Electricity: electrothermally or thermally actuated switches – Electrothermally actuated switches – Fusible element actuated
Reexamination Certificate
2000-03-02
2002-05-14
Vortman, Anatoly (Department: 2835)
Electricity: electrothermally or thermally actuated switches
Electrothermally actuated switches
Fusible element actuated
C337S190000, C337S187000, C338S0220SD, C338S224000
Reexamination Certificate
active
06388553
ABSTRACT:
FIELD OF THE INVENTION
This invention relates to current-limiting fuses containing a conductive polymer exhibiting a sharp increase in electrical resistance at a threshold current. The polymer is coupled in series with mechanical breaker contacts and is resistively heated to the threshold temperature in the event of a fault current, to limit current as the breaker opens. A commutation shunt resistance can be coupled in parallel with the polymer. The invention relates to conductive polymer current-limiting fuses which exhibit extremely low let-through values. More particularly, the conductive polymer current-limiting fuses are based on metal-filled elastomer material and exhibit let-through values less than 5,000 A
2
s with a switch current of 1.79 kA
p
, preferably less than 2,500 A
2
s, most preferably no more than 2,250 A
2
s. Several embodiments are disclosed, including an arrangement is which at least one electrode is in free surface contact with a planar polymer element and urged against the polymer element under a force.
BACKGROUND INFORMATION
For protecting circuits and loads as well as persons and property, electrical energy advantageously must be controlled nearly instantaneously to react to a sensed fault condition. For example, it is advisable promptly to disengage a load in the event of a short circuit or similar fault condition by cutting off the current supply. If the fault current is not quickly cut off, the potential for serious damage is increased. For this reason, short-circuit protection devices are routinely incorporated in power supply circuits for controllably interrupting a fault current. A circuit protective device may sense and respond to long term current overload, or to a ground fault or a short circuit. In the event of a short circuit. the protective device should operate as quickly as possible.
Devices for controlling current involve one or more means along a current supply conductor, in series with the load, which insert an insulating gap or large resistance when triggered. These include mechanically separable electrical contacts, fuses, thermistors with positive temperature coefficients and others. Elastomers for use in such devices are comprised of all polymers that exhibit elastic properties which are similar to those exhibited by natural rubber. Elastomers can be compressed or stretched within a relatively large permitted elastic area, and return to their original state when the load is removed. Electrically conductive elastomers are a class of rubber and plastics which have been made electrically conductive, either by the addition of metal mixtures or by orienting metal fibers under the influence of electric fields, or by the addition of different carbon mixtures or ceramics, for instance V2O3-material dispersed in the manner described in the article “V2O3 Composite Thermistors” by D. Loffat, et al, published in Proceedings of the Sixth IEEE International Symposium on Applications of Ferroelectrics, 1986, pages 673-676. In rubber, there is used several types of “carbon black”, for instance graphite, acetylene black, lampblack and furnace black with particle diameters ranging from 10-300 nm. Examples of appropriate rubber materials which become electrically conductive after adding metal mixtures or carbon mixtures are butyl, natural, polychloroprene, neoprene, EPDM, and the most important silicone rubber. Additives of metals and metal alloys in powder form suited as elastomer additives are silver, nickel, copper, silver-plated copper, silver-plated nickel, and silver-plated aluminum.
The most common types of carbon or metal-filled plastics are polyethylene and polypropylene. These are used at present for heating cables and for overload protectors, for instance the earlier mentioned polymer-based PTC-thermnistors. However, an electrically conductive filler impairs the mechanical properties of the plastic. The material becomes brittle and hard and is therewith not readily deformed. These materials are therefore less useful as pressure transducers with pressure contacts. A further limitation of carbon-filled plastics resides in their relatively high resistivity, typically one 1 Ohm cm and higher, which limits applications to low power. On the other hand, metal-filled plastics can be produced with significantly lower resistivity, lower than 0.5 Ohm cm, although voltage or tension stability becomes very poor, and consequently these materials are not suited as overload protectors.
Electrically conductive elastomers can be given very low resistances, for instance resistances of 2 mOhm cm or lower, by admixing metal powder. One advantage afforded by elastomers is that they are very soft in comparison with carbon-filled polyethylene and polypropylene, even when containing large quantities of electrically conductive filler. Such elastomers will have a typical Shore number of between 20-80, according American Standard ASTM D2240 (Q/C).
To function as a current limiting element includes at least one electrically conductive elastomeric body and two electrodes. The polymer composition of the elastomeric body may be of various kinds, examples of suitable elastomers including butyl, natural, polychlorpropylene, neoprene, EPDM and silicone rubber. The electroconductive powder material is preferably comprised of silver, nickel, cobalt, silver-plated copper, silver-plated nickel, silver-plated aluminum, lampblack, conductive soot or carbon black. The powder material will suitably have a particle size of 0.01-10 micro-meters and the powder filler is suitably present in an amount corresponding to 40-90% of the combined weight of the powder filler and elastomeric material. The resistivity of the electric elastomeric body will preferably lie within the range of 0.1 mOhm cm-10 Ohm cm. When the device includes more than one electrically conductive elastomeric body, the bodies may be made of mutually the same or mutually different elastomers and then with mutually the same or mutually different fillers and resistivity. The electrodes are of a conventional kind, for instance silver-plated copper. The electrodes are preferably oriented so that repulsion forces will occur between the electrodes when high currents pass therethrough. The pressure achieved on the electrodes, for instance with a known pressure device described in U.S. Pat. No. 3,914,727, or by a conventional spring mechanism for the on/off function of an electric switch, deforms the convex abutment surface of the elastomeric body, when the device includes such an abutment surface. This deformation will preferably reach at least 5%. A deformation of 5-30% is particularly preferred, as defined with a starting point from the distance between the bodies that borders on a considered elastomeric body, i.e. if the distance when the pressure is 0 and bordering bodies lie in abutment with the elastomeric body is d and if the distance changes to 0.7.multidot.d after the pressure has been applied, the body will have been deformed by 30%. Particularly preferred elastomeric bodies are those which have a hardness between 30-50 IRHD in accord with British Standard BS903/A26, although materials having both a lower and a higher hardness may conceivably be used.
Mechanical circuit breakers have the capacity to interrupt high currents.
However, the break-time of conventional circuit breakers is several milliseconds, with even the fastest circuit breakers taking 3 to 5 ms to open and interrupt fault currents. Due to various mechanical constraints and the continuation of conduction as an are is struck and extinguished between the separating contacts, traditional circuit-breaker technology leaves little potential for further reductions in break-times.
SUMMARY OF THE INVENTION
The present invention provides a compact, low cost, high power fuse, based on conductive polymers designed to be connected in series with a conventional mechanical circuit breaker. The combination is believed suitable for use at Type II protection levels, i.e., no damage to contactor contacts in a motor starter combination at high fault ratings. When combine
Hanna William K.
Shea John J.
Eaton Corproation
Moran Martin J.
Vortman Anatoly
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