Before any maintenance begins on powered equipment, the machine must be isolated from its energy source and the switch locked in the off position. The technician performing the work holds the only key. This procedure exists because a machine that appears dead can still hold stored energy in springs, hydraulic lines, or capacitors, and a coworker who does not know the machine is being serviced may restore power without warning. Records show that most serious injuries in maintenance shops happen not while a machine is running normally, but during servicing, when a guard has been removed and someone assumes the equipment is safe.