What is direct current? and application of DC current,circuit-breakers selection Criteria for direct current dc

 content:
                 Direct current (DC)

                 Main applications of direct current 

                 various factors for the interruption of direct current
                 selection of circuit-breakers for direct current


Direct current (DC) is the unidirectional flow of electric charge. Direct current is produced by sources such as batteries, power supplies, thermocouples, solar cells.Direct current may be obtained from an alternating current supply by using rectifier

direct current wave form

dc voltage wave form

Main applications of direct current
direct current (DC) used in many application due to its unique feature that is its storage facility

1) Emergency supply or auxiliary services:

the use of direct current is due to the need to employ a back-up energy
source which allows the supply of essential services such as protection
services, emergency lighting, alarm systems, hospital and industrial services,
data-processing centres etc., using accumulator batteries, for example.
 
2)  Electrical traction:

the advantages offered by the use of dc motors in terms of regulation and of
single supply lines lead to the widespread use of direct current for railways,
underground railways, trams, lifts and public transport in general.
 
3) Particular industrial installations:

there are some electrolytic process plants and applications which have a
particular need for the use of electrical machinery.
Typical uses of circuit-breakers include the protection of cables, devices and
the operation of motors.

Considerations for the interruption of direct current

Direct current presents larger problems than alternating current does in terms
of the phenomena associated with the interruption of high currents. 

Alternating currents have a natural passage to zero of the current every half-cycle, which corresponds to a spontaneous extinguishing of the arc which is formed when
the circuit is opened.This characteristic does not exist in direct currents, and furthermore, in order to extinguish the arc, it is necessary that the current lowers to zero.

The extinguishing time of a direct current, all other conditions being equal, is
proportional to the time constant of the circuit T = L/R.
It is necessary that the interruption takes place gradually, without a sudden
switching off of the current which could cause large over-voltages. This can be
carried out by extending and cooling the arc so as to insert an ever higher
resistance into the circuit.
The energetic characteristics which develop in the circuit depend upon the
voltage level of the plant and result in the installation of breakers according to
connection diagrams in which the poles of the breaker are positioned in series
to increase their performance under short-circuit conditions. The breaking
capacity of the switching device becomes higher as the number of contacts
which open the circuit increases and, therefore, when the arc voltage applied is
larger.
This also means that when the supply voltage of the installation rises, so must
the number of current switches and therefore the poles in series.

Criteria for the selection of circuit-breakers for dc
 
For the correct selection of a circuit-breaker for the protection of a direct current
network, the following factors must be considered:
1.the load current, according to which the size of the breaker and the setting
for the thermo-magnetic over-current release can be determined;
2.the rated plant voltage, according to which the number of poles to be
connected in series is determined, thus the breaking capacity of the device
can also be increased;
3.the prospective short-circuit current at the point of installation of the breaker
influencing the choice of the breaker;
4.the type of network, more specifically the type of earthing connection.