October 22, 2001

How to Determine Preload-Adjustment Factors for S&C Fuses.

In accordance with procedures described in ANSI Standard C37.41, minimum melting time-current characteristic curves for S&C fuses are based on tests starting at no initial load. When in service, of course, every fuse will be carrying a load that may approach or even exceed the ampere rating. This preloading raises the temperature of the fusible element and hence reduces the melting time for a given level of current.

To ensure precise coordination of fuses with other fuses or with automatic circuit reclosers, it is necessary that published minimum melting time-current characteristic curves be adjusted for the expected preloading condition. Preloading adjustment factors for S&C fuses can be found in the following S&C publications:

• Data Bulletin 210-195 (Type SMD Power Fuses)
• Data Bulletin 240-195 (Types SM, SML, SME, SMD-20, and SMD-40 Power Fuses)
• Data Bulletin 350-195 (Positrol Fuse Links)
• Data Bulletin 450-195 (Fault Tamer Fuse Limiters)

To determine the preload adjustment factor for a given fuse and for a given level of loading:

1. Calculate the load current in percent of the fuse ampere rating.
2. Find the point on the Preloading Adjustment Factor graph corresponding to the value calculated in Step 1 [i.e., enter the graph on the horizontal (“X”) axis].
3. Read off the corresponding preloading adjustment factor from the vertical (“Y”) axis.
4. Multiply the published minimum melting times, at various current values, by the adjustment factor determined in Step 3. NOTE: The fact that a fuse melts faster when it is carrying load current does not mean that it will clear a fault faster. The published maximum total clearing time-current characteristic curve is not adjusted for preloading or for any other factors.

Example: Determine the preload adjustment factor for 150E ampere SMD Power Fuse protecting a 12/16/20 MVA substation power transformer rated 115 kV operating at its top rating (i.e., 20 MVA). The full-load current of the transformer (top rating) is 100 amperes. This load current, expressed in percent of the primary fuse ampere rating is 66.7% (100/150 = 0.666). The preload adjustment factor, determined from Figure 1, below, or found in S&C Data Bulletin 210-195, is 0.90.

Figure 1. Preload adjustment curve for S&C Type SMD Power Fuses.

The preload adjustment factors found in the aforementioned publications are applicable only to S&C fuses since their derivation is dependent upon not only element construction but also the relationship of the minimum melting current to the ampere rating of the fuse.