Ri4Power Rated current of the busbar system Inc In accordance with IEC 61 439, the busbar system is referred As an effect in the opposite direction, it is possible to improve to as a circuit Inc in the low-voltage switchgear and controlgear the admissible rated busbar current by means of forced assembly. As described under “Rated current of switchgear convection. In contrast to a busbar system in the open air, a and controlgear assemblies” on page 45, particularly with higher airflow can be achieved in switchgear with the same low-voltage switchgear with a high rated current InA, the rated fan output, which cools the individual busbars and therefore current of the busbar system may be lower. However, for supports a higher current-carrying capacity. such a design to be admissible, it is necessary to prove In order to incorporate all the aforementioned effects within with a load flow calculation that the admissible rated current a low-voltage switchgear mathematically, major calculations of the busbar system is not exceeded in any operating are needed. The additional temperature rise caused by eddy scenario. If a busbar system is designed on the basis of currents or ring currents are particularly difficult to determine. the maximum possible current load, measures must be taken to ensure that the chosen busbar system also meets In accordance with IEC 61 439-1, the admissible values for the required short-circuit withstand capability. all busbar systems for the Ri4Power System have been deter- When calculating the requisite busbar cross-sections for a mined by testing with different busbar cross-sections inside the enclosure and different protection categories and cool- low-voltage switchgear with design certificate, it is not suffi- ing. The protection categories were selected in accordance cient to merely design to DIN 43 671. with the possible protection categories with Ri4Power. According to DIN 43 671, a rated current is calculated for In these tests, the admissible rated busbar currents were various copper sections and cross-sections with reference to calculated for two different temperature increases (30 K, a busbar system and measured in the open air. The admissi- 70 K). They included a maximum busbar temperature of ble current of a busbar was calculated at an ambient temper- 65 °C at 35 °C ambient temperature around the switchgear. ature of 35 °C and a busbar temperature of 65 °C. Using the Hence it is possible to achieve a comparable value to the correction factor diagram mentioned in this DIN standard, aforementioned DIN 43 671 and hence also to use the correc- this can also be converted to different ambient temperatures tion factor diagram. The admissible rated busbar currents and different busbar temperatures. were calculated for a busbar temperature considered by Rittal to be the maximum permissible of 105 °C at an ambi- Within a switchgear housing, however, other factors may ent temperature around the switchgear of 35 °C. This maxi- occur that influence the admissible busbar current. For mum value of 105 °C for the busbars is significantly below example, if a busbar system with a high current passes close by a steel strut, this will cause the steel strut to heat up, the temperature at which the copper material would soften. which in turn will cause additional warming of the busbar In most cases, the external dimensions of the low-voltage at this point. This effect is generated in the sheet steel by switchgear are also decisive. Given the model-based design induced circulating currents and ring currents and can of the main busbar system, in some main busbar system actually only be minimised by the use of non-ferro-magnetic variants, a restricted range of dimensions is available. materials in the immediate vicinity of the busbars. As a result By testing the possible busbar systems, all the possible influ- of these additional heating effects, the admissible busbar ences described in this chapter from the enclosure itself, the current compared with a busbar system measured in the protection category, the influence of the materials surround- open air may be reduced. ing the busbar system and the devices used have been If a busbar system with a higher rated current is fitted in an taken into account, thus guaranteeing reliable operation. enclosure with a protection category of IP 54 without the possibility for air convection, the interior temperature inside nc of the busbar systems areIf the requisite rated currents I the enclosure will be significantly increased. The ambient known, with due regard for the protection category and the type of cooling, it is possible to select the required busbar temperature around the enclosure may still correspond to the system from Tables 41 – 43 (see page 88). Once a busbar normal conditions, but the interior temperature of the switch- system has been selected, in a second stage it is necessary gear is likely to increase significantly depending on the cur- to check whether the short circuit withstand capability rent. If the heating effects from induction are disregarded, a comparable figure can be achieved, as demonstrated by requirements are met. a calculation using a correction factor diagram. The direct ambient temperature around the busbar inside the switch- gear is used instead of the ambient temperature around the switchgear. Rittal Technical System Catalogue/Power distribution 2 - 46
Technical System Catalogue Ri4Power
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