In case of a fire, before the firemen use the water hoses, they shut off the main electrical supply for the area. Explain why they do this.
Water may conduct electricity. If the electrical supply for the area is not shut off and water is poured over electrical appliances, then electricity may pass through water and harm the firemen. That is why, in case of a fire, the firemen shut off the main electrical supply for the area before they use the water hoses.