A generation and transmission utility in the Southern United States had a number of 18-kV and 24-kV generating stations in which 1940s-vintage S&C Type RR Potential Transformer Protective Combinations were applied to protect indoor potential transformers serving metering and other loads. These long-obsolete devices—installed in asbestos-wall potential transformer cubicles—feature a ½-A S&C Type DLC Liquid Power Fuse in series with 90- or 120-Ohm S&C Type RR Wire-Wound Current-Limiting Resistors that limit the extremely high 200- to 300-kA available fault current on the generator bus to a definitive low level.

Since the S&C Type DLC Liquid Power Fuse is no longer manufactured--and no conventional current-limiting fuse available today can handle such high fault currents at these voltages--the utility asked S&C to provide a functional equivalent.

The requirements were formidable. The replacement fuse would need to be capable of:

• Interrupting the very low currents limited by the resistors and neutral grounding transformer.
• Handling the high-frequency transient recovery voltage experienced under certain ground fault conditions.
• Matching the time-current characteristics of the S&C Type DLC Liquid Power Fuse.
• Quickly retrofitting in the existing resistored power fuse mountings with a minimum of rework, to eliminate disruption of the asbestos environment.

S&C's Power Systems Services engineers performed a site walk-down at several generating stations to determine the generator and potential transformer ratings, and assess the physical constraints. Transient analysis studies were then prepared to determine the proper testing parameters for the replacement fuse.

Based on these findings, S&C developed a special solid-material Type SMU-20 Fuse Unit with time-current characteristics closely matching those of the Type DLC Liquid Power Fuse. Performance of the LPF-20R Fuse Unit was verified through rigorous testing in S&C's electrical laboratory, using a test circuit closely matching that of the customer's generating stations.

To make field retrofit quick and easy, S&C's engineering team designed unique adjustable end fittings for the LPF-20R Fuse Unit. The replacement fuse neatly fits in the upper and lower contact assemblies of the resistored power fuse mounting without modification. The existing series-connected current-limiting resistors are unaffected by the retrofit.

Like the Type DLC Liquid Power Fuse it replaces, the LPF-20R Fuse Unit does not vent ionized gases when it operates--of particular importance in this application since the potential transformer is located directly below the fuse in the cubicle.