Page 164 - Handbook of Electrical Engineering

SWITCHGEAR AND MOTOR CONTROL CENTRES 147

• History of operation in similar plants and locations.
• Single or duplicate busbar system requirements.

Air-break circuit breakers are almost the same in design as the low voltage air-break devices
described above except that they are more robust and insulated for the high voltage. They are only
available in the 3-pole form, and up to about 24 kV is possible. They tend to be the most expensive and
require more frequent maintenance due to their exposed construction and relatively more complicated
mechanisms. Vacuum and sulphur hexafloride equipment is less expensive and tend to be preferred
to air-break equipment.
Sulphur hexafloride (usually referred to as SF6) gas is also used as the arc extinguishing
medium. SF6 circuit breakers are very robust, economical, small in size, and extremely reliable.
They require almost no major maintenance for at least 10 years of operation. The gas is contained
under a pressure slightly above atmospheric pressure and sealed in hermetically. There is no contact
with the outside air. Metalclad equipment is available up to 36 kV and SF6 has to a large extent
replaced all the air-blast equipment for distribution voltages up to 400 kV. At 11 kV the typical
ratings are 400 A to 2500 A with fault making duties up to about 25 kA.

Vacuum and SF6 devices were developed at about the same time as competitors. There is
little to choose between them since they are both simple mechanisms. Some engineers in the past
considered the possibility that the vacuum could be lost while the circuit breaker was in its ‘on’
state was a serious disadvantage. However, the technology has greatly improved and thousands of
vacuum circuit breakers are in service. Vacuum circuit breakers are limited to about 13.8 kV due
to insulation difficulties across the open contacts. Current ratings at 13.8 kV are limited to about
3000 A, with corresponding fault making duties up to 100 kA peak. SF6 equipment tends to be
preferred to vacuum equipment.

7.2.4 Forms of Separation

Cubicle type switchgear can be constructed in many different arrangements depending upon a variety
of requirements. For example the following aspects may be important for a particular plant, environ-
mental protection, ease of access to internal parts, ease of terminating cables, fixed or withdrawable
switching devices, maintainability and level of personnel skill, cost and economics, expected life
duration of the product, fitness for its purpose. The switchgear industry is very competitive and so it
is essential to clearly specify what is required in the form of assembly and its construction. Otherwise
a false or unsatisfactory decision may be made mainly based on a cost comparison.
The steel clading and compartments are necessary for support of the electrical and mechanical
components, and for providing a safety barrier for the personnel who operate and maintain the
switchgear. Safety risk has two main features. Firstly, electric shock and secondly, injury from
explosive faults and fires. A well-designed enclosure should ensure that these features are minimised.
The following discussion refers to IEC standards for low voltage switchgear in particular. However,
the basic concepts also apply to high voltage switchgear.
The main IEC standard for low voltage switchgear assemblies is IEC60439 which has seven
parts. Part 1 covers the basic requirements for internal separation, compartments, barriers and parti-
tions. It uses the IPXY notation of IEC60529 as a basis for the ingress protection, mainly concentrating
on ‘X’ for access by tools, fingers, hands, small particles and dust. The minimum value of ‘X’ used in