Answer / carl_ellis

The fail-safe feature in temperature measurement uses some means to detect an open circuit in the sensor input wiring. An open circuit could be a broken wire, loose connection or a failed sensor.

Should an open circuit be detected, the output signal representing temperature will drive either full scale up usually to a value higher than 20.0mA or full scale down to a value lower than 4.0mA in order to flag, alarm or indicate a failed input circuit.

The failsafe feature and setup for upscale or downscale under fault conditions is sometimes referred to as "thermocouple break". In heating applications, upscale break is usually chosen because an indicated very high temperature on a controller's input will drive a controller's output down to 0% output (assuming reverse control action), preventing an overheat, fire or potential damage situation.

Some failures, such as a shorted thermocouple extension wire or sensor drift can not be detected in with open circuit detection.

Carl Ellis
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