Protection
A temperature relay, ANSI device 49T, reads RTD sensors in motor windings, bearings or transformers and acts on set temperatures. Testing substitutes known resistances for each sensor and proves the readout, the alarm and trip levels and the failed-sensor response.
A device 49T relay measures actual temperature, usually through resistance temperature detectors, or RTDs, built into the equipment. In motors the sensors sit in the stator slots and in the bearings, and in transformers and generators they sit in windings, oil or cooling circuits. The relay passes a small current through each sensor, converts the resulting resistance into a temperature using the table for the sensor type, whether platinum, nickel or copper, and compares each reading with its own alarm and trip levels. Many units serve a dozen inputs or more, and some can group several inputs in a voting scheme. The field test does not heat anything. A precision resistance source, a decade box, stands in for each sensor, so the relay can be made to read any chosen temperature on every input.
Temperature is the quantity that actually damages insulation and bearings. Overload, blocked ventilation, high ambient conditions, a failing cooling fan or a bearing that is running dry will all raise it, and a current-based thermal model cannot see most of them. The 49T relay therefore adds the direct measurement, giving a warning before a trip and a trip before damage. Faults in the relay setup are easy to make and hard to notice: a sensor assigned to the wrong input type, a channel left unconnected, an alarm level entered in the wrong place, or a trip output that never reaches the breaker. Only a test that exercises each input and each output on purpose finds them, and it is usually done with the equipment out of service.
An RTD is a metal element whose resistance rises in a known, nearly linear way as its temperature rises. The relay measures that resistance, often with a three-wire connection that lets it cancel the resistance of the leads, and converts it to a temperature using the published relationship for the metal and the nominal resistance of the sensor. Because the sensor type fixes the table, a relay set for the wrong type reads a consistent but wrong temperature, which is why the type setting is checked as carefully as the levels. Alarm and trip comparators work on the converted temperature, with a small hysteresis and sometimes a delay so that a noisy reading does not chatter. A broken sensor reads as an open circuit, and a shorted lead reads as a very low resistance, so relays flag a failed input and can be set to ignore it or to act on it.
With the relay isolated and its outputs blocked, the technician records the identity, each input's assigned sensor type and location, and the alarm and trip levels for each. The sensor wiring is disconnected at the relay terminals, and a decade resistance box, with the lead arrangement the same as the sensor, is connected to one input at a time. Resistances corresponding to chosen temperatures are applied, taken from the published table for the sensor type, and the relay display or communication reading is compared with the temperature expected. The resistance is then raised through the alarm level and the trip level, and the point at which each output operates is noted, together with any delay. The reset is found by lowering it again. An open circuit and a short circuit are applied to confirm that the failed-sensor indication appears. The process is repeated for every input, and any voting logic is exercised. Finally the sensors are reconnected and the readout checked against ambient temperature.
The record lists each input tested with its sensor type and location, the substituted resistance and the temperature the relay displayed, together with the alarm and trip levels at which the outputs operated. It also records the failed-sensor response, any voting behavior, the resistance source and its calibration status, and the as-found and as-left settings. Adjustments made and the confirmation that each output reached its circuit are noted.
Results are compared with the published resistance-to-temperature relationship for the sensor and with the manufacturer's stated accuracy of the relay, and with the alarm and trip settings from the equipment study. The shapes of the criteria are a readout that agrees with the expected temperature on every input, a correct sensor type setting, alarm and trip outputs that operate at their set levels, a recognized failed-sensor condition and a final comparison of installed sensors with ambient temperature or with each other. Maintenance results are trended against earlier tests. 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 manufacturer's data.
The equipment classes and standard sections that call for this test, and the UltraDb forms that carry it.
It measures actual temperature through RTD sensors in motor windings, bearings or transformers, and gives an alarm and a trip at set levels. It protects against heating that current-based protection cannot see.
A decade resistance box replaces each sensor and supplies the resistance that corresponds to a chosen temperature. The relay then reads and acts as if the equipment were at that temperature.
Platinum, nickel and copper sensors have different resistance-temperature relationships. A relay set to the wrong type shows a steady but wrong temperature, so alarms and trips occur at the wrong real temperature.
A broken sensor looks like an open circuit and a shorted one like a very low resistance. The relay flags the input as failed and, depending on its settings, either ignores it or acts on it.
UltraDb references ASTM, ANSI, IEEE, NETA, NFPA, NEMA, FAA, TIA, and IEC standards descriptively, to identify the published methods its forms are built from. UltraDb and Verlecta are not affiliated with, endorsed by, certified by, or licensed by NETA, ASTM, ANSI, IEEE, NFPA, NEMA, FAA, TIA, IEC, or Megger. Standard names and section numbers are the property of their respective organizations. Always test to the edition your contract specifies. Where a standard is sold through the ANSI webstore, Verlecta may earn a commission on a purchase made through our link, at no extra cost to you. A commission never decides which standards we cite or how a form is built.