I te 

UNITED STATES DEPARTMENT OF AGRICULTURE 

Rural Utilities Service 

RUS Bulletin 1724E-220

RD-GD-2017-87

SUBJECT: Procurement and Application Guide for Non-Ceramic Composite Insulators, 

V0Ita2:e Class 34.5 kV and Above 

TO: All Electric Bonowers, Consulting Engineers,個d RUS Electric Staff 

EFFECTIVE DATE: Date of Approval. 

OFFICE OF PRIMARY INTEREST: Engineering Standards Branch;Office of Policy, 

Outreach, and Standards. 

FILING INSTRUCTIONS: This bulletin replaces RUS Bulletin 1724E-220, " Procurement 

and Application Guide for Non-Ceramic Composite Insulators, Voltage Class 34.5 kV and 

Above" issued March 17, 2005. 

AVAILABILITY: This bulletin can be accessed via the internet at: 

h tto: //www. rd. u sda. 2:ov /o u b Ii ca ti o ns/re2:ula ti o ns-2:u id elin es/bulletins/electric. 

PURPOSE: This bulletin provides a basis for procuring non-ceramic composite insulators for 

voltage class 34.5 kV and above. 

C .m s/rjt Christopher A. McLean 

Assistant Administrator, 

Electric Program 

Disclaimer: The contents of this guidance document does not have the force and effect of law and is not 

meant to bind the public in any way. This document is intended only to provide clarity to the public 

regarding existing requirements under the law or agency policies.
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ACKNOWLEDGMENTS 

The following current and former members of the Transmission Line Subcommittee of the 

National Rural Electric Cooperative Association, Transmission and Distribution Engineering 

Committee provided invaluable assistance in preparing this document: 

Ballard, Dominic, East Kentucky Power Coop., Winchester, KY 

Beadle, Bob, North Carolina EMC, Raleigh, NC 

Beckett, Thomas, Enercon, Kennesaw, GA 

Bertelsen, James, Dairyland Power Cooperative, La Crosse, WI 

Caldwell, Art, Georgia Transmission Corporation, Tucker, GA 

Harvey, Gary, East Kentucky Power Cooperative, Winchester, KY 

Johnson, Wilson, USDA, Rural Development Utilities Program, Washington, DC 

Kahanek, Bil, McCord Engineering, Inc., College Station, TX 

Lukkarila, Charles, Great River Energy, Maple Grove, MN 

McAndrew, Jeremy, South Mississippi Electric Power Assoc., Hattiesburg, MS 

Metro, Patti, National Rural Electric Cooperative Association, Arlington, VA 

Nordin, Bryan, Tri-State Generation & Transmission Association, Inc., Denver, CO 

Ruggeri, Erik, Power Engineers, Hailey, ID 

Shambrock, Aaron, South Central Power Company, Lancaster, OH 

Twitty, John, PowerSouth Energy Cooperative, Andalusia, AL 

Woodruff, Paul, Great River Energy, Maple Grove, MN 
 

 

 

 

 

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TABLE OF CONTENTS 

1 PURPOSE 7
 

2 STANDARDS 7
 

3 DEFINITIONS 11
 

4 GENERAL 14
 

5 DESIGN CONSIDERATIONS FOR SUSPENSION INSULATORS 21
 

6 DESIGN CONSIDERATIONS FOR POST INSULATORS 23
 

7 STATION POST INSULATORS 27
 

8 ENVIRONMENTAL CONSIDERATIONS 27
 

9 QUALITY ASSURANCE PROGRAM 30
 

10 TESTING 30
 

11 INSULATOR HANDLING 36
 

Exhibits
 

Exhibit A: SAMPLE SPECIFICATION A-1

Exhibit B: SI-METRIC CONVERSIONS B-1

INDEX: 

INSULATORS: Non-Ceramic Composite 

SPECIFICATIONS AND STANDARDS: Guide Specifications for Non-Ceramic Composite 

Insulators, Voltage Class 34.5 kV and Above 
 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

ABBREVIATIONS 

ANSI American National Standards Institute 

ASTM American Society for Testing and Materials 

ASQ American Society for Quality 

BIL Basic Insulation Level 

CBL Cantilever Breaking Load 

E Electrical 

ECR Electrical Chemical Resistant 

EPDM Ethylene Propylene Diene Monomer 

EPM Ethylene Propylene Monomer 

EPR Ethylene Propylene Rubber 

FRP Fiberglass-Reinforced Plastic 

IEC International Electrotechnical Commission 

IEEE Institute of Electrical and Electronics Engineers 

ISO International Organization for Standardization 

M&E Mechanical and Electrical 

MDCL Maximum Design Cantilever Load 

NESC National Electrical Safety Code 

psi pounds per square inch 

RCL Reference Cantilever Load 

RIV Radio-Influence Voltage 

RTL Routine Test Load 

RUS Rural Utilities Service 

SCL Specified Cantilever Load 

SML Specified Mechanical Load 

SR Silicone Rubber 

STL Specified Tensile Load 

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1 PURPOSE 

The objective of this guide is to assist users in developing specifications for procurement 

of non-ceramic composite insulators. Information in this bulletin will assist users not 

familiar with non-ceramic composite insulators and current standards in developing 

purchase specifications. 

This bulletin provides recommended design and manufacturing criteria to ensure 

acceptability of non-ceramic composite insulators performance on electrical facilities 

operating at voltages 34.5 kV and above. This guide is consistent with present day criteria 

already developed for non-ceramic composite insulator standards. 

Since this guide is general in nature, it may not address specific utility applications. 

Therefore, users should consider their own particular needs and select those criteria that 

are appropriate for a specific installation of a non-ceramic composite insulator. 

2 STANDARDS 

The latest editions of the following standards are relevant to non-ceramic composite 

insulators: 

a AMERICAN NATIONAL STANDARDS INSTITUTE (ANSI) 

ANSI C29.1 “Test Methods for Electrical Power Insulators” 

ANSI C29.2B “Insulators - Wet-Process Porcelain and Toughened Glass – 

Transmission Suspension Type” 

ANSI C29.11 “Composite Suspension Insulators for Transmission Lines – 

Test Methods” 

ANSI C29.12 “Composite Insulators – Transmission Suspension Type” 

ANSI C29.13 “Insulators - Composite - Distribution Deadend Type” 

ANSI C29.17 “Composite Insulators – Transmission Line Post Type” 

b AMERICAN SOCIETY FOR TESTING AND MATERIALS (ASTM) 

ASTM A153 “Standard Specification for Zinc Coating (Hot-Dip) on Iron 

and Steel Hardware” 

ASTM D412 “Standard Test Methods for Vulcanized Rubber and 

Thermoplastic Rubbers and Thermoplastic Elastomers— 

Tension” 
 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

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ASTM D495 “Standard Test Method for High-Voltage, Low-Current, Dry 

Arc Resistance of Solid Electrical Insulation” 

ASTM D570 “Standard Test Method for Water Absorption of Plastics” 

ASTM D573 “Standard Test Method for Rubber-Deterioration in an Air 

Oven” 

ASTM D575 “Standard Test Methods for Rubber Properties in 

Compression” 

ASTM D623 “Standard Test Methods for Rubber Property-Heat 

Generation and Flexing Fatigue in Compression” 

ASTM D624 “Standard Test Method for Tear Strength of Conventional 

Vulcanized Rubber and Thermoplastic Elastomers” 

ASTM D750 “Standard Practice for Rubber Deterioration Using Artificial 

Weathering Apparatus” 

ASTM D790 “Standard Test Methods for Flexural Properties of 

Unreinforced and Reinforced Plastics and Electrical Insulating 

Materials” 

ASTM D1149 “Standard Test Methods for Rubber Deterioration—Cracking 

in an Ozone Controlled Environment” 

ASTM D1499 “Standard Practice for Filtered Open-Flame Carbon-Arc Type 

Exposures of Plastics” 

ASTM D2240 “Standard Test Method for Rubber Property—Durometer 

Hardness” 

ASTM D2275 “Standard Test Method for Voltage Endurance of Solid 

Electrical Insulating Materials Subjected to Partial Discharges 

(Corona) on the Surface” 

ASTM D2565 “Standard Practice for Xenon Arc Exposure of Plastics 

Intended for Outdoor Applications” 

ASTM G21 “Standard Practice for Determining Resistance of Synthetic 

Polymeric Materials to Fungi” 

ASTM G154 “Standard Practice for Operating Fluorescent Light Apparatus 

for UV Exposure of Nonmetallic Materials” 

ASTM G155 “Standard Practice for Operating Xenon Arc Light Apparatus 

for Exposure of Non-Metallic Materials” 
 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

c 

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AMERICAN SOCIETY FOR QUALITY (ASQ) 

ASQ Z1.4 “Sampling Procedures and Tables for Inspection by 

Attributes” 

ASQ Z1.9 “Sampling Procedures and Tables for Inspection by 

Variables for Percent Nonconforming” 

d INSTITUTE OF ELECTRICAL AND ELECTRONICS ENGINEERS (IEEE) 

IEEE 4 “Standard for High-Voltage Testing Techniques” 

IEEE 539 “Standard Definitions of Terms Relating to Corona and 

Field Effects of Overhead Power Lines” 

IEEE 957 “Guide for Cleaning Insulators” 

IEEE C2 “National Electrical Safety Code” 

e INTERNATIONAL ELECTROTECHNICAL COMMISSION (IEC) 

IEC 62217 “Polymeric HV insulators for indoor and outdoor use - General 

definitions, test methods and acceptance criteria” 

IEC 60815-1 “Selection and dimensioning of high-voltage insulators 

intended for use in polluted conditions - Part 1: Definitions, 

information and general principles” 

IEC 60815-3 “Selection and dimensioning of high-voltage insulators 

intended for use in polluted conditions - Part 3: Polymer 

insulators for AC systems” 

IEC 61109 “Composite Insulators for A.C. Overhead Lines with a 

Nominal Voltage Greater than 1,000 V - Definitions, Test 

Methods and Acceptance Criteria” 

IEC 61466-1 “Composite String Insulator Units for Overhead Lines with a 

Nominal Voltage Greater than 1,000 V - Part 1: Standard 

Strength Classes and End Fittings” 

f INTERNATIONAL ORGANIZATION FOR STANDARDIZATION (ISO) 

ISO 9001 “Quality Management Systems” 
 

 

 

 

 

 

 

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g PURCHASE OF RELATED SPECIFICATIONS AND STANDARDS 

All ANSI and IEC standards may be purchased from: 

American National Standards Institute, Inc.
 

25 West 43rd Street, 4th Fl.
 

New York, NY 10036
 

Telephone: 1-212-642-4900
 

Website: www.ansi.org
 

All ASTM standards may be purchased from: 

American Society for Testing Materials
 

100 Barr Harbor Drive
 

West Conshohocken, PA 19428
 

Telephone: 1-610-832-9585
 

Website: www.astm.org
 

All ASQ standards may be purchased from: 

American Society for Quality
 

600 North Plankinton Avenue
 

Milwaukee, WI 53203
 

Telephone: 1-800-248-1946
 

1-414-272-8575 (outside the U.S. and Canada) 

Website: www.asq.org 

All IEEE standards may be purchased from:
 

Institute of Electrical and Electronics Engineers
 

445 Hoes Lane
 

PO Box 1331
 

Piscataway, NJ 08855-1331
 

Telephone: 1-800-678-4333
 

1-732-981-0060
 

Website: www.ieee.org
 

All ISO Standards may be purchased from: 

International Organization for Standardization
 

1 rue de Varembé
 

CH-1211 Geneva 20
 

Switzerland
 

Telephone: 4-122-749-0111
 

Website: www.iso.org
 
 

 

 

 

 

 

 

 

 

 

 

 

 

 

3 

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DEFINITIONS 

Cantilever Breaking Load (CBL) - A load specified by the manufacturer that represents 

the failing load of a non-ceramic composite post insulator. The failing load is the 

maximum load attainable under Cantilever Load testing and is greater than or equal to the 

Specified Cantilever Load (SCL). 

Chalking - A condition wherein filler material becomes exposed on the surface of rubber 

as a result of weathering (ultraviolet exposure). 

Core - The internal part of a composite insulator. The core is the mechanical loadbearing component of the insulator. The core consists mainly of glass fibers impregnated 

with a resin-based matrix so as to achieve maximum strength. It is also called a 

fiberglass-reinforced plastic (FRP) rod. 

Coupling Zone - The part of the metal end fittings of the insulator that transmits applied 

mechanical load to the line hardware, to the structure, or to accessories that are external 

to the insulator. The coupling zone does not include the interface between the core and 

the end fittings. 

Cracking - Any surface fracture of a depth greater than 0.1 mm. 

Crazing - The formation of surface microfractures of depths up to 0.1 mm resulting from 

ultraviolet exposure. 

Damage Limit - The long-term mechanical bending tests performed on line post 

insulators by the manufacturer to determine a damage limit for the fiberglass–resin core. 

Below the damage limit, the core exhibits elastic behavior where, with the test load 

removed, the core returns to its original position (possibly after several days) with no 

degradation. Above the damage limit some fibers begin to break and, when the test load 

is removed, the core does not return to its original position. If the test load is maintained, 

rod deflection begins to increase with time and more fibers break until eventually the 

insulator fails. 

Design Tests - Tests to evaluate the electrical design of the non-ceramic composite 

insulator that are required only on insulators of new design. Design tests may also be 

used to evaluate various mechanical characteristics. 

