A transformer changes a high-voltage supply into a low-voltage one, or vice versa:
Electricity is transferred from power stations to consumers through the wires and cables of the National Grid.
When a current flows through a wire some energy is lost as heat. The higher the current, the more heat is lost. To reduce these losses, the National Grid transmits electricity at a low current. This needs a high voltage.
Power stations produce electricity at 25,000 V. Electricity is sent through the National Grid cables at 400,000 V, 275,000 V and 132,000 V.
Step-up transformers are used at power stations to produce the very high voltages needed to transmit electricity through the National Grid power lines. These high voltages are too dangerous to use in the home, so step-down transformers are used locally to reduce the voltage to safe levels. The voltage of household electricity is about 230 V.