Low-voltage busbars

Low-voltage busbars are designed to carry electrical energy between the elements of an installation that supply it (generators, transformers, etc.) and distribute it (main distribution boards); they can also be part of the latter or of low-voltage switchgear assemblies subject to specific standards. They can also be used as distribution trunking, but this function is more generally entrusted to prefabricated trunking systems, which are also standardized.

In the case of transport, they are used in competition with insulated cables, but their main advantage is economic: for the same transported power, they cost considerably less, as metal (copper or aluminum) is better used.

This latter advantage is essentially due to better cooling conditions, since temperatures depend solely on environmental conditions and not on the limited resistance of cable insulation, which also acts as an obstacle to the outward transmission of internal heat.

On the other hand, they don't lend themselves well to complex pathways, as their shaping (or their accessories, in the case of prefabricated products) increases their cost.

In some cases, we may have to study several competing production methods; however, the solution chosen is not necessarily dominated by economic considerations, as other factors may come into play (adaptation, safety, local characteristics, limits of standardized assemblies, speed of supply or assembly, reuse, etc.), which may be decisive factors.

The main problems posed by the use of busbars can be grouped into two categories:

• the conditions of thermal equilibrium with their immediate environment, which determine the admissible currents, for an acceptable metal temperature; these are subject to the influence of many factors;

• mechanical design conditions, according to the stresses likely to be applied to them, both in normal service and in the event of a fault (short-circuit).

The installation regulations (NF C 15 100) give no indication of the permissible currents in busbars.

Numerous specifications from public authorities and industrial customers, technical documentation from electrical equipment manufacturers, and switchboards used by installers for decades – are all available, although their origin is not always known! – have put forward values that do not agree with each other.

Differences of up to 50% have been observed, almost certainly because the many parameters involved in the calculations were not assessed or chosen in the same way.

When we consult the technical literature, apart from a German...