Overvoltage Relay (Device 59) Testing: Pickup, Reset and Timing

What it is

Device 59 is the overvoltage relay, an element that watches the voltage delivered by voltage transformers and operates when it stays above a set level. It is applied on generators, buses, capacitor banks, motors and sensitive loads, where excess voltage endangers insulation and connected equipment. In electromechanical designs it is an induction disk or an attraction unit on a voltage coil. In solid-state and digital relays it is a measuring function, often with several stages: a low-set stage with a time delay that follows an inverse or definite-time characteristic, and a high-set stage that acts fast. The test confirms the pickup, the reset, the timing, the behavior of each stage and the output to the trip or alarm circuit, using the secondary voltage that the relay would see in service, phase-to-neutral or phase-to-phase as the relay connection requires.

Why it is done

Sustained overvoltage overstresses insulation, increases core saturation and heating in transformers and motors, shortens the life of lamps and electronics, and can damage capacitor banks. It follows a loss of load, a voltage regulator failure, switching surges or ferroresonance. The overvoltage relay is what turns such an event into a controlled trip. A pickup that drifted high lets the stress last longer than intended, one that drifted low causes nuisance trips on normal swings, and a wrong time delay can defeat coordination with other voltage protection. Settings entered incorrectly, a disconnected voltage input or an open output contact make the relay appear healthy while it protects nothing, which is why a real injection test is the only reliable proof.

The principle

A voltage element compares a measured magnitude with a setting. In the induction type, the torque on the disk grows with voltage, so the disk turns faster the higher the voltage rises above the setting, which produces an inverse time characteristic. In the attraction type, the coil pull overcomes a spring once the voltage is high enough. In digital relays, the input is sampled, filtered to the fundamental frequency and compared with the pickup, and a timer runs while the condition lasts and resets if the voltage falls back. The element also has a reset ratio, meaning it drops out at a level somewhat below its pickup so it does not chatter at the threshold. Since the element watches an upper limit, the test proceeds by raising the voltage from normal and noting where and how quickly the relay acts.

How it is performed

The relay is isolated from the voltage transformer secondaries so that the test set cannot back-feed a live bus, and the output contacts are blocked or the trip links opened so that testing does not open a breaker. A relay test set is connected to the voltage inputs in the connection the relay expects, with the phase sequence respected for three-phase relays. The as-found settings are recorded. Voltage is first applied at a normal level and then raised slowly until the output operates, noting the pickup, and then lowered to find the reset. To measure time delay, a step is applied from normal to a level chosen above pickup, and a timer runs from the step to the contact closure, with the same procedure repeated at other levels where the characteristic is inverse. Multi-stage relays are tested stage by stage, and the elements for each phase or phase pair are tested in turn. The trip links are then restored, the output is proven against the real trip or alarm circuit, and the settings are left as found unless the work order says otherwise.

What is recorded

The record states the relay type, the connection and stage tested, the pickup and reset voltages found, and the operating time measured at each level used for timing. It notes the voltage connection and phase sequence, the instrument and its calibration status, the state of the trip circuit and the as-found and as-left settings. Observations on mechanical condition, such as disk cleanliness and contact condition, are included for electromechanical units.

How results are judged

Pickup is compared with the setting from the protection study, allowing the tolerance published for that relay, and the reset is judged against the reset ratio the manufacturer gives. Timing is compared with the set delay or with the inverse curve published for the setting, at each level tested. Phase-to-phase comparison and comparison with earlier results on the same relay expose drift or a single failing element. ANSI/NETA ATS-2025 and ANSI/NETA MTS-2023 cover the overvoltage relay among the protective relay tests. For the tolerances and values, refer to your own licensed copy of those standards and to the relay instruction book.