Fracktal printers come with inbuilt feature to avoid accidents by over heating of the nozzle or heater cartridge. Due to some issue with printer if the heater cartridge heats beyond certain temperature it can lead to melting of hot end and also fire hazards. So the inbuilt programming in the firmware of the printer has features which make sure that uncontrolled heating of heater cartridge does not happens.
How the heater cartridge will over heat to dangerous levels?
The temperature control in 3D printers is done using input from the thermistor. The thermistor keeps continuously sensing the temperature and will pass it to the control board. Sometimes when thermistor fails to give correct reading, it may show that the nozzle has not reached the required temperature and in turn the heater goes on supplying heat which raise the temperature to dangerous levels causing accidents.
How thermal runaway protection helps to avoid this?
As nicely explained by a stack exchange user named Oscar :
"Thermal runaway protection is basically self-explaining; it is protection against the temperature getting out of control. Essentially, the firmware checks whether the measured output of the thermistor (What is a thermistor? A thermistor is basically a temperature sensor; it is an electrical component (more specific: a resistor) that has a large reduction of its resistance when heated; it is frequently used for measurement and control as you can link the resistance to the temperature via a table or a curve) is within an expected range for a certain target value within a certain time frame when heating the hotend or the heated bed.
E.g. When you request the hotend or heated bed to a certain temperature, the heater elements are being scheduled/switched on to increase the temperature. If the temperature increase as a result of scheduling the hotend or heated bed are not met in time (settings in the firmware configuration), the printer will halt and heating of the heater elements will stop. The printer needs to be reset after such a failure.
What triggers TRP?
Common problems that trigger the thermal runaway protection are:
a faulty thermistor,
an incorrectly placed thermistor (e.g. not making good enough contact with the heater block),
including falling out
a loose heater cartridge,
including falling out
faulty connectors,
faulty or partially broken wires,
basically, anything that interrupts either heating or the measurement of the signal."
Other causes for thermal runaway:
In short thermal runaway is triggered when the temperature reading and projected temperature values mismatch. So sometimes the printer gives this error when there is connectivity issue or bad heater cartridge as well. If the cartridge is faulty the printer will not reach the required temperature and the controller board will give thermal error in such case.
Trouble shooting check list:
If thermal runaway error occurs these can be the reasons.
1. Faulty Mosfet in control board.
2. FFC damaged and is not giving temperature to controller board.
3. Thermistor damaged or loose connection
4. Heater cartridge damaged or loose connection.
5. Faulty PCB's.
Troubleshooting Thermal runaway error is similar to diagnosing min temp error. You can refer this article to see the list of check you need to do.