Directional earth fault protection, device 67N, detects a fault to earth and also determines which direction that fault lies from the relay. A plain earth-fault relay, device 51N, measures the residual current - the leftover current when the three phases do not sum to zero - and trips when it is high enough, but it cannot tell an upstream fault from a downstream one. On parallel lines and ring circuits, where fault current can arrive from more than one path, that ambiguity causes mis-coordination. The 67N relay adds a direction decision by comparing the residual current against a zero-sequence reference, so it operates only for faults in the direction it is meant to protect.
Directional Earth Fault (67N) in one line: Directional earth fault protection (67N) detects an earth fault from the residual (zero-sequence) current and then decides whether the fault is forward or reverse by comparing that current against a zero-sequence voltage or current reference. This directional discrimination lets it coordinate on parallel and ring feeders, where a non-directional 51N element cannot tell an upstream fault from a downstream one.
In a healthy balanced three-phase circuit the three phase currents sum to zero, so there is no residual current. When a phase faults to earth, current returns through the earth path and the three phase currents no longer balance; their sum is the residual current, equal to three times the zero-sequence current. Earth-fault relays sense this residual, either from a dedicated core-balance CT that encircles all three conductors or by summing the outputs of the three phase CTs. A simple 51N element trips when the residual exceeds a threshold for a set time, which is enough on a plain radial feeder where fault current can only come from one direction.
The limitation appears the moment there is more than one source of fault current. On two parallel feeders sharing a common bus, an earth fault on one feeder is fed both from the source and, around the loop, through the healthy parallel feeder. A non-directional 51N on the healthy feeder sees residual current flowing through it toward the fault and cannot tell that this current is passing through on its way to a fault on the other feeder; it may trip and needlessly disconnect the healthy line. The same ambiguity plagues ring-main systems, where fault current can approach a relay from either side.
Direction resolves the ambiguity. If the relay knows not just that residual current is flowing but which way it is flowing relative to the line it protects, it can trip only for faults in its own protected direction - forward, down its line - and restrain for faults behind it or passing through it toward somewhere else. That single added piece of information, forward versus reverse, is what lets earth-fault protection coordinate correctly on parallel and ringed networks.
To decide direction, the relay needs a reference against which to compare the residual current - a stable quantity whose phase relationship to the fault current reveals which way the fault lies. This reference is called the polarizing quantity, and the process is called polarization. The most common choice is zero-sequence voltage, written 3V0, the residual voltage that appears during an earth fault. The relay compares the angle of the residual current against 3V0: for a forward fault the current has one phase relationship to the voltage, and for a reverse fault it is roughly opposite, so the relay can place the fault forward or reverse from that angle.
Zero-sequence voltage is obtained from a set of voltage transformers connected in a broken-delta arrangement or computed from wye-connected VTs, giving the relay the 3V0 it needs. Where suitable voltage is not available, an alternative is current polarization, using the current in a transformer or generator neutral-to-earth connection as the reference instead. The neutral current has a known direction during an earth fault, so comparing the line residual current against it yields the same forward-or-reverse decision without needing a residual voltage measurement.
There is also a specialized wattmetric method used on systems earthed through a high impedance or a resonant coil, where fault current is small and its phase angle relative to voltage is the key. Here the relay evaluates the real, in-phase component of the residual current with respect to 3V0 - effectively a zero-sequence power measurement - because in these systems the resistive component of the residual current is what reliably points to the faulted feeder. Choosing the right polarizing method depends on how the system neutral is earthed, which is why 67N settings are always considered alongside the earthing arrangement.
The payoff of directional earth-fault protection is selective coordination on networks that a non-directional scheme cannot handle. On parallel feeders, directional 67N relays are set to look down their own feeders, so only the relay on the truly faulted feeder sees a forward fault and trips, leaving the healthy parallel line in service. On a ring main, directional relays around the ring each look in a consistent direction, so a fault is cleared by opening the ring on just the two sides bounding the faulted section, isolating the fault while keeping the rest of the ring supplied. This selectivity is what makes meshed and looped distribution practical to protect.
The forward-reverse decision also underpins directional comparison and other coordination schemes, and it lets earth-fault grading be applied where simple time and current grading would break down. Because the relay restrains for reverse faults, it does not have to be graded against everything behind it, sharpening coordination and reducing the delays that a purely non-directional grading chain would impose. The result is faster, more targeted clearing of earth faults across complex feeder arrangements.
For operators watching a distribution network from a control room, the direction information the relay determines is valuable context, and it lives in the events the relay reports rather than in the platform. When a 67N element operates, it typically flags which direction the fault was and which feeder it isolated, and that event, sent over DNP3 or IEC 60870-5-104 to a cloud SCADA platform such as Merobix, tells operators not just that an earth fault occurred but where in the ring or parallel arrangement it was and which section was disconnected. Aggregating those directional earth-fault events across many feeders and sites lets engineers spot recurring earth-fault locations and confirm that coordination behaved as designed, turning individual relay decisions into a clear operational picture.
A 51N earth-fault element measures residual current and trips when it exceeds a threshold, but it cannot tell whether the fault is upstream or downstream of the relay. A 67N element adds a direction decision by comparing the residual current against a zero-sequence voltage or current reference, so it operates only for faults in the direction it protects. This lets 67N coordinate on parallel and ring feeders where a plain 51N would mis-operate.
A polarizing quantity is the stable reference the relay compares the residual fault current against to decide direction. The most common is zero-sequence voltage (3V0), the residual voltage that appears during an earth fault; the relay compares the current's angle to it to place the fault forward or reverse. Where suitable voltage is unavailable, the current in a transformer or generator neutral-to-earth connection can serve as the reference instead.
Wattmetric earth fault protection is used on systems earthed through a high resistance or a resonant (Petersen) coil, where earth-fault current is small and hard to detect by magnitude alone. It measures the real, in-phase component of the residual current relative to the zero-sequence voltage - effectively a zero-sequence power measurement - because that resistive component reliably points to the faulted feeder. The choice depends on how the system neutral is earthed.
This page references the protocol specifications published by the organizations below. Editions, product capabilities, and documentation change over time - confirm current requirements and specifications directly with the source.
Last reviewed: July 27, 2026. Merobix is not affiliated with, endorsed by, or sponsored by these organizations; their names are used only to identify the standards and products discussed.
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