Abstract: At approximately 10:00 a.m. on December 13, 2000, Employee #1 broke his upper left arm at the ball joint when he missed grabbing a rung while climbing down an 18-ft scaffold ladder. Employee #1 fell approximately 15 ft to the concrete ground. The ladder was attached to a scaffold on top of a large accumulator located in the Acid Unit of a large crude oil refinery. Employee #1 was transported to Harbor General Medical Center and was hospitalized for five days. He underwent a reconstructive surgery on the ball joint, which included the installation of a Polaris Nail with a Screw to reattach the arm to the ball joint. The employer of Employee #1 contracted instrumentation maintenance from the refinery. The worksite consisted of large accumulators used to recover and store acid used in the Alkylation Process in a crude oil refinery. The accumulators were approximately 15 ft tall. Scaffolding was built to access Oxygen Probes used in the vessels. This job was routine and Employee #1 did this and similar work throughout the Refinery. Employee #1 was not being supervised prior to the accident. Negligence during a routine job assignment was the causal factor contributing to the accident.