As you may know, employees are responsible for protecting the health and safety of their employees. Because of this, occupational health and safety regulations must be put in place, especially with electrical systems. When fixing a problem within an electrical system, power must be disconnected to the relevant part. This goes further than just disconnecting the main power, though. Reliable information is needed regarding the state of the installation before any work takes place, and measures to ensure against maloperation and malfunctions must also be taken. It is also important to protect against potential hazards from adjacent parts of the installation.
Electricians are familiar with these rules, and also understand that any precautions for safety that are taken are only as safe as the technologies being used.