When the Models AD2-P, AD2-PR and AD2-XHR are operating, a sample of air is drawn from the duct and passed through the sampling chamber by means of the input sampling tube. The air sample passes through the smoke detector mounted in the duct housing and is exhausted back into the duct through the outlet tube. This detector is equipped with cover removal switch (SW1) that instantly provides a trouble condition upon removal of the clear cover. For all testing and inspection with the cover removed, the cover removal switch (designated as SW1 on PCB) must be manually depressed to simulate normal operation. LED Indicator The AD2-P, AD2-PR and AD2-XHR contain an LED indicator (located on the smoke detector) capable of flashing either one of three distinct colors: green, yellow, or red. During each flash interval, the microprocessor-based detector checks the following: for smoke in its sensing chamber that its critical smoke sensing electronics are operating.