End Fittings, Metal - The metal attachment hardware that is connected to the insulator 

core that transmits the mechanical loads to the core at the ends of the insulator. 

EPM (Ethylene Propylene Monomer) - Sometimes used as a base polymer in nonceramic composite insulator rubber formulations. 

EPDM (Ethylene Propylene Diene Monomer) - Commonly used as a base polymer in 

non-ceramic composite insulator rubber formulations. 
 

 

 

 

 

 

 

 

 

 

 

 

 

 

3 

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DEFINITIONS (continued) 

EPR (Ethylene Propylene Rubber) - The generic term that includes both EPDM and 

EPM. 

Erosion - Nonconductive loss of material from the insulating surface caused by leakage 

current, corona discharge, or dry band arcing. Erosion can be uniform, localized, or treeshaped. Shallow surface traces, commonly tree-shaped, can occur on composite 

insulators, as on ceramic insulators, after arcing (flashover). Erosion is unobjectionable 

as long as it is non-conductive. When it becomes electrically conductive, it is known as 

tracking. 

Filler - A material that is used to improve the performance of the housing material. 

Grading Device - A device for controlling the potential gradient at the end fittings. 

Housing - The insulator component which is external to the core and contains both a 

sheath and weathersheds. The sheath–weathershed system protects the core and provides 

the required leakage distance. The weathersheds also provide wet electrical strength. 

Hydrolysis - A chemical process involving the reaction of a material with water in liquid 

or vapor form. It can lead to electrical and mechanical degradation (depolymerization). 

Hydrophobicity - The ability of a material to repel water. 

Interfaces - The surface between different materials. Examples of interfaces in composite 

insulators are as follows: 

• Glass fiber/impregnating resin • Weathershed/sheath 

• Filler/polymer • Housing/metal end fittings 

• Core/housing • Core/metal end fittings 

Lot - A number of insulator units in which each constituent component of the units (core, 

housing, metal end fittings, etc.) comes from the same production run and the units are 

assembled in the same manner and process. Each lot should, as far as is practicable, 

consist of units of product of a single type, grade, class, size, or composition 

manufactured under essentially the same conditions and in contiguous time periods. The 

units will have the same Specified Mechanical Load (SML) or Specified Cantilever Load 

(SCL) rating. The number of units in a lot should normally be set at some number not to 

exceed 500 units. 

Maximum Design Cantilever Load (MDCL) - The non-ceramic composite line post 

insulator cantilever load rating assigned by the manufacturer. The maximum load that 

can be applied to the post insulator every day with no deleterious effect on the service life 

of the insulator. Some manufacturers list this rating as the Reference Cantilever Load 

(RCL). 
 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

3 

Bulletin 1724E-220 

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DEFINITIONS (continued) 

Mechanical Load Test - A mechanical test for non-ceramic composite suspension 

insulators as specified in ANSI C29.11 and used to determine if a lot of insulators meet 

the Specified Mechanical Load (SML) requirements. The historical failure loads from 

this test justify the manufacturer’s choice of SML. Mechanical testing is also performed 

on every insulator to check its Routine Test Load (RTL) rating. 

Non-Ceramic Composite Insulator - An insulator unit that is made from material other 

than porcelain, glass, or other ceramic material. It consists of a load-bearing resinimpregnated fiberglass core, metal end fittings, and external elastomeric housing. 

Post Insulator - Primarily intended to be loaded in a cantilever mode, although specific 

applications may require loading in tension or compression. The most common types 

are: (1) horizontal line post where the insulator projects nearly horizontally from a 

structure and is loaded in bending by the conductor, (2) braced line post or horizontal Vee 

application, and (3) a station post insulator used as a bus support in an outdoor substation. 

Post insulators used in disconnect switch applications may also be loaded in torsion. 

Prototype Tests - Tests to evaluate and verify the suitability of materials, interfaces, 

prototype design, and method of manufacture. Prototype tests are performed only once 

and are considered valid for the whole class of insulators represented. 

Quality Assurance Program - A program that includes quality conformance testing and 

statistical control of the manufacturing process to ensure quality of the product. 

Quality Conformance Tests - Destructive or nondestructive tests that are used to verify 

insulator conformance to specific characteristics and determine acceptability of an 

insulator lot. 

Reference Cantilever Load (RCL) - A rating assigned by the manufacturer which 

indicates the maximum load that can be applied to the post insulator every day with no 

deleterious effect on the service life of the insulator. Some manufacturers list this rating 

as the Maximum Design Cantilever Load (MDCL). 

Routine Test - A test performed on every insulator from each lot to identify insulators 

with manufacturing defects. 

Routine Test Load (RTL) - The load applied to non-ceramic composite suspension 

insulators that is equal to or greater than 50 percent of the insulator Specified Mechanical 

Load (SML) rating. Also, considered to be the maximum continuous working load of the 

insulator. 

Silicone Rubber (SR) - Usually in the form of polydimethylsiloxane, it is used as a base 

polymer in non-ceramic composite insulator rubber formulations. It is known for its 

hydrophobic (water-repellent) properties. 
 

 

 

 

 

 

 

 

 

 

 

 

 

3 

Bulletin 1724E-220 

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DEFINITIONS (continued) 

Specified Cantilever Load (SCL) - A load specified by the manufacturer that represents 

the ultimate strength of a non-ceramic composite line post insulator under cantilever 

loading. The strength should be verified during cantilever load tests, and the historical 

failure loads (CBL) should justify the manufacturer’s choice of SCL. The SCL forms the 

reference point for selection of a composite line post insulator. It is not the maximum 

working load of the insulator. See MDCL. 

Specified Mechanical Load (SML) - A load specified by the manufacturer that represents 

the ultimate strength of a non-ceramic composite suspension insulator under tension. The 

strength should be verified during Mechanical Load Tests, and the historical failure loads 

should justify the manufacturer’s choice of SML. It forms the reference point for 

selection of a non-ceramic composite suspension insulator. It is not the maximum 

working load of the insulator. See RTL. The SML of an insulator may be reduced by the 

class of hardware used for the end fittings. 

Specified Tensile Load (STL) - The load specified by the manufacturer that represents 

the ultimate strength of a non-ceramic composite line post insulator under tension. The 

strength is verified by the same testing procedures used to determine the Specified 

Mechanical Load (SML) for composite suspension insulators. 

Suspension Insulator - Any insulator intended primarily to carry tension loads. It 

includes tangent, dead-end, and vee-string installations. 

Tracking - The formation of electrically conductive carbonized paths starting and 

developing on the surface of the insulating material and causing irreversible deterioration 

of surface material. These paths are conductive even under dry conditions. Tracking can 

occur on surfaces in contact with air and also on the interfaces between insulating 

materials. 

Treeing - The formation of micro-channels within the housing material causing 

irreversible internal deterioration. The micro-channels can be either conducting or nonconducting and can progress through the bulk of the material until electrical failure 

occurs. 

Ultimate Strength - An insulator’s tensile, compressive, or cantilever loading at which 

any part of the insulator fails to perform its function of providing mechanical support. 

Damage to the insulator core is likely to occur at loads lower than the insulator failing 

load. 

Weathershed - The part of the insulator’s housing which protrudes from the sheath and 

used to provide added leakage distance. 
 

 

 

 

4 

Bulletin 1724E-220 

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GENERAL 

a	 Strength and Loading. Actual line loads on an insulator consist of a combination 

of loads. The loads involved are quite different for a suspension insulator 

(tensile) as compared to a line post insulator (compressive, bending, and tensile). 

The non-ceramic composite insulator’s mechanical strength is determined by the 

fiberglass-reinforced resin core. The core strength depends on the type and ratio 

of glass fibers and resin, the polymerization process, and the core diameter. 

Insulator strength is directly related to core diameter and attachment strength of 

the metal end fittings. 

b Vertical, Longitudinal, and Transverse Loads. Because of the difficulty in 

acquiring new transmission line rights-of-way, and the need for efficient use of 

existing transmission line rights-of-way with minimal environmental impacts, 

utilities have focused on compact line designs utilizing line post insulators. Nonceramic composite insulators are flexible and respond well to line post 

configuration stresses that cause unbalanced longitudinal, vertical, and transverse 

loads such as unbalanced ice loads, unbalanced extreme wind loads, a 

combination of unbalanced wind and ice loads, sudden impacts (vehicular), line 

drops, and seismic activity. 

Insulator loading requirements are described in IEEE C2, “National Electrical 

Safety Code”. The NESC considers two weather loadings: (1) combined ice and 

wind loading and (2) extreme wind loading. Where both rules apply, the required 

loading should be the one that has the greater effect. The code requires that 

calculations be performed to determine the vertical, transverse, and longitudinal 

loading on line supports. The engineer should consider and accommodate these 

loads when choosing the insulator’s mechanical strength. 

(1)	 Vertical loads on an insulator should consist of the insulator’s own weight, 

associated hardware, plus the conductor, including radial ice loads. The 

effect of any difference in elevation of the supports should also be 

considered in the vertical load. 

(2)	 Transverse loads are horizontal loads on conductors created by wind 

pressure applied at right angles to the direction of the line against the 

projected area of the conductors and associated materials including the 

radial thickness of ice. At angles, the transverse load should be the vector 

sum of the transverse wind load and wire tension load. 

(3)	 Longitudinal loads occur at (1) changes in grades of construction, (2) 

deadends, and (3) unbalanced line tensions in adjacent spans created by 

asymmetric ice loading or sag loading of unequal spans. 

Strength Requirements. The NESC specifies the insulator’s strength 

requirements. Insulators should withstand all applicable loadings as specified in 

c 
 

 

 

 

 

Bulletin 1724E-220 

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the code except those of extreme wind loading without exceeding the following 

percentages of their rated ultimate strength: Cantilever (40 percent), Compression 

(50 percent), and Tension (50 percent). Proper allowance should be made for 

extreme wind loading. 

d	 Advantages and Disadvantages of Non-Ceramic Insulators 

Non-ceramic composite insulators are designed to withstand long-term 

mechanical and electrical stresses for the lifetime of the unit. They are used on 

overhead transmission line designs utilizing suspension insulators and line post 

insulators. Suspension insulators generally carry tension loads and can be applied 

in I-string, vee-string, and dead-end-type applications. Line post insulators are 

generally loaded in bending or compression and can be applied in horizontal post, 

vertical post, or braced post configurations. Occasionally, they are used for 

substation bus supports and disconnect switch applications. 

(1)	 Contaminated Environments. Non-ceramic composite insulators have 

improved flashover performance characteristics as compared to ceramic 

insulators with equivalent leakage distances, particularly in areas of heavy 

airborne contamination. Ceramic suspension insulators generally have a 

convoluted profile that allows the contaminant to more easily attach to the 

insulator surface. Non-ceramic composite insulators have simple, smooth 

shed profiles that enhance the performance of an insulator, especially in 

contaminated environments. 

Environmental contaminant magnitude, insulator leakage distance, and 

composition of weathershed material determine the flashover performance 

of the insulator. In general, utilities take a conservative approach and 

purchase non-ceramic composite insulators with leakage distances greater 

than or equal to ceramic insulators for the same application, thus allowing 

for additional protection from leakage current and flashovers. 

The type and composition of weathershed material affect the long-term 

contamination performance of an insulator. Silicone Rubber (SR) is 

hydrophobic and tends to resist wetting and causes beads of water to form 

from the moisture that comes into contact with the silicone material. If 

any environmental contamination adheres to the surface of silicone, the 

lightweight silicone chains in the SR material saturate the pollution layer 

and, upon wetting, form beads of water, and the contaminants roll off the 

weathersheds and clean the insulator. Therefore, it is recommended that 

SR insulators be utilized in contaminated environments. SR material 

greatly reduces the likelihood of pollution flashover. It should be noted 

that if continuous electrical discharge activity occurs, degradation in SR 

material may occur at a more rapid rate than with other insulating 

materials. This degradation will cause SR material to lose its 
 

 

Bulletin 1724E-220 

Page 17 

hydrophobicity thus greatly reducing the insulator’s performance in 

contaminated environments. 

Other types of insulator surface materials are categorized as hydrophilic 

and tend to wet easily, forming a thin water film over the surface of the 

insulator material. Ceramic and ethylene propylene rubber {EPR includes 

both ethylene propylene monomer (EPM) and ethylene propylene diene 

monomer (EPDM) materials} are hydrophilic. In contaminated 

environments, EPR may generally allow higher magnitudes of leakage 

current activity without significant degradation in the material. During 

foggy and light rain conditions, the insulators with hydrophilic materials 

in a contaminated environment may allow leakage currents to flow along 

the surface of the insulator, which can substantially increase the risk of 

flashover. Although EPR materials have exhibited better short-term 

flashover performance than ceramic insulators in contaminated 

environments, the risk of flashover may be reduced by increasing the basic 

insulation level (BIL) of the insulator or by utilizing insulators made of 

SR. Silicone additives have been combined with ethylene propylene 

rubber to create a material claiming better performance in contaminated 

environments than standard EPR materials. Hydrophilic insulator 

materials may be mechanically stronger than hydrophobic materials and 

may better resist leakage current activity. 

(2)	 Gunshot Damage. Non-ceramic composite insulators may be more 

desirable than ceramic insulators in areas of high vandalism because of 

gunshot damage. In-service non-ceramic composite insulators with a 

gunshot hole in the weathershed material will generally not pose a serious 

problem. However, bullets lodged in the fiberglass rod could create a 

serious problem, and these insulators usually are quickly removed from 

service. 

