In high impedence circuits, it is the voltage that remains after the main power is turned off. Use a grounding rod before working on the circuit to discharge high voltage capacitors.
Voltage at a point is defined as the work done by a unit positive charge carrying it from infinity to that point. while volage diffrance between two points is the work done bye unit positive charge...
In British standard voltage classified Low Voltage( up to 1000 v (1 kv) medium voltage 1001 v to 10000 v ( 1.01 kv to 10.99 kv) High voltage 11kv to 66 kv and Ehv 66.1kv and above.