A standby system automatically provides power to critical equipment if normal power is interrupted. Standby systems may be optional (Art. 702) or legally required (Art. 701). A glass plant, for example, would want a standby system so that molten glass doesn’t solidify in the equipment and thus destroy it. A chemical plant might be legally required to power systems that prevent an environmental release or a runaway reaction.

Emergency systems are legally required and classed as emergency by a governmental agency having jurisdiction. These systems automatically supply illumination and/or power essential for safety to human life.

Sometimes, an emergency system simply provides power for exit lighting and exit signs upon loss of the main power or in the case of fire. Its purpose isn’t to provide power for normal business operations, but rather to provide lighting and controls essential for egress to a safe area.