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# Using Circuit Breakers For Direct Current (DC) Applications

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## 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 (photovoltaicÂ plants, above all where accumulator batteriesÂ are used);
2. Electric traction (tram-lines, underground railways,Â etc.);
3. Supply of emergency or auxiliary services;
4. Particular industrial installations (electrolytic processes,Â etc.).

The basic element of a photovoltaic plant is the photovoltaicÂ cell constituted by semiconducting materialÂ (amorphous silicon or monocrystalline silicon). This cell,Â exposed to the rays of the sun, is able to supply a maximumÂ current Impp at a maximum voltage Vmpp, whichÂ a maximum power called Wp corresponds to.

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

For example, if a single cell can provide 5A at 35.5 Vd.c.,Â in order to reach the level of 100A at 500 Vd.c., it is necessaryÂ to connect 20 strings in parallel, each of themÂ constituted by 15 cells.

### Electric traction

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

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

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

It is very important that this power supply is guaranteedÂ since the auxiliary circuits may supply essential services,Â such as: air conditioning plants, internal and externalÂ lighting circuits, emergency brake systems, electrical heating systems, etc.

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

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

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