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Circuit breakers are designed to protect electrical circuits from overloads and faults by automatically interrupting the flow of electricity. They open when the current flowing through them exceeds their rated capacity, which prevents overheating and potential damage to the wiring and connected devices. This safety feature is vital for preventing electrical fires and equipment failure. The breaker's internal mechanism detects the excess current, which triggers the opening of the circuit, stopping the flow of electricity.

In scenarios where the circuit might be in standby mode, the breaker typically maintains its closed position, as no significant current is flowing. If the breaker can no longer operate, it might result from aging or failure, but it isn't a direct condition for automatic operation. When power is restored after an outage, the circuit breaker does not necessarily open; it may close automatically or require manual intervention, depending on the design and specifications of the system. Thus, the condition under which circuit breakers open primarily revolves around the need to protect the circuit from current levels that exceed safe operating limits, making the answer regarding current exceeding the breaker's capacity the most accurate.