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STANDARD|COMPOSITE INSULATORS FOR MEDIUM VOLTAGE LINES(3)

7.1.1.1 Material
Epoxy resin resistant to hydrolisys reinforced with glass fiber resistant to corrosion in order to prevent the risk
of brittle fractures.
7.1.2 Hydrophobic insulation housing
The hydrophobic insulation housing (sheath and sheds) protects the core from external agents providing
sealing and preventing the formation of a continuous film of water. The hydrophobic insulation housing
minimum thickness is 3mm. The housing provides the necessary creepage distance in order to get the
required insulation of the core surface.
In order to facilitate the integration with the environment the colour of the silicone-rubber shall be gray. Other
different colour shall be approved by Enel.
7.1.2.1 Material
Sheath and sheds shall be silicone-rubber (VMQ - Vinyl-Methyl-Polysiloxane, with filler additives) free of
EPDM or other organic rubbers.
7.1.2.1.1 Silicone rubbers
Silicone-rubbers recommended in this standard are:
- HTV (High Temperature Vulcanized – solid silicone rubber): This type of silicone rubber is solid
and vulcanizes at a high temperature (near 200ºC).
- LSR (Liquid Silicone Rubber): Two components of liquid silicone rubber are mixed to vulcanize at
a temperature between 100 and 200ºC.
Two fabrication processes are allowed for both silicone rubbers considered in the standard, molding process
or by assembling modules.
The characteristics of the silicone rubbers are described in the table below:

At every existing interface from the composite insulator, the adhesion strength of the interface (interface
resistance) shall be higher than the tear strength of the silicone.
Silicone-rubbers of insulators shall have a resistance to tracking and electric erosion with a classification of
Class 1A 4,5 according to IEC 60587 and shall resist the effects of corona discharges and ozone. It shall
withstand a low-current arc discharge for more than 300 seconds under the conditions indicated in standard
IEC 61621 and its volume resistivity shall be over 1010 ·m according to IEC 62631.
The silicone-rubber must be type V0 according to the IEC 60695-11-10.
Additionally the silicone rubber shall have highly hydrophobic features and shall be classified type WC1 as
specified in IEC TS 62073.
7.1.3 Fittings
The fittings transmit the mechanical stress from the ends of the core to the support and to the conductor.
There is a triple junction point located where the core meets the metal coupling and the silicone-rubber
coating ends (connection zone). Usually, the electrical field strength is concentrated in this point, so the
fabrication process shall be careful in this point. The connection zone must be water and air sealed to ensure
the total enclosure of the insulator.
The end fittings shall be directly attached to the core by radial or circumferential compression process for a
better load distribution.
7.1.3.1 Material
Hot dip galvanized forged steel:
High pollution:
- Minimum for individual sample: 500g/m², equivalent to 70m
- Minimum for the average of the whole sample: 600 g/m², equivalent to 85m
Very high pollution:
- Minimum for individual sample: 720g/m², equivalent to 100m
- Minimum for the average of the whole sample: 865 g/m², equivalent to 120m
7.1.3.2 Type of end fitting for string insulators
Standard IEC 61466 presents different types of end fittings. The ones selected for this standard are:


The end fittings must be only assembled by the insulator manufacturer, during the manufacturing process.
7.1.3.3 Type of end fitting for line post insulators
See local section for details.
7.2 Quality issues
The technical conformity of a particular design of composite insulator strings shall be determined by
accomplishing design tests, type tests, sample tests and routine test related in this document, but,
additionally, it is recommended that manufacturers take into account the conclusions given in the document
CIGRÉ 33-204. This document include some recommendations related to parting lines when removing the
insulator from the mould, distance between last shed and end fitting or the connection zone between core,
end fitting and housing.
On the other hand, IEC TS 60815-3 contains gives specific guidelines and principles of the behaviour of an
insulator in certain pollution environment. This guideline would be recommended in this type of situations.
Finally, IEC/TR 62662 is a technical report which gives guidance for production, testing and diagnostics of
polymer insulators with respect to brittle fracture of core materials.
8 MARKING
Each insulator shall be clearly and indelibly marked as specified in IEC 62217, indicating:
- The name or trademark of the manufacturer.
- The year of manufacture.
- The specified mechanical load (SML) / Maximum Design Cantilever Load (MDCL) – kN.
- Manufacturer reference for the insulator.
- Material maximum voltage
9 TESTING
The composite insulators must be according the standards IEC 62217 (for all types of polymeric insulators),
IEC 61109 (only for string composite insulators) and IEC 61952 (only for line post composite insulators).
Tests are divided into four groups:
- Design tests
- Type tests
- Sample tests
- Routine tests
Design and Type tests are performed once, during the TCA process. Sample and Routine tests shall be
carried out on every singular purchased lot, as an acceptance tests.
9.1.1 Design Tests
These tests are intended to verify the suitability of the designs, materials and methods of manufacture
(technology).
When happens any change in the design, these test must be repeated. IEC 62217 gives the general
recommendation for the criteria and IEC 61109 and 61952 the concrete rules.
The design of a composite insulator is defined by:
- Core and housing materials and their manufacturing method.
- When needed, end fitting material, design and method of attachment (excluding the other fittings of
the string).
- Thickness of the core housing.
- Core diameter.
Additional tests on the housing material in order to obtain certain mechanical and electrical properties:

9.1.1.1 Test on resistance of core against stress corrosion
This test will be performed at ambient temperature to confirm the mechanical resistance against corrosion
stress of the core. It is done according to the indications of the IEC Project 36-6-2 of WG 36-07 through the
Spanish transposition of the IEC/TR 62662 (UNE-CLC/TR62662 IN).
Test specimen
It will be used an insulator from the production line or a specimen with length between end fittings of at least
10 times the core diameter. The end fittings shall be identical to those used in the production.
The test is executed in the bare part of the rod, so the housing must be removed in the middle part of the
insulator in a minimum length of 150 mm. The visible core surface has to be smoothed by means of a fine
abrasive cloth (grain size 180). Remaining parts of the housing have to be completely removed. An acid
container made of polyethylene shall be arranged surrounding the visible core surface in such a way that
the liquid can simply be poured into the container and no acid comes into contact with the end fittings. The
size of the acid container shall be adapted in such a way that the core is surrounded by a liquid thickness
not less than 1 cm and a liquid level of not less than 4 cm. The container shall be covered to prevent liquid
evaporations greater than 5% of its volume during the test period.
Performance of the test
The insulator must be subjected to a tensile load applied between the metal parts along the test. The tensile
load must be increased rapidly but regular, from zero up to 70% of the specified mechanical load (SML) and
then must be maintained at this value for 96 h. Immediately after applying the load, a nitric acid of 1N
concentration must be poured into the acid container (i.e. 1 N = 63,01 g HNO3 per litre of solution). The acid
must not come into contact with the end fittings.

Test evaluation
The test is passed if no fracture of the core occurs during the 96 h. test (no failures occur, and
macroscopic inspection reveals no damage or change in the composite after the exposure).
9.1.2 Type tests
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......to be continued

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