Electricity: magnetically operated switches – magnets – and electr – Electromagnetically actuated switches – Automatic circuit-interrupting devices
Reexamination Certificate
2001-04-23
2002-07-16
Barrera, Ramon M. (Department: 2832)
Electricity: magnetically operated switches, magnets, and electr
Electromagnetically actuated switches
Automatic circuit-interrupting devices
C335S038000, C335S039000, C335S041000, C335S059000, C335S060000, C335S062000, C335S172000, C335S174000, C335S175000, C335S176000
Reexamination Certificate
active
06420948
ABSTRACT:
FIELD OF THE INVENTION
The present invention relates to the field of circuit breakers, and more particularly to multipole circuit breakers in which contact sets are paralleled in order to increase breaker capacity rating.
BACKGROUND OF THE INVENTION
In the field of electrical circuit breakers, it is well known to tie the mechanisms of a plurality of electrical poles, or independent circuit paths, together. In this case, it is often desired to provide a single control lever and a trip mechanism which operates the electrical contacts in synchrony. See, U.S. Pat. Nos. 5,565,828; 5,557,082, 4,492,941, and 4,347,488, expressly incorporated herein by reference.
A single pole circuit breaker is a device that serves to interrupt electrical current flow in an electrical circuit path, upon the occurrence of an overcurrent in the circuit path. On the other hand, a multipole circuit breaker is a device which includes two or more interconnected, single pole circuit breakers which serve to substantially simultaneously interrupt current flow in two or more circuit paths upon the occurrence of an overcurrent in any one circuit path.
In a multipole circuit breaker, typically the poles switch independent phases of AC current. Thus, two-pole and three-pole breakers are well known. In these systems, each pole is provided with a current sensing element to generate a trip signal, so that an overload on any phase circuit is independently sensed. In the event that an overload occurs, all of the phase circuits are tripped simultaneously. A manual control lever is provided which operates the phase circuits synchronously as well.
Conventional multipole circuit breaker arrangements thus include a trip lever mechanism associated with each pole of the multipole circuit breaker. Each trip lever includes a portion for joining it to adjacent trip levers. If any pole is tripped open by an overcurrent, the breaker mechanism of that pole causes the trip lever to pivot about its mounting axis. The pivotal motion of one lever causes all the interconnected trip levers to similarly pivot. Each lever may include an arm for striking the armature or toggle mechanism of its respective pole, and causing each pole to be tripped open.
In order to increase the capacity of a circuit breaker system, it has been proposed to parallel a set of contacts, each of which might be insufficient alone to handle the composite load. Thus, by paralleling two single pole circuit breaker elements, a higher capacity circuit breaker may be achieved. However, the art teaches that, preferably, a single contact set is provided having a larger surface area and greater contact force in order to handle a larger load. These larger load-handling capacity devices are typically dimensionally larger than lower load carrying designs. This is because, in part, many elements within a circuit breaker scale in size in relation with current carrying capacity, including the lugs, trip elements, trip mechanism, contacts and breaker arm.
In designing a trip element or system, the type of load must be considered. There are two main classes of trip elements; thermal magnetic and magnetohydraulic. These differ in a number of characteristics, and typically have different application in the art.
However, where such contact parallelization is employed, the contact ratings of the breaker should be derated from the sum of current carrying capacity of each of the contact sets. This is because a contact set having a lower impedance than others will “hog” the current, and may thus see a significantly greater proportion of the total current than 50%, resulting in overheating, and possible failure. Therefore, the art typically teaches that a pair of paralleled contact sets are derated, by for example about 25%, to ensure that each component will operate within its safe design parameters. Further, the contact resistance of a switch may change significantly with each closure of the switch. In parallel contact systems, it is known to employ both unitary thermal magnetic and multiple parallel-operating trip elements in multipole breakers. Thus, it is possible to design a circuit breaker with a specially designed trip element that controls an entire breaker system, or to parallel two entire breaker circuits of a multipole arrangement. In the later case, in order to equalize the current as much as possible between the circuits, a current equalization bar has been proposed. However, this does not compensate for unequal contact resistance, and nuisance tripping of the circuit breaker results when the unequal division of the current has caused enough current to pass through one of the current sensing devices to cause it to trip its associated mechanism.
Attempts have been made in thermal-type breakers to parallel the sets of contacts of a multipole breaker to achieve increased maximum current rating. In one case, exemplified by model QO12150 from Square-D Corp., a unitary thermal magnetic trip element was employed as a trip element for a set of two parallel contact sets, with a connecting member to trip both contact sets at the same time. In this case, the trip dynamics were defined by the thermal-magnetic trip element, and careful calibration of the thermal element was required. This design provided both contact sets within a common housing. Thus, while the internal parts were common with non-multipole arrangements, the housing itself was a special multipole breaker housing. The parallel breaker is housed in a shell that differs from single pole housings, with the parallel poles in a common space.
One typical known system is disclosed in U.S. Pat. No. 4,492,941, expressly incorporated herein by reference, provides electromagnetic sensing devices that are electrically connected at one of their ends to the load terminals. The load terminals are electrically connected in parallel with each other. A plurality of electromagnetic sensing devices are electrically connected at their other ends to each other and are electrically connected to all of the movable contacts which are themselves all electrically connected together. The stationary contacts are connected to line terminals that are also electrically connected in parallel with each other. Thus, the electromagnetic sensing devices are connected in parallel at both of their ends and the contact sets are also connected in parallel at both of their electrical ends, while the electromagnetic sensing devices, on the one hand, and the contact sets, on the other hand, are also in series with each other, thus seeking to equally divide the current among all of the electromagnetic sensing devices, even though the current may not be equally divided among all of the relatively movable contacts, because of varying contact resistances.
Another attempt to increase current carrying capability by paralleling contact sets using magnetohydraulic trip elements employed two parallel trip elements, each set for a desired derated value corresponding to half of the total desired current carrying capacity. For example, two 100 Amp breakers were paralleled (using a standard multipole trip bar) to yield a 150 Amp rated breaker, with 175% trip (about 250 Amps) rating, meeting UL 1077. The parallel set of breakers employed two side-by-side single breaker housings, with slight modifications, and thus did not require new tooling for housings and contact elements.
In this later case, it is difficult to comply with UL 489, which requires that the breaker trip at 135% maximum of rated capacity and 200% of rated capacity within 2 minutes, and that the breaker be capable of handling the specified loads without damage. For example, if the maximum expected deviation in contact resistance of the contact sets (which changes each time the contact is closed) could cause a current splitting ratio of 60%/40%, then in order to ensure reliable trip at 135% of total rated capacity, each trip element must be designed to trip at about 120% of rated capacity, which would lead to unreliability and nuisance trips because of insufficient margin.
Notwithstanding the foregoing attempts, i
Airpax Corporation, Inc.
Barrera Ramon M.
Milde & Hoffberg LLP
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