What is Safety Lockout?
A safety procedure which is used in industry and research settings to ensure that dangerous machines are properly shut off and not started up again prior to the completion of maintenance or servicing work. It requires that hazardous energy sources be "isolated and rendered inoperative" before any repair procedure is started. "Lock and tag" works in conjunction with a lock usually locking the device, or energy with the hasp, and placing in such position so hazardous energies can not be turned on. The procedure requires that a tag be affixed to the locked device indicating that it should not be turned on.
In the United States a lock selected by color, shape or size (e.g. red padlock) is used to designate a standard safety device, locking and securing hazardous energy. No two keys or locks should ever be the same. A person's lock and tag must not be removed by anyone other than the individual who installed the lock and tag unless removal is accomplished under the direction of the employer. Employer procedures and training for such removal must have been developed, documented and incorporated into the employer's energy control program.
-From Wikipedia
http://en.wikipedia.org/wiki/Lock_and_tag