What is a large single panel, frame, or assembly of panels containing switches and protective devices called?

The correct answer is a switchboard. A switchboard is a large assembly of panels that includes switches, protective devices, and monitoring equipment. It plays a crucial role in managing electrical power distribution, allowing for the control, monitoring, and protection of electrical circuits. Switchboards are designed to handle high voltage and high current, making them suitable for commercial and industrial applications where the distribution of electricity must be handled safely and effectively.

In contrast, a panelboard is generally a smaller version that contains circuit breakers or fuses for smaller loads and is typically found in residential or lighter commercial applications. A transformer is a device used to change the voltage of an electrical supply, and while a breaker refers to a device that interrupts the flow of electricity in a circuit to protect against overloads, it doesn't encompass the larger assembly functionality of a switchboard.