(3)	 Additional Benefits. Additional benefits of non-ceramic composite 

insulators include the following: 

(a)	 High strength-to-weight ratio creates ease of transportation and 

handling, thus reducing construction and maintenance costs. 

(b)	 The lighter weight allows less loading on structures. 

(c) Radio noise is reduced due to elimination of ball and socket joints. 

(d)	 Maintenance is reduced with hydrophobic materials, which 

generally require no cleaning of the insulator. 

(e)	 Ceramic line post insulators are brittle and do not deflect. This 

lack of resiliency may cause a cascading failure due to longitudinal 
 

 

 

 

 

 

 

 

 

 

Bulletin 1724E-220 

Page 18 

loading (shearing). Non-ceramic composite insulators are flexible 

and better protect the line from sudden impacts, line drops, wind 

gusts, asymmetric ice loading, and other longitudinal loads. 

e	 Composite Insulator Materials 

(1)	 Core: The internal core of a non-ceramic composite insulator consists of a 

fiberglass-reinforced plastic (FRP) rod. It is the primary mechanical loadbearing component of the insulator. The strength of the rod depends on 

the types of materials used, the diameter, and the percentage composition 

of glass fiber and impregnating resin. Generally, almost all manufacturers 

use E-glass (electrical) fibers and epoxy resin. ECR-glass (electrical 

chemical resistant) fiber is generally no longer used in the manufacturing 

process, but acid-resistant fiber is becoming available upon request. Acid 

resistant fiber may protect against brittle fracture caused by stress 

corrosion of the rod when the rod is exposed to moisture and corona 

discharge. Polyester and vinylester resins have been used but are 

uncommon. 

During fabrication, the individual glass fibers are equally tensioned to 

axially align each fiber parallel to the rod axis. The fibers are placed in a 

resin matrix such that each glass fiber is impregnated with the resin. The 

combination is then cured at a high temperature, allowing the individual 

fibers to bond to the resin matrix. This manufacturing technology is 

known as pultrusion. Generally, 60 to 70 percent of the FRP rod volume 

contains glass fibers. 

(2)	 Metal End Fittings: The strength of a composite insulator not only 

depends on rod materials and rod diameter, but also on the metal end 

fittings material type and the bonding procedures used to attach the metal 

end fittings to the FRP rod. Metal end fittings should be made from a 

good commercial grade of iron or steel and should be galvanized in 

accordance with ASTM A153, “Standard Specification for Zinc Coating 

(Hot-Dip) on Iron and Steel Hardware”. Metal end fittings may also be 

manufactured from a good grade of stainless steel. Aluminum is not 

recommended. 

Metal end fittings should be designed to effectively transmit the 

mechanical loads to the insulator core (FRP rod). The attachment of the 

metal end fittings to the FRP rod is important and should be performed 

with a symmetrically controlled crimping method that compresses the 

metal radially onto the rod without damage to the rod fibers or resin 

matrix. Excessive compression may crush the rod and cause de-bonding 

between the rod and the end fittings, which may reduce the mechanical 

strength of the insulator. Insufficient compression may cause slippage 
 

 

 

 

 

 

 

 

 

Bulletin 1724E-220 

Page 19 

between the rod and the end fittings. It is critical that the manufacturer 

provide the utility with documentation that details quality control 

procedures and crimping machine control mechanisms that prevent FRP 

rod damage and metal end fittings slippage. 

Metal end fittings should be properly sealed to prevent moisture contact 

with the FRP rod. This seal should never be broken during testing or 

otherwise. 

ANSI C29.11, “Composite Suspension Insulators for Overhead 

Transmission Lines - Tests” defines the following testing procedures for 

composite suspension insulators to confirm the integrity of the metal end 

fittings interface: 

Prototype Tests:	 Power Frequency Voltage Test
 

Sudden Load Release Test
 

Thermal Mechanical Test
 

Water Penetration Test
 

Steep Front Impulse Test
 

Sample Test:	 Mechanical Load Test
 

Galvanizing Test
 

Routine Tests:	 Tension – Proof Test 

The tensile strength of metal end fittings attached to the FRP rod should 

equal or exceed the specified mechanical load (SML) rating defined for 

the composite suspension insulator. Similar testing procedures to confirm 

the integrity of the metal end fittings interface for composite line post 

insulators are specified in ANSI C29.17, “Composite Insulators – 

Transmission Line Post Type”. 

(3)	 Housing: The housing is composed of a sheath and weathersheds, both 

are made of a polymeric compound. The sheath covers the fiberglass core 

to protect it from the external environment while the weathersheds provide 

the required leakage distance and electrical insulation strength. 

It is important when considering housing materials to determine which 

type of material is best suited for the insulator application. Housing 

materials are generally made from EPR or SR. An alloy material of 

EPDM and silicone additive is also available. The weathersheds and 

sheath may be bonded together during the vulcanization process or molded 

into one piece by injection molding. The sheath is generally bonded to the 

rod, but a void-free silicone grease interface is also available. The housing 

may be either bonded to the metal end fittings or sealed to prevent 

moisture ingress to the FRP rod. 
 

 

 

 

 

 

 

Bulletin 1724E-220 

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Experience has shown that the performance of different formulations 

varies for a given environment. Therefore, the utility should obtain 

written verification from the manufacturer that the insulator housing 

material considered will provide satisfactory performance in the particular 

environment to which the insulator is to be subjected by the purchaser. SR 

housing materials have exhibited the best short-term flashover 

performance and are generally recommended in areas with heavy 

contamination. 

(4)	 Grading Devices: When the electrical field created by a high voltage 

ionizes the air and discharges, it creates corona. Corona activity can 

degrade the composite insulator housing and core. Generally, corona 

activity occurs at the surface of line conductors and insulators, and at the 

energized hardware. Corona activity creates energy loss, radio influence 

voltage (RIV) audible noise. 

The electrical field distributed along a ceramic suspension insulator string 

is capacitively graded by the individual insulators’ metal caps and pins. 

Composite insulators have no metal components along their length, 

resulting in high electrical field magnitudes concentrated around the 

energized metal end fittings. In addition, corona activity generally 

increases at energized locations when environmental contaminants are 

present. Experience has shown that composite insulators subjected to 

continuous contamination may be damaged from corona activity 

concentrated along the molding seam of the insulator and occurring at the 

energized end of the housing of the insulator. Therefore, it is important to 

consider the addition of grading devices at the end fittings when using 

composite insulators to evenly dissipate electrical stress. Grading devices 

are generally shaped like large metal rings and are designed to alter the 

distribution of electric fields at the ends of the insulator. These metal 

rings, called corona rings, are designed to either reduce or eliminate 

corona discharge activity under wet and dry conditions. 

The IEEE 987, “Guide for Application of Composite Insulators”, specifies 

when grading devices are required for composite insulators. In general, 

suspension and dead-end insulators operating at voltages above 345 kV 

require grading devices at both ends. Voltages of 345 kV to 230 kV 

(inclusive) require grading devices at the energized end. Phase-to-phase 

insulators require grading devices at or above 161 kV, and line post 

insulators require grading devices above 230kV. These recommendations 

may not prevent serious dry or wet corona activity, and although 

manufacturers generally specify when grading rings are required, the 

decision should be made by the utility. 
 

 

 

 

 

 

 

5 

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Electrical field magnitudes will vary depending on the geometrical 

configuration of the insulator pole-top unit. Reduced phase spacing due to 

compaction of lines, voltage uprating of existing lines, contaminated 

environments, weather, and high altitudes may increase corona activity 

and possibly degrade the insulator materials. Therefore, it is 

recommended that grading devices be used on the energized end of all 

suspension insulators and dead-end insulators operating at a voltage of 115 

kV and above, and on both ends of suspension insulators operating at 345 

kV and above (including horizontal vee and braced post configurations), 

and at the energized end of all line post insulators operating at 230 kV and 

above. In environments subjected to long periods of contamination or salt 

air, or where the geometry of the structure hardware arrangement 

enhances the probability of corona activity, the utility should consider 

using corona rings on the energized end of all suspension insulators 

operating at 69 kV and line post insulators operating at 115 kV. In some 

expected severe cases the electrical field stresses may need to be 

calculated by a field plot and appropriate remedial measures should be 

added to the design. 

Corona rings may reduce the strike distance of the insulator. Care should 

be taken when replacing ceramic insulators with non-ceramic composite 

insulators to ensure that the addition of corona rings does not reduce the 

leakage distance of the insulator to undesirable levels, possibly increasing 

the probability of flashover. If the utility considers there is a possibility of 

flashover or corona damage then it is prudent to require the manufacturer 

to verify acceptable performance of the grading devices by conducting 

radio influence voltage and visual corona tests with the insulator, 

hardware, and conductor assembly that will be used. A visual corona test 

method using a light amplifier is provided in Section 10.c of this bulletin. 

If the utility or manufacturer does not have the test capabilities, there are 

several independent laboratories available that can perform these tests. 

DESIGN CONSIDERATIONS FOR SUSPENSION INSULATORS 

a	 Mechanical Considerations: Mechanical ratings for non-ceramic composite 

suspension insulators are similar to those for ceramic suspension insulators. The 

difference is non-ceramic suspension insulators are given a SML rating and 

ceramic suspension insulators are given a combined Mechanical and Electrical 

(M&E) rating. These two ratings are directly interchangeable for comparing their 

mechanical strengths. The maximum continuous working load applied to a nonceramic suspension insulator should not exceed 50 percent of the SML rating. 

(1)	 Tension Loads. Suspension insulators are usually subjected to tension 

loads. Normally suspension insulators do not have compression, bending, 

or combined load ratings. For applications such as vee-string or horizontal 
 

 

 

 

 

 

 

 

 

c 

Bulletin 1724E-220 

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vee installations where combined loading may occur, the manufacturer 

should be consulted to provide combined load ratings. 

(2)	 Twisting. When non-ceramic composite suspension insulators are 

installed horizontally, they may be subjected to twisting. The torsional 

loading capacity of a non-ceramic suspension insulator depends on the 

combined torsional strength of the rod and the method of attaching the end 

fittings to the rod. The manufacturer should be consulted regarding 

torsional loading, since long conductor dead-end spans may twist the 

insulator, exceeding its torsional strength. In the case where a single 

insulator is used to deadend a single conductor, a twist may inadvertently 

be placed on the insulator during tensioning. Composite insulators have 

various torsional capabilities based on their fiberglass rod characteristics. 

However, this torsional capability is small. It becomes necessary to watch 

the insulator during line stringing because long insulators can twist 360 

degrees. When some degree of twisting is unavoidable, be certain it is 

within the manufacturer’s limits. Otherwise, the use of parallel units or 

stress-relief hardware may be required. 

b	 Electrical Considerations: Ceramic and non-ceramic suspension insulators having 

the same section length may not have the same dry arc distance. The ability of an 

insulator to protect against flashovers could be affected by a reduction in dry arc 

distance. A reduction in the dry arc distance can be attributed to the length of the 

end fittings and placement of grading rings. Generally, the dry arc distance is the 

linear length of the polymeric material between the metal end fittings or the linear 

length between the grading rings. To maintain similar lightning performance and 

critical impulse flashover protection of ceramic insulators, the replacement nonceramic composite insulator should have the same dry arc distance. Care should 

be taken to ensure proper clearances are maintained between phases and adjacent 

structures. Swing angle should also be considered. 

The short-term flashover performance of non-ceramic composite insulators is 

better than that of ceramic insulators. Where wet switching surge or 

contamination is a primary concern, non-ceramic composite insulators generally 

provide improved performance over ceramic insulators. 

Insulator Interchangeability and Replacement: The mechanical working loads 

suspension insulators are required to withstand are specified in NESC Rule 277. 

Suspension insulators should not be loaded in tension beyond 50 percent of their 

rated ultimate strength except under extreme wind loading. 

The ultimate mechanical strength of a ceramic (porcelain or glass) suspension 

insulator is the “Combined Mechanical and Electrical Strength” as defined in 

ANSI C29.1, “Test Methods for Electrical Power Insulators”. ANSI C29.2B, 

“Insulators - Wet-Process Porcelain and Toughened Glass – Transmission 

Suspension Type”, requires the manufacturer to subject each assembled ceramic 
 

 

 

 

 

 

 

 

 

 

6 

Bulletin 1724E-220 

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suspension insulator to a “Tension-Proof Test” load of 50 percent of its ultimate 

mechanical strength. This value is the maximum working load of the insulator. 

All insulators that fail the test do not meet the requirements of ANSI C29.2. 

The ultimate mechanical strength of a non-ceramic composite suspension 

insulator under tension is the SML rating as defined in ANSI C29.11. The 

manufacturer’s selection for the value of the SML of an insulator is based on its 

historical failure load record. ANSI C29.11 requires the manufacturer to subject 

each assembled suspension insulator to a “Tension-Proof Test” load equal to or 

greater than the routine test load (RTL) rating. The RTL is a rating equal to 50 

percent of the SML. This is the maximum working load of the insulator. 

