Author Topic: Rating Definitions Applied to Medium Voltage Fuses  (Read 161 times)

Offline mahzuba

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Rating Definitions Applied to Medium Voltage Fuses
« on: October 19, 2014, 05:10:03 PM »
Expulsion fuses are defined as follows:
Expulsion fuse

A vented fuse in which the expulsion effect of the gases produced by internal arcing, either alone or aided by other mechanisms, results in current interruption.
EATON's medium voltage expulsion fuses

EATON’s medium voltage expulsion fuses provide full-range fault protection for both indoor and outdoor, medium voltage distribution systems

In addition, medium voltage fuses are further classified as power fuses or distribution fuses as follows:

Power fuse

Defined by ANSI C37.42-1996 as having dielectric withstand (BIL) strengths at power levels, applied primarily in stations and substations, with mechanical construction basically adapted to station and substation mountings.

Distribution fuse

Defined by ANSI C37.42-1996 as having dielectric withstand (BIL) strengths at distribution levels, applied primarily on distribution feeders and circuits, and with operating voltage limits corresponding to distribution voltages.

    These are further subdivided into distribution current limiting fuses and distribution fuse cutouts, as described below.

Current-limiting fuses interrupt in less than _ cycle when subjected to currents in their current-limiting range. This is an advantage as it limits the peak fault current to a value less than the prospective fault current as described above for low voltage fuses. This provides current-limiting fuses with high interrupting ratings and allows them to protect downstream devices with lower short-circuit ratings in some cases.
However, the same technologies that combine to give medium voltage current-liming fuses their current-limiting characteristics can also produce thermal issues when the fuses are loaded at lower current levels. For this reason, the following definitions apply to current-limiting fuses.

Backup current-limiting fuse

A fuse capable of interrupting all currents from its maximum rated interrupting current down to its rated minimum interrupting current.

General purpose current-limiting fuse

A fuse capable of interrupting all currents from the rated interrupting current down to the current that causes melting of the fusible element in no less than 1h.

Full-range current-limiting fuse

A fuse capable of interrupting all currents from its rated interrupting current down to the minimum continuous current that causes melting of the fusible elements.

Due to the limitations of backup and general purpose current limiting fuses, current-limiting power fuses have melting characteristics defined as E or R, defined as follows:
E-Rating

The current-responsive element for ratings 100 A or below shall melt in 300 s at an RMS current within the range of 200% to 240% of the continuous-current rating of the fuse unit, refill unit, or use link. The current responsive element for ratings above 100 A shall melt in 600 s at an RMS current within the range of 220% to 264% of the continuous-current rating of the fuse unit, refill unit, or fuse link.
R-Rating

The fuse shall melt in the range of 15 s to 35 s at a value of current equal to 100 times the R number. Similarly, distribution current-limiting fuses are defined by given characteristic ratings, one of which is the C rating, defined as follows:
C-Rating

The current-responsive element shall melt at 100 s at an RMS current within the range of 170% to 240% of the continuous-current rating of the fuse unit. A typical time-current curve for an E-rated current-limiting power fuse is shown in Figure 1.

The fuse in Figure 1 is a 125E-rated fuse. Note that the curve starts at approximately 250 A for a minimum melting time of 1000 s.
Care must be taken with backup and general-purpose current-limiting fuses so that the load current does not to exceed the E- or R-rating of the fuse. Failure to do this can result in the development of a hot-spot and subsequent failure of the fuse and its mounting. For fuses enclosed in equipment, this can have disastrous consequences since failure of the fuse and/or its mounting can lead to an arcing fault in the equipment.

Note that the boundary of the characteristic, denoting the minimum-melting current, should be further derated to take into account pre-loading of the fuse (consult the fuse manufacturer for details). Note that, as with low voltage fuses, the current-limiting fuse characteristic does not extend below .01 seconds since the fuse would be in its currentlimiting range below this interrupting time.