Which animals cause electrical disruptions most commonly?

Electrical disruptions caused by squirrels are common and widespread, and can involve the disruption of power grids. It has been hypothesized that the threat to the internet, infrastructure and services posed by squirrels may exceed that posed by cyber-attacks.

Squirrels damage electrical distribution facilities by tunneling, by chewing through electrical insulation, or by simultaneously coming into contact with two conductors at different electrical potentials. Typically the animal is killed by the passage of current through its body. Prevention is complicated by the ability of squirrels to bypass plastic animal guards, gnaw through insulation and squeeze through small openings into substations.

In 1987 a 90-minute loss of power to Nasdaq's automated trading computer, caused by a squirrel, affected twenty million trades. Nasdaq was shut down for about 30 minutes again in 2014 by a squirrel-induced power outage.