Always disconnect the power source before attempting maintenance on or servicing the machine.
Always turn the main disconnect switch to the OFF position before removing or servicing any circuit boards, switches, fuses, cables, and components within the Power and/or CNC control cabinets.
Post a warning on or near the disconnect switch telling others not to turn ON the power to the machine whenever working in an area away from the switch.
If it becomes necessary to make any tests or checks with live circuits, be extremely careful and follow all established safety practices. Remember, the line side of the disconnect switch may be hot even though the switch is in the OFF position.
Always wear the static control wrist strap when working inside the CNC control cabinet. Avoid flexing the printed circuit boards (PCBs, also called printed wiring boards or PWBs) whenever they are handled, or removed, or inserted in the machine. Hairline cracks in the printed circuit etching may result from rough handling, causing problems that are often intermittent and difficult to find. Avoid touching the components (devices) installed on a printed circuit board because they may become damaged by static
Source: IMTS Connected Factory (Community Member)