The flame detector has an ultraviolet light sensor that is activated when it "sees" flame. This causes the detector to appear as a closed circuit when sensed by the PLC I/O.
There is a classic problem in the industry that tempts boiler operators to bypass (short circuit across the terminals) the flame detectors since a faulty reading can shut down an operating boiler. Modern flame detectors usually include diagnostic mechanisms to alarm on their own failure, and often to detect if they have been bypassed. Boiler safety systems also are usually programmed to detect true flame-out conditions by using the state from multiple flame detectors and other sensors, and to detect any bypassed flame detectors.
We used to do some oven work for a customer. They had thermocouples near
every flame detector. I don't recall what the setting was but when the
temperature was above a certain temperature the control system ignored that flame detector on the theory that it was so hot that there was no way the gas could not be burning and there was no reason to shut down on a false trip of the flame detector.
