Electricity: electrical systems and devices – Safety and protection of systems and devices – With specific circuit breaker or control structure
Patent
1995-09-25
1997-11-18
Gaffin, Jeffrey A.
Electricity: electrical systems and devices
Safety and protection of systems and devices
With specific circuit breaker or control structure
361 93, 361 62, H02H 300
Patent
active
056893973
DESCRIPTION:
BRIEF SUMMARY
BACKGROUND OF THE INVENTION
The present invention relates generally to apparatuses for disconnecting branches of a low voltage supply network under short circuit conditions, and more particularly to an apparatus for disconnecting branches of a low voltage supply network under short circuit conditions, in which power consumers are connected in the branch to phase conductors and to a corresponding neutral conductor, a switching element is connected in the incoming circuit to the consumer in order to switch it on and/or off, and switching elements for short-circuit breaking being provided in the phase conductor and/or the neutral conductor, to which switching elements a bridging circuit with a switching contact is connected, and in which a single circuit-breaker is also provided in the phase conductor.
Usually current-limiting circuit-breakers or line safety switches are used for the short-circuit protection of low voltage branches. The switching devices limit the pass-through currents to typically 1/3 to 1/2 of the current amplitude of the prospective short-circuit current. However, sensitive electrical devices, such as contactors or relays, are not able to tolerate the short-circuit effects of such high pass-through currents. Thus, grounding contacts or relay contacts may open dynamically in response to electrodynamic forces at high currents and become welded together when they close again as a result of contact fusing caused by arcs.
Various possibilities for limiting the effects of short circuits on the load side of protective devices by bridging a low-voltage branch are already well known in the art. For example, German A-40 40 359 describes a hybrid bridging device, which limits the Joulean heat integral to about 1/20 of the value of current-limiting line safety switches on the load side of the protective circuit. This very high protective action is necessary when using semiconductor switches in low-voltage branches, for example to control consumers, since overload currents of a short duration must not exceed ten times the value of the rated current in order not to destroy the semiconductor. However, this requires considerable outlay for equipment, since the hybrid bridging device needs a semiconductor switch with a higher rated current and a current-detecting and driving device. In addition, EP-A-0 504 463 proposes bridging low-voltage branches, where the circuit-breaker is provided with a tap of the arc chute and the switching arc starts up the bridging. In this case, the short-circuit protection depends on the operating speed of the switch mechanism and on the transit speed of the switching arc. Therefore, only a relatively small reduction in the Joulean heat integral is achieved on the load side of the protective device, which amounts more or less to half of the pass-through Joulean heat integral.
Furthermore, German A-41 10 335 discloses a device for short-circuit protection, in which is connected a semiconductor switch in incoming circuit to the sensitive consumer, and the semiconductor switch connected in series to a mechanical switch and contains a bridging branch circuit leading to the neutral conductor. The bridging branch circuit is thereby provided with a bridging switch, which is switched on instantaneously by the short-circuit current and bridges the consumer and an additional circuit-breaker. By this means, the consumer and the sensitive semiconductor switch are removed from load, it being necessary in any case for line safety switches to be present in the phase conductor and in the neutral conductor, which entails additional expenditure.
In addition, U.S. Pat. No. 2,924,752 discloses an arrangement for disconnecting three-phase low-voltage supply networks using a three-pole circuit breaker, in which a bridging circuit with a bridging current branch is provided, by way of which a part of the short-circuit current is conducted past the magnetic drive via switching contacts. Due to the special design of the bridging current branch, this arrangement is intended exclusively for multipole net
REFERENCES:
patent: 2924752 (1960-02-01), Scott, Jr.
patent: 3864604 (1975-02-01), Pfanzelt
Jaehner Wilfried
Pohl Fritz
Gaffin Jeffrey A.
Jackson Stephen
Siemens Aktiengesellschaft
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