Abstract: On July 11, 2002, Employee #1, a pipe layer, and Employee #2, a laborer, were working in a trench manually digging dirt out of the trench in preparation of realigning a sewer main. A large excavator was clearing out a section of dirt, above another section of the sewer pipe. There was a concrete slab, approximately 8 ft by 20 ft bridged over the sewer pipeline. The concrete bridge was a barrier to protect underground utilities. The excavator was clearing dirt underneath the concrete bridge, when the bridge collapsed into an unshielded section of the sewer line below. The concrete bridge fell and struck a piece of PVC electrical conduit, severing the live 4000-Volt line inside the conduit. This electrical line shorted out and caused an explosion of a nearby underground electrical vault, which caused a plume of smoke that both employees inhaled. The inhalation of smoke by the employees caused both to become short of breath, disoriented, and nauseated. Both employees were hospitalized and treated for smoke inhalation.