Electricity + Control 2019

ELECTRICAL PROTECTION + SAFETY

The IEC 61439 series of standards is applicable to low voltage switch- gear and controlgear assemblies. In this short series of tutorials, Johan Basson looks at some of the terminologies and explains them so the reader may gain better understanding of their meaning. Extracts from the standard itself are used extensively. This third tutorial focuses on the topic of the ‘rated diversity factor (RDF)’ in electrical switchgear and controlgear assemblies. Understanding the terminology in IEC 61439 Johan Basson, Managing Director - JB Switchgear Solutions

An installation showing functional units within an assembly.

I n general all circuits within an assembly are in- dividually capable of carrying their rated current continuously but, the current carrying capacity of any circuit may be influenced by adjacent cir- cuits. Thermal interaction can result in heat being imported from, or exported to, circuits close by. Cooling air available to a circuit may be at a tem- perature well above the ambient temperature due to the influence of other circuits. In practice, not all circuits within an assembly are normally required to carry rated current con- tinuously and simultaneously. Within a typical ap- plication the type and nature of loads differ appre- ciably. Some circuits will be rated on the basis of inrush currents and intermittent or short duration loads. A number of circuits may be heavily loaded while others are lightly loaded or switched off. To provide assemblies in which all circuits can be operated at rated current continuously is there- fore unnecessary and would incur an inefficient VALUES OF RATED DIVERSITY FACTOR Number of main circuits Rated diversity factor 2 and 3 0,9 4 and 5 0,8 6 to 9 inclusive 0,7 10 and above 0,6 Table 1.

use of materials and resources. The standard rec- ognises the practical requirements of assemblies by assigning a rated diversity factor – as defined in IEC 61439-1 chapter 3. 8. 11. In stating a rated diversity factor, the assembly manufacturer is specifying the ‘average’ loading conditions for which the assembly is designed. The rated diversity factor confirms the per unit value of rated current to which all the outgoing circuits, or a group of outgoing circuits, within the assembly, can be continuously and simultaneously loaded. In assemblies where the total of the rated currents of the outgoing circuits operating at rated diversity fac- tor exceeds the capacity of the incoming circuit, the diversity factor applies to any combination of outgo- ing circuits used to distribute the incoming current. Rated diversity factor (RDF) The rated diversity factor is the per unit value of the rated current, assigned by the assembly manu- Notes 1.The assumed loading of the outgoing circuits can be a steady continuous current or the thermal equivalent of a varying current. The rated diversity factor is applicable with the assembly operating at rated current. 2. The rated diversity factor recognises that multiple func- tional units are in practice not fully loaded simultane- ously or are intermittently loaded.

1. In general all circuits with- in an assembly are individ- ually capable of carrying their rated current con- tinuously but, in practice, not all circuits within an assembly would normally be required to carry their rated current continuously and simultaneously. Take Note! 1

16 Electricity + Control

JUNE 2019