Thermocouple Burnout: Introduction
Thermocouples are among the most widely used temperature sensors in the process industry. You will find them in furnaces, boilers, reactors, kilns, heat exchangers, and almost every high-temperature application. They are simple, rugged, and suitable for harsh environments.
However, one common weakness of thermocouples is that they usually fail in an open-circuit condition. This failure is known as thermocouple burnout.
If burnout is not detected properly, it can lead to false temperature readings, unstable control loops, and even unwanted plant trips. That is why thermocouple burnout detection is an essential feature in temperature instruments, PLCs, and control systems.
This article explains thermocouple burnout detection in a clear and practical way, covering how it works, why it is needed, and how instruments respond when a thermocouple fails.
What Is Thermocouple Burnout?
A thermocouple consists of two dissimilar metals joined at one end, called the hot junction. When this junction is exposed to temperature, a small millivoltage is generated due to the Seebeck effect. The other ends are connected to the measuring instrument.
Thermocouple burnout occurs when:
The hot junction physically breaks
One of the thermocouple wires opens
The compensation cable becomes disconnected
In all these cases, the thermocouple becomes an open circuit, and no valid temperature signal reaches the instrument.
What Is Thermocouple Burnout Detection?
Thermocouple burnout detection is a design feature in temperature instruments that forces the output to a known, predictable state when the thermocouple circuit opens.
Instead of allowing random noise to dominate the input, the instrument deliberately drives the measurement either high or low to clearly indicate sensor failure.
This controlled behavior is called burnout mode.
Basic Principle of Burnout Detection Circuit
A simple burnout detection circuit includes:
A very high-value resistor (typically mega-ohms)
A small internal voltage source
A selectable switch for burnout direction
The resistor is connected so that it has negligible effect during normal operation, but becomes effective when the thermocouple opens.
Why Open Thermocouples Are Dangerous
Modern temperature instruments have very high input impedance. This means they draw almost no current from the thermocouple.
When a thermocouple circuit opens:
The instrument input becomes electrically “floating”
Nearby electrical noise couples into the input
Noise from motors, VFDs, heaters, and power cables appears as voltage
The instrument interprets this noise as temperature
As a result, the display may show:
Rapidly fluctuating temperature
Extremely high temperature
Random unstable readings
This false indication can mislead operators and control systems.
Typical Causes of Thermocouple Burnout
Thermocouple burnout does not happen randomly. It usually occurs due to practical field conditions.
Excessive process temperature
When the hot junction is exposed to temperatures beyond its rated limit, the thermocouple materials oxidize or melt, leading to an open circuit.Mechanical damage
Vibration, bending, or poor installation can cause the thermocouple wire to break over time.Compensation cable failure
If the extension or compensation cable is cut, loose, or damaged, the circuit becomes open even if the sensor is intact.Hot work near cables
Cutting, welding, grinding, or chipping near thermocouple cables can burn or damage insulation and conductors.Aging and corrosion
Long exposure to high temperature and corrosive environments weakens thermocouple materials, eventually causing burnout.
Consequences of Thermocouple Burnout
When burnout is not detected, serious operational issues can occur.
Erroneous temperature readings
Electrical noise may appear as false temperature, causing incorrect process decisions.Unstable process control
Temperature controllers may hunt or oscillate due to fluctuating inputs.Unwanted plant trips
Safety logic may trip equipment based on false high or low temperature signals.Safety risks
Operators may take wrong corrective actions based on incorrect temperature indication.
Because of these risks, burnout detection is not optional—it is essential.
High Burnout vs Low Burnout Selection
The choice between high and low burnout depends on process safety philosophy.
High burnout selection
Used when a high temperature reading is safer, such as shutting down heaters or furnaces.Low burnout selection
Used when a low temperature indication is safer, such as preventing cold-start damage.
The SPDT switch or software configuration allows engineers to select the safest response.
PLC and SCADA Integration
Modern PLCs continuously monitor thermocouple input values.
If the input suddenly goes beyond normal limits
Or jumps to a configured burnout value
The PLC recognizes it as thermocouple failure
The PLC then:
Generates a thermocouple burnout alarm
Displays fault status on SCADA
Allows maintenance teams to act quickly
This prevents blind operation and improves plant reliability.
What we learn today?
Thermocouple burnout is a common and unavoidable failure mode in industrial environments. What matters is how the control system reacts when it happens.
Thermocouple burnout detection ensures that when a sensor fails open, the instrument responds in a controlled, predictable, and safe manner. By forcing the signal high or low instead of allowing noise to dominate, burnout detection protects processes, equipment, and people.
For any serious temperature measurement application, burnout detection is not just a feature—it is a necessity.
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