https://www.westernpower.co.uk/downloads/4093 https://www.westernpower.co.uk/downloads/4093 is a pdf for one of the local power distribution companies that incl

https://www.westernpower.co.uk/downloads/4093 https://www.westernpower.co.uk/downloads/4093 is a pdf for one of the local power distribution companies that includes an explanation. The short answer is that they have frequency-sensitive relays that automatically disconnect some or all of the feeders downstream of 132kV substations as frequency drops through various specified limits.