When insulators are expected to be subjected to tension and compression loading 

it is important to check that the insulators selected are intended for such use. If 

insulators are expected to be subjected mainly to tension loading applications but 

may occasionally be subjected to compression loads, it is important to check to be 

certain the insulators have compression load carrying ability satisfactory for the 

expected load. On some types of tension insulators, the grip of the metal end 

fittings to the fiberglass rod have a low compression load carrying ability and may 

not be suitable for installation where compression loading will be expected. It is 

especially important to check to be certain acceptable compression loading ability 

exists on insulators designed for tension used as phase-to-phase spacers. 

Non-ceramic composite suspension insulators come in a wide variety of metal end 

fittings including oval eye, Y-clevis, socket, ball, clevis, and tongue. Oval eye 

and Y-clevis end fittings are generally not available with ceramic insulators. 

When replacing ceramic suspension insulators with composite suspension 

insulators, careful consideration should be given to the required insulator length to 

maintain proper sag/tension requirements. 

DESIGN CONSIDERATIONS FOR POST INSULATORS 

a	 Mechanical Considerations: The mechanical characteristic by which non-ceramic 

composite line post insulators are rated is the specified cantilever load (SCL). 

The SCL represents the ultimate strength of a composite post insulator under 

cantilever loading. 

(1) Maximum Design Cantilever Load (MDCL). The maximum working load 

allowed for line post insulators is specified in NESC Rule 277. Except 

under extreme wind loading, loading of line post insulators should not be 

allowed to exceed 40 percent of the manufacturer specified cantilever 

load. 

The maximum design cantilever load (MDCL) or reference cantilever load 

(RCL) rating of a non-ceramic composite post insulator is defined as the 

maximum cantilever load that can be applied to a post insulator every day 
 

 

 

 

 

 

 

 

 

Bulletin 1724E-220 

Page 24 

with no deleterious effects on the service life of the insulator. The MDCL 

or RCL load rating generally represents 40 percent, but may be as much as 

50 percent (per ANSI C29.17), of the insulator’s ultimate cantilever 

strength and is determined by the manufacturer’s specified failing loads, 

which are based on cantilever load testing. 

Post insulator deflection table values are based on the insulator loaded at 

its MDCL rating. An insulator’s specific deflection values should be 

included when determining clearances to ground. Generally, deflections 

are linear in this range, and a linear relationship can be assumed when 

evaluating deflections at lower loading values. 

(2)	 Simultaneous Loading. It should be noted that the MDCL or RCL rating 

is for a single cantilever load applied to the insulator and should not be 

used solely to determine line post insulator loading limits. Line post 

insulators placed in service on overhead line applications are actually 

subjected to more than just a cantilever load along one axis. Horizontal 

line posts in actual use are subjected to vertical, transverse, and 

longitudinal loads simultaneously. Vertical, transverse, and longitudinal 

loads each contribute to the total bending moment, or total applied stress 

on the insulator’s core. Manufacturers provide combined loading 

application curves, which represent the mechanical strength limits of a 

non-ceramic line post insulator is when subjected to simultaneous loading. 

The loading curves are used to determine how a specific insulator’s 

combined loading requirements compare with its cantilever (bending) 

strength. The combined loading application curves are used during the 

engineering stage to evaluate whether the mechanical strength of a specific 

insulator is suitable for the specific line loading criteria. 

(3)	 Loading Calculations. The actual expected loading of an insulator should 

be calculated first and the results compared with the application curves. 

Insulator loading calculations are based on conductor type, weight and 

diameter. Conductor loading calculations are generally performed for 

three types of conditions: (1) everyday loading, (2) maximum wind or 

extreme wind loading, and (3) combined ice and wind loading. The 

loading requirements are specified in the NESC. 

Everyday loading is generally a vertical load determined by conductor 

weight. Transverse loading is created by horizontal wind pressure against 

the projected surface area of the conductor. Extreme wind loading creates 

a large transverse load on the insulator rod either in compression or 

tension. It should be noted that a composite post insulator’s strength is 

most limiting in compression. Combined ice and wind loading generally 

causes a larger vertical load than with everyday loading due to the weight 

of ice on the conductor. 
 

 

 

 

Bulletin 1724E-220 

Page 25 

(4)	 Applying Loads. Combined loading application curves based on the 

MDCL of a non-ceramic composite post insulator are used in determining 

the suitability of a specific insulator for a particular installation. Using the 

manufacturer’s application curves that are based on the insulator’s MDCL 

rating, first plot the everyday insulator loading that was calculated using 

the specific line design criteria and location where the insulator will be 

installed. Next, plot the maximum wind loading for the same installation 

site. Similarly plot the insulator’s combined ice and wind loading on the 

application curve. It is now possible to determine whether the 

manufacturer’s mechanical values of the insulator will be able to sustain 

the expected simultaneous loading without exceeding 40 percent of the 

insulator’s ultimate cantilever strength rating. 

It should be noted that the insulator’s allowable vertical and compressive 

load limits are dependent on the insulator’s longitudinal loading. 

Longitudinal loading on line post insulators is generally caused by 

unbalanced line tension created from asymmetric ice or sag loading. As 

the longitudinal load requirement decreases, the allowable vertical and 

compressive load limits increase. 

Actual conductor loading may exceed values given in the NESC as a result 

of local weather conditions or exceptional loading conditions. Care 

should be taken to evaluate the insulator strength based on realistic 

expected conductor loading conditions for the intended installation. 

(5)	 Damage Limit. To ensure long-term performance of composite line post 

insulators under extreme ice or extreme wind loading conditions, it is 

recommended that the manufacturer’s specified “damage limit” rating of 

the insulator used be greater than the expected cantilever (bending) load 

for realistic worst-case conditions in the particular line under construction. 

If the “damage limit” of the insulator is exceeded, the core could be 

irreversibly damaged. If cantilever loads remain below the “damage 

limit” during extraordinary conditions. The composite line post insulator 

should provide satisfactory performance without any decrease of strength 

over time. 

To establish the “damage limit,” the manufacturer determines the 

insulator’s maximum allowable cantilever or bending moment (stress). 

The manufacturer does this by testing specific rod sizes and rod lengths 

under various bending loads and time durations. The insulator is subjected 

to various magnitudes of bending loads for specific time periods and the 

resulting deflection values are recorded. Low stress bending loads are 

those loads to which a specific insulator design can be subjected to, incur 

deflection, and return to the original unloaded position after the bending 

load is removed. High stress bending loads are those loads on a specific 

insulator that cause the rod to deflect in such a way that the rod fibers 
 

 

 

 

 

 

 

 

c 

Bulletin 1724E-220 

Page 26 

become damaged (permanent deformation) and upon removal of the load 

the insulator does not return to the original unloaded position. Deflection 

variations with time are plotted for increasing applied bending stresses in 

the embedded cross-section of the rod and the “damage limit” or high 

stress load that causes permanent undesirable deflection of the rod is 

established. The “damage limit” is a zone between the elastic-type 

behavior and plastic-type behavior of the rod. 

According to manufacturers, the damage limit for a fiberglass–resin core 

can occur at approximately 50 percent of the core’s ultimate cantilever 

strength. The engineer should be aware that different manufacturer’s 

damage limit ratings vary, and the specific manufacturer should be 

consulted regarding how values are determined for the intended insulator. 

A combined loading application curve based on the insulator’s strength 

below its damage limit should be evaluated for line post insulator loading 

applications under extreme wind, extreme ice, or exceptional loading 

conditions. For specific extreme loading design applications, some 

manufacturers may formulate a special or non-standard fiberglass–resin 

core with a higher damage limit rating. The manufacturer should be 

contacted for the availability of this design option. 

Care should be taken when using the manufacturer’s combined loading 

application curves. While some manufacturers supply separate application 

curves based on the MDCL and damage limit ratings mentioned above, 

others may supply only one combined loading application curve. It is 

important to know which rating the application curve is based. 

b	 Electrical Considerations: Non-ceramic composite line post insulators can be 

obtained with the same or better electrical characteristics as the porcelain line 

posts they are intended to replace, and no electrical interchangeability problems 

are known. The engineer should, however, be certain that added accessories 

(corona rings, etc.) do not adversely affect the electrical characteristics of the base 

insulator design. 

Insulator Interchangeability and Replacement: The maximum working load for 

line post insulators, except under extreme wind loading, is specified in the NESC. 

Line post insulators should not be subjected to loads that exceed the following 

percentages of their rated ultimate strength: 40 percent cantilever, 50 percent 

compression, and 50 percent tension. The rated ultimate strength of ceramic line 

post insulators should be determined as specified in ANSI C29.17. The ratings 

are determined from the recorded values obtained during ultimate cantilever 

strength and tensile load testing. 

(1)	 Tensile Loading. The manufacturer is required to subject each assembled 

insulator to a routine tensile load test using a load of not less than 50 

percent of the rated specified tensile load (STL). This ANSI test load is 
 

 

 

Bulletin 1724E-220 

Page 27 

the recommended working load for a ceramic line post insulator subject to 

loads in tension and it doubles as a metal end fittings crimping verification 

test which checks the integrity of the core and end fittings connection. 

(2)	 Cantilever Loading. Presently, no routine cantilever test is required by the 

standard for a ceramic insulator, but the customer may require that the 

manufacturer perform a cantilever breaking load (CBL) sample test to 

confirm the manufacturer’s choice of the SCL rating. 

Non-ceramic composite line post insulators are rated by their ultimate 

strength and generally either by their MDCL or RCL. The ultimate 

cantilever strength is defined as the SCL, and this rating is based on the 

statistical results of historical cantilever load testing of composite line post 

insulators. The SCL is not the maximum working load of the insulator. 

The maximum working load of the insulator is defined as the MDCL or 

RCL, and this load may be applied in cantilever to the line post insulator 

every day with no harmful effect on its service life. The MDCL or RCL 

rating generally represents 40 percent of the insulator’s SCL rating 

specified by the manufacturer. It is recommended that the line post 

insulator’s working load, except under extreme ice or wind loading 

conditions, should not exceed 40 percent of the insulator’s SCL rating. 

(3)	 Compression Loading. When applying line post insulators in 

compression, it is important to check that they are suitable for such use. It 

is especially important to check the suitability of line post insulators used 

on inside angle structures, or as struts on braced post applications. 

(4)	 Combined Loading. Care should be given to select a non-ceramic 

composite line post insulator strength based on the simultaneous combined 

loading as seen by the insulator during its service life. Manufacturers 

generally provide application curves for line post insulators that detail the 

simultaneously combined compressive, vertical, and longitudinal load 

capability of the insulator. Subjecting non-ceramic composite insulators 

to a simultaneous combination of loads may reduce the vertical load 

carrying capability of the insulator. 

(5)	 Other Replacement Considerations. Non-ceramic composite line post 

insulators can be obtained with the same or better electrical characteristics 

as the porcelain posts they are intended to replace, and no electrical 

interchangeability problems are known. The engineer should, however, be 

certain that added accessories (corona rings, etc.) do not adversely affect 

the electrical characteristics of the base insulator design. 
 

 

 

 

 

 

 

 

Bulletin 1724E-220 

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7 STATION POST INSULATORS 

Ceramic station post insulator ratings are specified in standard ANSI C29.19, “Solid Rod 

Fiberglass Insulators (Apparatus Post Type)”. Non-ceramic station post insulators may 

not have the same electrical characteristics as ceramic insulators of the same length. 

Compression ratings should be dependent on the compressive forces expected for the 

installation and an insulator rated to be able to withstand these forces should be selected. 

Compression ratings are generally determined by the ceramic insulator that is to be 

replaced. Torsion ratings are a concern with switching applications. Combined load 

ratings should be considered and mounting dimensions should be thoroughly reviewed. 

The manufacturer should be contacted for information on interchangeability. 

8 ENVIRONMENTAL CONSIDERATIONS 

The engineer should give careful consideration to the environment in which the insulator 

will be placed. Environments of particular concern include those prone to fog, salt spray, 

high humidity, airborne industrial contaminants, and aerially applied agricultural 

fertilizers and sprays. 

a	 Leakage Distance: The engineer should be confident that the insulator leakage 

distance is adequate for the specific application environment. The leakage 

distance is the shortest path along the surface of the insulator between the metal 

end fittings. The required leakage distance will vary with the type and amount of 

contamination present. In many cases, the utility’

s experience will determine the 

leakage distance requirements for non-ceramic composite insulators. A guide that 

is generally used by utility companies for the selection of insulators in polluted 

environments is IEC 60815, “Guide for the Selection of Insulators in Respect of 

Polluted Conditions”. This guide defines pollution level categories for ceramic 

insulators. It is recommended that this guide also be used when selecting nonceramic insulators in polluted environments. Tables 1 and 2, below, are from IEC 

60815 may be used to determine the required leakage distance for insulators in 

polluted environments. 

(1)	 Specific Leakage Distance: The minimum “Specific Leakage Distance” of 

ceramic insulators in regard to pollution level is provided in IEC 60815. 

The “Specific Leakage Distance” is defined as the ratio of the leakage 

distance of the insulator divided by the rms phase-to-phase maximum 

voltage for the equipment expected on the electrical system to which the 

insulator will be installed. It is recommended that non-ceramic composite 

insulator leakage distance be equal to or greater than that recommended for 

ceramic insulators. The “Specific Leakage Distance” values recommended 

are provided as a guideline, and environments of severe pollution may 

require “Specific Leakage Distance” values greater than 1.22 inches/kV. If 

a ceramic insulator installed in the field has performed satisfactorily, then a 

non-ceramic composite insulator with similar leakage distance may be used 

as a replacement. If ceramic insulator performance has proven to be 

problematic, then the leakage distance requirement for a non-ceramic 

replacement insulator should be increased. 
 

