What does noise in a power line indicate?

Voltage below normal levels for a sustained period describes a sag or undervoltage condition, which is a drop in the power signal amplitude, not an interference event.

Power line noise is defined as random or periodic electromagnetic interference superimposed onto the normal power waveform, caused by sources such as motors, fluorescent lighting, switching power supplies, or radio frequency interference. Unlike other power anomalies, noise does not significantly alter the RMS voltage level but distorts the clean sine wave. This distortion can cause data corruption or erratic behavior in sensitive computing and electronic equipment.

A momentary high voltage condition is classified as a spike or transient surge, a brief overvoltage event, which is distinct from noise superimposed on the line.

A prolonged loss of power is a blackout or power outage, the complete absence of voltage, which is entirely different from noise riding on an active power line.