The results of full-scale fire tests are used to determine optimum distributions of fire sensors for the protection of conveyor belt haulageways in underground mines. Empirical relationships are derived that relate the spacing of fire sensors to the rate of development of the fire hazard. For this application, a critical fire size is defined as that size of fire for which belt ignition and subsequent flame spread down the belt is assured. Detection and alarm, regardless of sensor type or sensing methodology, must be achieved at some point in time before the critical fire size is reached. Based upon this approach, it is found that the horizontal spacing for products-of-combustion sensors is in the range of 300 to 600 meters, while the required thermal sensor spacing is found to be 4 meters or less. Additional design criteria are discussed for product-of-combustion fire sensing systems.