What must the maximum continuous operating voltage (MCOV) of an SPD exceed?

The maximum continuous operating voltage (MCOV) of a surge protective device (SPD) is crucial for ensuring that the device can handle the typical voltage levels present in a power system without failing or being damaged. When selecting an SPD, it is important for the MCOV to exceed the upper tolerances of the utility power system. This ensures that the SPD can safely operate under normal conditions without being activated unnecessarily due to transients.

Exceeding the upper tolerance means that the SPD will not engage its protection capabilities for regular voltages, thus avoiding frequent interruptions or faults. If the MCOV is set too low, the SPD could activate too easily, resulting in frequent disruptions to the electrical supply and potentially damaging the connected devices.

In contrast, the other options focus on various ratings and standards that, while important in selecting an SPD, do not specifically address the essential requirement of sustaining voltage levels without activation. The lower tolerance, national safety standards, and the device's nominal rating all play roles in the overall compatibility and safety of an SPD installation, but they do not directly inform the necessary operational voltage handling represented by the MCOV.