How to Test Burnout Detection on a Temperature Loop
Burnout detection is the temperature loop's answer to a dead sensor: when the element opens, the transmitter drives its output to a deliberate failure value instead of freezing on a plausible number. Configuring it is easy; proving it actually works, end to end through the alarms and interlocks, is the part that gets skipped. This procedure is for commissioning and for periodic proof testing on loops where a failed sensor has consequences.
Test Temperature Burnout Detection in one line: To test burnout detection, first confirm the configured fail direction is the safe direction for this loop, then, with the process in a safe state and the loop owner's authorization, disconnect one sensor lead to simulate an open element and verify the transmitter output goes to its configured failure value, the control system alarms, and any interlock acts as designed. Restore the sensor and confirm the reading recovers cleanly.
What You Need
Have the loop documentation stating the intended fail direction and failure value, the transmitter configuration tool, a milliamp meter, and - most importantly - authorization and a process state where forcing this loop to its failure value is safe. The background on why the feature exists and how transmitters detect an open element is in thermocouple burnout detection; the same concept applies to RTD inputs.
Confirm the Configured Direction Is the Safe Direction
Before testing the mechanism, challenge the choice. Upscale burnout makes a dead sensor look hot, which is right when high temperature drives the process toward shutdown, as on a fired heater where a hot-looking failure cuts fuel. Downscale is right where low readings trigger the protective response. The wrong direction is worse than none: a heater loop with downscale burnout responds to a dead sensor by adding heat. Verify the loop sheet, the transmitter setting, and any input-card setting all agree.
On smart transmitters, also note the failure current values in use. The widely adopted NAMUR NE 43 convention signals failure at 3.6 mA or below for downscale and 21 mA or above for upscale, keeping failure currents clearly outside the normal measuring range so the receiving system can distinguish a dead sensor from a merely extreme reading.
Simulate the Open Sensor and Watch the Whole Chain
With authorization in hand, lift one sensor conductor at the head or the marshalling terminals to create a genuine open circuit. Watch three places in sequence: the transmitter output should drive to the configured failure current within its detection time, the control system point should annunciate a sensor-failure or out-of-range alarm rather than displaying a believable temperature, and any interlock fed by the point should act, or be verified inhibited, exactly per the test plan.
Measure the actual output with the milliamp meter rather than trusting the display, and note what the historian recorded; a loop whose failure value is quietly clamped back into range by input-card scaling will look healthy at the transmitter and lie upstream. This is the single most common gap this test finds: the transmitter behaves, and the system above it hides the evidence.
Verifying the Result and Common Mistakes
Restore the conductor, confirm the reading returns to a sane process value with no lingering alarm latching that should have cleared, and record the as-found configuration, the measured failure current, and the observed system response. On loops with redundant sensors, repeat for each input, since burnout on the selected sensor and burnout on the backup may be handled differently by the selection logic.
Common mistakes: testing by unplugging the transmitter instead of opening the sensor circuit, which tests loop-power failure rather than sensor failure; forgetting that a shorted thermocouple does not trigger burnout at all because the circuit is still continuous, so it reads the temperature at the short instead - a failure mode burnout detection cannot catch; and leaving a test-inhibited interlock inhibited afterward. Confirm the general failure behavior of the loop matches what operators have been told it does, and update training if it does not - the practical role of the transmitter in all this is covered in what a temperature transmitter does.
Frequently Asked Questions
Does burnout detection catch a shorted thermocouple?
No. Burnout detection watches for an open circuit or, on some transmitters, an out-of-family resistance. A shorted thermocouple circuit remains electrically continuous and simply reports the temperature at the short point, which is often a plausible number. Shorts have to be caught by rate-of-change or deviation alarms, redundant sensor comparison, or diagnostic-capable transmitters, not by burnout.
What failure current should the output go to?
Whatever the loop documentation specifies, typically following NAMUR NE 43: at or below 3.6 mA for downscale failure, at or above 21 mA for upscale. The specific value matters less than two properties: it must sit clearly outside the normal 4-20 mA measuring band, and every system consuming the signal must interpret it as sensor failure rather than as a valid extreme temperature.
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