Protection
A directional overcurrent relay, device 67, trips on high current only when it flows in the protected direction. Testing checks the direction decision, pickup and timing using injected current and a polarizing reference.
A directional overcurrent relay, ANSI device 67, is an overcurrent element that is supervised by a direction decision. It compares the phase of the operating current with a reference quantity, usually a voltage, and permits tripping only when the current flows in the direction the scheme designates as forward. The test confirms that this decision is made correctly and that the overcurrent element behind it picks up and times as intended. It is performed on electromechanical, solid-state and microprocessor relays that protect looped or parallel feeders, ring buses, sources that can feed a fault from both ends, and ground circuits where current can return through more than one path. The directional unit and the overcurrent unit may be separate elements in one case, or one function in a numerical relay, and each is exercised on its own as well as together.
Where a fault can be fed from more than one side, a plain overcurrent relay cannot tell a fault on its own feeder from a fault behind it, so it may trip a healthy circuit or fail to coordinate with its neighbors. The direction element is what makes selective clearing possible on loops and parallel lines. If that element has the wrong polarity, a reversed connection, a wrong angle setting or a failed polarizing source, the relay can trip for a reverse fault, dropping healthy load, or it can block for a fault it should clear, which leaves the fault to a slower backup. These errors are common after wiring changes and relay swaps, and stay hidden until a fault occurs. A test with a known direction exposes them while the circuit is out of service.
The relay needs the current to be judged and a reference whose phase does not change with fault direction. For phase elements it is normally a voltage that keeps a stable phase relationship with the fault current. For ground elements it may be a sequence quantity or neutral current. The direction element measures the angle between current and reference. Its characteristic is a half-plane around a design angle called the maximum torque angle, so current that falls on the forward side produces operating torque and current on the reverse side produces restraint. Because the output is weak near the boundary, many relays need a minimum polarizing voltage or memory. When the direction element permits, the overcurrent element runs its own characteristic, instantaneous, definite time or an inverse-time curve. Testing finds where the direction decision flips as the injected angle is rotated, then checks pickup and timing in the permitted direction.
The relay is isolated from its current and voltage transformers, from the trip circuits and from any breaker-failure or lockout paths, in line with the owner's safety plan, and the settings are read from the current setting sheet. The case is inspected and the relay identification noted. An insulation measurement between the relay circuits and the frame is taken at the test level the standard assigns, after the reading has settled. A relay test set then supplies both a current and a voltage, so that the phase angle between them can be controlled. With a healthy polarizing voltage applied, the current is stepped up to find pickup, and the angle is swept in each direction to find the points where the relay stops and starts operating, which locates the boundaries and the angle of maximum sensitivity. The tester confirms operation in the forward region and a block in the reverse region, and tests each phase and the ground element in turn, with the correct polarity for each. The overcurrent element is then timed from the set pickup at one or more multiples, with the direction permitted, and the contact is watched at the trip output. Where the relay uses memory voltage or a polarizing current from a separate transformer, those inputs are tested too. Everything is returned to service settings, and an end-to-end check confirms that the trip signal reaches the breaker.
The record covers the relay identification and settings in use, the current and voltage transformer ratios, the polarizing source and its connection, the measured boundaries of the direction decision and the angle of maximum sensitivity, the pickup values found, the operating times observed, the insulation reading, the instrument used, and the as-found and as-left condition. Any setting change made is noted with the reason, so that a later review can tell a relay that drifted from one that was reset.
Results are judged against the relay's own settings and the manufacturer's published tolerance and curve data, not against a single universal number. The direction decision should flip near the designed angle and keep the forward and reverse regions distinct. Pickup should repeat within the relay's stated accuracy, and each timed point should land on the curve for the selected time dial and curve family. Phases and elements are compared with one another, and a result that differs from earlier tests of the same relay is investigated even if it remains inside tolerance. The insulation reading is read against the table for the circuit voltage. The governing acceptance and maintenance frameworks, ANSI/NETA ATS-2025 and ANSI/NETA MTS-2023, name the directional overcurrent function among the protective relay tests; refer to your licensed copies of both, and to the manufacturer's data, for the values and criteria.
The equipment classes and standard sections that call for this test, and the UltraDb forms that carry it.
It is the phase angle between current and polarizing reference at which the direction element is most sensitive. The forward region is a half-plane built around that angle. Testing sweeps the injected angle to find where the relay starts and stops operating and confirms these points match the relay's settings and documentation.
Current alone cannot say which way a fault lies, because its phase shifts with the fault location. The voltage provides a reference with a stable phase, so the relay can compare the two. Without a healthy reference, many relays rely on memory or block operation.
Reversed current transformer polarity, swapped phases, a wrong polarizing connection and a wrong angle setting are the usual findings. Each can make the relay trip for a reverse fault or block for a forward one. Injecting a known direction at the relay terminals exposes them before the circuit is energized.
You can time the overcurrent element alone if the direction supervision is satisfied or disabled by test settings, but that proves only half the function. A complete test also confirms that the relay blocks in the reverse direction and operates in the forward one, with settings returned afterward.
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