The Personnel Safety System includes Run/Safe boxes which are located throughout Hall A, and approximately every 100 feet in the linac. A run/safe box has three positions: Safe, Operational, and Unsafe. When the hall is in Restricted Access, the run/safe box will be in the Safe position. While in this position, the PSS prevents delivery of beam to the hall. Before beam can be delivered, the hall must be swept to ensure that no one is left inside. During the Sweep, each run/safe box is moved to the Operational position in preparation for Beam Permit. After the sweep has been completed and the hall is placed in the RF Power Permit state, the run/safe box will show Unsafe. Each box has an emergency stop button. If you see the box in the Unsafe position, you are in danger of receiving high levels of ionizing radiation. Immediately press the emergency stop button, exit the hall, and call the Machine Control Center Crew Chief at extension 7050.