Electrical resistors – Resistance value responsive to a condition – Magnetic field or compass
Patent
1996-06-17
1998-08-11
Walberg, Teresa J.
Electrical resistors
Resistance value responsive to a condition
Magnetic field or compass
H01C 710
Patent
active
057932786
DESCRIPTION:
BRIEF SUMMARY
FIELD OF THE INVENTION
The present invention relates to a limiter for current limiting which changes from a low resistance to a high resistance when an overload occurs. More specifically, the present invention relates to a current limiter having a thermoplastic resistive member and flat, metal electrodes.
BACKGROUND INFORMATION
Low-voltage circuit-breakers are often connected in series with so-called limiters in order to significantly increase the short-circuit switching capacity in low-voltage electrical networks and to significantly limit the cut-off currents. Such limiters are designed to transition rapidly, in case of a short circuit, from a low-resistance state to a high-resistance state and contribute with their voltage requirement to rapid current limiting and disconnection.
For use as a central limiter, which is connected ahead of multiple branches in order to provide short-circuit protection to the branches, it especially would be desirable to have the limiter function go into effect only for high short-circuit currents, which, for example, circuit-breakers present in the branches cannot handle, and to have no limiter function for moderate or decaying short-circuit currents. This would prevent undesired, longer lasting voltage interruptions which could lead to uncontrolled circuit states, e.g., in contactors or relays.
Switching devices with a simple electromechanical design are commonly used as limiters. The contacts of such devices open dynamically due to electrodynamic forces and such devices normally have no latching mechanism and no triggering system. Their arc-drop voltage plateau lies in the range of the line voltage amplitude. Together with the arc-drop voltage of the circuit-breaker connected in series, the short-circuit current is brought to a rapid decay and the disconnection time is shortened. Electric arc limiters with this design have problems with contact welding, which can be managed in technical terms only with special contact materials and/or with a special contact mechanism.
PCT International Application WO-A-91/12643 and European Patent Application EP-A-0 487 920 describe limiters which take advantage of the PTC (positive temperature coefficient) effect. In these devices, high-current resistors are used which consist essentially of a polyethylene layer filled with carbon black which exhibits the PTC effect. In such a high-current resistor usable as a protective element, the polymer resistive member should be connected with its surface areas to electrodes in order to guarantee the PTC effect. A pressure device is present which exerts a pressure perpendicularly on the electrodes and the surface areas of the resistive member of the conductive polymer layer.
Moreover, German Published Patent Application DE-A-37 07 494 describes a resistive component having two metal electrodes between which a material having PTC characteristics is arranged in which a roughening of the electrode surfaces is created by electrolytic deposition. Here, irregular peaks from 0.5 .mu.m to 500 .mu.m can exist. In the method of operation of such a PTC element, a permanent, unchanging connection of the electrode surfaces to the PTC material is presupposed in order to realize constant resistance values of the component as a function of temperature.
Finally, U.S. Pat. No. 4,377,541 describes a method for manufacturing varistors from polycrystalline metal oxide for low-voltage applications in which a molding press is used in which the press matrices have projections with a regular structure. Thus, in the powder-metallurgical manufacture, a profile is introduced into the varistor disk in order to achieve a low breakdown voltage. Such varistors should be suited particularly to applications at low power levels with low currents and low voltages, a non-linearity of the electrical conductivity of the metal oxide being taken advantage of. The non-linearity of the varistor material is based on its semiconductor characteristics and is distinguished fundamentally from the pattern of the electrical characteristics of
REFERENCES:
patent: 2785316 (1957-10-01), Kingsbury
patent: 3243753 (1966-03-01), Kohler
patent: 3835434 (1974-09-01), Kahn
patent: 4377541 (1983-03-01), Bobik
patent: 4426339 (1984-01-01), Kamath et al.
patent: 4482801 (1984-11-01), Habata et al.
patent: 4833305 (1989-05-01), Mashimo et al.
patent: 4959632 (1990-09-01), Uchida
patent: 5379022 (1995-01-01), Bacon et al.
patent: 5382938 (1995-01-01), Hansson et al.
Jaehner Wilfried
Pohl Fritz
Easthom Karl
Siemens Aktiengesellschaft
Walberg Teresa J.
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