Which of the following best describes a 21 device?

A 21 device is best described as a Distance Relay. Distance relays are protective devices used in electrical power systems, particularly in transmission lines. They determine the distance to a fault based on the impedance of the line and operate to isolate the fault by measuring the voltage and current at their terminals.

Distance relays are particularly effective because they can differentiate between faults that are within their zone of protection and those that are not. This capability relies on the principle that the impedance measured decreases as distance to the fault decreases, allowing for precise operation based on the specific characteristics of the transmission line.

In contrast, the other types of relays corresponding to the incorrect options serve different functions; for example, power factor relays are concerned with maintaining the power factor within acceptable limits, time overcurrent relays focus on detecting excessive current levels over a specific time frame, and AC reclosing relays are used to close circuit breakers after a fault has cleared. Each of these devices serves unique protective roles in the electrical system, but they do not encompass the principal function of a 21 device.