This information notice is intended to alert addressees to potential problems caused by inadequate programs to control activities related to fuses. It is expected that recipients will review the information for applicability to their facilities and consider actions, as appropriate, to avoid similar problems. These deficiencies included inadequate root cause evaluations of blown fuses, inadequate verification of design information for installed and replacement fuses, inadequate identification and labeling of fuses, and lack of coordination between fuses and circuit breakers. Fuses are relied upon to isolate electrical faults to ensure minimum disturbance to the system. Fuses are also used as electrical isolation devices between a class 1E power supply and non-class 1E electrical equipment.
Proper protection is achieved through the careful selection of the type and the rating of the fuse.
If an oversized fuse is selected that has a higher capacity than the upstream fuse or breaker, the oversized fuse could make the entire bus unavailable in case of a fault within the circuit. In addition, a faulted circuit with an oversized fuse can degrade the voltage of a bus and impart excessive heat to adjacent cables. On the other hand, if an undersized fuse is selected, the undersized fuse could lead to a loss of the safety function of equipment by prematurely opening the protected circuit. Therefore, when selecting fuses, it is important to consider the voltage, current, interrupting rating, fuse type, and coordination with other fuses and circuit breakers. The loss of the annunciators apparently resulted from blown fuses in the power supplies for the annunciators.