Global challenges Study Guide

The National Grid is the network of cables and wires that distributes electricity from power stations to homes and businesses across the country. Electrical power is generated at power stations, typically at voltages of around 25,000 V. To transmit this power efficiently over long distances, the voltage is significantly increased by step-up transformers, often to 400,000 V. This high voltage reduces the current for a given amount of power (Power = Voltage × Current), which in turn minimises energy losses due to heating in the transmission cables. The resistance of the cables causes some energy to be dissipated as heat (Power loss = Current² × Resistance), so by reducing the current, these losses are substantially decreased.

Once the high-voltage electricity reaches substations near populated areas, step-down transformers are used to reduce the voltage in stages. Initially, it might be reduced to 132,000 V, then further to 33,000 V or 11,000 V for local distribution networks. Finally, at residential substations, the voltage is stepped down to 230 V for safe and practical use in homes and offices. This whole process ensures that electricity is delivered safely and efficiently from the point of generation to the end user.

In a mains circuit, the live wire carries alternating potential difference from the supply, typically around 230 V in the UK. Even when a switch in a circuit is open, the appliance itself is isolated, but the live wire leading up to the switch remains at a high potential relative to the earth. This means that if a person touches the live wire, a potential difference exists across their body. If the person is also earthed (e.g., standing on the ground or touching an earthed object), a large current will flow through their body to the earth. This current can cause an electric shock, which can lead to serious injury or death by stopping the heart or causing severe burns. The purpose of a switch is to break the circuit to the appliance, preventing current from flowing through it, but it does not disconnect the live wire from the mains supply at the point before the switch, nor does it make the live wire harmless. Therefore, it is crucial to always assume that a live wire is dangerous, even if a switch is open. Any direct connection between the live wire and the earth, whether through a person or an unintended short circuit, creates a path for a very large current to flow, triggering safety devices like fuses or circuit breakers to cut off the supply, but not before a dangerous current may have occurred.