One way to a fire is to cool the area around it. A Class fire involves flammable liquids, gases, and other chemicals. A Class fire involves live electric equipment. The two types of electric power sources are alternating current (AC) and direct current (DC). Of these two types, power sources cause more severe shocks. The three elements of the fire triangle are , , and . How often should the batteries in a smoke detector be changed? Electricity always follows the path of least . How often should a portable fire extinguisher undergo a complete maintenance check? Water and water-based solutions are best used to extinguish a Class fire. Poor lifting techniques are a frequent cause of injuries.