Most 3D printer firmware includes some sort of thermal runaway protection, a feature that monitors the heaters and shuts the printer down if it notices something gone wrong. You should always enable the thermal runaway protection in the firmware.

However, most printers use MOSFETs to switch power to the heating element. Unfortunately, when MOSFETs fail, they usually fail closed (i.e. conducting). This means that, even if the firmware detects something has gone wrong, it won’t be able to do anything about it. Solid State Relays (SSRs) can fail in the same way. This can be dangerous if you have a powerful high wattage (mains-powered) heated bed.

This device provides extra protection against such failure by monitoring the temperature independently and control the printer power with its own thermistor. It is primarily designed for monitoring the heated bed, especially those powered by mains voltage. It also can be used for monitoring other parts of the printer that heat is of concern, such as the hotend or the step motor drivers to prevent hardware damage or fire.

There are many sensors can be used for measuring the temperature. In the design, the same type of 100K 3950 NTC thermistor as that is commonly found on 3D printers is used. Therefore the device can be also used as a thermistor tester.”