What is the purpose of interlocks fitted to circuit breaker?

Interlocks fitted to breakers suggest that a load is supplied from two separate sources. An example would be where one breaker is supplied from a utility source and the other breaker will be supplied from a generator source. To prevent both breakers from being on at the same time to supply the load, an interlock is placed across both breakers. This allows only one breaker to be on at a time. Manual interlocks allow a person to make the decision as to which power source will supply the load. Automatic transfer switches have built in interlocks and will sense where the source power is coming from. This type of system switches from one electrical source to another without any human intervention.