The national grid is the network of power lines (cables hung from pylons or buried underground), transformers and substations that links power stations to the places that use electricity. It lets electrical power be moved all over the country.
Electrical power is transferred from the power stations at a high voltage. The potential difference (voltage) of the main power lines can be as much as 400 000 V. Remember that this high voltage is applied across the lines, not along them.
High voltage is not safe or useful in a house. So in each locality the voltage is reduced to a lower value before it reaches homes, where the supply is about 230 V.
