Many serious accidents have happened when someone thought a machine or the power to it was safely off. “Lock-out tag-out” is a way to protect yourself and others by ensuring that machines remain completely, temporarily off. Without a lock-out tag-out system there is the possibility that a machine will unexpectedly start up, either because of stored energy which was not correctly released or through the actions of someone starting the process without realizing that it isn’t safe to do so.
The lock-out tag-out standard requires that hazardous energy sources be “isolated and rendered inoperative” before maintenance or servicing work can begin. These energy sources include electrical (either active current or stored as in a capacitor), pneumatic, hydraulic, mechanical, thermal, chemical, and the force of gravity. It is important to remember all of the energy sources must be “isolated and rendered inoperative.” Overlooking an energy source has proved fatal on several occasions.
via Lock-out Tag-out.