oSIST prEN IEC 61462:2022

Composite hollow insulators - Pressurized and unpressurized insulators for use in electrical equipment with AC rated voltage greater than 1 000 V AC and D.C. voltage greater than 1500V - Definitions, test methods, acceptance criteria and design recommendations

This International Standard applies to composite hollow insulators consisting of a load-bearing insulating tube made of resin impregnated fibres, a housing (outside the insulating tube) made of elastomeric material (for example silicone or ethylene-propylene) and metal fixing devices at the ends of the insulating tube. Composite hollow insulators as defined in this standard are intended for general use (unpressurized) or for use with a permanent gas pressure (pressurized). They are intended for use in both outdoor and indoor electrical equipment operating on alternating current with a rated voltage greater than 1 000 V a.c. and a frequency not greater than 100 Hz or for use in direct current equipment with a rated voltage greater than 1 500 V d.c.
The object of this standard is:
- to define the terms used;
- to prescribe test methods;
- to prescribe acceptance criteria.
Hollow insulators are integrated into electrical equipment which is electrically type tested as required by the applicable equipment standard. So, it is not the object of this standard to prescribe dielectric type tests because the withstand voltages and flashover behaviour are not characteristics of the hollow insulator itself but of the apparatus of which it ultimately forms a part.
All the tests in this standard, apart from the thermal-mechanical test, are performed at normal ambient temperature. This standard does not prescribe tests that may be characteristic of the apparatus of which the hollow insulator ultimately forms a part.
Composite hollow insulators are intended for use in electrical equipment, such as, but not limited to:
- HV circuit-breakers,
- switch-disconnectors,
- disconnectors,
- station posts,
- disconnecting circuit breakers,
- earthing switches,
- instrument- and power transformers,
- bushings,
- cable terminations.
Additional testing defined by the relevant IEC equipment standard may be required.

Verbundhohlisolatoren - Druckbeanspruchte und drucklose Isolatoren für den Einsatz in elektrischen Betriebsmitteln mit Bemessungsspannungen über 1 000 V - Begriffe, Prüfverfahren, Annahmekriterien und Konstruktionsempfehlungen

Isolateurs composites creux - Isolateurs avec ou sans pression interne pour utilisation dans des appareillages électriques de tensions alternatives assignées supérieures à 1 000 V et de tensions continues supérieures à 1 500 V - Définitions, méthodes d’essai, critères d’acceptation et recommandations de conception

Votli kompozitni izolatorji - Tlačni in breztlačni izolatorji za električno opremo z naznačeno izmenično napetostjo, višjo od 1000 V, in enosmerno napetostjo, višjo od 1500 V - Definicije, preskusne metode, merila sprejemljivosti in priporočila za načrtovanje

General Information

• Status: Not Published
• Public Enquiry End Date: 08-Jun-2022
• ICS: 29.080.10 - Insulators
• Technical Committee: IZL - Insulators
• Current Stage: 4020 - Public enquire (PE) (Adopted Project)
• Start Date: 23-Mar-2022
• Due Date: 10-Aug-2022
• Ref Project: prEN IEC 61462:2022 - Composite hollow insulators - Pressurized and unpressurized insulators for use in electrical equipment with AC rated voltage greater than 1 000 V AC and D.C. voltage greater than 1500V - Definitions, test methods, acceptance criteria and design recommendations

RELATIONS

• Revised: SIST EN 61462:2008 - Composite hollow insulators - Pressurized and unpressurized insulators for use in electrical equipment with rated voltage greater than 1 000 V - Definitions, test methods, acceptance criteria and design recommendations
• Effective Date: 12-Jan-2021

Standards Content (sample)

SLOVENSKI STANDARD
oSIST prEN IEC 61462:2022
01-junij-2022

Votli kompozitni izolatorji - Tlačni in breztlačni izolatorji za električno opremo z

naznačeno izmenično napetostjo, višjo od 1000 V, in enosmerno napetostjo, višjo

od 1500 V - Definicije, preskusne metode, merila sprejemljivosti in priporočila za

načrtovanje

Composite hollow insulators - Pressurized and unpressurized insulators for use in

electrical equipment with AC rated voltage greater than 1 000 V AC and D.C. voltage

greater than 1500V - Definitions, test methods, acceptance criteria and design
recommendations

