The present invention relates to a cooling device for a circuit breaker and to a circuit breaker which comprises such cooling device as well as to a switchboard which comprises such circuit breaker.
As known, low-voltage breaking devices (that is for applications with nominal voltages up to 1000V AC/1500V DC), such as automatic circuit breakers, disconnectors, and contactors, commonly referred to as “switching devices” and hereinafter collectively referred to as circuit breakers, are devices designed for allowing correct operation of specific parts of electrical systems and installed loads.
Such devices are usually installed inside distribution switchboards located in electrical systems. Distribution switchboards comprise suitable cells or cubicles arranged for connecting the devices to the electrical power distribution lines. Distribution lines are normally constituted by systems of conductors, such as bus bars and/or cables. The use of appropriate distribution switchboards, in addition to improving the practicality, ergonomics of use, and the aesthetic appearance of the systems, contributes to the maintenance over the time of adequate safety conditions and correct functionality of all installed parts.
The choice of the devices to be used and their related installation configurations, have to be compatible with the technical characteristics of the distribution switchboard. Such compatibility relates to electrical, dimensional, mechanical, and thermal aspects. For circuit breakers, there are three main installation configurations in the switchboards.
In particular, a first installation configuration for circuit breakers is the so-called fixed execution wherein the electrical terminals of the circuit breaker are directly and stably connected to the conductors of the distribution lines. Such connection is normally done by using clamps or screws.
A second installation configuration for circuit breakers is the so-called plug-in execution, wherein special adapter devices are used which are mechanically connected to the switchboard and stably connected to the conductors of the distribution lines by means of their own electrical terminals; each circuit breaker is mechanically coupled to a corresponding adapter device and, by means of appropriate plug-in electrical terminals, it realizes the electrical connection to the distribution line; the plug-in coupling normally includes mechanisms of the plug-socket type.
A third installation configuration for the circuit breakers is the so-called withdrawable execution; it is substantially an evolution of the preceding removable configuration, wherein accessory elements are added as guiding and/or support and/or movement means for facilitating plugging and withdrawal operations of the circuit breaker.
Of these three installation configurations, the first one is the simplest and least expensive, but it is only suitable for solutions that are definitive and in any case non-flexible; on the other hand, the configurations of the removable and withdrawable type offer a greater flexibility. These in fact allow (once the adapter is secured in the switchboard) very quick and totally safe installation or removal of the circuit breaker and, above all, without having to intervene directly on the distribution lines.
Installations of circuit breakers of the removable and withdrawable type do have at least one drawback compared to the fixed-type installation. In order to realize the plug-in junction (plug/socket), it is in fact necessary to introduce at least one additional electrical connector element. Considering the assembly made up of the circuit breaker and its related adapter, it is in fact possible to schematize each of its poles or branches as an electrical chain constituted by elements placed in series with each other. In such electrical chain, each element contributes to an increase in the electrical resistance (or analogously to a deterioration of the overall conductivity) and thus constitutes a potential source of heat due to the Joule effect.
The undesired heat is generated both in the various conducting sections (for example made of copper) and, above all, at each of the present electrical couplings. The various junctions present, and in particular the plug/socket plugs and the main contacts of the circuit breaker, which by their nature cannot be soldered, in fact introduce other micro-discontinuities where conspicuous localized increases of electrical resistance can be found. In practice, the most critical energy dispersion peaks due to the Joule effect, with consequent undesirable heat production, tend to occur in these areas.
As can be seen, the heat that is generated due to these dispersions contributes to the increase in the temperature of the system consisting of circuit breaker, cubicle and switchboard. But, since the temperature of the circuit breaker and the temperature of the switchboard should be maintained within predefined operating limits, any undesired increase of electrical resistance in the conducting branches of the system consisting of the circuit breaker and its related adapter compels limiting the power that can be drawn by a device. In addition, the temperature can negatively influence the operation of the circuit breakers.
The fraction of the actually usable maximum load (compared to the theoretical nominal capacity) is generally expressed in the form of “derating” coefficients that are based on the overall effective conditions of installation. Such installation conditions take account of the combination of the characteristics of the circuit breaker, the adapter, the cubicle, the switchboard, the external environment, etc.
Besides the constraints associated with derating, it is therefore desirable to maintain the operating temperature of the circuit breakers at low levels; it is well known in fact that the higher is the operating temperature, the lower is the life span of the circuit breaker (or of its more sensitive components).
Many solutions have been introduced by various manufacturers in order to reduce the electrical resistance of the poles of the circuit breakers and the electrical contact resistance of the electrical coupling between the circuit breaker and the adapter, and/or in order to improve the overall thermal efficiency of the switchboard.
Although these known solutions certainly provide some technical benefits, there is room and necessity for further improvements.