Electricity: electrical systems and devices – Safety and protection of systems and devices – Circuit interruption by thermal sensing
Patent
1997-10-16
1999-04-20
Gaffin, Jeffrey
Electricity: electrical systems and devices
Safety and protection of systems and devices
Circuit interruption by thermal sensing
361 58, 338 22R, H02H 504
Patent
active
058962646
DESCRIPTION:
BRIEF SUMMARY
TECHNICAL FIELD
The present invention relates to a device for current limitation and protection against short-circuit currents, which comprises at least one polymer-based electrically conducting body and at least two electrodes, at least one of the contact surfaces of the electrically conducting body making contact freely with an electrode or another electrically conducting body.
BACKGROUND OF THE INVENTION
European patent EP 0 487 920 describes a device of the above-mentioned type in the form of a PTC element which at least comprises one polymer-based electrically conducting body, the resistivity of which exhibits a positive temperature coefficient and electrodes for conducting current through the body. At least one of the two contact surfaces of the polymer body makes free contact with one of the electrodes or another electrically conducting body. The polymer body and the electrodes are retained by resilient and pressure-applying devices. This results in to the creation of a contact pressure in a contact surface where the polymer body freely makes contact with an electrode or another electrically conducting body. Since current transition between the electrode and the polymer body only takes place at certain contact points, a current displacement arises at the contact surface resulting in a voltage drop near the contact surface. As a result of the difference in resistivity between the polymer body and the electrode, this voltage drop occurs substantially in the body which has the highest resistivity, that is, the polymer body.
At current intensities below rated current, a low contact resistance is maintained at the contact surface in that the contact pressure which is applied to the contact surface by the pressure-applying devices ensures current transition at a sufficient number of contact points. At short-circuit currents, the temperature at the contact points is increased. This increase in temperature results in a local melting and/or gasification of the polymer material at some of these points, whereby the resistance is increased. Finally, a layer of gasified polymer/carbon arises near a freely contacting contact surface, whereby a strong increase of the resistance across the layer is obtained, that is, the current-limiting device trips. Since the contact pressure is maintained by the pressure-applying devices, the original contact pressure and the original contact resistance are essentially resumed across the contact surface in connection with the gas pressure decrease. One problem is that the gasification has normally arisen at both contact surfaces of the polymer body before the current-limiting device trips. Therefore, it is difficult, using constructive changes such as cooling electrodes, etc., to increase the ability of the current-limiting device to withstand overcurrents for a limited period of time, i.e, currents with a current intensity is normally up to 10 times the rated current, while at the same time maintaining the ability of rapidly limiting short-circuit currents. Overcurrents of the order of magnitude of 10 times the nominal current occur frequently, for example when starting electric machines.
To maintain a low contact resistance at a normal current transition and to ensure that essentially the original contact and the original contact resistance are formed again after the current-limiting device has tripped as a result of a short-circuit current, it is desirable that a great contact pressure act on the contact surface since a high contact pressure ensures a large number of contact points and hence a large effective contact surface and a small current displacement. However, at the same time, the current-limiting function of the device is deteriorated since the limited current displacement results in a low energy development in the layer. In this way, with increasing contact pressure, a larger level of power must be supplied to the polymer body before the current-limiting device trips. This means that an increased contact pressure extends the time to a trip at short-circuit curre
REFERENCES:
patent: 2720573 (1955-10-01), Lundqvist
patent: 5057674 (1991-10-01), Smith-Johannsen
patent: 5166658 (1992-11-01), Fang et al.
patent: 5382938 (1995-01-01), Hansson et al.
patent: 5602520 (1997-02-01), Baiatu et al.
Banghammar Lars
Bijlenga Maria
Hansson Tomas
Jeppsson Ola
ABB Research Ltd.
Gaffin Jeffrey
Sherry Michael J.
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