How the device that shuts the gas off when a flame goes out actually senses the flame.
A flame supervision device, sometimes called a flame failure device, is a burner safety control that cuts off gas after flame loss. It stops unburnt gas flowing for long, though it takes a short time to respond. CENWAT expects you to know, in principle, how the two common types sense the flame.
The common job is to prove the flame while firing and close the gas valve after flame loss, such as a blown-out pilot or a burner that fails to light. Check the allowed response time in the appliance instructions.
A thermocouple works on heat. Its tip sits in the flame, and the heated junction of two dissimilar metals generates a very small voltage. That voltage energises a coil that holds the gas valve open, so when the flame goes out the tip cools, the voltage falls away and the valve closes. This is why a thermocouple system has a short delay before it shuts off, as the tip has to cool.
Flame rectification works on electricity. A flame will conduct a small electric current and, because of the way it conducts, it turns an alternating signal into a mostly one-way (rectified) current. The control detects that rectified current as proof of a real flame, and reacts very quickly if it disappears.
For fault finding, the practical point is that each type has recognisable symptoms: a thermocouple system that will not stay lit, or a rectification system that locks out, each point you towards the sensing path, the electrode or the connections.
Test and replace flame supervision components to the manufacturer instructions for the specific appliance, and never defeat or bypass the device.