Verbundhohlisolatoren - Druckbeanspruchte und drucklose Isolatoren für den Einsatz in

elektrischen Betriebsmitteln mit Bemessungsspannungen über 1 000 V - Begriffe,
Prüfverfahren, Annahmekriterien und Konstruktionsempfehlungen

Isolateurs composites creux - Isolateurs avec ou sans pression interne pour utilisation

dans des appareillages électriques de tensions alternatives assignées supérieures à 1

000 V et de tensions continues supérieures à 1 500 V - Définitions, méthodes d’essai,

critères d’acceptation et recommandations de conception
Ta slovenski standard je istoveten z: prEN IEC 61462:2022
ICS:
29.080.10 Izolatorji Insulators
oSIST prEN IEC 61462:2022 en,fr,de

2003-01.Slovenski inštitut za standardizacijo. Razmnoževanje celote ali delov tega standarda ni dovoljeno.

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oSIST prEN IEC 61462:2022
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oSIST prEN IEC 61462:2022
36/540/CDV
COMMITTEE DRAFT FOR VOTE (CDV)
PROJECT NUMBER:
IEC 61462 ED2
DATE OF CIRCULATION: CLOSING DATE FOR VOTING:
2022-03-18 2022-06-10
SUPERSEDES DOCUMENTS:
36/509/CD, 36/531A/CC
IEC TC 36 : INSULATORS
SECRETARIAT: SECRETARY:
Sweden Mr Dan Windmar
OF INTEREST TO THE FOLLOWING COMMITTEES: PROPOSED HORIZONTAL STANDARD:
TC 17,SC 17A,SC 17C
Other TC/SCs are requested to indicate their interest, if
any, in this CDV to the secretary.
FUNCTIONS CONCERNED:
EMC ENVIRONMENT QUALITY ASSURANCE SAFETY

SUBMITTED FOR CENELEC PARALLEL VOTING NOT SUBMITTED FOR CENELEC PARALLEL VOTING

Attention IEC-CENELEC parallel voting
The attention of IEC National Committees, members of
CENELEC, is drawn to the fact that this Committee Draft
for Vote (CDV) is submitted for parallel voting.
The CENELEC members are invited to vote through the
CENELEC online voting system.

This document is still under study and subject to change. It should not be used for reference purposes.

Recipients of this document are invited to submit, with their comments, notification of any relevant patent rights of

which they are aware and to provide supporting documentation.
TITLE:

Composite hollow insulators - Pressurized and unpressurized insulators for use in electrical

equipment with AC rated voltage greater than 1 000 V AC and D.C. voltage greater than 1500V -

Definitions, test methods, acceptance criteria and design recommendations
PROPOSED STABILITY DATE: 2026
NOTE FROM TC/SC OFFICERS:

Copyright © 2022 International Electrotechnical Commission, IEC. All rights reserved. It is permitted to

download this electronic file, to make a copy and to print out the content for the sole purpose of preparing National

Committee positions. You may not copy or "mirror" the file or printed version of the document, or any part of it, for

any other purpose without permission in writing from IEC.
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oSIST prEN IEC 61462:2022
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1 CONTENTS

2 FORE WORD ........................................................................................................................ 4

3 INTR ODUCTION .................................................................................................................. 6

4 1  Scope ............................................................................................................................ 7

5 2  Normative references ..................................................................................................... 8

6 3  Terms and definitions .................................................................................................... 8

7 4  Relationships of mechanical loads ................................................................................ 12

8 4.1  Loads from outside the insulator .......................................................................... 12

9 4.2  Pressures ............................................................................................................ 12

10 5  Marking ........................................................................................................................ 12

11 6  Classification of tests .................................................................................................... 12

12 6.1  Design tests ........................................................................................................ 12

13 6.2  Type tests ........................................................................................................... 14

14 6.3  Sample tests ....................................................................................................... 14

15 6.4  Routine tests ....................................................................................................... 14

16 7  Design tests ................................................................................................................. 15

17 7.1  General ............................................................................................................... 15

18 7.2  Tests on interfaces and connections of end fittings ............................................... 15

19 7.3  Tests on shed and housing material ..................................................................... 17

20 7.4  Tests on the tube material .................................................................................... 17

21 7.5  Tests on the tube material and housing ................................................................ 18

22 8  Type tests (only mechanical tests) ................................................................................ 18