 

 

 

 

 

 

 

 

 

 

 

Bulletin 1724E-220 

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Table 1 

Pollution Level Examples of Typical Environments 

I – Light 

• Areas without industries and with low density of houses equipped with 

heating plants 

• Areas with low density of industries or houses but subjected to frequent 

winds and/or rainfall 

• Agricultural areas 

• Mountainous areas 

All these areas shall be situated at least 10 km to 20 km from the sea and 

shall not be exposed to winds directly from the sea. 

II – Medium 

• Areas with industries not producing particularly polluting smoke and/or 

with average density of houses equipped with heating plants 

• Areas with high density of houses and/or industries but subjected to 

frequent winds and/or rainfall 

• Areas exposed to wind from the sea but not too close to the coast (at least 

several kilometers’ distance) 

III – Heavy 

• Areas with high density of industries and/or large cities with high density 

of heating plant producing pollution 

• Areas close to the sea or in any case exposed to relatively strong winds 

from the sea 

IV – Very 

Heavy 

• Areas generally of moderate extent, subjected to conductive dusts and to 

industrial smoke producing particularly thick conductive deposits 

• Areas generally of moderate extent, very close to the coast and exposed 

to sea spray or to very strong and polluting winds from the sea 

• Desert areas, characterized by no rain for long periods, exposed to strong 

winds carrying sand and salt, and subjected to regular condensation 

1 Use of fertilizers by spraying, or the burning of crop residues, can lead to a higher pollution level due to dispersal 

by wind. 2 Distances from the sea coast depend on the topography of the coastal area and on the extreme wind conditions. 

Table 2 

Pollution Level Minimum Nominal “Specific Leakage Distance” 

Inches/kV (mm/kV) 

I ­ Light 

II – Medium 

III – Heavy 

IV – Very Heavy 

0.63 (16) 

0.79 (20) 

0.98 (25) 

1.22 (31) 
 

Bulletin 1724E-220 

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b	 Insulator Material Composition: The composition of non-ceramic insulator 

weathershed materials is based on ethylene EPR or SR. A combination of EPDM 

and silicon additives is available. Laboratory testing and field installations have 

shown that these non-ceramic insulating materials exhibit improved short-term 

flashover performance over ceramic insulators in contaminated environments. 

(1)	 EPR Insulators. Generally, EPR insulators are hydrophilic and allow a 

thin film of water to form on the surface. EPR materials may exhibit 

hydrophobic properties initially due to residues left on their surface from 

the manufacturing process. Once the insulator is exposed to the 

environment, these residues wear off, allowing the surface material to wet 

out. This may lead to greater leakage current activity under contaminated 

environments and reduce the flashover performance of the insulator. 

(2)	 SR Insulators. SR insulators provide a hydrophobic weathershed surface 

by resisting surface wetting and forming water beads from moisture 

contact. A continuous water layer cannot form, thereby reducing the 

probability of leakage currents and associated discharge activity. When 

environmental contamination adheres to the surface of silicone, the 

lightweight silicone chains in the SR material saturate the pollution layer. 

Upon wetting, this saturated pollution layer mixes with the beads of water 

that form. This mixture eventually becomes heavy enough so that the 

water along with the contaminant rolls off the weathersheds thus cleaning 

the insulator. SR materials have exhibited superior flashover performance 

and aging characteristics compared to those of EPR and are recommended 

for areas of heavy contamination. It should be noted that excessive 

electrical discharge activity may cause degradation in the SR material to 

occur at a more rapid rate than with other insulator materials. This 

degradation will cause the SR material to lose its hydrophobicity thus 

greatly reducing the insulator’s performance in contaminated 

environments. 

c	 Contaminated Environment Performance: Non-ceramic composite insulators 

have performed well in contaminated environments and have been used to correct 

numerous flashover problems. In severely contaminated environments where 

heavy airborne pollution is present, a periodic inspection program should be 

performed to detect actual damage to the insulator. If non-ceramic composite 

insulators will be located in a contaminated environment, then it is recommended 

that test results be obtained from the manufacturer documenting the performance 

of the specific insulator elastomeric formulation in contaminated areas similar to 

the proposed installation environment. 

QUALITY ASSURANCE PROGRAM 

The utility should ask the manufacturer to furnish a description of its Quality Assurance 

Program including fabrication, testing, and inspection. Any material (i.e., rubber), 

9 
 

 

 

 

 

 

Bulletin 1724E-220 

Page 31 

components (i.e., rod) or hardware (i.e., end fittings) the manufacturer has had fabricated 

by others should also be included. Manufacturing methods and material composition 

documentation should be acquired and kept for future reference. It is recommended that 

the manufacturer have a Quality Assurance Program conforming to ISO 9001:2000, 

“Quality Management Systems - Requirements”, or the latest equivalent. 

10 TESTING 

When procuring non-ceramic composite insulators, the engineer should specify the 

electrical and mechanical parameter values and the testing the utility requires the 

manufacturer to conduct in order to demonstrate product conformance to the utility’s 

specifications. Although testing is specified in the ANSI standards, use of the standards 

by manufacturers is completely voluntary. Therefore, in procuring non-ceramic 

composite insulators it is recommended utilities require manufacturers specify and 

specifically cite the ANSI standards along with the minimum test parameter values the 

utility will accept for demonstrating compliance. 

a	 Composite Suspension Insulator Testing: ANSI standards C29.11 and C29.12, 

“Insulators - Composites - Suspension Type”, describe important characteristics 

and testing evaluations for non-ceramic composite suspension insulators intended 

for use on overhead transmission lines 70 kV and above. ANSI C29.12, 

“Insulators – Composite – Suspension Type”, specifies materials and testing 

applicable to non-ceramic composite suspension insulators, and ANSI C29.11 

specifies the specific testing methods and procedures. 

A discussion of the ANSI tests that should be specified when procuring nonceramic composite suspension insulators is provided in Section 10.a of this bulletin. 

(1)	 Prototype Tests: Prototype tests verify the suitability of design, materials, 

and manufacturing methods. They are described in ANSI C29.11, 

Sections 4.1 and 7. Prototype testing should be considered valid for the 

whole class of insulators represented by the specific prototype design. 

Prototype tests are performed only once for each particular type of 

insulator design and the results are documented in a test report. It is 

recommended that the test reports be obtained from the manufacturer prior 

to procurement of a class of insulators to verify successful completion of 

the prototype testing. The testing is done in four parts and is described in 

Sections 7.1, 7.2, 7.3, and 7.4 of ANSI Standard C29.11. The testing 

should include the following: 

• Tests on Interfaces and Connection of End Fittings 

 Test Specimen – Three insulators should be tested 

 Power Frequency Voltage Test 

 Sudden Load Release Test 

 Thermal Mechanical Test 

 Water Penetration Test 

 Verification Tests 
 

 

 

 

 

 

 

Bulletin 1724E-220 

Page 32 

o	 Visual 

o	 Steep Front Impulse Voltage Test 

o	 Power Frequency Voltage Test 

o	 Dye Penetration Test 

•	 Core Time – Load Test 

 Test Specimen – Six insulators should be tested 

 Determination of Average Failing Load of the Core 

 Core Time Load Test 

•	 Housing Tracking and Erosion Test 

 Test Specimen – Two insulators should be tested 

 The Test Chamber, Conditions, and Evaluation shall be performed 

as specified in ANSI C29.11 

•	 Core Materials Test 

 Dye Penetration Test 

 Water Diffusion Test 

See ANSI C29.11 for a complete description of the testing and acceptance 

criteria. 

(2)	 Electrical Design Tests: Electrical design tests verify the insulator’s 

performance characteristics based on its size and shape. The tests are 

performed only once on complete, full-scale insulators of a specific 

electrical design. The design testing that should be performed is defined 

in ANSI C29.12. The procedures describing how the testing will be 

performed are documented in ANSI C29.11 and C29.1. 

It is recommended that the test reports be obtained from the manufacturer 

prior to procurement of a specific voltage class of insulators so the utility 

can verify completion of design testing as required by ANSI C29.12. This 

testing should include the following: 

•	 Low-Frequency Dry Flashover Test 

•	 Low-Frequency Wet Flashover Test 

•	 Critical Impulse Flashover Tests – Positive and Negative 

•	 Radio-Influence Voltage (RIV) 

If grading rings are to be purchased, then the utility should request that the 

manufacturer provide reports of successful electric field modeling or 

testing for the specific insulator design to be purchased. 

(3)	 Quality Conformance Tests: ANSI C29.12 specifies the quality 

conformance testing that should be performed on non-ceramic composite 

insulators. The specific testing procedures are defined in ANSI C29.11 
 

 

 

 

 

Bulletin 1724E-220 

Page 33 

and C29.1. The test procedures are identified as sample tests in C29.11. 

The insulators are selected from the production lot at random with the 

purchaser having the right to make the selection. Testing is required on 

each lot. The tests verify that the insulators conform to the requirements 

of the ANSI standards. ANSI C29.12 requires the following tests be 

performed on at least three insulators from each lot: 

 Dimensional Tests 

 Galvanizing Test 

 Specified Mechanical Load Test 

Some ambiguity exists between ANSI C29.12 and C29.11 regarding 

sample selection quantity. ANSI C29.11 was completed in the 1980s in an 

attempt to get a usable standard out to the public. ANSI C29.12 was 

published in 1997 and is a more recent standard. Because of the 

ambiguities, it is recommended that the utility include in its specific 

requirements the test quantity of insulators to be subject to Quality 

Conformance Testing (sample tests). It is recommended that for lots of 

more than 200 insulators, the utility specify at least three samples be tested 

for both the dimensional tests and galvanizing test. At least one sample 

should be mechanical load tested. For lots of 200 insulators or less, the 

number of samples tested should be determined by agreement between the 

utility and manufacturer. The utility should feel comfortable that the 

sample quantity tested is sufficient to represent quality conformance for all 

insulators procured. The utility should also determine an acceptance 

criteria based on the number of tested samples. In the event one insulator 

or metal part fails to comply with the requirements of a sample test, a retest procedure is described in ANSI C29.11, paragraph 9.6. 

(4)	 Routine Testing: Routine testing should be performed on every insulator 

produced to help eliminate procurement of insulators with manufacturing 

defects. ANSI C29.12 specifies two routine tests: 

•	 Tension – Proof Test 

•	 Visual Examination 

The test procedures are defined in ANSI C29.11. 

(5)	 Development of ANSI Standards: The development of ANSI standards 

regarding non-ceramic composite insulators is an ongoing process. 

Testing methods for electrical power insulators are defined in ANSI 

C29.1. Testing methods included in ANSI C29.1 have been generally 

utilized for ceramic insulators. In an attempt to define testing methods 

and procedures for composite suspension insulators, ANSI C29.11 was 

developed. ANSI C29.11 is considered the test standard. ANSI C29.12 

was developed to define the tests that are required to be performed to meet 
 

 

 

 

 

Bulletin 1724E-220 

Page 34 

minimum requirements for the manufacturing and procurement of 

composite suspension insulators. ANSI C29.12 is considered a product 

standard, similar to ANSI C29.2 for ceramic insulators. ANSI is presently 

developing standards for composite station post insulators (ANSI C29.19). 

b	 Composite Line Post Insulator Testing: ANSI C29.17 describes the test 

procedures applicable to non-ceramic elastomeric composite line post insulators 

intended for use on overhead lines in electric power systems energized at 70 kV 

and above. 

A discussion of the ANSI tests that should be specified when procuring nonceramic composite line post insulators is provided in Sections 10.b of this 

bulletin. 

(1)	 Prototype Tests: The purpose of prototype testing is to verify the 

suitability of product design, materials, and method of manufacture. 

ANSI C29.17 specifies the design characteristics for the various classes of 

non-ceramic composite line-post insulators. The prototype test is 

performed only once for a class of insulators. When changes in a design 

occur that exceed the classification limits, prototype testing should be 

repeated in accordance with the standard. Prototype testing is performed 

in five parts. The test specimen should pass each part of the tests in 

sequence. It is recommended that the utility obtain a copy of the prototype 

test report from the manufacturer prior to insulator procurement to verify 

successful completion of prototype testing as specified in the ANSI 

standard. Prototype testing should include the following tests: 

(a) Tests on Interfaces and Connection of End Fittings 

•	 Test specimens – three insulators should be tested 

•	 Pre-stressing 

 Thermal – mechanical test (Cantilever Load Test at 50 percent 

of SCL) 

 Water Immersion Test 

•	 Verification Tests (should be completed within 48 hours following 

removal from water immersion test) 

 Visual Examination 

 Steep-Front Impulse Test 

 Power Frequency Voltage Test (a fourth reference insulator 

should also be tested) 

(b)	 Assembled Core Load Tests 

•	 Core Time – Load Test 

 Test Specimens – three insulators should be tested. 

 Core Time–Load Test 

 Evaluation 

 Visual Examination 

 Dissection and Dye Penetration 
 

 

 

 

 

 

 

 

Bulletin 1724E-220 

Page 35 

•	 Tensile Load Test 

 Test Specimens – three insulators should be tested 

 Tensile Test 

(c)	 Evaluation Housing Tracking and Erosion Tests 

• Aging or Weathering Test –two insulators should be tested 

(d)	 Core Material Tests 

•	 Dye Penetration Test (ten samples) 

•	 Water Diffusion Test (six samples) 

(e)	 Flammability Test 

(2)	 Electrical Design Tests: Design tests are performed on full-scale 

insulators to establish the electrical capabilities of an insulator class. 

