How to Determine the Peak-Load Capability of S&C Fuses.

S&C fuses can be subjected to currents in excess of their ampere ratings without permanently damaging either the fusible element or the arc-interrupting medium . . . the solid-material (boric acid) liner in the case Types SM, SML, and SMD Power Fuses, or the sheath in the case of Positrol Fuse Links. The maximum current to which a fuse can be subjected is referred to as its “peak-load capability.” For solid-material power fuses, peak-load capabilities are published for continuous, daily, and emergency duties. The daily and emergency capabilities are further delineated by time (e.g., ½ hour, 1 hour, 2 hours, etc.). For Positrol Fuse Links, peak-load capability values are published for continuous and 8-hour emergency duties. Note that the published peak-load capability for a given fuse is not the same as its minimum melting current, which is generally 200% or more of its ampere rating. Fuses subjected to currents in excess of their published peak-load capabilities may be damaged in such a way as to impair future operation, particularly on low-magnitude faults. See Figure 1.

TCC Curve.

The continuous, daily and emergency peak-load capability values for S&C fuses can be found in the following S&C publications:

Data Bulletin 210-190 (Type SMD Power Fuses)
Data Bulletin 240-190 (Types SM, SML, SME, SMD-20, and SMD-40 Power Fuses)
Data Bulletin 350-190 (Positrol Fuse Links)
Data Bulletin 450-190 (Fault Tamer Fuse Limiters)

Loading fuses to their maximum capabilities is not generally recommended since the result can be loss of coordination with other protective devices. This is because published minimum melting time-current characteristic curves for fuses are based on no preloading. The excessive preloading produced by overloading fuses to the peak-load values listed in the aforementioned publications can cause a reduction in melting time by as much as 70% of the published values. In transformer-primary fusing applications where the principal objectives are maximum transformer protection and maximum backup protection for secondary-side equipment and circuits, it may be expedient to forego complete coordination with other protective devices for the duration of such peak-load conditions.

Peak-load capabilities are based on a 24-hour average ambient temperature of 30°C (25°C for Positrol Fuse Links). Peak-load capabilities are increased approximately 0.5% for each degree of ambient temperature below 30°C (or 25°C), and decreased approximately 0.5% for each degree of ambient temperature above 30°C (or 25°C).