23 8.1  General ............................................................................................................... 18

24 8.2  Test specimens ................................................................................................... 18

25 8.3  Preparation of the test specimen .......................................................................... 19

26 8.4  Internal pressure test ........................................................................................... 20

27 8.5  Bending test ........................................................................................................ 21

28 9  Sample tests ................................................................................................................ 22

29 9.1  Selection and number of insulators ...................................................................... 22

30 9.2  Testing ................................................................................................................ 23

31 9.3  Verification of dimensions .................................................................................... 23

32 9.4  Mechanical tests .................................................................................................. 23

33 9.5  Galvanizing test ................................................................................................... 24

34 9.6  Re-test procedure ................................................................................................ 24

35 10  Routine tests ................................................................................................................ 25

36 10.1  General ............................................................................................................... 25

37 10.2  Visual examination............................................................................................... 25

38 10.3  Routine mechanical test ....................................................................................... 25

39 10.4  Routine pressure test ........................................................................................... 25

40 10.5  Routine tightness test .......................................................................................... 26

41 11  Documentation ............................................................................................................. 26

42 Annex A (Normative) Tolerances of form and position ......................................................... 31

43 Annex B (informative) General recommendations for design and construction ..................... 34

44 B.1 Guidance for design ...................................................................................................... 34

45 B.2 Guidance for the maximum service pressure ................................................................. 34

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46 B.3 Guidance on sample testing of tube material ................................................................. 34

47 B.4 Guidance for the temperature required by the equipment manufacturer .......................... 35

48 B.5 Guidance for the mechanical loads required by the equipment manufacturer .................. 35

49 B.6 Summary of the tests .................................................................................................... 36

50 Annex C (informative) Principles of damage limit and use of reversible and irreversible

51 strain caused by internal pressure and/or bending loads on composite hollow

52 insulator tubes .............................................................................................................. 39

53 C.1 Introductory remarks ..................................................................................................... 39

54 C.2  Definition ...................................................................................................................... 39

55 C.3 Example of determining the strain tolerance .................................................................. 39

56 Annex D (informative) Principle sketch of hollow insulators design assembly ....................... 42

57 Annex E (informative) Type tests on tapered (conical) insulators .......................................... 43

58 Bibliography ...................................................................................................................... 46

60 Figure 1 – Thermal-mechanical pre-stressing test – Typical cycles ....................................... 27

61 Figure 2 – Thermal-mechanical pre-stressing test – Typical test arrangement ...................... 28

62 Figure 3 – Test arrangement for the leakage rate test .......................................................... 29

63 Figure 4 – Examples of sealing systems for composite hollow insulators .............................. 30

64 Figure A.1 – Parallelism, coaxiality and concentricity ........................................................... 31

65 Figure A.2 – Angular deviation of fixing holes: Example 1 .................................................... 32

66 Figure A.3 – Angular deviation of fixing holes: Example 2 .................................................... 32

67 Figure A.4 – Tolerances according to standard drawing practice .......................................... 33

68 Figure B.1 – Relationship of bending loads .......................................................................... 38

69 Figure B.2 – Relationship of pressures ................................................................................ 38

70 Figure C.1 – Position of strain gauges for pressure load and bending load ........................... 40

71 Figure C.2 – Strain/time curve, reversible elastic phase ....................................................... 41

72 Figure C.3 – Strain/time curve, irreversible plastic phase, damage limit ................................ 41

73 Figure D.1.- Interface description for insulator with housing made by modular assembly ....... 42

74 Figure D.2.- Interface description for insulator with housing made by injection molding

75 and overmolded end fitting ............................................................................................ 42

76 Figure E.1 Illustration of tapered insulators in bending. ........................................................ 44

77 Figure E.2. Illustration of axial membrane stress along the insulator when the length of

78 the cylindrical parts is changed. .................................................................................... 45

79 Table 1 – Mechanical loads applied to the insulator ............................................................. 12

80 Table 2 – Pressures applied to the insulator ........................................................................ 12

81 Table 3 – Tests to be carried out after design changes ........................................................ 13

82 Table 4 – Sample sizes ....................................................................................................... 22

83 Table 5 – Choice of re-test procedure .................................................................................. 24

