What condition do transmission circuit breakers respond to?

Transmission circuit breakers are specifically designed to respond to conditions in the electrical system that could indicate a fault or an excessive demand beyond safe operating levels. The option indicating "abnormally high amperage" is the correct choice because circuit breakers monitor the electrical current flowing through a circuit and are triggered to open (disconnect the circuit) when the current exceeds predefined thresholds. This situation typically signifies an overload, short circuit, or some other type of fault that could damage equipment or create safety hazards.

Understanding how circuit breakers function is crucial for maintaining the integrity and safety of electrical systems. While high voltage is a pressing concern in transmission systems, circuit breakers primarily react to overcurrent situations which can arise from excessive amperage. Low impedance and normal current are not conditions that would trigger a circuit breaker, as a low impedance would typically indicate a functioning path for current flow without an indication of fault, and normal current levels fall within the expected operational range that would not necessitate a response from the breaker.