Electricity can seem like a miracle--flip a switch and you have light, heat or entertainment. But it can also lead to tragedy. Each year more than 300 people die in electrical-related fires and more than 1,000 are injured. Electricity causes fires by providing a heat source that causes oxygen and fuel to ignite.
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A short circuit occurs when a live wire touches a ground or common wire, energizing the circuit. If the circuit is fused correctly this will blow the fuse, cutting off power to the circuit. In cases where the circuit is not correctly fused, the electricity will continue to pass through the wires and they will overheat. If combustible material is near the overheated wires, the heat can be sufficient to start a fire.
Electrical wiring is rated as to its capacity for current. Most wiring is capable of carrying more current than it is rated for. This underestimation is done for safety reasons. However, sometimes a circuit is overloaded, which means more current is being drawn through the wires than it is capable of handling safely. If the heat can dissipate from the wires this doesn't create a problem. However, if the wires are encased in a closed space that contains the heat, and is located near combustible materials it can cause a fire.
If exposed wires make contact with water for a period of time it can accumulate salts, which conduct electricity even better than water itself does. This conduction can create heat buildup and ignite surrounding combustibles. This scenario can occur when electrical boxes get wet or damp.
An electrical spark can occur any time an electrical current is open or closed, as when a switch is turned off or on. A spark is usually fleeting and won't cause damage, unless it occurs in the presence of combustible gasses. In situations where this type of buildup of gasses is expected to occur, specialized switches and contacts should be installed to prevent the creation of spark.