Protection
A phase sequence relay, ANSI device 47, watches three-phase voltage for reversed rotation, a lost phase or unbalance. Testing applies correct and reversed sequence and reduced single-phase voltage, and proves the relay output, its pickup and its delay.
A device 47 relay monitors the three-phase supply voltage and operates when the sequence is wrong or the voltages are unbalanced. Its job differs from that of an undervoltage relay, which cares about level, because this relay cares about the pattern of the three phases. Two quantities are commonly watched: the phase order, whether the voltages rise in the expected rotation, and the balance, whether the voltages are close to equal and spaced evenly. Many units are normally energized and drop out on a fault, so that loss of control power also gives a safe response. They are applied on motor control centers, elevators and cranes, pumps, compressors and generator or automatic transfer schemes. The field test uses a three-phase voltage source whose sequence and amplitudes can be controlled.
Wrong rotation, for example after maintenance or a utility reconnection, makes a motor run backward. A pump or compressor driven in reverse can be damaged within seconds, a fan may move air the wrong way, and a hoist can move the wrong direction with a load. A lost or heavily reduced phase causes single-phasing of three-phase motors, with overheating and failure, and in transfer schemes a source with a missing phase must be rejected. The relay is the supervision that blocks closing or trips the circuit. Since a sequence fault appears only after rewiring or an event, the element sits idle for years, and a mistake in how it was wired may go unnoticed until a test applies each condition on purpose.
Three balanced voltages are equal in size and spaced evenly around the circle, and the order in which they reach their peaks defines the rotation. Reverse the order of two phases and the rotation reverses. Analyzed as symmetrical components, a correct supply has only the positive sequence component, while reversed rotation shows up as a supply that is entirely negative sequence, and a lost or low phase produces a mixture of both. The relay uses this idea, either with a small network that discriminates between the two sequences or with digital calculation. Level thresholds and a delay can be added so that momentary disturbances do not drop the output. Since the quantity is a ratio between sequences and not a voltage magnitude, the test needs to vary both the sequence and the size of individual phases.
With the relay isolated from the live bus and its output connected only to a lamp, meter or timer, or its trip path blocked, the technician records the identity, the type of output and the settings for unbalance level and delay. A three-phase voltage test set is connected to the relay inputs, and the set is first brought up with the correct rotation and equal voltages, where the output should take its healthy state. The rotation is then reversed, and the output should change state in the set time. Returning to correct rotation, one phase is lowered slowly until the output changes, which gives the unbalance or phase-loss pickup, and the process is repeated for each phase in turn. A complete loss of one phase is also applied. The delay is timed by switching the set suddenly from a healthy to a faulted condition, and the reset behavior is observed when the condition is cleared. Where a relay is applied with a contactor or transfer control, the response of that circuit is observed as well.
The record gives the relay identity, the type of output and the settings under test. It lists the sequence conditions applied and the response of the output to each, the pickup found when each phase was reduced, the timed delay and the reset. It names the test set and its calibration status and gives the as-found and as-left settings. It also notes whether the response of the controlled circuit was confirmed and any adjustment made.
Results are compared with the settings issued for the installation and with the manufacturer's data, since acceptable unbalance depends on the load and the supply. The shapes of the criteria are an output that responds correctly to the right and wrong rotation, a pickup that agrees with the setting on every phase inside the manufacturer's stated accuracy, a delay that matches the setting, a reset that is stable and an output that acts on the controlled circuit as intended. Maintenance results are also compared with the previous test. The scope of relay testing is described in ANSI/NETA ATS-2025 for acceptance and ANSI/NETA MTS-2023 for maintenance. For tolerances and clause detail, consult your own licensed copy of those standards.
The equipment classes and standard sections that call for this test, and the UltraDb forms that carry it.
It checks that three-phase voltages arrive in the correct rotation and are reasonably balanced. If the order is reversed or a phase is lost, it blocks the circuit or trips, protecting motors and driven equipment.
An undervoltage relay responds to the voltage level. A phase sequence relay responds to the order and balance of the phases, so it can act even when every phase reads a normal level.
Holding the output energized while conditions are healthy means a loss of control power also gives the protective state. That fail-safe behavior is worth confirming during the test.
The test set swaps the order of two phases of the injected voltage, or is set to produce a negative sequence. The output should change state in its set time and return when correct rotation is restored.
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