Which of the following is not considered to be electric system security data?

Electric system security data refers to information that is critical for ensuring the stability and reliability of the power system. This typically includes data on the operational status of various assets and facilities within the electric grid.

Transmission line status provides information about whether transmission lines are operational, helping operators understand the grid's current configuration and capability. Generator status informs operators about the availability and output of power plants, which is essential for maintaining balance in generation and load. The status of automatic voltage control facilities offers insights into voltage levels across the network, which is crucial for maintaining system reliability.

In contrast, Available Transfer Capability (ATC) refers to the maximum amount of electrical power that can be transferred from one area to another while maintaining reliability. Although ATC is vital for planning and operational purposes and is related to system reliability, it does not directly reflect the current operational status of system elements. As a result, ATC is not categorized as electric system security data, making it the correct answer in this context.