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Fuses are commonly used to provide medium voltage (MV) protection to distribution transformers. Unfortunately, there are certain deficiencies introduced when primary-side fuses are used. In paper mill power systems, given the unique organization of the paper mill loads around individual feeders to facilitate load shedding, it may be advantageous to provide transformer differential relay protection from the primary of the transformer to the load side of the main secondary breaker instead of primary fuse protection. This protection can then trip either a primary-side grounding switch (with fuses) or a primary-side feeder circuit breaker using a fiber or a WiMAX (Worldwide Interoperability for Microwave Access) connection. Protection against overload is provided by a secondary main circuit breaker and by design. [ABSTRACT FROM AUTHOR]

When one undertakes an arc flash study, one must develop a model based on the power system of interest. The model requires estimates of conductor lengths and other data. To ensure that the arc flash study that one is performing does not underestimate the magnitudes of the short circuit currents present within the power system, and to allow for limitations in the procedure by which one develops the model, typically visual inspection, one must carefully estimate the lengths of conductors. Computing “safety margins” can help to avoid errors, and maintenance switches can address the uncertainties involved. The final solution may be software that permits the percentage estimates for impedance elements to be built into the model for various equipment and used to compute arc flash levels on the secondaries of low-voltage circuit breakers. [ABSTRACT FROM AUTHOR]