Restricted earth fault (REF) protection — why this gives more sensitive earth-fault detection than ordinary differential protection

The transformer differential protection (87T) guide covers percentage-differential protection, which compares the total current entering and leaving a transformer. This article covers an additional protection function focused specifically on earth faults: restricted earth fault (REF), referred to in practice by the ANSI code 64N (for a high-impedance voltage-operated relay) or 87N (for a current-differential implementation).

Why "restricted", and why separate from the phase differential

REF does not compare phase currents; instead it compares the sum of the three phase currents at the winding terminals (which is nearly zero under healthy conditions, because the three phase currents cancel each other) with the current flowing through that same winding's star-point earth connection. An earth fault inside the protected winding creates an imbalance between these two quantities; an earth fault outside that winding — for example, further out in the network — leaves both quantities balanced, and the relay does not operate. This strict limitation to the zone between the phase CTs and the star-point CT gives the protection its name: restricted earth fault.

Why REF is more sensitive than 87T for a fault near the star point

With phase differential protection (87T), an internal fault is seen via the change in phase current that the fault causes. For an earth fault close to the star point of a star-connected transformer winding, however, the effective voltage between that point and the star point is small — a large part of the winding sits between the fault and the line terminals — so the resulting fault current, as seen from the phase side, stays relatively low and can easily be missed by 87T protection, especially in the lower segment of the restraint characteristic. REF, however, measures directly the current flowing to earth via the star point, and is therefore significantly more sensitive to exactly this type of fault, regardless of where along the winding the fault occurs.

The high-impedance principle: a stabilizing resistor prevents unwanted operation

The most common REF implementation is the high-impedance principle: the phase CTs and the star-point CT are connected in parallel across a relay with a stabilizing resistor in series. During a heavy external fault, one of the CTs involved can saturate (as also covered in the guide on CT protection class and knee-point voltage), producing an apparent voltage across the relay circuit. The stabilizing resistor is sized so that this apparent voltage during an external fault stays below the relay's operating threshold, while a genuine internal fault — where the full fault current is driven through the relay circuit instead of via a healthy, low-impedance loop — does produce a voltage well above the threshold.

Why the CT knee-point voltage is critical here

For a stable high-impedance REF relay, the current transformers used must have a sufficiently high knee-point voltage relative to the set relay voltage — commonly the knee-point voltage of each CT involved is required to be at least twice the actual relay operating voltage, so that none of the CTs already saturates during an external fault before the stabilizing resistor can do its job. As covered in the guide on CT protection class and knee-point voltage, knee-point voltage is therefore relevant not only for differential protection in general, but is specifically a design requirement for a correctly functioning high-impedance REF scheme.

Note: alongside the high-impedance principle, a low-impedance, current-biased REF scheme also exists (functionally comparable to an 87T differential relay, but applied to the star-point current instead of the phase current); the choice between the two and the exact setting values follow from the system study and the relay manufacturer, not from a fixed standard value.

Why REF complements, rather than replaces, 87T protection

REF only protects the zone between its own CTs — usually one winding and its associated connection cable — and does not see phase-to-phase faults without earth involvement, nor faults outside its own zone. Phase differential protection (87T) protects the whole transformer against phase-to-phase faults and against most earth faults, but is less sensitive to an earth fault close to the star point. A complete protection concept for a power transformer therefore typically applies both, supplemented by the Buchholz relay (see the related guide) for slowly developing, low-energy faults that neither electrical protection detects fast enough.

Practical relevance

When assessing the protection concept of an earthed-star-point power transformer, it is important to recognise that REF serves a specific purpose — sensitive earth-fault detection within a bounded zone — and that the stability of a high-impedance REF scheme depends directly on the knee-point voltage of the CTs used relative to the relay setting; replacing a CT without reassessing this ratio can undermine the stability of the scheme.

Common mistakes

Assuming 87T differential protection detects an earth fault just as sensitively as REF — an earth fault close to the star point produces a low phase current that 87T protection can miss, while REF does detect it.
Replacing REF CTs without re-checking the knee-point voltage relative to the relay setting — a CT with too low a knee-point voltage undermines the stability of the high-impedance scheme during a heavy external fault.
Treating the stabilizing resistor as a fixed, universal component — its value is calculated for the specific CTs, cable length and relay setting of that scheme, and must be reassessed if any of those factors changes.
Thinking that REF protects the whole transformer — REF only protects the zone between its own CTs, not the transformer as a whole.