84 Table B.1 – Loads/stress and classification of tests ............................................................. 36

85 Table B.2 – Example of pressure/bending values – Practical relationship of the values ......... 37

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88 INTERNATIONAL ELECTROTECHNICAL COMMISSION
89 ____________
91 COMPOSITE HOLLOW INSULATORS –
93 PRESSURIZED AND UNPRESSURIZED INSULATORS
94 FOR USE IN ELECTRICAL EQUIPMENT WITH AC RATED VOLTAGE
95 GREATER THAN 1 000 V AC AND D.C. VOLTAGE GREATER THAN 1 500 V
96 –
97 DEFINITIONS, TEST METHODS, ACCEPTANCE CRITERIA AND
98 DESIGN RECOMMENDATIONS
100
101 FOREWORD

102 1) The International Electrotechnical Commission (IEC) is a worldwide organization for standardization comprising

103 all national electrotechnical committees (IEC National Committees). The object of IEC is to promote

104 international co-operation on all questions concerning standardization in the electrical and electronic fields. To

105 this end and in addition to other activities, IEC publishes International Standards, Technical Specifications,

106 Technical Reports, Publicly Available Specifications (PAS) and Guides (hereafter referred to as “IEC

107 Publication(s)”). Their preparation is entrusted to technical committees; any IEC National Committee interested

108 in the subject dealt with may participate in this preparatory work. International, governmental and non-

109 governmental organizations liaising with the IEC also participate in this preparation. IEC collaborates closely

110 with the International Organization for Standardization (ISO) in accordance with conditions determined by

111 agreement between the two organizations.

112 2) The formal decisions or agreements of IEC on technical matters express, as nearly as possible, an international

113 consensus of opinion on the relevant subjects since each technical committee has representation from all

114 interested IEC National Committees.

115 3) IEC Publications have the form of recommendations for international use and are accepted by IEC National

116 Committees in that sense. While all reasonable efforts are made to ensure that the technical content of IEC

117 Publications is accurate, IEC cannot be held responsible for the way in which they are used or for any

118 misinterpretation by any end user.

119 4) In order to promote international uniformity, IEC National Committees undertake to apply IEC Publications

120 transparently to the maximum extent possible in their national and regional publications. Any divergence

121 between any IEC Publication and the corresponding national or regional publication shall be clearly indicated in

122 the latter.

123 5) IEC provides no marking procedure to indicate its approval and cannot be rendered responsible for any

124 equipment declared to be in conformity with an IEC Publication.

125 6) All users should ensure that they have the latest edition of this publication.

126 7) No liability shall attach to IEC or its directors, employees, servants or agents including individual experts and

127 members of its technical committees and IEC National Committees for any personal injury, property damage or

128 other damage of any nature whatsoever, whether direct or indirect, or for costs (including legal fees) and

129 expenses arising out of the publication, use of, or reliance upon, this IEC Publication or any other IEC

130 Publications.

131 8) Attention is drawn to the Normative references cited in this publication. Use of the referenced publications is

132 indispensable for the correct application of this publication.

133 9) Attention is drawn to the possibility that some of the elements of this IEC Publication may be the subject of

134 patent rights. IEC shall not be held responsible for identifying any or all such patent rights.

135 This new edition cancels and replaces the previous edition. It constitutes a technical revision

136 and has the status of an International Standard since 2007.
137
138
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139 The text of this standard is based on the following documents:
FDIS Report on voting
To be completed/FDIS To be completed/RVD
140

141 Full information on the voting for the approval of this standard can be found in the report on

142 voting indicated in the above table.

143 This publication has been drafted in accordance with the ISO/IEC Directives, Part 2.

144 The committee has decided that the contents of this publication will remain unchanged until

145 the maintenance result date indicated on the IEC web site under "http://webstore.iec.ch" in the

146 data related to the specific publication. At this date, the publication will be

147 • reconfirmed,
148 • withdrawn,
149 • replaced by a revised edition, or
150 • amended.
151
152
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153 INTRODUCTION

154 Composite hollow insulators consist of an insulating tube bearing the mechanical load

155 protected by an elastomeric housing, the loads being transmitted to the tube by metal fittings.

156 Despite these common features, the materials used and the construction details employed by

157 different manufacturers may vary.

158 Some tests have been grouped together as "Design tests" to be performed only once for

159 insulators of the same design and material. The design tests are performed in order to eliminate

160 designs and materials not suitable for high-voltage applications.

