Custom Search
|
|
|
||
 member and fuse holder.
4.5.2 Rating. Fuses are rated in terms of continuous-current-carrying capability,
current-interrupting capability, and voltage.
4.5.2.1 Continuous-Current Rating. The continuous current rating of a fuse is the
designated limit in rms alternating current, or direct current, that it can carry continuously
without deteriorating or exceeding the limit of permissible temperature rise. The continuous
current rating of a fuse is normally selected as equal to or slightly greater than the
current-carrying capacity of the circuit that it protects. One major disadvantage of fuses is that
considerably more current, than the system continuous-current rating, is required to melt the
fusible element.
4.5.2.2 Interrupting Rating. The interrupting rating denotes the maximum symmetrical
fault current permitted at the fuse location. Generally, both symmetrical and asymmetrical rms
ratings are given.
4.5.2.3 Voltage Rating. The voltage rating of a fuse is the nominal system voltage
application. Associated with the voltage rating is the maximum design voltage, marked on the
nameplate, which is the highest system voltage for which the fuse is designed to operate.
4.5.3 Low-Voltage Fuses. Low-voltage fuses are those rated 600 V and below and are
generally classified as either plug type or cartridge type. Low-voltage fuses are used primarily
on indoor circuits, or devices, and are enclosed in a metal cabinet.
4.5.3.1 Cartridge-Type Fuses. Most cartridge-type fuses consist of a fusible link enclosed
in a cylindrical cartridge with contact ferrules or knife blades at each end. End contacts slip into
fuse clips or pressure contacts. Cartridge fuses are made for two voltages: (1) 250 V and lower,
and (2) 600 V maximum. Class G, H, J, or K fuses have ferrule-type contacts up to 60 A and
Class H, J, K, or L have knife blade-type contacts from 70 to 600 A with Class L fuses available
to 6000 A. Cartridge fuses may be of the one time type or renewable. The one time fuse is a unit
assembly and must be completely replaced after an interrupting operation. Renewable fuses have
provisions for the replacement of the fusible member. There are several types of cartridge fuses
available for use on a power system; such as, dual element, current-limiting, and
high-interrupting capacity. Selection depends upon the type of circuit or equipment to be
protected. Fuses with very little time delay, short time delay, and long time delay are available in
standard cartridge type. In addition, special fuses having current limiting ability may be used
where short-circuit currents are 10,000 A or greater. Class H fuses are rated at 10,000 A
interrupting capacity (A.I.C.). Class G, J, K and L are rated from 50,000-200,000 A.I.C. These
fuses have a long time delay in the overcurrent range and very fast operation in the range of short
circuit or fault current. Such a fuse provides protection for the equipment and the circuit while
withstanding starting inrush current of motors.
4-22
|
 
|
|
|
||