Design tests are performed only once for a class of insulators. It is 

recommended that the utility obtain a copy of the test report be obtained 

prior to insulator procurement to verify successful completion of testing as 

required by ANSI C29.17. The following design tests are required: 

•	 Low Frequency Wet Flashover Test on one insulator 

•	 Low Frequency Dry Flashover Test on one insulator 

•	 Lightning Critical-Impulse Tests: positive and negative on one 

insulator 

•	 Radio-Influence Voltage and Visible Corona Test on one insulator 

(3)	 Sample Tests: These tests verify conformance of the insulators purchased 

to the manufacturing requirements. The insulators are selected from the 

production lot at random with the purchaser having the right to make the 

selection. It is recommended that for lots of more than 200 insulators, the 

utility specify that at least three samples from each lot be tested for both 

Dimensional Tests and Galvanizing Tests. At least one sample from each 

lot should be tested to verify the Cantilever Strength, and one sample from 

each lot should be tested to verify the Tensile Strength. For lots of 200 

insulators or less, the number of samples tested should be determined by 

agreement between the utility and manufacturer. Testing is required on 

each lot. The utility should feel secure that the sample quantity tested is 

sufficient to represent quality conformance for all insulators procured. 

The utility should also determine an acceptance criteria based on the 

number of tested samples. 

ANSI C29.17 requires the following sample tests be performed: 

•	 Verification of Dimensions 

•	 Galvanizing Test 

•	 Verification of Cantilever Strength Test 

 Cantilever Strength Verification 

 Cantilever Breaking Load (CBL) Test 

•	 Specified Tensile Load Test 

•	 Retest Procedure of Sample Tests (if required) 
 

 

 

 

 

 

 

Bulletin 1724E-220 

Page 36 

(4)	 Routine Tests: Routine testing should be performed on every insulator 

produced to verify the final quality acceptance of the manufactured 

insulators. These routing tests will help to eliminate the procurement of 

insulators with manufacturing defects. The following tests should be 

performed: 

•	 Tensile Load Test 

•	 Visual Examination 

(c)	 Insulator Assembly Visual Corona Testing (Suspension or Post Insulators): It is 

recommended that a sample of each type of insulator with appropriate hardware 

and corona grading devices attached be tested. The test unit should duplicate the 

field application unit including the conductor arrangement and insulator assembly 

dimensions and hardware material. Each conductor may be simulated by using 

either the actual conductor type or a conductive buss bar no larger than the 

conductor plus 0.10 inches. 

The insulator assemblies and grading rings, if any, should be subjected to a 60 Hz 

voltage to determine that the corona extinction voltage level is not less than 1.15 

times the maximum line-to-ground voltage rating of the insulator under clean, dry 

conditions, with ground plane (12 feet x 20 feet +) at a maximum of 15 feet from 

the assemblies. The 15 percent voltage factor accounts for variation in design and 

application, but not moisture. The corona detection device should be a light 

amplifier with a gain of 30,000 or more with F/2.8 or faster lens. The test 

specimen image when viewed should be no smaller than 20 percent of the light 

amplifier screen (described in “IEEE Transactions on Power Apparatus and 

Systems”, PAS Vol. 100, No. 2, February 1981, page 885). The device should be 

able to detect light generated by corona in the visual spectrum. 

Proper operation of the light amplifier should be demonstrated in a dark 

laboratory prior to testing. The test insulator should be energized just above the 

corona threshold and the detection of glow corona verified. 

Continue testing in a dark laboratory with a source voltage higher than 1.15 times 

the insulator’s maximum line-to-ground voltage (high enough to create corona), 

voltage should be lowered to 1.15 times L-G voltage. The test specimen should 

be viewed from at least two directions approximately 180° apart on the plane of 

the building floor that has the best view of the flat plane or planes of the test 

specimen. There should be no visible corona (through the night viewing device) 

on the test specimen and grading rings. The effects of continuous corona on the 

housing surface may cause insulator housing damage. 

An insulator with the weathershed material partially encapsulating the metal end 

fittings should have the encapsulating material removed for the corona test. 
 

 

 

 

 

 

 

Bulletin 1724E-220 

Page 37 

(d)	 Insulator Assembly Modeling Test (Suspension or Post Insulator): When grading 

rings are required it is recommended that the utility request a digital finite 

analysis electric field modeling test of the exact configuration to which the nonceramic insulator will be applied. This is to verify the effectiveness of grading 

rings. The model should be three dimensional with results containing drawings 

depicting the electric field in various colors, each of a different voltage level. The 

result of this study should show that the voltage field surrounding the non-ceramic 

insulator is not greater than 0.45 kV/mm along the entire length of the insulator, 

with the effected hot end of the insulator being a critical location. The threshold 

at which corona may or may not be present is defined as 0.45 kV/mm. 

11 INSULATOR HANDLING 

Handling practices for non-ceramic composite insulators should be different from those 

used for ceramic insulators. A single cut, puncture, or tear in the polymeric housing 

material covering the FRP rod may expose the rod to moisture and eventually cause 

mechanical failure of the insulator. A single cut or tear in the weathershed material, 

particularly near the hot end fittings, may cause excessive corona activity, resulting in 

eventual degradation of the housing material and possible exposure of the rod to 

moisture. 

Insulators should be properly stored and transported to the job site in their original 

shipping containers. If the insulators are stored outside of their shipping containers 

(which may result because of prolonged exposure of the shipping container to weather 

and eventual deterioration of the container), they should never be lying down against the 

ground, walked on, stacked on top of each other, or transported hanging over the tailgate 

of a pickup truck. Composite insulators may also be damaged by rodents during storage, 

and proper safeguards should be taken. 

Care should be taken when unpacking insulators to ensure that the polymeric housing is 

not cut or punctured. Insulators should be inspected for damage after removal from 

shipping containers. 

Composite insulators should never be lifted by their weathersheds. Long suspension 

insulators should be carried by two workers and, if lifting with a rope or strap, attach it 

only to the insulator end fittings. Long suspension insulators should also be supported in 

the middle by workers to avoid bending since suspension insulators are generally not 

designed for bending or compressive loads. 

If composite insulators are removed from their original shipping containers and then 

transported to the job site, extreme care should be taken at all times to avoid damaging 

the polymeric housing material. No sharp objects, materials, tools, or corona rings 

should be placed either on top, or situated below the insulators. Composite insulators 

should not be tied down with rope over the housing material during transport or 

transported on overhead racks of line crew trucks where bending of the insulator occurs. 
 

Bulletin 1724E-220 

Page 38 

During installation, the composite insulator should not be subjected to any loads that it 

would not normally see in service. The insulator should be thoroughly inspected before 

and after installation. Damaged insulators should not be used and should be marked for 

inspection by qualified utility personnel. 
Bulletin 1724E-220 

Exhibit A 

Page A-1 

EXHIBIT A
 

TECHNICAL PROCUREMENT SPECIFICATIONS FOR 

NON-CERAMIC COMPOSITE INSULATORS
 

(SAMPLE SPECIFICATION)
 
Bulletin 1724E-220 

Exhibit A 

Page A-2 

Instructions for the use of Exhibit A 

Exhibit A is a sample purchase specification for non-ceramic composite insulators. It does not 

include front-end documents or specifications for construction. The user of this specification 

must add front-end documents including general conditions and any supplemental instructions to 

the bidders. This specification was written for the procurement of non-ceramic composite 

suspension and line post insulators. Although it may be used to procure station post insulators, 

it is intended to be used to procure line post insulators. If the utility wishes to use this document 

for the procurement of station post insulators, additional information may be required regarding 

insulators compression and torsion ratings, depending on the application. 

If non-ceramic composite insulators are competitively bid, use of this Procurement Guide should 

eliminate ambiguities that might cause confusion between the owner and manufacturer. The 

owner is responsible for completing this specification by adding additional information, 

drawings, and supplemental requirements as appropriate. It may be necessary to modify this 

specification in order to consider special applications or preferences of the owner. 

When replacing existing ceramic insulators, Attachment A can be used with the Specification to 

provide the manufacturer with information on the existing insulators. The manufacturer should 

supply all information requested on this form regarding the proposed replacement insulators and 

return the form with the manufacturer’s proposal. The owner may use this form to compare the 

existing insulator details with the manufacturer’s proposed insulator details. The owner may 

also use this form to specify minimum requirements for the proposed insulators. Attachment A, 

Insulator Replacement Information Form, is provided at the end of this specification. 
 

 

Bulletin 1724E-220 

Exhibit A
 

Page A-3
 

TABLE OF CONTENTS 

1 SCOPE A-5
 

2 DEFINITIONS A-5
 

3 CODES AND STANDARDS A-8
 

4 GENERAL REQUIREMENTS A-9
 

5 INSPECTION AND TESTING A-12
 

6 DOCUMENTATION A-16
 

7 PACKAGING AND SHIPPING A-17
 

8 DRAWINGS A-18
 

ATTACHMENT A: INSULATOR REPLACEMENT INFORMATION FORM A-19
 
Bulletin 1724E-220
 

Exhibit A
 

Page A-4
 

BLANK PAGE
 
 

 

 

 

 

 

 

 

Bulletin 1724E-220 

Exhibit A 

Page A-5 

TECHNICAL PROCUREMENT SPECIFICATIONS FOR
 

NON-CERAMIC COMPOSITE INSULATORS
 

1 SCOPE 

This purchase specification covers the technical aspects of manufacture, testing and 

preparation for delivery of non-ceramic composite insulators. It does not include frontend documents or specifications for construction. The user of this specification must add 

front-end documents including general conditions and any supplemental instructions to 

the bidders. This specification was written for the procurement of non-ceramic 

composite suspension and line post insulators. Although it may be used to procure 

station post insulators, it is intended to be used to procure line post insulators. If the 

utility wishes to use this document for the procurement of station post insulators, 

additional information may be required regarding insulators compression and torsion 

ratings, depending on the application. 

If non-ceramic composite insulators are competitively bid, use of this Procurement Guide 

should eliminate ambiguities that might cause confusion between the owner and 

manufacturer. The owner is responsible for completing this specification by adding 

additional information, drawings, and supplemental requirements as appropriate. It may 

be necessary to modify this specification in order to consider special applications or 

preferences of the owner. 

When replacing existing ceramic insulators, Attachment A, Insulator Replacement 

Information Form, can be used with the Specification to provide the manufacturer with 

information on the existing insulators. The manufacturer should supply all information 

requested on this form regarding the proposed replacement insulators and return the form 

with the manufacturer’s proposal. The owner may use this form to compare the existing 

insulator details with the manufacturer’s proposed insulator details. The owner may also 

use this form to specify minimum requirements for the proposed insulators. Attachment 

A is provided at the end of this specification. 

2 DEFINITIONS 

Cantilever Breaking Load (CBL) - A load specified by the manufacturer that represents 

the failing load of a non-ceramic composite post insulator. The failing load is the 

maximum load attainable under Cantilever Load testing and is greater than or equal to the 

Specified Cantilever Load (SCL). 

Core - The internal part of a composite insulator. The core is the mechanical loadbearing component of the insulator. The core consists mainly of glass fibers impregnated 

with a resin-based matrix so as to achieve maximum strength. Also called a fiberglassreinforced plastic (FRP) rod. 
 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

2 

Bulletin 1724E-220 

Exhibit A 

Page A-6 

DEFINITIONS (continued) 

Design Tests - Tests to evaluate the electrical design of the non-ceramic composite 

insulator that are required only on insulators of new design. May also be used to evaluate 

various mechanical characteristics. 

End Fittings, Metal - The metal attachment hardware that is connected to the insulator 

core that transmits the mechanical loads to the core at the ends of the insulator. 

EPDM (Ethylene Propylene Diene Monomer) - Commonly used as a base polymer in 

non-ceramic composite insulator rubber formulations. 

EPR (Ethylene Propylene Rubber) - The generic term that includes both EPDM and 

EPM. 

Grading Device - A device for controlling the potential gradient at the end fittings. 

Housing - The insulator component which is external to the core and contains both a 

sheath and weathersheds. The sheath–weathershed system protects the core and provides 

the required leakage distance. The weathersheds also provide wet electrical strength. 

Lot - A number of insulator units in which each constituent component of the units (core, 

housing, metal end fittings, etc.) comes from the same production run and the units are 

assembled in the same manner and process. Each lot should, as far as is practicable, 

consist of units of product of a single type, grade, class, size, or composition 

manufactured under essentially the same conditions and in contiguous time periods. The 

units will have the same Specified Mechanical Load (SML) or Specified Cantilever Load 

(SCL) rating. The number of units in a lot should normally be set at some number not to 

exceed 500 units. 

Maximum Design Cantilever Load (MDCL) - The non-ceramic composite line post 

insulator cantilever load rating assigned by the manufacturer. The maximum load that 

can be applied to the post insulator every day with no deleterious effect on the service life 

of the insulator. Some manufacturers list this rating as the Reference Cantilever Load 

(RCL). 

Mechanical Load Test - A mechanical test for non-ceramic composite suspension 

insulators as specified in ANSI C29.11 and used to determine if a lot of insulators meet 

the Specified Mechanical Load (SML) requirements. The historical failure loads from 

this test justify the manufacturer’s choice of SML. Mechanical testing is also performed 

on every insulator to check its Routine Test Load (RTL) rating. 

