For describing the voltage of power lines, cables and overhead wires.
For describing the voltage of any high voltage power line and cable. This can be high-voltage transmission line or even a overhead wire of train tracks, or on a street for trolley busses. Used for e.g. powerlines and railway electrification.
Any value should be in Volts without unit itself and without thousand note/delimiter. Value for a 15 kV wire is "15000". Not "15 kV", nor "15,000" or combinations of this.
The ITO Map electricity layer shows power-lines by voltage, allowing for quick analysis of the OSM data. The voltage levels shown are currently designed with the UK power grid in mind, but at a minimum the layer will still show whether a voltage=* is present or not.
How to get the voltage of a line/cable without touching it? Simple safety rule for transmission lines is this. The length of the isolator (separating wires from tower) is 1 meter per 100 000 Volt. 380 kV wires have isolators of ~4 meters long. Following this rule a 25 kV overhead wire use 0.25 m long isolators.
A high-voltage transmission line with a voltage of 110 kV would use the following keys and values.
An electrified railway track in Germany with overhead power supply would use these keys and values.
Nearly the same for a subway with separate rail for power supply.
A street with overhead wires for trolley busses.
A distribution substation, stepping-down voltages for use by houses.