Fires are categorized into different classes with a different extinguishers to fight each type of fire an untrained person is  expected to look at the fire, evaluate and then process to fight the fire at the time of crises this can case confusion wasting of time and we extremenly dangerous if a class A extinguisher which is a water  based is used on an electric fire it can lead to instant electrocution if a class A extinguisher is use on an oil / petrol based fire the oil will float on the water and the fire will instantly speed out if a classA and class B extinguisher is use on a class A it will have a no effects whatsover.