Power failures and blackouts are not common in the UK, although they do still occur. The most common causes are cable theft and severe weather bringing down power lines. To avoid power outages, cables ring cities, from substation to substation, with current transformers monitoring power leaving and arriving at each substation. In normal operations the current should remain identical whilst the voltage will fluctuate. When an abnormal event happens, such as a tree falling on a power line, it will affect the power balance between substations that can be detected. If a cable falls and earths for example, the current will climb at the delivering substation, while the receiving substation will get nothing. In this scenario, the sub stations need to be switched out very quickly to isolate the line and avoid further damage to the network, and every fraction of a second counts.
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