Fire combustion requires oxygen, heat, and a combustible material to reach ignition temperature. Fires are classified into different types based on the material burning: Class A for ordinary materials, Class B for oils, Class C for gases, Class D for metals, and Class E for electrical fires. Different fire extinguishers are used depending on the class of fire, such as soda acid for Class A fires, foam for Class B, and dry chemical or carbon dioxide for Class C, D, and E fires. An early warning fire system includes smoke detectors, heat detectors, and manual call points connected to a main panel.