How to Determine Ambient Temperature 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 with the fuse at an ambient temperature of 25°C. When in service, of course, fuses may operate in an ambient temperature other than 25°C. This difference in ambient temperature will affect the melting time of the fuse slightly.

For typical coordination requirements this shift in minimum melting time due to high or low ambient temperatures may be ignored. Nonetheless, where temperature ranges are extreme, it may be desirable to consider the actual shift in the fuse’s melting time. Ambient temperature adjustment factors for S&C fuses can be found in the following S&C publications:

Data Bulletin 210-200 (Type SMD Power Fuses)
Data Bulletin 240-200 (Types SM, SML, SME, SMD-20, and SMD-40 Power Fuses)
Data Bulletin 350-200 (Positrol Fuse Links)
Data Bulletin 450-200 (Fault Tamer Fuse Limiters)

To determine the adjustment factor for a given fuse, and for a given ambient temperature (other than 25°C):

Find the point on the Ambient Temperature Adjustment Factor graph corresponding to the ambient temperature (i.e., enter the graph on the horizontal (“X”) axis).
Read off the corresponding ambient temperature adjustment factor from the vertical (“Y”) axis.
Multiply the published minimum melting times — for the fuse selected — at various current values by the adjustment factor determined in Step 2.

NOTE: The fact that a fuse melts faster (or slower) when it is operating at an elevated (or reduced) ambient temperature does not mean that it will clear a fault faster (or slower). The published maximum total clearing time-current characteristic curve is not adjusted for unusual ambient temperatures, or for any other factors.

Example: Determine the ambient temperature 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 ambient temperature is +40°C. The ambient temperature adjustment factor, determined from the Figure 1 below, or found in S&C Data Bulletin 210-200, is 0.96.

Figure 1. Ambient-temperature adjustment curve.

The ambient temperature adjustment factors found in the aforementioned publications are applicable only to S&C fuses, since their derivation is dependent upon the design and construction of the fusible elements.