What can be defined as a separately derived system?

A separately derived system is best defined as a system that can operate independently. This concept is important in electrical systems as it specifically refers to a setup where the power supply is distinct and independent from the main utility grid. Such systems can include backup generators or transformers that provide localized power without relying on public utilities.

This definition highlights the critical aspect of independence, which allows a separately derived system to operate autonomously during a failure of the primary power source or in remote locations. The ability to function without connection to the main grid is essential for various applications, such as ensuring safety and providing continuity of power in critical infrastructure.

While the option mentioning a transformer that derives its power from another source does touch on the function of transformers, it does not encapsulate the idea of independence as clearly as the correct definition. The other options involve connections to the utility grid or a reliance on public utilities, which contradicts the essential characteristic of a separately derived system being able to operate independently of the utility infrastructure.