If a protective relay is designed to de-energize a faulted circuit in less than one minute, a ____ percent insulation level power cable is required.

The requirement for a 100 percent insulation level power cable is based on the need for a proper safety margin to handle potential faults in a circuit. When a protective relay is designed to de-energize a faulted circuit quickly—specifically in less than one minute—the insulation that supports this operation must ensure reliable performance under fault conditions.

Using a 100 percent insulation level means that the insulation is rated to handle the nominal voltage of the system directly. This is crucial because during fault conditions, the voltage stress on the insulation can briefly exceed normal operating levels. Having insulation rated at 100 percent provides adequate protection against dielectric breakdown and ensures that the system can withstand transient voltages without failure for the short duration before disconnection.

Insulations rated higher than 100 percent—the other options—are typically used for additional safety margins or specific applications where higher voltage levels are consistently encountered. However, when the criteria is specifically for systems designed to quickly isolate a fault, the standard insulation level corresponding directly to the system's operating voltage is sufficient and more appropriate. Thus, a 100 percent insulation level is the correct choice in this context.