Which of the following is true regarding optical time domain reflectometers (OTDR)?

The assertion that OTDR measurements can help identify the possible cause and location of a fault on an active fiber is accurate because OTDRs are designed to send pulses of light into a fiber and then measure the time it takes for the light to reflect back. By analyzing the reflection patterns, technicians can determine the location of faults, splices, and connectors along the fiber. This capability is especially valuable in diagnosing issues such as breaks or excessive loss points, making it a critical tool for maintaining the integrity of fiber optic networks.

In the context of the other options: while OTDRs are often used with dark fibers to measure loss, they might not be effective in identifying issues on an active fiber in real-time, as they typically work on non-active connections. They can test long spans effectively, depending on their specifications, particularly those designed for longer distances, contradicting the notion that they are only useful for short spans. Furthermore, OTDRs often include built-in calibration features to enhance accuracy, reducing the need for manual calibration under many circumstances.