PM820-1 PM820-2 ABB Control system module spare parts

PM820-1 PM820-2 ABB Control system module spare parts

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Main applications of direct current:

• 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.

• 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.

• 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.