161 The relevant design tests defined in IEC 62217 are applied for composite hollow insulators;

162 additional specific mechanical tests are given in this standard. The influence of time on the

163 electrical and mechanical properties of the complete composite hollow insulator and its

164 components (tube material, housing material, interfaces, etc.) has been considered in specifying

165 the design tests in order to ensure a satisfactory lifetime under normal service conditions. These

166 conditions may also depend on the equipment inside or outside the composite hollow insulators;

167 however, this matter has not been covered in this standard. Test methods not specified in this

168 standard may be considered for specific combinations of materials and specific applications,

169 and are a matter of agreement between manufacturers and users. In this standard, the term

170 “user” in general means the equipment manufacturer using composite hollow insulators.

171 Composite hollow insulators are used in both a.c. and d.c. applications. Before the

172 appropriate standard for d.c. applications will be issued, the majority of tests listed in this

173 standard can also be applied to d.c. insulators. In spite of this, a specific tracking and erosion

174 test procedure for d.c. applications as a design test is still being considered to be developed.

175 Some information about the difference of a.c. and d.c. material erosion test can be found in

176 the CIGRE Technical Brochure 611. For the time being, the 1 000 h a.c. tracking and erosion

177 test of IEC 62217 is used to establish a minimum requirement for the tracking and erosion

178 resistance, for both a.c. and d.c.

179 This standard distinguishes between design tests and type tests because several general

180 characteristics of a specific design and specific combinations of materials do not vary for

181 different insulator types. In these cases results from design tests can be adopted for different

182 insulator types.

183 Pollution tests according to IEC 60507 or IEC 61245 are not included in this standard since

184 they are designed for non-polymeric items. Specific pollution tests for polymeric insulators are

185 still under consideration.

186 The mechanical characteristics of composite hollow insulators are quite different compared to

187 those of hollow insulators made of ceramics. In order to determine the onset of mechanical

188 deterioration of composite hollow insulators under the influence of mechanical stress, strain

189 gauge measurements are used.

190 This standard refers to different characteristic pressures which are used for design and testing

191 of composite hollow insulators. The term "maximum service pressure" (MSP) is equivalent to

192 the term "design pressure" which is used in other standards for ceramic hollow insulators;

193 however, this latter term is not used in this standard in order to avoid confusion with "design"

194 as used in "design tests".

195 General recommendations for the design and construction of composite hollow insulators are

196 presented in Annex B.
197
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198 COMPOSITE HOLLOW INSULATORS –
199
200 PRESSURIZED AND UNPRESSURIZED INSULATORS
201 FOR USE IN ELECTRICAL EQUIPMENT WITH A.C. RATED VOLTAGE
202 GREATER THAN 1 000 V AND D.C. VOLTAGE GREATER THAN 1 500 V –
203 DEFINITIONS, TEST METHODS, ACCEPTANCE CRITERIA AND
204 DESIGN RECOMMENDATIONS
205
206
207
208 1 Scope

209 This International Standard applies to composite hollow insulators consisting of a load-bearing

210 insulating tube made of resin impregnated fibres, a housing (outside the insulating tube) made

211 of elastomeric material (for example silicone or ethylene-propylene) and metal fixing devices

212 at the ends of the insulating tube. Composite hollow insulators as defined in this standard are

213 intended for general use (unpressurized) or for use with a permanent gas pressure

214 (pressurized). They are intended for use in both outdoor and indoor electrical equipment

215 operating on alternating current with a rated voltage greater than 1 000 V a.c. and a frequency

216 not greater than 100 Hz or for use in direct current equipment with a rated voltage greater

217 than 1 500 V d.c.
218 The object of this standard is:
219  – to define the terms used;
220  – to prescribe test methods;
221  – to prescribe acceptance criteria.

222 Hollow insulators are integrated into electrical equipment which is electrically type tested as

223 required by the applicable equipment standard. So, it is not the object of this standard to

224 prescribe dielectric type tests because the withstand voltages and flashover behaviour are not

225 characteristics of the hollow insulator itself but of the apparatus of which it ultimately forms a

226 part.

