Only a few amps are needed to disrupt transformer operation, but over 100 amps have been measured in the grounding connections of transformers in affected areas. Most of Hydro Quebec's neighboring systems in the United States came close to experiencing the same sort of outage. The Hydro Quebec outage resulted from the linked malfunction of more than 15 discrete protective-system operations. Induced currents produced extreme heat that damaged insulation in transformers like this one. The Quebec outage did not, in this instance, cascade beyond the province's borders.
Extensive blackouts are the nightmare of the power industry.
Once power is interrupted in large metropolitan areas, diversity of electric use on the network is lost. When power is restored, all thermostatically controlled electric loads come back on simultaneously. Such a blackout is also likely to cause transient voltage stresses and permanent damage to network equipment such as high-voltage breakers, transformers, and generation plants, which makes them unavailable for restoring power. Hours or days may pass before power can be restored. This figure does not account for the potential disruption of critical services such as transportation, fire protection, and public security.