Electricity: electrical systems and devices – Safety and protection of systems and devices – High voltage dissipation
Reexamination Certificate
2003-12-30
2004-09-21
Leja, Ronald (Department: 2836)
Electricity: electrical systems and devices
Safety and protection of systems and devices
High voltage dissipation
C361S120000
Reexamination Certificate
active
06795290
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The invention is directed to a surge arrestor having a middle electrode and at least one outer electrode. An electrically conductive spring clip is secured to the middle electrode and exerts a spring force on the outer electrode.
2. Description of the Related Art
Surge arrestors of the type previously described are usually employed for securing telecommunication devices against briefly occurring over-voltages as resulting, for example, from lightning strikes. By triggering the surge arrestor, the outer electrode is shorted to the middle electrode by an arc. As soon as the occurrence of the over-voltage has ended, the arc vanishes and the gap between middle and outer electrode functions as an insulator again.
Arrestors can be equipped with additional functions in order to be able to maintain the previously described protective function even given an outage of a surge arrestor. Mechanisms are known in this context for securing the arrestor given a thermal overload (fail safe) in which a fusible element of solder material or an insulating foil as well is arranged between the spring clip and the outer electrode; this arrangement, given an excessively high temperature, releases the movement of the spring clip which then bridges and, thus, shorts the gap of the arrestor between middle electrode and outer electrode.
Another fault that may occur in an arrestor is the loss of tightness of the arrestor, this resulting in that the trigger voltage of the arrestor rises greatly. The arrestor then no longer triggers at the original trigger voltage and can thus also no longer generate heat that activates the fuse mechanism. In order to also secure the protective function in this situation, the arrestors can be additionally equipped with a fuse given looseness (vent safe). Each spark gap of the arrestor has an additional voltage-limiting component part connected in parallel to it. This can be a varistor or a semiconductor (for example, a break-over diode). This assures that the protective function is preserved even given a malfunctioning or leaking arrestor since, in this case, either the additional, voltage-limiting component part itself protects, shorts the arrestor, or triggers a thermal short-circuit mechanism due to heating.
The highest demands made of the fault-protection mechanisms arise given a leaking arrestor. For example, the American specification Telcordia 1361 prescribes a test in which a leaking 3-electrode arrestor is applied to an alternating voltage of 1000 V at which a maximum current of 30 Amperes per break can flow. In one arrestor version that envisions the use of varistors as voltage-limiting component parts, switching powers of 30 kW per break must be governed. This high electrical power necessarily leads to sparking and burn-up due to arc that harbors a fire hazard sine the arrestor are usually installed in a plastic housing.
U.S. Pat. No. 5,388,023 discloses arrestors of the species initially described in which a fusible element is arranged between the spring clip and the outer electrode. In the normal operating case, the fusible element must prevent a short circuit and, accordingly, must be fashioned in a solid manner with a minimum thickness. In case of a fault, the fusible element releases the electrical contact between the spring clip and the varistor or the outer electrode. This fusible element has a relatively large mass to be fused, as a result of which the transition of the fusible element from the solid into the molten phase lasts a long time and, accordingly, the triggering of the melt fuse is delayed. The risk of sparking thus rises greatly.
SUMMARY OF THE INVENTION
It is therefore an object of the present invention to provide a surge arrestor that provides a fast trigger mechanism in case of a fault.
This object is inventively achieved by a surge arrestor, comprising a middle electrode; an outer electrode; an electrically conductive spring clip secured to the middle electrode and configured to exert a spring force on the outer electrode; an electrical component part arranged between the spring clip and the outer electrode, the electrical component part being non-conductive at a trigger voltage of the surge arrestor and configured to generate heat given a flow of current; a fusible mass; a spacer element; and an electrically conductive contact element that is secured to the spacer element with the fusible mass and that is spaced from the outer electrode, the spring clip lying against the contact element, and the contact element being pressed against the outer electrode by the spring clip when the fusible mass melts. Advantageous developments are described below.
The invention provides a surge arrestor that comprises a middle electrode and at least one outer electrode. An electrically conductive spring clip is secured to the middle electrode and exerts a spring power on the outer electrode. An electrical component part that generates heat given a flow of current is arranged between the spring clip and the outer electrode. The spring clip lies against an electrically conductive contact element that is secured to a spacer element by a fusible mass and that is spaced from the outer electrode. The fusible mass adheres both to the contact element as well as to the spacer element. When the mass melts, the contact element is pressed against the outer electrode by the spring clip.
The inventive surge arrestor has the advantage that the isolation between the spring clip and the outer electrode required in the normal operating case of the arrestor is effected by the spacing of the contact element from the outer electrode by the spacer element. The fusible mass is only required in order to secure the contact element to the spacer element and can therefore be provided in a slight amount that need only assure that the contact element is held by the spacer element.
The electrical component part thus insulates at the trigger voltage that the arrestor has in normal operation.
In an advantageous embodiment of the invention, the electrical component part can be a varistor. Given standard, static trigger voltages of approximately 350 V of arrestors that are present during normal operation, such a varistor still has a high impedance of approximately 0.5 M&OHgr;, so that it practically represents an insulation between the spring clip and the outer electrode. In case of over-voltage, however, the resistance of the varistor is diminished, so that the protective function can be assumed by the varistor in case the trigger voltage of the arrestor rises to extremely high values (>1000 V) due to a fault in the arrestor. Over and above this, a varistor is an electrical component that generates heat when current flows and can itself therefore trigger a thermal protective mechanism.
In another embodiment of the invention, however, the component part can be a semiconductor component.
In one embodiment of the surge arrestor, the spring clip and the contact element are two different component parts, where the spring clip lies against the contact element and thus presses it against the outer electrode.
In another embodiment, the contact element is integrated as one piece in the spring clip. Here, too, the spring clip lies against the contact element since contact element and spring clip directly adjoin one another.
In one embodiment of the invention, the spacer element is in the form of a pin. The contact element comprises a disk that has a hole into which the spacer element projects.
This embodiment of the invention has the advantage that it can be especially easily and simply manufactured. The fastening of the contact element to the spacer element can, given a corresponding dimensioning of the pin or hole, be produced with a very slight amount of fusible compound, this yielding the advantage of a fast trigger mechanism.
In another embodiment of the invention, the spring clip likewise comprises a hole through which the spacer element projects when the compound has melted. This avoids impeding the trigger mechanism due to a mechanical
Bobert Peter
Boy Juergen
Fuhrmann Klaus-Dieter
Epcos AG
Leja Ronald
Schiff & Hardin LLP
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