An ECCS also may be used after a "partial" (incomplete) SCRAM to help bring a runaway reaction under control.
Each power plant has multiple independent ECCS systems, any one of which should be adequate to cool the core.
ECCS systems are nuclear safety-grade components.
ECCS systems can be powered by plant power (as long as the generator is on-line), offsite power, or the plant's Emergency Diesel Generators (another nuclear safety-grade system).
ECCS systems typically activate automatically upon occurrence of a LOCA (along with other automatic plant actions), to restore cooling as fast as possible so as to prevent a nuclear meltdown.