A procedural operation whereby all sources of energy are removed from a device undergoing modification, maintenance, or repair. The exact procedures will vary depending on the venue and the device. For more information, see publication NFPA 70E, Standard for Electrical Safety in the Workplace®, available from NFPA's website. See also http://www.osha.gov/SLTC/controlhazardousenergy/index.html. Even something as mundane and seemingly harmless as swapping a dimmer module (a very common troubleshooting procedure) now requires completely powering down the rack.