Undervoltage Relay (Device 27) Testing: Pickup, Timing and Trip Check

What it is

A device 27 undervoltage relay watches a voltage, usually taken from a potential transformer on a bus, feeder or motor, and operates when that voltage stays below its setting for long enough. It is found in switchgear and motor control centers, in automatic transfer and load-shedding schemes, on generator and utility interconnections, and anywhere a sag must be separated from normal operation. The relay may be electromechanical, with a disk or an armature that drops out as the coil voltage falls, or solid-state or microprocessor-based, where the setting lives in firmware. The field test applies a known voltage to the relay terminals from a calibrated source and observes the point and speed at which the output contact changes state, together with the identification and insulation checks that accompany every relay.

Why it is done

Low voltage is a quiet hazard. Motors that keep drawing power through a deep sag overheat, stall or fail to restart in an orderly way, and sensitive control equipment drops out unpredictably. An undervoltage element is the protection that sheds load, blocks a restart, starts a bus transfer or isolates a generator from a collapsing system. If its pickup has drifted, its time delay is wrong or its contact does not reach the trip circuit, the scheme either operates for harmless disturbances and drops a process, or stays silent during a real collapse. Because the element sits idle for years, only a deliberate test proves it will act, and the test also catches setting errors introduced during commissioning or a firmware change.

The principle

The element compares the applied voltage with an internal reference. In an electromechanical unit the coil torque falls as voltage falls until a spring overcomes it and the contact moves, so the relay operates on the way down and resets on the way back up. Reset happens at a higher voltage than operation, and the gap between the two is the dropout ratio, which keeps the relay from chattering near its setting. In a digital relay the sampled voltage is processed to a magnitude and compared with a threshold, and a timer then runs. The timing may be definite, a fixed delay once the threshold is crossed, or inverse, where a deeper sag trips faster. Many relays supervise the element with a block that prevents operation when voltage is absent altogether, so that a de-energized bus is not treated as a fault. Pickup, dropout and delay are therefore three separate behaviors, and each is verified on its own.

How it is performed

With the relay isolated from the live system and the trip outputs disabled or blocked, so that no breaker moves unintentionally, the technician identifies the unit and records its settings from the nameplate or the settings file. A three-phase test set, or a single-phase source for single-voltage elements, is wired to the voltage inputs, and the output contact is monitored with the set's timer input. Voltage is first applied at a healthy level, then lowered slowly in small steps until the contact changes state, which gives the pickup point. Voltage is then raised slowly until the contact resets, which gives the dropout point. For timing, the set steps the voltage from normal to a chosen fraction of the setting and measures the interval to contact operation, repeated at more than one level on an inverse characteristic. Where the scheme allows, the contact is then connected to the real trip or transfer circuit with the breaker in the test position to confirm that the output reaches its destination. Instrument accuracy, source stability and waveform purity all affect the result and are noted.

What is recorded

The record states the relay identity and setting group, the pickup and dropout voltages found, the timing results at each applied level, and whether any blocking or supervision feature was exercised. It names the test set and its calibration status, the ambient conditions where they matter, and the as-found and as-left state of the settings. It also notes whether the output contact was proven through to the trip or transfer circuit, and any adjustment made, so a later test can be compared with this one.

How results are judged

Results are compared with the setting prepared by the protection study, not with a universal figure, because the correct pickup depends on the system and the equipment protected. The shapes of criteria are a measured pickup that agrees with the setting inside the manufacturer's stated accuracy, a dropout that sits in the published relationship to pickup, a time delay that agrees with the setting or with the published curve, and a contact that operates cleanly without chatter. Maintenance results are also trended against the previous test and the acceptance baseline. 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 and the relay manufacturer's data.