Which measure indicates poor voltage stability when operating beyond limits?

The measure that indicates poor voltage stability when operating beyond limits is voltage collapse. This phenomenon occurs when the voltage levels in an electric power system drop to a point that they can no longer maintain stability, often leading to widespread outages or system failure. Voltage stability is critical in ensuring that electrical systems can handle variations in load demand, generation, and other operational stresses without leading to drastic voltage drops.

When a system operates beyond its voltage stability limits, it can no longer sustain adequate voltage levels across the grid, resulting in voltage collapse. This typically happens when there is a sudden increase in load, a failure of voltage regulating equipment, or inadequate reactive power support in the system. Such situations highlight the need for robust voltages across the network to prevent collapse and disruptions.

In contrast, increased energy efficiency, enhanced system reliability, and decreased load demand do not reflect indicators of poor voltage stability. In fact, these measures typically contribute to improved system performance and stability by optimizing resource use, enhancing the reliability of services, and reducing the stress on the electrical system, respectively. Thus, voltage collapse is a specific failure outcome linked directly to inadequate voltage management under operating stress, making it the most accurate choice.