Limiter indicator

Limiters for protection against fault currents in electrical distribution networks the limiter having externally visible indicator for indicating that a limiter element has fused and which visible indicator may be reset and reused after a fused limiter is replaced.

BACKGROUND OF THE INVENTION 
The present invention relates to limiters for protection against fault 
currents in electrical distribution networks, and particularly to limiters 
having indicators for revealing that a fault current has vaporized a 
fusible element of the limiter. 
The chief purposes of network protection are to interrupt fault currents 
before damage occurs to cable insulations and associated equipment, and to 
eliminate service interruptions. There are two principal kinds of fault 
currents: high energy arcing faults and sustained faults. The arcing fault 
is more common and can sputter briefly before clearing. The arcing fault 
can also be of such duration as to "roast" the insulation. A sustained 
fault occurs when two conductors contact solidly with each other and 
induce the flow of heavy short circuit currents. Suitable protection in 
the form of limiters is required to prevent damage to cable insulation, 
combustible fumes, fire and explosion caused by fault currents. 
Accordingly limiters are installed at each end of cable sections and are 
selected according to specific time-current characteristics to avoid 
unnecessary outages. In this way limiters act to confine fault current 
damage to the section of cable where the fault occurs. 
When limiters are installed in electric distribution networks, particularly 
underground networks, the limiter element and cable ends are securely 
covered. The insulating covering, with joints wrapped in tape, prevents 
easy identification of a particular limiter which has operated, among many 
in the distribution network. The task of finding a limiter which has 
operated or fused involves the tedious and time consuming task of 
uncovering and recovering a series of limiter assemblies. 
Many techniques for blown fuse indication have been developed and include 
spring loaded indicators, visual windows, color changes and pressure 
dependent indication. All pressure indicators thus far have been 
destructive, single use operating devices. Expulsion fuses discharge a 
weighted indicator designed to remove energy from the circuit and relieve 
internal pressure. The total fuse must be replaced. U.S. Pat. No. 
4,016,521 to Seybold discloses an overpressure limiter indicator including 
a plastic wall which melts and is forced out by internal pressure caused 
by limiter fusing. The Seybold device allows a single use because the unit 
including fusible link and housing is permanently altered to indicate 
limiter operation and must be removed and replaced for the circuit to be 
reset. 
SUMMARY OF THE INVENTION 
The present invention comprises an insulating sleeve with externally 
visible means for indicating that a limiter has fused or operated and 
which may be reset and reused after a fused limiter is replaced. 
In a preferred embodiment of the invention, the insulating sleeve is formed 
of a suitable insulating material such as molded rubber and is provided 
with a molded-in blister to act as an indicator. The blister is positioned 
on the sleeve for location adjacent the limiter when the sleeve insulating 
assembly is installed in a network. When installed, the blister is 
manually pressed inwards for providing visual indication that the limiter 
within the insulating sleeve is functioning normally. 
When a fault current occurs and the limiter fuses, the element is 
vaporized. Vaporization causes increased internal gas pressure within the 
sleeve assembly forcing the blister outward thereby providing external 
visual indication that the limiter element has fused. 
A bright coloring of the blister allows for easy visual identification of 
the faulty element. 
In an alternate embodiment of the invention, the blister is a separately 
made insert which is fitted through a prepared hole in the insulating 
sleeve wall. The blister has a domed top wall which is depressed manually 
during insulating sleeve assembly. The insert blister operates on the same 
principle as the molded-in blister and pops out when vaporization of the 
element occurs within the insulating sleeve. 
The limiter indicator according to the invention may be reset and reused so 
that the fusible link is the only portion of the limiter assembly 
requiring replacement. Once the fused link is replaced, the original 
components of the limiter including the shell and insulator sleeve with 
indicator are reassembled, and the indicator reset. 
OBJECTS OF THE INVENTION 
It is an object of the invention to provide an insulating sleeve assembly 
for network limiters with means for visual indication of limiter operation 
that is resettable and reusable. 
It is another object of the invention to provide an insulating sleeve 
assembly for network limiters with an integral blister normally depressed 
which changes shape to give visual indication of limiter fusing. 
It is a further object of the invention to provide an insulating sleeve 
assembly for network limiters with a separate blister insert for visual 
indication of limiter operation. 
It is a further object of the invention to provide an insulating sleeve 
assembly for network limiters with a brightly colored blister for ease of 
visual indication of limiter fusing. 
Other and further objects of the invention will occur to one skilled in the 
art with an understanding of the following detailed description of the 
invention or upon employment of the invention in practice.

DETAILED DESCRIPTION OF THE INVENTION 
Referring now to the drawing the limiter 10 according to the invention 
comprises a fusible limiter link 12 having axially aligned, outwardly open 
receptacles 14 for receiving and crimping cable ends 16 forming part of a 
distribution network. The receptacles are physically and electrically 
connected at their inner closed ends by means of an integral strip 18 
defining a fusible element. The fusible element vaporizes in the event a 
fault current occurs in connected cables 16 in excess of the element's 
time-current characteristic. 
The fusible limiter link is encased in a generally cylindrical open ended 
composition shell halves 20a, 20b having spaced cavities 20c and internal 
fingers 20d for accommodating and positioning the fusible element 18. 
A pair of insulating sleeves 22a, 22b preferably formed of rubber entirely 
cover the cable, fusible link, and composition shell subassembly. A 
suitable connecting joint such as a circular dovetail 24 is provided at 
the inner ends 25 of cooperating sleeves. 
The limiter assembly is shown in FIG. 2. 
In accordance with the invention, one of the insulating sleeves 22a is 
provided with means 26 for visually indicating fusion of the limiter 
element with such means being resettable and reusable after the occurrence 
of one or more fault currents through the limiter assembly. As shown in 
FIGS. 3-6, one of the insulating sleeves is provided with indicating means 
in the form of a gas impervious molded-in blister or projection 28 
preferably in the form of an inverted cylindrical cup with upstanding 
cylindrical wall 29 attached at its open end 30 to the sleeve wall by 
means of a ring 32. The blister includes an outer dome 33 which together 
with the wall 29 define an interior chamber 34 communicating with the 
interior of the limiter assembly 36. The diameter, wall thickness, and 
height of the blister are selected to enable the blister at full height to 
project above the outer surface 38 of its host sleeve, and to be supple 
enough to be manually depressed or folded in on itself and remain in 
depressed configuration shown by solid lines of dome 33 in FIG. 5 until 
activated by a fused element. In practice, the sleeve part 22a and the 
projection 28 are molded integral of a suitable rubber or like 
composition. The outer surface of the blister may have a brightly colored 
coating 40 to aid visual indication of blister position and limiter 
condition. 
A modification of the invention is shown in FIG. 6 in which the means for 
visually indicating fusion is a separately molded blister 28 comprising an 
annular rim 42 and a generally cylindrical side wall 44 terminating in a 
dome 46. The blister defines an interior chamber 48 in communication with 
the interior 36 of the limiter assembly. The modified blister is inserted 
in a receptor hole 50 provided in the insulating sleeve wall 38. 
In operation, the limiter assembly is installed between adjacent cables 
comprising part of a distribution network. The normal position of the 
indicating blister is the depressed position shown in full lines in FIGS. 
5 and 6. This position is imparted to the blister by the installer who 
simply presses the blister in upon itself. In the event a fault current 
occurs exceeding the time-current characteristic of the limiter, the 
element vaporizes creating internal gas pressure within the limiter 
assembly which enters the blister chamber causing the blister to pop out 
to a full height or blown position of dash lines in FIGS. 5 and 6. 
The operation of the modified blister is substantially the same.