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In case of a fire, before the firemen us...

In case of a fire, before the firemen use the water hoses, they shut off the main electrical supply for the area. Explin why they do this.

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The water used to extinguish fire, have lot of minerals and dissolved salts, which make it conducting. Before throwing water into the burning houses, the mains of electricity of the area is shut off because then the water may conduct huge current and firemen get electrocuted.
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