Electrical strength of insulating materials - Test methods - Part 1: Tests at power frequencies

This part of IEC 60243 provides test methods for the determination of short-time electric strength of solid insulating materials at power frequencies between 48 Hz and 62 Hz. This standard does not cover the testing of liquids and gases, although these are specified and used as impregnates or surrounding media for the solid insulating materials being tested. NOTE Methods for the determination of breakdown voltages along the surfaces of solid insulating materials are included.

/

Rigidité diélectrique des matériaux isolants - Méthodes d'essai - Partie 1: Essais aux fréquences industrielles

La CEI 60243-1:2013 fournit les méthodes d'essai pour la détermination de la rigidité diélectrique de courte durée des matériaux isolants solides, aux fréquences industrielles comprises entre 48 Hz et 62 Hz. La présente norme ne couvre pas la manière d'effectuer l'essai des liquides ou des gaz bien que ceux-ci soient spécifiés et utilisés comme imprégnants ou comme milieu ambiant pour l'essai des matériaux isolants solides. NOTE: Les méthodes pour déterminer les tensions de claquage en surface des matériaux isolants solides sont indiquées.

Električna prebojna trdnost izolacijskih materialov - 1. del: Preskušanje pri omrežnih frekvencah

Ta del standarda IEC 60243 podaja preskusne metode za določitev kratkoročne električne moči trdnih izolacijskih materialov pri obratovalnih frekvencah med 48 Hz in 62 Hz. Ta standard ne zajema preskušanja tekočin in plinov, čeprav so ti določeni in se uporabljajo kot sredstva za impregnacijo ali sredstva okrog trdnih izolacijskih materialov, ki se preskušajo. OPOMBA Metode za določitev prebojnih napetosti na površinah trdnih izolacijskih materialov so vključene.

General Information

Status
Published
Publication Date
22-Jul-2013
Current Stage
6060 - National Implementation/Publication (Adopted Project)
Start Date
22-Jul-2013
Due Date
26-Sep-2013
Completion Date
23-Jul-2013

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SLOVENSKI STANDARD
SIST EN 60243-1:2013
01-september-2013
1DGRPHãþD
SIST EN 60243-1:2001
(OHNWULþQDSUHERMQDWUGQRVWL]RODFLMVNLKPDWHULDORYGHO3UHVNXãDQMHSUL
RPUHåQLKIUHNYHQFDK

Electrical strength of insulating materials - Test methods - Part 1: Tests at power

frequencies

Rigidité diélectrique des matériaux isolants - Méthodes d'essai - Partie 1: Essais aux

fréquences industrielles
Ta slovenski standard je istoveten z: EN 60243-1:2013
ICS:
17.220.99 Drugi standardi v zvezi z Other standards related to
elektriko in magnetizmom electricity and magnetism
29.035.01 Izolacijski materiali na Insulating materials in
splošno general
SIST EN 60243-1:2013 en

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

---------------------- Page: 1 ----------------------
SIST EN 60243-1:2013
---------------------- Page: 2 ----------------------
SIST EN 60243-1:2013
EUROPEAN STANDARD
EN 60243-1
NORME EUROPÉENNE
July 2013
EUROPÄISCHE NORM
ICS 17.220.99; 29.035.01 Supersedes EN 60243-1:1998
English version
Electric strength of insulating materials -
Test methods -
Part 1: Tests at power frequencies
(IEC 60243-1:2013)
Rigidité diélectrique des matériaux Elektrische Durchschlagfestigkeit von
isolants - Méthodes d'essai - isolierenden Werkstoffen -
Partie 1: Essais aux fréquences Prüfverfahren -
industrielles Teil 1: Prüfungen bei technischen
(CEI 60243-1:2013) Frequenzen
(IEC 60243-1:2013)

This European Standard was approved by CENELEC on 2013-04-30. CENELEC members are bound to comply

with the CEN/CENELEC Internal Regulations which stipulate the conditions for giving this European Standard

the status of a national standard without any alteration.

Up-to-date lists and bibliographical references concerning such national standards may be obtained on

application to the CEN-CENELEC Management Centre or to any CENELEC member.

This European Standard exists in three official versions (English, French, German). A version in any other

language made by translation under the responsibility of a CENELEC member into its own language and notified

to the CEN-CENELEC Management Centre has the same status as the official versions.

CENELEC members are the national electrotechnical committees of Austria, Belgium, Bulgaria, Croatia, Cyprus,

the Czech Republic, Denmark, Estonia, Finland, Former Yugoslav Republic of Macedonia, France, Germany,

Greece, Hungary, Iceland, Ireland, Italy, Latvia, Lithuania, Luxembourg, Malta, the Netherlands, Norway, Poland,

Portugal, Romania, Slovakia, Slovenia, Spain, Sweden, Switzerland, Turkey and the United Kingdom.

