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STANDARD|COMPOSITE INSULATORS FOR MEDIUM VOLTAGE LINES(2)

STANDARD|COMPOSITE INSULATORS FOR MEDIUM VOLTAGE LINES(2)
3 SERVICE CONDITIONS
The service conditions to be considered for the material included in this standard are:
- Maximum Ambient Air Temperature: + 50 ºC.
- Minimum Ambient Air Temperature: - 40 ºC.
- Maximum relative humidity: 95%. For higher values see IEC 60071-1 and IEC 60071-2.
- Maximum altitude above mean sea level: 1.000 m (*). For higher values see IEC 60071-1 and IEC
60071-2
- Maximum solar radiation: 1.000 W/m2
Note: Insulation capacity depends on these service conditions, it can be considered the selection of a Um value equal to
or higher than the expected level for the nominal voltage when the equipment has to be installed at an altitude
higher than 1.000 m in order to compensate the decrease of withstand voltage of the external insulation.
3.1 Specific service conditions for Colombia
The reference altitude is 2.700 m.
4 DEFINITIONS
4.1 Insulator
Device designed to support and insulate a conductive element.
4.2 Composite insulator
Insulator made of at least two insulating parts, namely a core and a housing equipped with end fittings.
Composite insulators, for example, can consist either of individual sheds mounted on the core, with or
without an intermediate sheath, or alternatively, of a housing directly moulded or cast in one or several
pieces onto the core.
4.3 String insulator unit
Insulator of which the end fittings are suitable for flexible attachment to other similar string insulator units or
to connecting accessories.
4.4 Line post insulator
Rigid insulator intended to be subjected to cantilever, tensile and compressive loads, constructed with one or
more insulating materials and assembled on a metal base that is intended to be mounted rigidly on a
supporting structure.
4.5 Creepage distance
Shortest distance through the insulator surface between conductive parts of both sides of the insulator which
support the service voltage.
4.6 Arcing distance
Shortest distance in the air between conductive parts of both sides of the insulator which support the service
voltage.
5 TECHNICAL REQUIREMENTS
5.1 Mechanical Load
5.1.1 String insulators: Specific Mechanical Load (SML)
Three levels of specific mechanical load are specified:

5.1.2 Line Post insulator
Four levels of specific cantilever load are specified:

5.2 Electrical requirements
5.2.1 Standardized insulation levels
The standardized insulation levels are specified according to IEC 60071-1:

For countries of South America, it could be used higher electrical requirements due to their specific
conditions:

 
5.2.2 Creepage distance and arcing distance
The minimum creepage distance of an insulator depends on its maximum material voltage and the pollution
level . Standard IEC 60071-2 establishes a rule to get a value for this minimum creepage distance:

On the other hand, the minimum arcing distance is related to the lighting impulse withstand voltage.
Standard IEC 61466-2 shows a table relating the insulation level and the minimum arcing distance for
standardized composite string insulators:

As a rule, the application of IEC 60071 (parts I and II) based upon the real voltage and the real service
conditions will give the definition of the needed insulator.
5.3 Other dimensions
5.3.1 Total length
The total length is defined considering the minimum distance phase-ground from the supports.
5.3.1.1 Line post insulators
The minimum total length phase-ground shall be higher than 300 mm.
5.3.2 String insulators: Maximum diameter
According to IEC 61466, the maximum diameter from the isolating zone of the string insulators is 200 mm.
6 DESIGNATION
6.1 String insulators
The string insulators are designated as follows (see IEC 61466-1 and 2):
- Letters CS
- Specific Mechanic Load in kN
- Two letters representing both end fittings, first one for the insulator upper side.
- Two numbers separated by a slash, the first one represents the lighting impulse withstand voltage in
kV and the second one the minimum creepage distance in mm
- A dash followed by the total length in mm (this last information doesn’t appear in the IEC 61466)
6.2 Line post insulators
Line post insulators are designated as follows:
- Letters CLP
- Specific Cantilever Load in kN followed by dash.
- Two numbers separated by a slash, the first one represents the lighting impulse withstand voltage in
kV and the second one the minimum creepage distance in mm
6.3 Designation Example
CS 70 EB 125/600-455
Composite string insulator, Specific mechanic load equal to 70 kN, with an eye for the support-side and a ball
for the conductor side, lighting impulse withstand voltage equal to125 kV and minimum creepage distance of
600 mm. Total length equal to 455 mm.
7 CONSTRUCTION CHARACTERISTICS
7.1 Construction
There are three important parts in an insulator:
- Dielectric Insulator Core
- Hydrophobic insulation housing
- Fittings
The insulator shall be designed and assembled to ensure that no moisture, water or external substances
reach the core.
7.1.1 Dielectric Insulator Core
It transmits the mechanical stresses produced by conductors to the support and provides the necessary
electrical insulation.
......to be continued

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