Unwanted signal from a building's electrical wiring. It matters because a reading at the power-line frequency can come from the wiring rather than from the device's own pulse. The power line runs at 60 Hz in North America and at 50 Hz across most of Europe, Asia and Africa. See Deliberate Frequency.
Also known as: Mains Frequency, Mains Interference
A building's wiring runs at 50 or 60 Hz. A setting that lands on one of those values may be picking up the power line rather than producing a designed pulse, so it is flagged.