227 All the tests in this standard, apart from the thermal-mechanical test, are performed at normal

228 ambient temperature. This standard does not prescribe tests that may be characteristic of the

229 apparatus of which the hollow insulator ultimately forms a part.
230

231 Composite hollow insulators are intended for use in electrical equipment, such as, but not

232 limited to:
233  HV circuit-breakers,
234  switch-disconnectors,
235  disconnectors,
236  station posts,
237  disconnecting circuit breakers,
238  earthing switches,
239  instrument- and power transformers,
240  bushings,
241  cable terminations.
242

243 Additional testing defined by the relevant IEC equipment standard may be required.

244
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245 2 Normative references

246 The following referenced documents are indispensable for the application of this document.

247 For dated references, only the edition cited applies. For undated references, the latest edition

248 of the referenced document (including any amendments) applies.

249 IEC 60060-1: High-voltage test techniques – Part 1: General definitions and test requirements

250 IEC 62155: Hollow pressurized and unpressurized ceramic and glass insulators for use in

251 electrical equipment with rated voltages greater than 1 000 V

252 IEC 62217: Polymeric insulators for indoor and outdoor use with a nominal voltage >1 000 V-

253 General definitions, test methods and acceptance criteria
254 3 Terms and definitions

255 For the purposes of this document, the following terms and definitions apply.

256 3.1
257 composite hollow insulator

258 insulator consisting of at least two insulating parts, namely a tube and a housing

259 Note 1 to entry - The housing may consist either of individual sheds mounted on the tube, with or without an

260 intermediate sheath, or directly applied in one or several pieces onto the tube. A composite hollow insulator unit is

261 permanently equipped with fixing devices or end fittings
262 [IEV 471-01-08, modified]
263 3.2
264 tube (core)

265 central internal insulating part of a composite hollow insulator which provides the mechanical

266 characteristics
267 Note 1 to entry – The housing and sheds are not part of the core.

268 Note 2 to entry – The tube is generally cylindrical or conical, but may have other shapes (for example barrel). The

269 tube is made of resin impregnated fibres.

270 Note 3 to entry – Resin impregnated fibres are structured in such a manner as to achieve sufficient mechanical

271 strength. Layers of different fibres may be used to fulfil special requirements.

272 [IEV 471-01-03, modified by the addition of a synonym]
273 3.3
274 fixing device
275 end fitting

276 integral component or formed part of an insulator, intended to connect it to a supporting

277 structure, or to a conductor, or to an item of equipment, or to another insulator

278 Note 1 to entry – Where the end fitting is metallic, the term “metal fitting” is normally used.

279 [IEV 471-01-06, modified by the addition of a synonym]
280 3.4
281 coupling

282 part of the fixing device which transmits load to the hardware external to the insulator

283 [IEC 62217, section 3]
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284 3.5
285 connection zone

286 zone where the mechanical load is transmitted between the insulating body and the end fitting

287 [IEC 62217, section 3]
288 3.6
289 housing

290 external insulating part of composite hollow insulator providing necessary creepage distance

291 and protecting tube from environment

292 Note 1 to entry – If an intermediate sheath is used it forms a part of the housing

293 [IEC 62217, section 3]
294 [SOURCE: IEV 471-01-09, modified]
295 3.7
296 shed (of an insulator)

297 insulating part, projecting from the insulator trunk, intended to increase the creepage distance

298 Note 1 to entry – The shed can be with or without ribs.
299 [IEV 471-01-15]
300 3.8
301 insulator trunk
302 central insulating part of an insulator from which the sheds project
303 Note 1 to entry – Also known as shank on smaller insulators.
304 [IEV 471-01-11]
305 3.9
306 creepage distance

307 shortest distance or the sum of the shortest distances along the surface on an insulator

308 between two conductive parts which normally have the operating voltage between them

309 Note 1 to entry – The surface of any non-insulating jointing material is not considered as forming part of the

310 creepage distance.
311 [IEV 471-01-04, modified]
312 3 . 10
313 arcing distance

314 shortest distance in the air external to the insulator between the metallic parts which normally

315 have the operating voltage between them
316
317 Note 1 to entry – The term “dry arcing distance” is also used.
318 [IEV 471-01-01]
319 3 . 11
320 tracking

321 process which forms irreversible degradation by formation of conductive paths (tracks)

322 starting and developing on the surface of an insulating material
323 Note 1 to entry – These paths are conductive even under dry conditions.
324 [IEC 62217, section 3]
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325 3 . 12
326 erosion

327 irreversible and non-conducting degradation of the surface of the insulator that occurs by loss

328 of m
...