What is defined as two or more switch contacts connected in a circuit, or set of circuits, to control a load?

The correct choice is control string. In HVAC systems and electrical engineering, a control string refers to the arrangement of two or more switch contacts that work together to manage a particular load or set of loads within a circuit. This concept is integral to how electrical systems function, as it allows for the coordinated operation of multiple components, enhancing both efficiency and safety.

In practice, a control string can enable features such as multiple switches controlling a single light fixture or coordinating different systems within a heat pump, ensuring that they operate in harmony. This coordinated switching can optimize performance while also allowing for complex control schemes suited to specific operational needs.

The other options do not accurately describe the arrangement of switch contacts controlling a load. For instance, a load circuit pertains to the part of the circuit responsible for delivering power to a device or load, but does not inherently involve the organization of multiple switch contacts. Similarly, a power circuit refers to the electrical path designed to supply power, while a signal line generally conveys information rather than controlling circuits. Thus, control string is the most precise terminology to describe the function of two or more switch contacts connected in a circuit.