How would three-hour power cuts work if enacted in Great Britain?

People in England, Scotland and Wales are braced for the possibility of rolling power cuts this winter after a warning on Thursday from National Grid. The electricity and gas system operator has said households could face a series of three-hour power cuts if Vladimir Putin shuts off gas supplies from Russia, and Britain experiences a cold snap akin to 2018’s “beast from the east”.

Although National Grid has labelled the scenario “unlikely”, the emergency plan has prompted memories of persistent power outages in the 1970s and brought into focus the process by which people are cut off.

What is rota disconnection?

The system of rota disconnection, or rota load-shedding, is designed to equally share out the available power in a country or region through strategic shutdowns. In Great Britain, consumers in different parts of the country would be notified a day in advance of a three-hour block of time during which they would lose power. Households in different areas would then be cut off at different times or days, with the frequency rising depending on the severity of the supply shortage. The process is in legislation under the Electricity Supply Emergency Code.

Related: National Grid warns households could face three-hour power cuts this winter

How would it work?

There are 14 licensed areas of the country; within these, there are smaller areas on different circuits that have a timetable for cutoffs. The aim is to reduce power usage by about 5% through the three-hour disconnections. Consumers would typically be notified with a text message, similar to when there is a planned outage for maintenance work. An emergency public information campaign by National Grid and the government would be deployed across radio, billboards and social media platforms to urge people to use less energy. Liz Truss has so far resisted calls to ask people to use less energy.

What has to happen for cutoffs to be introduced?

The starting principle in the energy industry is to avoid disruption to consumers as long as it is safe to do so. With this in mind, the business department has spent months discussing usage with businesses, including large manufacturers, which use a lot of energy, about how to change their shift patterns away from times of peak demand. Households are now involved too, through a new “demand flexibility service”, which incentivises consumers not to turn on appliances when demand is high, such as during early evenings.

However, if demand is still stretched after this intervention, and standby coal plants have been fired up, then consumers may start to experience “brownouts”, in which the electrical voltage drops. For households, that means lights may flicker more. At that point, rota disconnection could be employed.

Why would the King be involved?

Arcane legislation shows that the process cannot be enacted until the monarch gives their approval. An order in council would need to be approved by King Charles, taken on the advice of his privy council, after a recommendation by the business secretary.

Who would be prioritised?

Businesses can apply for protected site status to avoid disconnection. Certain manufacturers, such as steel plants where coal-fired blastfurnaces run constantly, are already exempt if shutdowns for three-hour periods would cause “significant financial damage”. Other designated sites with exemptions include hospitals, oil refineries, gas terminals, electricity generators, water treatment plants, armed forces bases and telecoms sites. Transport would be supported too, with airports, railways and ports allowed to run smoothly.