Abstract: At 4:15 a.m. on November 20, 2018, Employees #1, #2, #3, and #4, electrical line workers employed by an electrical contractor specializing in high voltage lines, were working at an underground electrical vault. Employees #1, #2, and #3 were working inside the vault, and Employee #4 was outside the vault. They were completing the final phase of a Voltek Vault replacement that included changing out dead break elbows with load break elbows and replacing 3-way junctions with 4-way junctions along one of two 12-kilovolt circuits, the circuit on the vault's south side that was de-energized and grounded. Employee #1 was working on the north side of the vault, racking and zip-tying energized high voltage electric cables, when an arc flash occurred. All four employees were injured in the flash. Employees #3 and #4 were transported to one hospital, where they were treated and released. Employees #1 and #2 were transported to another hospital, where Employee #2 was treated and released. Employee #1, who was closest to the origin of the flash, was critically injured. Employee #1 was transferred to a burn center, where he was hospitalized for several weeks for treatment of second and third degree burns to his face and hands.