What is a unit of an electrical system that carries or controls electric energy as its principal function?

The term "device" in the context of electrical systems refers to any unit that carries or controls electric energy as its primary function. Devices can include various electrical components such as switches, relays, circuit breakers, and transformers. These units are essential for managing the flow of electricity in a circuit, making them integral to the safe and efficient operation of electrical systems. Devices can perform a variety of functions, from merely carrying electricity to actively controlling it, which aligns perfectly with the definition provided in the question.

In contrast, other terms such as conduit body, fitting, and hickey refer to components that serve different purposes within an electrical system. A conduit body is used to facilitate the routing of electrical wiring, a fitting connects wiring or pipes, and a hickey typically refers to a specific type of bend in a conduit, but none of these primarily function to carry or control electric energy like a device does. Understanding these distinctions helps clarify the role of devices in electrical systems and why they are categorized as such.