Time Overcurrent Relay (Device 51) Testing: Curves, Pickup and Timing

What it is

Device 51 is the time overcurrent element, the workhorse of feeder, motor, transformer and generator backup protection. Unlike an instantaneous element, it delays its trip according to an inverse time-current characteristic: the larger the current, the shorter the wait. In an electromechanical relay it is an induction disk that turns against a drag magnet and closes a contact after travelling a distance set by the time dial, with a tap block that sets the pickup. In solid-state and digital relays the same behavior is produced by a mathematical curve, selected from a family of shapes such as moderately inverse, very inverse and extremely inverse, with a time multiplier and a pickup setting. The test verifies the pickup and the timing at chosen points on the curve, and confirms that the relay will coordinate with the devices upstream and downstream.

Why it is done

Time overcurrent protection is what lets a power system ride through harmless overloads and starting currents while still clearing sustained faults, and its value depends on timing that matches the coordination study. A relay that is too fast may trip for a transient that downstream devices should have cleared, dropping more load than necessary. One that is too slow may let a conductor or winding overheat past its thermal withstand, or let a fault outlast the clearing time that arc flash calculations assumed. Disk relays suffer from bearing friction, dirt, magnet drift and spring fatigue, and digital relays can carry the wrong curve family or multiplier. Only a timing test at real currents shows whether the relay will behave as the study expects.

The principle

An inverse-time element behaves like an integrator: the nearer the current is to the pickup, the more slowly the element accumulates toward operation, and the further above pickup it is, the faster it gets there. In the induction disk, the torque grows with the square of the current while the drag magnet opposes motion in proportion to speed, so the disk travels at a speed that rises steeply with current, and the time dial sets how far it must travel. In a digital relay, the firmware evaluates a published equation of time against the multiple of pickup and integrates toward a trip threshold, and many models also emulate the reset of a disk, with a slow decay of accumulated travel when the current falls. Near pickup the timing is long and sensitive, so tests are taken at several multiples of the pickup, usually a low, a medium and a high one, to show the shape of the curve and not a single point.

How it is performed

The relay is isolated and its current transformer secondaries are shorted before any lead is changed. A relay test set injects current into the relay secondary. The as-found tap, time dial or time multiplier, and curve selection are recorded. The pickup is found first by raising the current slowly until the disk starts to move in an electromechanical relay or until the pickup indicator operates in a digital one, and the dropout or reset is noted. Timing tests then apply a step of current at each chosen multiple of pickup, starting a timer when the current is applied and stopping it at contact closure, and the intervals are compared with the manufacturer's published curve for that dial position. On a disk relay the disk is allowed to return to its reset position between runs, since a partly traveled disk shortens the next time. Injection levels are kept within the continuous and short-time ratings of the coil. Phase elements and the ground element are tested separately, and the trip output is proven against the breaker trip circuit or a substitute indicator.

What is recorded

The record states the relay type, curve family, tap and time dial or multiplier, the pickup and reset found, and the operating time measured at each tested multiple of pickup, together with the injection method and the instrument used with its calibration status. It notes any mechanical observations on a disk relay, such as cleaning, drag magnet adjustment or bearing condition, and the as-found and as-left settings, so that later tests show how the timing has moved over the years.

How results are judged

Each measured time is compared with the value on the manufacturer's published curve for the setting in service, allowing the tolerance given for that relay, and with the time the coordination study expects. The test points should line up along the curve in a consistent way, since a relay that is right at one multiple and wrong at another points to a calibration problem and not to a simple setting error. Phase-to-phase comparison and comparison with earlier results on the same relay reveal drift. ANSI/NETA ATS-2025 and ANSI/NETA MTS-2023 cover the time overcurrent element 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.