Página 578 - BURNDY

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For proper protection, BURNDY
limiters are
designed with time-current characteristics
approximating the insulation damage curve of
the cable with which they will be used. Figure
4 shows time-current characteristic curves
for a range of standard (250 volt) limiters,
superimposed on insulation damage curves
for several cable sizes. Although the limiter
curve crosses the insulation damage curves,
in practice the limiters will blow before the
insulation can deteriorate. The insulation
damage characteristics represent three phas-
es equally loaded in a duct. Since low-current
faults seldom affect more than one phase at a
time, the rate of heat generated in the conduit
is much less than for a balanced 3-phase
fault, and the time to reach the damage point
is appreciably longer. Practical experience
confirms that limiters provide protection dur-
ing low-current, as well as high-current faults.
The limiter is essentially a compression-type
electrical connector with its center section
accurately formed to provide a fusible ele-
ment. This fusible element is enclosed in a
molded composition shell and the assembly
encased in an insulated sleeve.
Interrupting capacities are as follows:
Standard Limiters . . 30,000 amps at 250V
Limiters . . . . . . . . 20,000 amps at 250V
The protection probably lies in the fact that
the fault impedance reduces the actual fault
current to a value considerably less than
Replaceable-Link Limiters
Replaceable-link limiters, which provide
faster time-current characteristics (Figure 5),
are used in smaller networks, on the fringes of
larger networks, at points where radial feed-
ers leave a network, and for fusing service
cables. As its name implies, this limiter is also
distinctive in that its fusible link is replace-
Current in Amperes
Figure 5
Blue highlighted items are industry standard and most frequently ordered.
US: 1-800-346-4175
Canada: 1-800-387-6487