Non-Ceramic Composite Insulator - An insulator unit that is made from material other 

than porcelain, glass, or other ceramic material. It consists of a load-bearing resinimpregnated fiberglass core, metal end fittings, and external elastomeric housing. 
 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

2 

Bulletin 1724E-220 

Exhibit A 

Page A-7 

DEFINITIONS (continued) 

Post Insulator - Primarily intended to be loaded in a cantilever mode, although specific 

applications may require loading in tension or compression. The most common types 

are: (1) horizontal line post where the insulator projects nearly horizontally from a 

structure and is loaded in bending by the conductor, (2) braced line post or horizontal Vee 

application, and (3) a station post insulator used as a bus support in an outdoor substation. 

Post insulators used in disconnect switch applications may also be loaded in torsion. 

Prototype Tests - Tests to evaluate and verify the suitability of materials, interfaces, 

prototype design, and method of manufacture. Prototype tests are performed only once 

and are considered valid for the whole class of insulators represented. 

Quality Conformance Tests - Destructive or nondestructive tests that are used to verify 

insulator conformance to specific characteristics and determine acceptability of an 

insulator lot. 

Reference Cantilever Load (RCL) - A rating assigned by the manufacturer which 

indicates the maximum load that can be applied to the post insulator every day with no 

deleterious effect on the service life of the insulator. Some manufacturers list this rating 

as the Maximum Design Cantilever Load (MDCL). 

Routine Test - A test performed on every insulator from each lot to identify insulators 

with manufacturing defects. 

Routine Test Load (RTL) - The load applied to non-ceramic composite suspension 

insulators that is equal to or greater than 50 percent of the insulator Specified Mechanical 

Load (SML) rating. Also, considered to be the maximum continuous working load of the 

insulator. 

Silicone Rubber (SR) - Usually in the form of polydimethylsiloxane, it is used as a base 

polymer in non-ceramic composite insulator rubber formulations. It is known for its 

hydrophobic (water-repellent) properties. 

Specified Cantilever Load (SCL) - A load specified by the manufacturer that represents 

the ultimate strength of a non-ceramic composite line post insulator under cantilever 

loading. The strength should be verified during cantilever load tests, and the historical 

failure loads (CBL) should justify the manufacturer’s choice of SCL. The SCL forms the 

reference point for selection of a composite line post insulator. It is not the maximum 

working load of the insulator. See MDCL. 

Specified Mechanical Load (SML) - A load specified by the manufacturer that 

represents the ultimate strength of a non-ceramic composite suspension insulator under 

tension. The strength should be verified during Mechanical Load Tests, and the historical 

failure loads should justify the manufacturer’s choice of SML. It forms the reference 

point for selection of a non-ceramic composite suspension insulator. It is not the 
 

 

 

 

 

 

 

 

 

 

3 

Bulletin 1724E-220 

Exhibit A 

Page A-8 

maximum working load of the insulator. See RTL. The SML of an insulator may be 

reduced by the class of hardware used for the end fittings. 

Specified Tensile Load (STL) - The load specified by the manufacturer that represents 

the ultimate strength of a non-ceramic composite line post insulator under tension. The 

strength is verified by the same testing procedures used to determine the Specified 

Mechanical Load (SML) for composite suspension insulators. 

Suspension Insulator - Any insulator intended primarily to carry tension loads. It 

includes tangent, dead-end, and vee-string installations. 

Ultimate Strength - An insulator’s tensile, compressive, or cantilever loading at which 

any part of the insulator fails to perform its function of providing mechanical support. 

Damage to the insulator core is likely to occur at loads lower than the insulator failing 

load. 

Weathershed - The part of the insulator’s housing which protrudes from the sheath and 

used to provide added leakage distance. 

CODES AND STANDARDS 

The manufacturer shall conduct all testing in accordance with the most recent codes and 

standards as indicated in this section: 

a American National Standards Institute (ANSI) 

ANSI C29.1 “Test Methods for Electrical Power Insulators” 

ANSI C29.11 “Composite Suspension Insulators for Overhead 

Transmission Lines - Tests” 

ANSI C29.12 “Insulators – Composites - Suspension Type” 

ANSI C29.17 “Composite Insulators – Transmission Line Post Type” 

ANSI Y14.5.1M “Mathematical Definition of Dimensioning and Tolerancing 

Principals” 

b American Society for Testing and Materials (ASTM) 

ASTM A153 “Standard Specification for Zinc Coating (Hot-Dip) on Iron 

and Steel Hardware” 
 

 

 

 

 

 

 

 

 

 

4 

Bulletin 1724E-220 

Exhibit A 

Page A-9 

GENERAL REQUIREMENTS 

The design, fabrication, processes, tolerances, and inspection of non-ceramic composite 

insulators shall conform to the following: 

a	 Language and Units 

(1)	 All correspondence, literature, drawings, and markings shall be in the 

English language. 

(2)	 Dimensioning and tolerancing shall conform to ANSI Y14.5.1M, 

“Mathematical Definition of Dimensioning and Tolerancing Principals”. 

Dimensions shall be in the U.S. customary units, unless SI (metric system) 

units are requested. If fabricating in SI units, both U.S. and SI units shall 

be shown on the drawings. Conversion dimensions shall be 1 inch = 25.4 

mm. Conversion dimensions may be rounded off to the nearest 1/32 of an 

inch (0.794 mm) provided the rounded dimension falls within the design 

limits. 

b	 Design and Material Requirements 

(1)	 Core: The internal core shall consist of a FRP rod consisting of E-glass 

fibers and epoxy-based resin. If required, an acid-resistant fiber may be 

used. All fibers of the insulator core shall be fully and completely coated 

with the epoxy resin matrix. During fabrication of the insulator core, the 

glass strands in the insulator core shall not be allowed to touch one 

another without resin matrix surrounding each individual fiber. Fibers 

shall be continuous between rod ends and oriented parallel to the rod axis. 

Glass content shall equal or exceed 60 percent by volume of the 

fiberglass-epoxy mixture. All areas in the rod not occupied by fiberglass 

strands shall be filled with epoxy. All fiberglass strands and epoxy shall 

be fully bonded together. The insulator core shall be mechanically and 

electrically sound, free from voids, foreign substances, and manufacturing 

flaws. The core shall have the same diameter throughout the entire length 

of the rod. 

(2)	 Housing: The core of the non-ceramic composite insulator shall be 

completely covered by a continuous housing consisting of a sheathweathershed system. The weathersheds and sheath shall be bonded 

together during the vulcanization process or molded into one piece by 

injection molding. The sheath shall be bonded to the rod or have a voidfree silicone interface. The housing shall be bonded to the metal end 

fittings or be properly sealed to prevent moisture or contaminant ingress to 

the FRP rod. The housing shall fully protect the fiberglass rod for the 

service life of the insulator. The housing shall be smooth and free from 

imperfections. 
 

 

 

 

Bulletin 1724E-220 

Exhibit A 

Page A-10 

The housing shall be made of a polymer compound consisting of either 

ethylene-propylene-diene monomer (EPDM), silicone rubber (SR), or a 

combination of EPDM and silicone oil compounds formed during 

vulcanization. The material shall be strong both electrically and 

mechanically. The housing shall have a thickness of not less than 3.0 mm 

along the entire length of the housing. The core/housing interface shall be 

manufactured so as to prevent leakage current flow over the surface of the 

core. The color and consistency of the material shall be uniform. 

The manufacturer shall determine if additional leakage distance is required 

based on the long-term performance of the housing material and the 

proposed insulator environmental conditions. The manufacturer shall 

provide the owner documentation which demonstrates satisfactory test 

results of specific formulations subjected to applicable environmental 

contamination together with satisfactory performance data of insulators 

(manufactured of the specified formulation) which have been installed on 

existing utility lines energized at the same voltage and in the same type of 

proposed environment. The manufacturer will define if or when insulators 

may require washing and the specific washing procedures required to 

protect insulators from mechanical damage. 

(3)	 Metal Parts: The metal end fittings shall be designed to transmit the 

mechanical load to the core and the end fittings shall maintain uniform and 

consistent mechanical strength. 

Materials and methods used in the fabrication of metal parts shall be 

selected to provide good toughness and ductility. Metal end fittings and 

bases shall be made from a quality commercial grade of iron or forged 

steel and shall be hot-dipped galvanized in accordance with ASTM A153, 

“Standard Specification for Zinc Coating (Hot-Dip) on Iron and Steel 

Hardware”. 

Metal end fittings and bases shall be uniform and without sharp edges or 

corners and shall be free of cracks, flakes, slivers, slag, blow-holes, 

shrinkage defects, and localized porosity. The attachment to the FRP rod 

shall be performed with a symmetrically controlled crimping method that 

compresses the metal radially onto the rod without damage to the rod 

fibers or resin matrix while providing a strength equal to or greater than 

the defined and specified ultimate strength of the insulator. 

Each insulator shall be permanently sealed at the interface between the 

metal end fittings and the housing to ensure that no moisture or foreign 

materials shall enter. The metal end fittings and the housing of non­
 

 

 

 

 

 

 

 

c 

Bulletin 1724E-220 

Exhibit A 

Page A-11 

ceramic composite insulators after complete assembly shall be coaxial 

with one another and with the core, resulting in no eccentric loading. 

Cotter keys shall be grade 304L stainless steel (minimum grade). 

Bolts, nuts, and spring lockwashers shall be made of steel and hot-dipped 

galvanized in accordance with ASTM A153, unless otherwise specified. 

Bolts may have either rolled or cut threads and shall have thread 

engagements capable of developing the specified strength of the unit. 

Nuts may be re-threaded after galvanizing to ensure clean threads, but 

bolts shall not be threaded or re-threaded after galvanizing. Threaded 

holes and nut threads shall be tapped oversize to closely fit those of the 

galvanized bolt with no unnecessary looseness, but free enough to permit 

the nut to be turned on freely with the fingers over the entire thread length. 

(4)	 Grading Devices: Non-ceramic suspension insulators operated at 115 kV 

and above, and line post insulators operated at 230 kV and above, shall 

require a grading device attached to the end fittings of the energized end 

of the insulator. The size and placement of the metallic grading device 

shall be designed to prevent corona along the entire length of the insulator. 

At voltages of 345 kV and above, grading rings should be provided at both 

ends of a suspension insulator. All grading rings and brackets shall be 

designed as an integral part of the insulator assembly with a positive 

mounting system that allows mounting in only one position. The design 

of the grading ring shall be such that the ring can only be mounted with its 

orientation towards the weathersheds for maximum RIV and corona 

control. Grading rings shall be designed in such a manner that the rings 

can be readily installed and removed with hot line tools without 

disassembling any other part of the insulator assembly. 

Materials and Workmanship 

(1)	 Materials specified in the design and fabrication requirements of this 

specification shall be unused, recently manufactured, and free of defects or 

irregularities. 

(2)	 All components of the same design and designation shall be identical, and 

like components shall be interchangeable. 

(3)	 Owner will accept offers only from manufacturers who intend to isolate 

insulators for lot control (if the order is large enough to contain lots) 

during manufacturing, quality control, and shipping. 
 

 

 

 

 

 

 

 

 

 

 

 

5 

Bulletin 1724E-220 

Exhibit A 

Page A-12 

d	 Markings 

(1)	 All suspension insulators shall be clearly and permanently marked in 

accordance with ANSI C29.11, “Composite Suspension Insulators for 

Overhead Transmission Lines - Tests”, with the following information: 

•	 Manufacturer’s Name or Trademark 

•	 Year Manufactured 

•	 Specified Mechanical Load (SML) in pounds 

•	 Routine Test Load (RTL) in pounds and identified by the word 

“TEST” or “RTL” 

(2)	 All line post insulators shall be clearly and permanently marked with the 

following information: 

•	 Manufacturer’s Name or Trademark 

•	 Year Manufactured 

•	 Specified Cantilever Load (SCL) in pounds 

•	 The marking should also include the Maximum Design Cantilever 

Load (MDCL) or Reference Cantilever Load (RCL) in pounds 

(3)	 A serial number and lot number (if the order is large enough to contain 

lots) shall be required on each insulator. The manufacturer shall keep 

documented records describing the insulator information, serial numbers, 

and lot numbers for each insulator throughout its useful life for future 

reference. 

INSPECTION AND TESTING 

a	 General 

(1)	 Non-ceramic composite insulator testing shall be performed in accordance 

with procedures as specified in ANSI C29.11 and C29.12, “Insulators ­

Composites - Suspension Type”, for suspension insulators and ANSI 

C29.17, “Composite Insulators - Transmission Line Post Type”, for line 

post insulators. The testing standards and procedures as defined in ANSI 

C29.11, C29.12, and ANSI C29.17 shall be strictly adhered to. Although 

these standards are for composite insulators intended for use on overhead 

transmission lines 70 kV and above, the owner requires the same 

characteristic testing as defined in ANSI C29.11, C29.12, and C29.17 for 

all non-ceramic composite insulators the owner will procure. 
 

 

 

 

 

 

 

 

 

 

 

Bulletin 1724E-220 

Exhibit A 

Page A-13 

(2)	 The manufacturer should provide the owner with certified ultimate 

cantilever strength documentation (Specified Cantilever Load) based on 

historical Cantilever Load Tests for each classification of line post 

insulator procured under this request. 

