An 87 device provides differential protection. On which of the following would it be least effective?

Differential protection is a method used to detect faults by comparing the current entering and exiting a piece of equipment. If the two currents differ beyond a predetermined threshold, this indicates an issue like an internal fault, prompting the protection system to trip the device.

In the case of feeders, differential protection is less effective because feeders typically carry current from a source to various loads. The nature of the feeder's configurations and the distribution of loads can lead to significant changes in current that are not indicative of faults. There are nominal current variations as different loads are switched on or off. This can create conditions where false trips might occur, or conversely, real faults may not be detected if the current difference falls within acceptable limits.

In contrast, differential protection is particularly effective for transformers, generators, and motors. For transformers and generators, the internal geometry and operational consistency allow for reliable fault detection. Likewise, with motors, differential protection can effectively identify failures like phase-to-phase or phase-to-ground faults due to uniformity in their operational parameters.