Página 579 - BURNDY

Versión de HTML Básico

Canada: 1-800-387-6487
US: 1-800-346-4175
Blue highlighted items are industry standard and most frequently ordered.
Limiter Variations
The Limiter Lug provides a fusible connection
between a cable and a flat surfaced terminal
of a transformer or other apparatus. The
Limiter Tap incorporates a Limiter Lug assem-
bly, modified to terminate cable to a ring bus.
The straight Limiter is made for installation in
a single conductor cable. The Molimiter is a
Limiter designed so that one end is crimped
onto a cable and the other fits the clamping
element of a MOLE™ outlet. The Limiter
HYCRAB™ connector is essentially a
HYCRAB™ with a fusible section in each of
its outlets.
Network Protector Fuses
Type Y and Z Network Protector Fuses pro-
vide back-up protection in case the protector
breaker fails to operate during a primary fault.
The fuse time-current curves (Figure 7), are
similar to those of the limiter, thus permitting
correct fuse-limiter coordination for complete
network protection.
Design and Construction
The fusible element for a Type Y or Type Z
Fuse is a tin-plated copper bar with reduced
section, encased in an arc-resistant molded
transite enclosure. One-piece construction
eliminates possibility of joint failure and
assures maximum reliability.
Limiter-Fuse Coordination
To isolate a fault before it can cause extensive
damage, and without interrupting service in
other sections of the network, limiters and
fuses must clear at the proper time and in
proper sequence, depending on the fault's
location in the primary or secondary system.
When a primary fault occurs, the fuse should
clear before any limiters blow. For a second-
ary fault, limiters should clear the fault before
the network protector fuse opens. Failure of
limiters and network protector fuses to func-
tion in proper sequence could cause cascad-
ing of other Fuses, or clearing of secondary
faults by Fuses rather than limiters.
Premature blowing of Limiters not in the fault-
ed section could cause unnecessary service
interruption in sections remote from the fault.
To assure the coordinated functioning of
fuses and limiters throughout a system, prop-
er rating must be selected. The four-step
“Coordination Study” (Figure 8) used in a 4-
parallel cable feed system from the protector
to the first secondary junction is a typical
example of how to select proper ratings.
1. Plot the damage characteristic curve of
the cable insulation in the system. Curves
for Class L620 (260° C. or 500° F.), appear
in (Figure 5).
2. Plot the time-current characteristic curve
of the limiter for the cable size in the sys-
3. Plot the time-current characteristic curve
of the same limiter in Parallel secondary
mains, assuming it carries 40% of total
backfeed current. Allowing for the possi-
bility of unequal current distribution of
secondary mains, the “40% Cable Limiter
Curve” provides a conservative basis for
selection network protector fuses.
4. Select a fuse with its time-current charac-
teristics (Figure 7) lying between the lim-
iter curves plotted in steps 2 and 3.
This procedure avoids the selection of fuses
so light that they might overheat the network
protector or clear unnecessarily, possibly
cascading other fuses in the network; or so
heavy that transformer secondaries might be
damaged or limiters blow before the fuse.
Proper limiter-fuse coordination, facilitated by
the use of fuses and limiters that are precise-
ly matched, assures effective protection with-
out unnecessary interruption.
Current in Amperes
Figure 8
Amperes in Thousands
Figure 7