Circuit-breakers inside LV Switchboards

Dissipation of the heat generated inside switchboards

After an analysis of the main heat sources and of the measures to limit heat generation, the modalities through which switchboards can dissipate the heat towards the outside are described now. Many of these considerations derive from the Std. IEC/TR 60890, which gives formulas and tables where constructional characteristics and installation modalities are in relation with the temperature-rise at the same power loss. In particular, this article shall take into consideration the switchboard ventilation, the surfaces of the switchboard and their positioning, the form of internal separation of the switchboard and the degree of protection of the switchboard. 

2.2.1 Switchboard ventilation 

To increase the switchboard cooling, it is important that a good circulation of air inside the switchboard is realized  and maintained. To this purpose, for example, the possible ventilation openings are to be properly dimensioned and positioned. As regards dimensioning, IEC/TR 60890 for temperaturerise assessment inside low voltage switchgear and controlgear assemblies prescribes for the enclosures with ventilation openings that the cross section for air outlets is 1.1 times the cross section of the inlets. This requirement is due to the greater volume of hot air (going out of the switchboard) in comparison with the cold air (going into the switchboard). When disregarding this prescription, the air inlet surface of the switchboard is not fully exploited. 

As regards the positioning of the ventilation openings, these are to be located so that the “draught chimney” effect is achieved: an opening shall be positioned at the bottom of the switchboard, on the front part, the other one shall be positioned at the top, on the rear part, or on the “roof” of the switchboard. It is important to remind that any openings at mid height could reduce the “draught chimney” effect, thus causing a reduction of the air “draught”. The equipment inside the switchboard shall be positioned so that the circulation of the air is not excessively impeded by a reduction of the section for the air flow. In case of withdrawable circuit-breakers, particular attention shall be paid to prevent the obstruction of the ventilation openings in the fixed part of the circuit-breaker . 

2.2.2 Side surfaces and positioning of switchboards

 It is necessary to take into account that a switchboard exchanges heat with the surrounding environment through its surfaces (top, bottom and side walls) and therefore, at the same power dissipation level by the internal components, the larger the exchange surface towards the outside and the better the exchange conditions depending on the installation modality are, the greater amount of heat is released. For example, the switchboard should be positioned so that the air circulation around its external surface is facilitated or, however, is little impeded, thus improving heat exchange. IEC/TR 60890, which, as already said, suggests a method for the temperature-rise assessment inside switchboards, does not consider the real external geometric surface of the switchboard, but introduces the concept of effective cooling surface “Ae”, intended as the sum of the individual surface areas (top, front, side, ....) “A0 “ multiplied by the surface factor “b”. This factor takes into account the heat dissipation of the individual surfaces according to the type of installation of the enclosure, that is the different capacity of dissipating heat according to the surface positions and to their being either exposed or covered. The values of the parameter “b” related to the different surface types are shown in the table below .

                                                                     Ae =∑ (A0 x b)

2.2.3 Forms of internal separation of switchboards 

With separation form it is meant the type of division provided for the different circuits inside the switchboard. Separation is obtained by means of metallic or insulating barriers or partitions. For further information about the different forms of separation reference shall be made to the  prescriptions of Std. IEC 60439-1. As evident, remarkable separation forms tend to limit air circulation inside the switchboard, thus affecting the temperature inside the switchboard itself. 

2.2.4 Switchboard protection degree 

The degree of protection IP shows the protection of the enclosure against access to hazardous parts, against ingress of solid foreign objects and ingress of water. The code IP is the identification system of the degrees of protection based on the prescriptions of the Std. IEC 60529. The degree of protection of a switchboard affects its capacity of dissipating heat: the higher the degree of protection is, the less heat is dissipated by the switchboard. Therefore, the choice of high degrees of protection is not recommended when they are unnecessary. Besides, it should be kept in mind that a defined degree of protection may be reached through different modalities. For example, the protection against the vertical fall of drops of water (IPX1) can be realized by such modalities so as not to affect heat dissipation and so as to keep the “chimney effect” inside the switchboard. 

This article has been extracted from circuit-breakers inside LV switchboards by ABB 

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