Using Circuit Breakers For Direct Current (DC) Applications

Circuit breakers in direct current

This technical application paper is intended to explain the main aspects of the most important applications of circuit breakers in direct current. The main purpose is giving precise information by means of tables providing a rapid choice of the protection / disconnection device and paying particular attention to the installation characteristics (fault typologies, installation voltage, earthing arrangement).

There are also some annexes giving further information about direct current, and more precisely:

• Information about the distribution systems in compliance with the IEC 60364-1
• Calculation of the short-circuit current in d.c. in compliance with the IEC 61660-1
• Circuit breakers and disconnectors for applications up to 1000 Vd.c.

DC Applications

In the low voltage field, direct current is used for different applications, which, in the following pages, have been divided into four macro families including:

1. Conversion into other forms of electrical energy (photovoltaic plants, above all where accumulator batteries are used);
2. Electric traction (tram-lines, underground railways, etc.);
3. Supply of emergency or auxiliary services;
4. Particular industrial installations (electrolytic processes, etc.).

The basic element of a photovoltaic plant is the photovoltaic cell constituted by semiconducting material (amorphous silicon or monocrystalline silicon). This cell, exposed to the rays of the sun, is able to supply a maximum current I_mpp at a maximum voltage V_mpp, which a maximum power called W_p corresponds to.

More photovoltaic cells are connected in series to form a string to raise the voltage level. By connecting more strings in parallel, the current level is increased.

Electric traction

The particular torque-speed characteristic curve and the ease with which the speed itself can be regulated have led to the use of d.c. motors in the field of electric traction. In the present scenario, direct current is used above all in urban transport, that is trolleybuses, trams, underground railways with a supply voltage of 600V or 750V, up to 1000V.

Direct current supply gives also the great advantage of having the contact line consisting of a single conductor since the rails provide the return conductor.

The use of direct current is not limited to vehicle traction only, but direct current represents a supply source for the auxiliary circuits onboard vehicles; in such cases, accumulator batteries are installed, which constitute an auxiliary power supply source to be used if the external one should fail.

The applications of circuit-breakers in d.c. circuits for electric traction in general can be summarized as follows:

• Protection and operation of both overhead and rail contact lines;
• Protection of air compressors on board underground and train cars;
• Protection of distribution plants for services and signaling systems;
• Protection of d.c. supply sources (accumulator batteries)
• Protection and operation of d.c. motors.