Fuses, Motor Protection

R-rated fuses provide required short circuit protection for medium voltage motors, motor controllers and associated circuitry. These components have limited ability to absorb the energy of large short circuit currents. Medium voltage motor controllers contain overload relays which provide both overload protection and locked rotor protection to the motor. The controllers are also intended to interrupt low value short circuits within the capability of the motor controller. This protects the medium voltage fuse from sustained over currents which are less than their minimum interrupting rating. NEMA standards for R-rated medium voltage power fuses require that they operate within 15 to 35 s when subjected to an RMS current 100 times the R-rating.