Most dc systems require some form of overcurrent and/or short circuit protection. These systems include dc motor drives and controllers, semiconductor components, telecommunications switching stations (both power and signal), and transit substations. Battery-powered applications from automobiles and lift trucks to Uninterruptible Power Supply (battery backup) systems also require dc overcurrent protection. As with any fuse selection, the three elements of system voltage, normal load current, and available short-circuit current must be considered. For proper application, the fuse's ratings must equal or exceed the system parameters. However, the task of selecting overcurrent protection for dc systems is not always straightforward. Manufacturer's published data are usually based on ac ratings, and the burden often falls on the user to determine the dc applicability. Furthermore, the manufacturer's dc test data may not necessarily apply to the dc system at hand. Factors including circuit time constant, voltage, and available short-circuit current may preclude the use of certain ''dc rated'' fuses. This paper will address the procedures used to determine the dc ratings for fuses. Also, the issues pertinent to insuring a proper dc fuse application will be discussed.
