A unique study on fire safety in hard rock underground mines with focus on heat release rates of mining vehicles is presented. A literature inventory was conducted with respect to fires in underground hard rock mines, which revealed that the most common fire cause in underground mines was flammable liquid sprayed onto hot surface and the most common fire object was a vehicle. A major concern was the lack of documented fire experiments in mining vehicles and heat release rate curves. It also revealed the limited research carried out on fire safety and fire development on vehicles found in hard rock underground mines. In order to fill the gap of knowledge lack on heat release rates, fire experiments were carried out on wood cribs and wooden pallets in a model-scale tunnel with longitudinal ventilation where the distance between the fuel items were kept constant as well as varied. Different ignition criteria were applied in the ensuing calculations. It was found that the critical heat flux criterion generally showed very good agreement with the corresponding results of performed fire experiments but tended to have too short ignition times when the distance between the fuel items was increased. The ignition temperature criterion generally performed poorly compared with the measured results, but it was found that the accuracy improved considerably as the distance between the fuel items and the amount of energy accumulated on the fuel surface was increased. As a final approach, two full-scale fire experiments were carried out in an operative underground mine using a wheel loader and a drilling rig respectively. The resulting heat release rates of the experiments were compared with calculated overall heat release rates applying the different ignition criteria. It was found that the critical heat flux criterion resulted in ignition times very close to the observed ignition times. The ignition temperature criterion resulted in surface temperatures that never achieved the corresponding ignition temperatures. Some difficulties were experienced when calculating the heat release rate curve of the wheel loader, as it was difficult to accurately predict the mechanical failure of a significant part initiating the highly significant fire in the hydraulic oil. Additional heat terms were added to the heat balance, where the added flame radiation term was found to have a large impact on the output results while the heat loss terms were found to have very little effect.