Stock material or miscellaneous articles – Composite – Of silicon containing
Reexamination Certificate
2001-05-31
2003-11-11
Dawson, Robert (Department: 1712)
Stock material or miscellaneous articles
Composite
Of silicon containing
C524S405000, C524S424000, C524S430000, C524S436000, C524S444000, C524S449000, C524S450000, C524S456000, C524S492000, C524S493000, C524S497000, C528S015000, C528S031000, C528S032000
Reexamination Certificate
active
06645637
ABSTRACT:
This Application claims priority under 35 U.S.C §119 and/or 365 to application Ser. No. 00810495.2 filed in Europe on June 7, 2000; the entire content of which is hereby incorporated by reference.
The present invention relates to an extinguishing medium for quenching electric arcs in electrical machines, preferably in overcurrent-protection elements, such as fuses in general, for example household fusible cutouts, high-voltage/high-breaking-capacity fuses (h.v.h.b.c. fuses) in the distribution network or substrate fuses, which can be used from electronics to high-voltage engineering or in repeating fuses, for example in PTC elements (PTC=positive temperature coefficient). The extinguishing medium according to the invention consists of a pasty to solid silicone matrix, which is filled with selected mineral fillers, and has a substantially improved quenching characteristic. The extinguishing medium according to the invention provides a substantially improved switching characteristic in said applications, for example h.v.h.b.c. fuses, which contain an extinguishing medium according to the invention.
TECHNICAL FIELD
Extinguishing media for quenching electric arcs in electrical machines, for example in fuses, are known per se. The function of the extinguishing medium in electrical fuses is for the extinguishing medium to absorb sufficient energy from the electric arc, or to cool the electric arc so strongly, that it is quenched during the current zero crossing. Sand is generally used as an extinguishing medium. The switching characteristic of a fuse that contains sand is therefore substantially influenced by the composition of the extinguishing sand and by its average grain size distribution and grain shape. Very different extinguishing sands are therefore employed by various manufacturers.
PRIOR ART
U.S. Pat. No. 4,444,671 or U.S. Pat. No. 5,406,245 disclose the use, for cooling electric arcs in electrical fuses, of organic compounds and their optional application to the fuse wire as a coating. For instance, it has been proposed to use liquid polymers such as polyurethanes, polyacrylates, melamine-formaldehyde resins, and mixtures of such polymeric compounds, or hexamethylenetetramine. In this case, the polymer decomposes in contact with the hot electric arc, and this quenches the arc. However, the use of said compounds generally has the disadvantage that degradation phenomena occur. In addition, these compounds frequently produce electrically conductive decomposition products when they decompose in the electric arc, and the environmental compatibility of these decomposition products is often questionable. Furthermore, the dielectric strength of the fuse is impaired after the current has been switched off, so that thermal re-striking of the electric arc must be reckoned with.
DESCRIPTION OF THE INVENTION
Two working ranges must be taken into account for an h.v.h.b.c. fuse, namely the one in the case of small overcurrents up to 10 I
n
(I
n
=rated current, also denoted I
nominal
) and the one in the case of large fault currents. Large fault currents are relatively simple to deal with by introducing weak points into the current path, in a manner which is known per se. When a sufficiently high potential difference is produced via the root voltage, these weak points cause quenching of the electric arcs at the current zero crossing.
For small overcurrents, a very different switching response takes place in conventional fuses. At the center of the fuse, a tin particle (M-spot, Metcalf effect) is applied to the fuse wire. When the fuse is heated by an overcurrent, the tin diffuses into the silver. The resulting intermetallic AgSn
2
phase has a significantly lower melting point than the basic material (silver) and melts at the point where enough tin has diffused into the silver wire. An electric arc is formed at this point. This electric arc is then quenched by the extinguishing medium, generally quartz sand, owing to the absorption of energy which takes place when the sand melts. In order to provide enough energy to melt the sand, the fault current must generally be at least three times higher than the rated current of the fuse. In the event of smaller currents, on the one hand the electric arc cannot grow correspondingly since the energy is not sufficient to melt the roots on the fuse element (wire) but, on the other hand, the electric arc cannot be quenched because the energy is not sufficient to melt the fuse sand to the required extent. The electric arc therefore continues to burn stably over a defined path within the fuse. The heat energy which is then delivered in a locally limited way leads to a heightened thermal gradient within the fuse in the region where the electric arc is burning, which may cause the fuse to explode. In order to make it possible, in spite of this, to interrupt currents that lie between the rated current and the minimum switch-off current of about 3I
N
, it is necessary to improve the cooling of so-called low-current electric arcs.
It has now been found that silicone polymers, preferably in pasty to solid form, which contain mineral compounds that are known per se in a suitable form and concentration as fillers, represent excellent extinguishing media for quenching electric arcs in electrical fuses. Using the extinguishing media according to the invention, it is possible to interrupt or quench electric arcs which are produced by currents that are below the minimum switch-off current of about 3I
N
and currents that are significantly smaller than the rated current, without occurrence of the disadvantages described above. For instance, electric arcs at 0.67 times the rated current I
N
can be quenched using the extinguishing medium according to the invention.
Using the extinguishing medium according to the invention, even very fine particles of media that have a cooling effect can be positioned in large amounts directly and permanently in the vicinity of the expected electric arc. The cooling power is significantly improved owing to the large surface area of the fine particles, with scarcely any conductive and no highly toxic decomposition products being produced during the oxidation of the silicone by the electric arc. By using the extinguishing medium according to the invention for electric arc quenching, the dimensions of fuses, such as e.g. h.v.h.b.c. fuses, can be significantly reduced with the same performance. In addition, the distance between parallel fuse wires, which is currently at least about 16 mm, can be reduced greatly to about 1 mm when using the extinguishing medium according to the invention, without causing a short-circuit between spiral turns of the fuse wire during or after the switching process. This offers the possibility of fitting a significantly longer wire inside the fuse, for the same standardized dimensions, with spiral winding of the fuse wires. The length of the wire, which is identical to the electrical insulation path after the fuse has been tripped, determines the maximum voltage for which the fuse can be used. When the extinguishing medium according to the invention is employed, it is possible to increase the 36 kV voltage, which is currently counted as an upper limit, and to produce fuses for up to 110 kV or more with a compact structure. The improved cooling and arc quenching according to the invention also reduces costs when producing h.v.h.b.c. fuses, since e.g. the hitherto used fuse body can be configured for significantly lower pressures.
The present invention is defined in the patent claims. In particular, the present invention relates to an extinguishing medium in pasty to solid form for quenching electric arcs, consisting of a silicone polymer or a mixture of such silicone polymers, characterized in that this silicone polymer or the mixture of the silicone polymers contains at least one mineral compound or a mixture of such compounds in powder form as a filler, preferably with an average grain size in the range of from 500 nm to 500 &mgr;m and in a concentration of at least 10 percent by weight, expre
Kaltenborn Uwe
Rocks Jens
Skryten Pal Kristian
ABB Research Ltd
Burns Doane Swecker & Mathis L.L.P.
Dawson Robert
Zimmer Marc S.
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