Systems installed to protect loads that could create hazards when stopped are known as _____.

The term "legally required standby systems" refers to systems that are specifically mandated by codes or regulations to ensure that critical loads remain powered during a power outage or other disruptions. These loads could create hazards if they were to stop functioning unexpectedly, such as emergency lighting, fire alarm systems, and life safety equipment. This classification emphasizes the importance of maintaining power to these systems to protect public safety and comply with legal standards.

In contrast, optional standby systems are not required by law but can be installed to provide backup power to non-essential systems, whereas emergency backup systems often refer more broadly to any system providing emergency power. Primary service systems typically address the main electrical service functioning under normal conditions and do not encompass backup requirements. Thus, the focus of legally required standby systems on safeguarding critical functions underlines their role in safety and compliance, making them the appropriate choice for systems that protect against potential hazards when normal power is lost.