HOLT Electricians Practice Exam

If the dual-element fuse for short-circuit ground-fault protection is not capable of carrying the load, what is the maximum it can exceed?

1. 150% of motor current

2. 200% of motor current

3. 225% of motor full-load current rating

4. 250% of motor full-load current rating

The correct answer is: 225% of motor full-load current rating

The correct answer is based on the guidelines for dual-element fuses, which are designed to protect motors from overloads and short-circuit conditions. For effective short-circuit ground-fault protection, a dual-element fuse may be rated to carry a load that does not exceed 225% of the motor's full-load current rating. This rating is critical because it allows the fuse to accommodate the inrush current of the motor during startup without blowing unnecessarily, while still providing protection against excessive currents that could indicate a fault condition. The choice of 225% aligns with the National Electrical Code (NEC) requirements for motor protection. This standard ensures that the protection device is adequate for typical motor starting conditions and will act appropriately during fault conditions. Thus, having the capacity to exceed the full-load current rating by 225% provides a balance between operational necessity during startup and safety during overloads. The other options present higher percentages, which exceed the established standards and could lead to insufficient protection for the motor. Using a fuse that exceeds these parameters could result in significant damage to the motor or the connected circuit in the event of a fault, which is why those percentages are not acceptable for dual-element fuses in this context.