallels the phase CTs and the neutral CT and places a high-impedance voltage-operated relay across the junction, often with a metrosil or varistor to limit voltage. During through faults the CT secondaries circulate current among themselves and little appears across the relay; during an internal fault the imbalance forces current through the relay's high impedance, developing a voltage that operates it. This approach is inherently stable against CT saturation on external faults but demands that all CTs in the scheme have matched ratios and adequate knee-point voltage.
A low-impedance, or numerical biased, scheme instead digitizes each CT current separately and computes the differential and restraint quantities in the relay. It applies a bias characteristic so that error currents from CT mismatch and saturation are tolerated at high through-current while genuine internal faults still operate the element. Low-impedance REF is more flexible about CT matching, can share CTs with other functions, and is what most modern numerical transformer relays provide as an integrated 64REF element.
The practical trade-off is between simplicity and flexibility. High-impedance REF is time-proven, easy to reason about, and very secure against external-fault CT saturation, but rigid about CT selection and wiring. Low-impedance REF folds neatly into a multifunction relay, coexists with differential and overcurrent elements, and adapts through settings, at the cost of relying on the relay's algorithms to stay stable under saturation. Both aim at the same target: fast, sensitive detection of earth faults confined to the transformer winding zone.
Transformer differential (87T) compares currents on all windings to detect any internal fault, but it loses sensitivity for earth faults close to a star-point neutral where fault current is small. Restricted earth fault (64REF) looks only at earth current, comparing the neutral CT against the phase-CT residual, so it stays sensitive right up to the neutral. They are complementary: differential gives broad coverage, REF adds sharp earth-fault sensitivity near the star point.
It is restricted because the protection zone is confined to the winding between the phase CTs and the neutral CT. Faults outside that boundary keep the neutral and phase-residual currents in balance, so the scheme ignores them, while faults inside the boundary create an imbalance that trips it. The CTs define the exact limits of what the scheme protects.
High-impedance REF parallels the CTs into a voltage-operated relay across a high impedance, which is very secure against CT saturation but requires matched CTs and dedicated wiring. Low-impedance, or numerical biased, REF digitizes each CT separately and computes a differential with a bias characteristic, which tolerates CT mismatch and integrates into a multifunction relay. Both detect earth faults inside the winding zone; they differ in wiring and CT requirements.
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