Abstract: On December 18, 2003, Employee #1 was a noncontract employee working as a systems operator on the roof of Pacific Hall at the University of California, San Diego (UCSD) along with a maintenance worker and a mechanic. They were replacing the fan belts on fume hood exhaust fan EF4. A sheet metal guard had been removed and the electrical motor was locked out and tagged out. The fan pulley was blocked out with a piece of wood wedge between the pulley spokes. EF4 was connected to a common plenum that also fed exhaust fans EF1, EF2, and EF3, which were still running. Although EF4 was switched off, the air movement due to fans EF1, EF2, and EF3 created a negative pressure on Exhaust Fan 4. The negative pressure exerted on EF4 would make the fan and pulley system of EF4 rotate at high speeds if it were not blocked out, even with the electricity turned off. When the belts had been changed, the electrical shut-off switch was turned back on, and the wood wedge was removed from the pulley. The electrical power was not restored at the same time that the wood wedge was removed, though, so EF4's belt and pulley system started rotating in the reverse direction because of the vacuum created by the other operating fans. Employee #1 instinctively put his gloved hand on the pulley in attempt to stop the improper rotation. Employee #1's glove and hand were pulled in by the belt and pulley, and the tip of Employee #1's right thumb was amputated through the glove. Employee #1's two coworkers were within 3 to 5 feet of Employee #1 and witnessed the accident. Employee #1 was hospitalized. This accident could have been prevented had additional steps been followed. All the fans should have been turned off, locked out and tagged out. The sheet metal guard should have been replaced before the lockout/tagout was removed, and only then should all the fans have been re-energized. The employer did not have all fans turned off during the maintenance procedure. The pulley involved in this accident was the 18-in. diameter top pulley on EF4. An 0.625 in.-wide drive belt connects this pulley to a 5.25 in. bottom pulley. The top pulley is 82 inches above the ground. The fan is a Snyder General exhaust fan system, with serial number 4XA00382, driven by a 1760 rpm electric motor.