What causes electrical blackouts?

A power outage - or blackouts to use a dramatic sounding American term - is when power stops flowing to end users such as businesses or residents. It can be hyper-local - as simple as a fuse blowing in a domestic distribution board that blocks the circuit and shuts off all the power to a single residence - or on a grid network level, across streets and towns. The cause is usually one of two things – an issue with the cables that carry power along the grid, or an issue with the generators responsible for generating electricity. 

An issue with a cable could be caused by numerous factors - It might have been damaged, perhaps it's been accidentally severed during proximate maintenance work (roadworks cutting underground cables is a particular risk). Alternatively, an aerial cable could short circuit if, for example, a tree falls on them. In these instances, the affected cable will need to repaired or replaced – and until then, no power can flow along that section of the network. In most cases there are multiple pathways and redundancies for power to flow along - it's rare that a single cable failure can cause more than localised disruption, but if the homes or businesses who depend on that electricity can’t get it via another path on the network, then they'll experience a blackout.

Issues with the machinery generating power are less common, but more serious and can take longer to fix. In practice, most power grids have multiple sources feeding the grid, meaning that if one generator fails, others can pick up the slack. However, this increases the load on the remaining machines – and if that load increases above a certain amount, then the other generators will automatically shut down to prevent damage. This may happen if demand for electricity is high – for instance, in hot weather when lots of people are running air conditioning units and fans – and can cause a cascade of failures across the power grid as generators trip in succession. This is referred to as a rolling blackout.

Blackouts can also be intentionally triggered by governments or energy companies. This may be due to the need to conserve energy when supply outstrips demand – such as during the 1970s in the UK when coal miners were engaged in industrial action –  or for public safety (for instance, in 2018 a series of gas explosions in Massachusetts led officials to switch off electricity to prevent additional explosions. 

Unplanned blackouts can never be fully prevented, though there are measures that can be taken to keep networks safe – a large part of running a Power Grid is centred around this. Regular testing of service-aged cables through Partial Discharge and Tan Delta testing can help identify weak spots and likely areas of failure to reduce the risk of wider damage to the network.