Rule 14 of 40 · Chapter II — The Body and the Current
Discharge stored energy first
Why this rule exists
Turning off the source does not empty the circuit. Capacitors in power supplies, motor drives, and lighting ballasts can hold a dangerous charge for minutes after power is removed, and large ones can deliver a lethal jolt long after the plug is pulled. Inductors, batteries, and long cable runs store energy too. Lockout keeps the source off; it does nothing to drain what is already inside. A reading taken too soon can even climb as a capacitor bleeds back up. You have to discharge stored energy deliberately, then verify.
In practice
After de-energizing and locking out, wait the manufacturer's specified discharge time before opening energy-storing equipment. Discharge large capacitors through a proper resistive discharge tool, never by shorting them with a screwdriver, which can weld the tip and hurl sparks. Verify the discharge with a meter at the terminals. Treat disconnected batteries and UPS units as live sources in their own right. For elevated loads, springs, and pressure, restrain or release that mechanical energy too. Only after everything is drained and verified do you proceed.
When it doesn't apply
High-energy capacitor banks and drive DC buses require specific discharge procedures and tools; guessing is dangerous. Some equipment has bleeder resistors that self-discharge, but do not trust them without verifying, since bleeders fail open. If you do not know the discharge procedure, that is your signal to stop and consult the manual or a qualified technician.