INSULATORS AND BUSHINGS MOUNTING

Outdoor EHV Cable Sealing End

How Insulators & Bushings Standard and Quality Ensure for Underground Cable Project

The important to select and obligate the insulator and bushing technical specification complying the used materials to manufacture followed standard requirements. The cable sealing end termination, Surge Arrester, Cross-bonding grounding is the major point for an underground power transmission cable project where insulator is used. Also others type of insulator like pin/post/disc insulator, suspension insulator etc. are used.

The designated manufacturer followed the international standard to manufacture insulators and bushings. Porcelain or glass insulators and bushings should comply with the requirements of IEC 168, IEC 137, IEC 305, IEC 273, IEC 433 and IEC 815.

Porcelain for insulating purposes should comply with the requirements of BS 1598. Each porcelain insulator should bear the manufacturers mark and batch identification, which should be applied before firing. The clamping surfaces of all porcelain insulators should be accurately grounded and should be free of glaze.

Insulator and Bushing Test Standard

Insulators and bushings should satisfy the test requirements of IEC 168-Post, IEC 233-Hollow, IEC 383-Overhead Line. The design of insulators should be such as to minimize radio interference (RFI), and tests will be required as proposed in IEC 437 or equivalent to limit RFI to CISPR or CCITT recommended limits, or equivalent National Standards or Regulations.

Insulator and Bushing Materials Property

The post Insulator may made of good commercial grade wet process porcelain. The porcelain must be sound, thoroughly vitrified and free from defects and blemishes that might adversely affect the life of the Insulator. The exposed parts of the porcelain be smoothly glazed and specified (say brown) in color.

Insulators and bushings of organic molded or resin-bonded material should comply with the requirements of IEC 660 as appropriate. They should have a durable non-hygroscope surface finish with a high anti-tracking index. Precautions should be taken during manufacture and assembly of insulators of this type to exclude all moisture.

The Comparative Tracking Index (CTI) should be determined on all organic material insulators, and other insulating material as directed by the Engineer. The test method on any electrical materials intended for use outdoors or in servers ambient conditions should be in accordance with IEC 587, and materials not exposed to such conditions should be tested in accordance with IEC 112.

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Insulator and Bushing Material Identification

Insulator and bushings of molded or resin bonded material should be identified with the name of trade mark of the manufacturer, the type of Insulator and the batch & year of manufacture. Such marking should not impair the electrical properties of the surface finish.

Test on Insulators and Bushings

Type test, Sample test and Routing test are the main three test is mainly performed on insulator.

Type Test	Sample Test	Routing Test
Withstand Test Flash-over test Dry One-minute test Dry flash-over test One-minute Rain test Wet flash-over test Visible discharge test	Temperature cycle test Mechanical loading test Electro-mechanical test Over voltage test Porosity test	Mechanical Routine Test Mechanical load of 20% in excess of the maximum working load of the insulator is applied after suspending the insulator for one minute. There should be no mechanical failure of the insulator.

Properties of Insulating Materials

Properties of some insulating materials are as below:

Material	Conductivity (σ), S/m	Permitivity ( εr)
Water(distilled) Porcelain Glass Air SF6	2.10-4 10-10 10-10 .. ..	80 6 5 1.0006 1

Insulator and Bushing Mounting

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Insulators and bushings should be mounted, and the method of attaching connections be such that there is no likelihood of their being mechanically over stressed during normal tightening of the mounting and connection fixings. Similar provision should be made to accommodate expansion and contraction of the connections having regard in the temperature likely to be attained during fault conditions. 

Mountings should be of sufficient strength and rigidity to withstand the forces created by the passage of maximum prospective short-circuit current with full asymmetry, without permanent damage or permanent deflection sufficient to reduce electrical performance of insulation strength.