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The GFCI is a fast-acting circuit breaker which senses small imbalances in the circuit caused by current leakage to ground, and in a fraction of a second shuts off the electricity. However, the GFCI will not protect the employee from line-to-line contact hazards such as a person holding two "hot" wires or a hot and neutral wire in each hand. The GFCI does provide protection against the most common form of electrical shock hazard - the ground fault. It also provides protection against fires, overheating, and destruction of wire insulation. GFCI 's can be used successfully to reduce electrical hazards on construction sites. Tripping of GFCI 's, interruption of current flow, is sometimes caused by wet connectors and tools. It is good practice to limit exposure of connectors and tools to excessive moisture by using watertight or sealable connectors. Providing more GFCI 's on shorter circuits can prevent tripping caused by the cumulative leakage from several tools or by leakages from extremely long circuits. (Adapted from OSHA 3007 ; Ground-Fault Protection on Construction Sites-1987) . Fire Prevention During equipment operation, periodic vapor-concentration measurements should be taken with an explosimeter or combustimeter. If at any time the vapor concentrations exceed 20% of LEL, the Site Safety Officer, or designated field worker, should immediately shut down all operations. Only approved metal cans will be used to transport and store flammable liquids. All gasoline and diesel-driven engines requiring refueling must be shut down and allowed to cool before filling. Smoking is not allowed during any operations within the work area in which petroleum products or solvents in free- floating,- dissolved or vapor forms, or other flammable _ - ---liquids may be present. No open flame or spark is allowed in any area containing petroleum products, or other flammable liquids. 5 -27-