Ri4Power Selection and dimensioning of the main busbar system Parameters for selection of the main busbar system The core element for the distribution of electrical power in In most cases, the external dimensions of the low-voltage low-voltage switchgear is generally the main busbar system. switchgear are decisive. Due to the model-based design Several points must be taken into account when selecting the of the main busbar system, in some main busbar system busbar system. variants, a restricted range of dimensions are available. The decisive criteria for selection of a main busbar system After selecting a busbar system, it is necessary to check are: that the other criteria for the busbar system are also met, The rated current of the system InA 5.3.1, such as rated voltage etc. see page 38 The rated peak withstand current Ipk 5.3.4, see page 39 The rated short-time withstand current Ipk 5.3.5, see page 39 The degree of protection 8.2, see page 40. Rated peak withstand current Ipk and rated short-time withstand current Icw Short-circuit response Rated current InA i pk (Ipk) = Icw · √2 · χ kA i DC Icw kA InA A Compared with short- circuit currents, the rated current InA shown on the left is several times smaller. Initial short-circuit current Short-time current The rated peak withstand current Ipk 5.3.4 and the rated Table 1: Root-mean-square value of the short-circuit current short-time withstand current Icw 5.3.5 are the principal values for making a statement on the mechanical stability Root-means-square value Icw cos ϕ n of a busbar system during an electrical short-circuit. of the short-circuit current – / <= 5 kA 0.7 1.5 The forces arising during a short-circuit are generally several times higher than the actual weight force of the busbar sys- 5 kA 10 kA 0.5 1.7 tem. For one thing, different force effects occur during the 10 kA 20 kA 0.3 2 short-circuit which may act between the individual strands, 20 kA 50 kA 0.25 2.1 conductors and the enclosure. The above diagram shows the 50 kA – 0.2 2.2 development of a short-circuit current with indication of the various current values. The short-time current stresses the busbar system by caus- At the start of the short-circuit, the peak short-circuit current ing a large temperature rise in the busbars, as well as via the Ipk generates the greatest force effect acting between the interaction between the magnetic field and the associated components of the busbar system. Once the initial short- interaction between the attracting and repelling forces result- circuit current has receded, only the root-mean-square value ing from this. The rated short-time withstand current Icw is of the short-circuit current can be measured. The ratio generally given as a value relating to a short-circuit period between the peak short-circuit current and the continuous of 1 second. In some cases or countries, the data may also short-circuit current depends inter alia on the level of short- need to be given for 3 or 5 seconds. In such cases, a 3-sec- circuit current. The Table 1 indicates the ratio pursuant to ond value may be calculated from the available data using Table 7 of IEC 61 439-1. This ratio between the surge current the formula I12 · t1 = I22 · t2. and the short-time current applies to most application cases. Using the values rated peak withstand current Ipk and rated short-time withstand current Icw it is possible to define the mechanical and thermal stability of a busbar system sub- jected to the short-circuit. 2 - 43 Rittal Technical System Catalogue/Power distribution
Technical System Catalogue Ri4Power
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