## Technical Application Paper

Direct current, which was once the main means of distributing electric power, is still widespread today in the electrical plant supplying particular industrial applications. The advantages in terms of settings, offered by the employ of d.c. motors and by supply through a single line, make direct current supply a good solution for railway and underground systems, trams, lifts and other transport means.

 (photo credit: ABB circuit-breakers for direct current applications)

In addition, direct current is used in conversion plants (installations where different types of energy are converted into electrical direct energy, e.g. photovoltaic plants) and, above all, in those emergency applications where an auxiliary energy source is required to supply essential services, such as protection systems, emergency lighting, wards and factories, alarm systems, computer centers, etc.. Accumulators - for example – constitute the most reliable energy source for these services, both directly in direct current as well as by means of uninterruptible power supply units (UPS), when loads are supplied in alternating current.

This Technical Application Paper is intended to explain to the readers the main aspects of the most important applications in direct current.

#### Generalities on Direct Current

Knowing the electrical characteristics of direct current and its differences in comparison with alternating current is fundamental to understand how to employ direct current.

For definition, the electric current called “direct” has a unidirectional trend constant in time.

As a matter of fact, by analyzing the motion of the charges at a point crossed by a direct current, it results that the quantity of charge (Q) flowing through that point (or better, through that cross section) in each instant is always the same. The sources which can provide direct current are batteries or dynamos; besides, through a rectifying process it is possible to convert an alternating current into a direct current. However, a “pure” direct current, that is a current which does not present any periodic fluctuation, is generated exclusively by batteries (or accumulators).

In fact,the current produced by a dynamo can present small variations which make it not constant in time; nonetheless, from a practical point of view, this is considered a direct current.

In a d.c. system, respecting the current direction has a remarkable importance; therefore it is necessary to connect correctly the loads by respecting the polarities, since, in case of a wrong connection, operation and safety problems could arise. For example, if a d.c. motor were supplied by reversing the polarities, it would rotate in the reverse direction and many electronic circuits when supplied in the wrong way could also suffer irreversible damages.

This article was extracted from "Circuit-breakers for direct current applications" by ABB