The logic behind calling 1000 V to 72.5
kV a medium-voltage range is not obvious unless it is compared with
the maximum North American grid voltages of 800 kV and more common
transmission system voltages of 60 to 500 kV.
Nevertheless, medium-voltage circuit
breakers can protect AC generators, some transmission and
subtransmission lines, and distribution substations.
The industry classifies circuit
breakers in the following way:
# Medium-voltage power (1000 V to 72.5
kV)
# Low-voltage power (1000 V and below)
# Industrial molded case (600 V and
below)
# Miniature or branch circuit (240 V
and below)
All circuit breakers are
electromechanical devices that make and break currents under normal
conditions and make, carry for a specified time, and break currents
under abnormal conditions such as short circuits. Circuit breakers,
like transformers and batteries, are made in a wide range of ratings.
Because the higher-voltage circuit
breaker contacts can be damaged or destroyed by the burning action of
electric arcs when the contacts of a high-voltage circuit are opened,
various methods have been developed to provide an appropriate
quenching medium around the contacts that will assist in
extinguishing any arcs formed as rapidly as possible.
The names of mediums used for
extinguishing the arc are included in the descriptions of the circuit
breaker. For example, there are oil circuit breakers, air-blast
circuit breakers, and magnetic-air circuit breakers.
The selection of the appropriate method
for quenching the arcs depends on the cost-effectiveness and
availability of sources of and means for providing air blasts,
insulating gas, insulating oil, magnetic fields, or vacuums.
The two basic designs for high-voltage
circuit breakers are oil and oil-less. The oil type circuit breaker
had been the most popular for outdoor service up to 362 kV, but the
air-blast and gas-type versions have been gaining in popularity.
At 550 and 800 Kv, oil-less breakers
predominate. For new indoor applications magnetic-air and vacuum
circuit breakers predominate, along with some gas-type. Indoor
magnetic-air, air-blast, and vacuum breakers have been adapted for
outdoor use in the 2.5- to 34.5-kV range by protecting them with
metal covers.
It is essential that the correct
circuit breakers, fuses, and switches be selected for each power
control application because of their importance in the design and
function of the overall electrical system.
Immediately upon sensing a short
circuit or break in the supply line, fuses and circuit breakers must
isolate the sections of the electrical network where the fault
occurred, to prevent further damage while permitting the remainder of
the network to remain operational.
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