(3)	 A code shall be selected by the manufacturer for the purpose of identifying 

the “lot” or the insulator production run. The units shall use the same type 

of polymeric and rod materials. The insulator units shall have the same 

ultimate strength rating. The end fittings shall be attached in the same 

manner. 

(4)	 The number of insulator units in a lot should normally be set at some 

number not exceeding 500 units. Sample sizes for quality conformance 

testing shall conform to the requirements of ANSI C29.12 for suspension 

insulators and ANSI 29.17 for line post insulators. The test insulators 

shall be marked with their corresponding lot number. 

b	 Prototype Testing 

(1)	 Prototype testing is generally performed only once for each particular type 

of insulator design and is considered valid for the whole class of insulators 

represented. 

(2)	 The manufacturer shall provide the owner with copies of certified testing, 

evaluation and acceptance reports detailing conformance to the specific 

tests noted below which were conducted in accordance with procedures 

specified in ANSI C29.11 for suspension insulators and ANSI C29.17 for 

line post insulators: 

• Tests on Interfaces and Connection of End Fittings 

• Assembled Core Load Tests 

• Housing Tracking and Erosion Tests 

• Aging or Weathering Test 

• Core Material Tests 

• Flammability Test 

(3)	 In addition to the testing reports conforming to ANSI C29.11 and ANSI 

C29.17 for prototype testing, the manufacturer shall provide the owner 

with reports documenting the testing procedures and results of an 

accelerated aging test similar to the CIGRÉ 5000 hour test performed on 

insulators of the particular type of design as represented by the prototype. 

(4)	 For non-ceramic suspension insulators, the manufacturer shall provide the 

owner with reports documenting the torsion strength testing as specified 

by ANSI C29.1, “Test Methods for Electrical Power Insulators”. 
 

 

 

 

 

 

 

 

 

 

 

c 

Bulletin 1724E-220 

Exhibit A 

Page A-14 

Design Testing 

(1)	 Design testing is generally performed only once on complete, full-scale 

insulators of a specific electrical design. The manufacturer shall provide 

the owner with copies of certified testing, evaluation, and acceptance 

reports detailing conformance of all insulators procured under this request 

to the provisions of ANSI C29.11 and C29.12 design tests of suspension 

insulators and ANSI C29.17 design tests for line post type insulators. The 

reports shall include the results from the following tests: 

• Low-Frequency Dry Flashover Test 

• Low-Frequency Wet Flashover Test 

• Critical Impulse Flashover Test, Both Positive and Negative 

• Radio-Influence Voltage (RIV) Test and Visible Corona Test 

(2)	 In addition to the above electrical design tests, if a particular insulator 

design requires a grading device, the manufacturer will provide the owner 

with certified testing reports that the insulator design with applicable 

grading device has successfully passed a grading device test. The grading 

device test performed should be similar to the following described test: 

Grading devices shall be tested using a mechanical shaker with at 

least a one-inch stroke at the grading device and a frequency of no 

less than three cycles per second for a duration of 2,000,000 

cycles. Movement shall be along the long axis of the insulator. 

The test shall reflect the manufacturer’s recommended method for 

attaching the grading device to the insulator. The grading device 

shall be attached to the insulator and the insulator attached to the 

shaker in a vertical position. The test shall be considered 

successful if no movement is detected in the ring with respect to 

the insulator and there is no physical damage to the grading device 

and the attachment assembly. 

(3)	 The manufacturer shall provide the owner with documentation that the 

insulator design with applicable grading devices will minimize or 

eliminate corona discharge activity under wet and dry conditions for the 

type of pole-top configuration proposed. The effects of corona discharge 

activity on insulator life shall be negligible. 

(4)	 Under normal use and all but the worst environments, the insulator should 

not require washing. However, if the environment specified by the owner 

at the beginning of this request is, in the opinion of the manufacturer, such 

that cleaning is required, the manufacturer should provide the owner with 

certified documentation that the insulator has passed a high-pressure wash 

test. The test performed should be similar to the following described test: 
 

 

 

 

 

 

 

Bulletin 1724E-220 

Exhibit A 

Page A-15 

A pressure sprayer with a minimum nozzle pressure of 600 psi 

and a minimum flow rate of 7 liters per minute shall be applied 

on the underside of each weathershed at a distance that is 

similar to but less than the actual field application for 15 

seconds minimum. Ordinary tap water must be used. After 

spraying, a low-frequency wet flashover test shall be conducted 

in accordance with ANSI C29.1, Section 4.3. A visual 

inspection test shall then be performed with careful attention 

given to any dislodgement between the weathersheds and sheath 

and the end fittings seals. A dye penetration test conforming to 

ANSI C29.11 shall be performed after spraying on the section 

of insulator containing the end fittings and filler material. 

Dislodgment of weathersheds or failure to pass visual 

inspection, dye penetration test, or low-frequency wet flashover 

test shall constitute failure. 

d	 Quality Conformance Testing (Sample Tests) 

(1)	 ANSI C29.12 specifies the quality conformance tests that shall be 

performed on non-ceramic composite suspension insulators. ANSI 

C29.17 specifies the quality conformance tests (sample tests) that shall be 

performed on non-ceramic composite line post insulators. For each lot, 

the quantity of units to be tested shall be as defined in the respective ANSI 

standards. The number of insulator units in each lot shall not exceed 500 

units. Insulators shall be selected from each lot at random with the owner 

having the right to make the selection. The manufacturer shall provide the 

owner with certified documentation that details the results of the following 

test for all samples tested: 

• Dimensional Tests 

• Galvanizing Test 

• Specified Mechanical Load Test (suspension insulators) 

• Cantilever Load Test (line post insulators) 

• Specified Tensile Load Test (line post insulators) 

(2)	 The owner must be assured that the sample quantity tested is sufficient to 

represent quality conformance for all insulators procured. If one insulator 

or metal part fails to comply with the requirements of a sample test as 

defined by ANSI standards, the retest procedure as described in the 

standards shall be performed. If one insulator or metal part fails a sample 

test, a new sample equal to twice the original quantity submitted to that 

test shall be re-tested. If two or more insulators or metal parts fail to 

comply with any of the sample tests, or if any failure occurs during the re­
 

 

 

 

 

 

Bulletin 1724E-220 

Exhibit A 

Page A-16 

testing, the complete lot shall be withdrawn for offer to this owner by the 

manufacturer at no cost to the owner. 

e	 Routine Testing 

(1)	 Routine testing shall be performed on every insulator unit produced for 

procurement under this request and shall be performed in accordance with 

the requirements of ANSI C29.12 for suspension insulators and ANSI 

C29.17 for line post insulators. The manufacturer shall provide the owner 

with certified documentation that details the results of the following tests: 

• Tension – Proof Test 

• Visual Examination 

f	 Certified Test Reports 

(1)	 Certified test reports shall include information that allows the owner to 

clearly identify the units tested. The following information shall be 

included on the test reports: 

• Purchase Order Number 

• Shipment Date 

• Destination 

• Catalog Numbers 

• Utility’s Code Number 

• Specified Mechanical Load of the Insulator’s Core 

• ANSI Class End Fittings 

• Number of Lots in the Shipment 

• Number of Insulators in Each Lot 

• Lot Code Numbers 

• Date of Testing 

• Name of Individual(s) Documenting the Tests 

(2)	 The owner’s inspector shall be allowed to witness all special design 

testing, quality conformance testing, and routine testing performed for 

each lot of insulators ordered and shall have the right to make an 

inspection before authorizing shipment of the insulators. 

DOCUMENTATION 

The manufacturer shall submit a complete bid proposal to the owner including the 

following documentation: 

• Insulator Unit Cost Information 

• Applicable Drawings with Associated Information as Specified in the RFP 

6 
 

 

 

 

 

 

7 

Bulletin 1724E-220 

Exhibit A 

Page A-17 

•	 Ultimate Strength of Insulator (Suspension SML Rating, Line Post SCL Rating) 

•	 Maximum Continuous Working Load Ratings of Insulators (Suspension RTL Rating, 

Line Post MDCL Rating) 

•	 Warranty Information 

•	 Certified Prototype and Design Testing Reports 

•	 Applicable Deflection and Combined Loading Application Curves 

•	 Documentation of Insulator Performance in Proposed Installation Environment 

•	 Insulator Washing Requirements and Procedures, If any 

•	 Grading Device Information 

•	 Testing Reports as Specified in the RFP 

•	 Delivery Date and Location 

•	 Packaging and Shipping Details 

•	 Special Application and/or Design Details 

•	 Handling and Installation Details 

PACKAGING AND SHIPPING 

a	 Insulator Packaging 

(1)	 The packaging of insulators and any grading devices shall provide proper 

protection for both products from damage during handling, shipping, and 

storage. If the insulators are to be stored outdoors, they shall be packaged 

in wooden containers completely enclosed on all sides with plywood and 

internal wooden supports constructed so each layer of insulators is selfsupported by their own end fittings and not supported by a lower layer of 

insulators. 

(2)	 Wooden containers shall be treated to resist degradation using treating 

materials that have been determined not to harm the environment or the 

enclosed insulator in any manner. Weathersheds shall not rest against 

adjacent insulator weathersheds such that bending of the weathershed 

occurs. The number of insulator units per container shall be reasonable 

and the containers shall be designed for forklift handling. The containers 

shall be strong enough to support stacking of the containers to a maximum 

level of three containers high. The insulator containers shall never be 

stacked during transport since insulators and containers may become 

damaged due to sudden impacts. If insulators will be stored indoors, 

particleboard containers are acceptable. 

(3)	 The containers shall be marked, in English, with the number of insulators 

contained, manufacturer’s name, insulator catalog number, insulator 

description, owner’s purchase order number, and production lot number. 
 

 

 

 

 

 

 

 

 

 

8 

Bulletin 1724E-220 

Exhibit A 

Page A-18 

b Grading Device Packaging 

(1)	 The appropriate number of grading devices shall be packaged in a separate 

enclosed container and shipped in the same wooden container as the 

insulators they attach to. If the grading devices are large, or the grading 

device container may damage the insulators during shipment, the grading 

devices may be shipped separately. If shipped separately, the grading 

device container and its associated insulator container must be clearly 

marked with easy to read matching identification. 

(2)	 Prior to the insulator delivery, the manufacturer shall supply the owner 

with documentation explaining the use of matching identification number 

grading ring installation, insulator lot numbers and insulator identification 

numbers. 

DRAWINGS 

Drawings for each type of proposed insulator shall be submitted with the proposal. Each 

drawing shall clearly show the following dimensions (including manufacturing 

tolerances) and ratings: 

• Connection Length 

• Number of Sheds 

• Dry Arc Distance 

• Leakage Distance 

• Approximate Weight 

• Low-Frequency Dry Flashover 

• Low-Frequency Wet Flashover 

• Critical Impulse Flashover Positive 

• Critical Impulse Flashover Negative 

• For Suspension Insulators: 

1. Specified Mechanical Load 

2. Routine Test Load 

• For Line Post Insulators: 

1. Specified Cantilever Load 

2. Maximum Design Cantilever Load or RCL 

• Rod Material 

• Housing Material 

• End Fitting Material 

• Cross-Sectional View of Insulator Unit with Dimensions 

• Size and Dimension Details for Both End Fittings 

• Insulator Description 

• Manufacturer’s Catalog Number 

• Manufacturer’s Drawing Number 

• Grading Ring Dimensions and Orientation 

• Insulator Color 

All drawings shall be approved by the owner before fabrication begins. 
 

 

Bulletin 1724E-220 

Exhibit A 

Page A-19 

ATTACHMENT A
 

 INSULATOR REPLACEMENT INFORMATION FORM

 MINIMUM REQUIREMENTS

 Post Suspension

 REQUIREMENTS Existing Proposed Existing Proposed

 Manufacturer

 Catalog Number *

 Structure/Pole Type

 System Voltage

 END FITTINGS 

Structure End 

Energized End

 DIMENSIONS

 Length, in. 

Leakage Distance, in. 

Dry Arc Distance, in. 

Number of Bells/Sheds 

Diameter of Bells/Sheds, in.

 Core Diameter, in.

 CORONA RING 

Structure End Ring, diameter 

Energized End Ring, diameter

 MECHANICAL VALUES

 Combined M & E Strength, lbs 

SML Rating, lbs 

RTL Rating, lbs

 SCL Rating, lbs

 STL Rating, lbs 

MDCL Rating, lbs

 ELECTRICAL VALUES 

Low-Frequency Dry Flashover, kV 

Low-Frequency Wet Flashover, kV 

Critical Impulse Flashover 

Positive, kV 

Negative, kV 

Basic Impulse Level (BIL), kV 

NOTE to Owner: 

* If possible, supply existing insulator drawings with mounting details in bid proposal.
Bulletin 1724E-220
 

Exhibit A
 

Page A-20
 

BLANK PAGE
 
 

Bulletin 1724E-220 

Exhibit B 

Page B-1 

EXHIBIT B
 

METRIC CONVERSION FACTORS
 

TO CONVERT FROM TO MULTIPLY BY 

Foot (ft) meter (m) 0.3048 

Inch (in) centimeter 2.5400 

Pound per cubic foot (pcf) (lb/ft3) kilogram per cubic meter (kg/m3) 16.0185 

Pound per square inch (psi) (lb/in2) kilogram per square meter (kg/m2) 703.0696 

Degrees Fahrenheit (°F) degrees Celsius (°C) 5/9(X°-32)