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Basic terminology

Basic terminology and definitions The alternating current used for example to power lighting and motor operations regularly changes strength and direction in a sinusoidal variation. The current strength grows from zero to a maximum value, then falls to zero, changes direction, grows to a maximum value in the opposite direction to then become zero again. The current has then completed a period. The period T is the time in seconds under which the current has gone through all its values. The frequency states the number of complete cycles per second.
When speaking about current or voltage it is usually the root mean square value that is meant. With a sinusoidal current the relation for the current's respective voltage's root mean square value is:
Voltage under 50V is called extra low voltage. Voltage under 1000V is called low voltage. Voltage over 1000V is called high voltage. Standard voltages at 50Hz are 230/400V and 400/690V.