Which term describes operation at a substantially constant load for an indefinitely long period of time?

The term that describes operation at a substantially constant load for an indefinitely long period of time is "Duty, Continuous." In the context of electrical systems, continuous duty means that the equipment is designed to operate without interruption, managing heat dissipation and ensuring efficiency over time. This characteristic is essential in applications where reliability and stability of performance are critical, such as in industrial machinery or large electrical installations.

Continuous duty ratings consider factors such as thermal stability and the ability of the equipment to handle sustained loads without risk of overheating or mechanical failure. This term is distinctly contrasted with the other options, as they refer to different operational characteristics. For instance, short-time duty involves operation for a limited duration, intermittent duty entails cycling between active and inactive states, and varying duty suggests changes in load conditions over time, none of which fulfill the criteria of sustained, constant operation as outlined in the definition of continuous duty.