CLASSIFICATION OF FUSES by IEEE

Fuses for high voltage as defined in IEEE Std C37.40-1993 refers to fuses with rating above 1000 V. It covers covers fuses used in both medium- and high-voltage systems. 

Fuses may also be classified as power fuses and distribution fuses, depending on the intended location of the fuses in the power system.

In addition, fuses may be classified for

Outdoor application only, or
Application indoors or in enclosures

Fuses using new technologies have been available since the early 1980s, but have not yet been addressed by standards.

POWER FUSES
According to ANSI C37.42-1996, a power fuse is identified by the following characteristics:

— Dielectric withstand [i.e., basic impulse insulation level (BIL)] strengths at power levels
— Application primarily in stations and substations
— Mechanical construction basically adapted to station and substation mountings

Power fuses have other characteristics that differentiate them from distribution fuses in that they are available in higher voltage ratings, higher continuous-current ratings, higher interrupting-current ratings, and in forms suitable for indoor and enclosure application and for all types of outdoor applications.

A power fuse consists of a fuse support (commonly called a mounting) plus a fuse unit or, alternately, a fuse holder that accepts a refill unit or fuse link.
Power fuses are rated as E or R depending on their melting characteristics. They are defined as follows:
E rating
.
The current-responsive element for ratings 100 A or below shall melt in 300 s at a root-mean-square (rms) current within the range of 200% to 240% of the continuous-current rating of the fuse unit, refill unit, or fuse 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. When interchanging E-rated fuses of one manufacturer with another, coordination should be carefully checked because the time-current characteristics (TCCs) may be different. The same guidance applies to R-rated fuses. E-rated fuses are available as both expulsion and current-limiting, and R-rated fuses are available only as current-limiting.

Power fuses employing new technology, such as vacuum or sulfur hexafluoride (SF6) as the interrupting medium, have recently been developed. In general, these fuses have melting and clearing characteristics similar to expulsion fuses. Other new technology fuses feature built in sensing and electronics to develop special melting characteristics. 

These fuses ordinarily carry current through a bus bar. When the built-in sensing calls for operation, current is transferred almost instantaneously to a current-limiting fuse section for interruption by physically separating the bus bar element sufficiently to prevent restrike from the system voltage and the transient impulse created during the current-limiting interruption process. Some designs can take a command from other protective systems located remotely from the fuse.

DISTRIBUTING CURRENT LIMITING FUSES
According to IEEE Std C-37.40-1993, a distribution current-limiting fuse contains a fuse
support and a current-limiting fuse unit and is identified by the following characteristics:

a) Dielectric withstand (BIL) strengths at distribution levels
b) Application primarily on distribution feeders and circuits
c) Operating voltage limits corresponding to distribution system voltages

The specification for current-limiting distribution fuses is detailed in ANSI C37.47-1981. Depending on the melting time characteristics, they may be given a C rating, which is defined as follows: The current-responsive element shall melt in 1000 s at an rms current within the range of 170% to 240% of the continuous-current rating of the fuse unit.

The C rating specifies but one point on the TCC curve. While interchanging a C-rated fuse of one manufacturer with another, coordination should be carefully checked because the TCCs may be different.

The current-limiting fuse unit may be a disconnecting type or it may fit into a set of clips. Some fuses rated up to 15.5 kV also have circuit interrupters so they can be used to disconnect a live circuit. 

The principle application is in underground distribution systems and they are used for the protection of pad-mounted transformers supplying residential areas or small commercial or industrial plants. Another application is in small, enclosed capacitor banks.

Both the clip and disconnecting styles may be used to provide an open point on loop feeds. Backup fuses and some general purpose and full range fuses are also used in overhead systems where current limitation is desired. These types are also applied as under-oil fuses in distribution transformers for the same reason. 

Because current-limiting fuses do not emit any exhaust, they are also used in enclosures or vaults. Special types are used for the protection of individual capacitors in outdoor capacitor banks.

DISTRIBUTION FUSE CUTOUTS
According to IEEE Std C37.40-1993, a distribution fuse cutout is defined by the following
characteristics:

a) Dielectric withstand (BIL) strength at distribution levels
b) Application primarily on distribution feeders and circuits
c) Mechanical construction basically adapted to pole or crossarm mounting, except for
distribution oil-fused cutouts
d) Operating voltage limits corresponding to distribution system voltages

Characteristically, a distribution fuse cutout consists of a special insulating support and a fuse holder. The fuse holder, normally a disconnecting type, engages contacts supported on the insulating support and is fitted with a simple inexpensive fuse link. This type of fuse is normally an expulsion fuse; the holder is lined with a gas-evolving material, historically bone fiber. Interruption of an overcurrent takes place within the fuse holder by the action of deionizing gases liberated when the lining is exposed to the heat of the arc established when the fuse link melts in response to an overcurrent.

Distribution fuse cutouts were developed many years ago for use in overhead distribution circuits. They are commonly applied on such circuits along with distribution transformers supplying residential areas or small commercial or industrial plants. Fuse cutouts provide protection to the distribution circuit by de-energizing and isolating a faulted transformer. They are also used for protecting pole-mounted capacitor banks used for power factor correction or voltage regulation.

ANSI C37.42-1996 details the specifications for the distribution cutouts and fuse links. Distribution fuse cutouts are available up to a continuous current of 200 A at 15 kV and up to 100 A at 38 kV. 

Reference: IEEE HIGH VOLTAGE FUSES (1000 V THROUGH 169 KV) Std 242-2001

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