CENELEC
European Committee for Electrotechnical Standardization
Comité Européen de Normalisation Electrotechnique
Europäisches Komitee für Elektrotechnische Normung
Management Centre: Avenue Marnix 17, B - 1000 Brussels

© 2013 CENELEC - All rights of exploitation in any form and by any means reserved worldwide for CENELEC members.

Ref. No. EN 60243-1:2013 E
---------------------- Page: 3 ----------------------
SIST EN 60243-1:2013
EN 60243-1:2013 - 2 -
Foreword

The text of document 112/237/FDIS, future edition 3 of IEC 60243-1, prepared by IEC/TC 112

"Evaluation and qualification of electrical insulating materials and systems" was submitted to the

IEC-CENELEC parallel vote and approved by CENELEC as EN 60243-1:2013.
The following dates are fixed:
(dop) 2014-01-30
• latest date by which the document has
to be implemented at national level by
publication of an identical national
standard or by endorsement
• latest date by which the national (dow) 2016-04-30
standards conflicting with the
document have to be withdrawn
This document supersedes EN 60243-1:1998.

EN 60243-1:2013 includes the following significant technical changes with respect to

EN 60243-1:1998:

The significant technical change with respect to the previous edition is that the current version now

includes an option for testing elastomeric materials.

Attention is drawn to the possibility that some of the elements of this document may be the subject of

patent rights. CENELEC [and/or CEN] shall not be held responsible for identifying any or all such

patent rights.
Endorsement notice

The text of the International Standard IEC 60243-1:2013 was approved by CENELEC as a European

Standard without any modification.

In the official version, for Bibliography, the following note has to be added for the standard indicated :

IEC 60674-2 NOTE Harmonised as EN 60674-2.
---------------------- Page: 4 ----------------------
SIST EN 60243-1:2013
- 3 - EN 60243-1:2013
Annex ZA
(normative)
Normative references to international publications
with their corresponding European publications

The following documents, in whole or in part, are normatively referenced in this document and are

indispensable for its application. For dated references, only the edition cited applies. For undated

references, the latest edition of the referenced document (including any amendments) applies.

NOTE When an international publication has been modified by common modifications, indicated by (mod), the relevant EN/HD

applies.
Publication Year Title EN/HD Year
IEC 60212 - Standard conditions for use prior to and EN 60212 -
during the testing of solid electrical insulating
materials
IEC 60296 - Fluids for electrotechnical applications - EN 60296 -
Unused mineral insulating oils for
transformers and switchgear
IEC 60455-2 - Resin based reactive compounds used for EN 60455-2 -
electrical insulation -
Part 2: Methods of test
IEC 60464-2 - Varnishes used for electrical insulation - EN 60464-2 -
Part 2: Methods of test
IEC 60684-2 - Flexible insulating sleeving - EN 60684-2 -
Part 2: Methods of test
IEC 60836 - Specifications for unused silicone insulating EN 60836 -
liquids for electrotechnical purposes
IEC 61099 - Insulating liquids - Specifications for unused EN 61099 -
synthetic organic esters for electrical
purposes
ISO 293 - Plastics - Compression moulding of test EN ISO 293 -
specimens of thermoplastic materials
ISO 294-1 - Plastics - Injection moulding of test EN ISO 294-1 -
specimens of thermoplastic materials -
Part 1: General principles, and moulding of
multipurpose and bar test specimens
ISO 294-3 - Plastics - Injection moulding of test EN ISO 294-3 -
specimens of thermoplastic materials -
Part 3: Small plates
ISO 295 - Plastics - Compression moulding of test EN ISO 295 -
specimens of thermosetting materials
ISO 10724 Series Plastics - Injection moulding of test EN ISO 10724 Series
specimens of thermosetting powder
moulding compounds (PMCs)
---------------------- Page: 5 ----------------------
SIST EN 60243-1:2013
---------------------- Page: 6 ----------------------
SIST EN 60243-1:2013
IEC 60243-1
Edition 3.0 2013-03
INTERNATIONAL
STANDARD
NORME
INTERNATIONALE
Electric strength of insulating materials – Test methods –
Part 1: Tests at power frequencies
Rigidité diélectrique des matériaux isolants – Méthodes d'essai –
Partie 1: Essais aux fréquences industrielles
INTERNATIONAL
ELECTROTECHNICAL
COMMISSION
COMMISSION
ELECTROTECHNIQUE
PRICE CODE
INTERNATIONALE
CODE PRIX U
ICS 17.220.99; 29.035.01 ISBN 978-2-83220-696-6

Warning! Make sure that you obtained this publication from an authorized distributor.

Attention! Veuillez vous assurer que vous avez obtenu cette publication via un distributeur agréé.

® Registered trademark of the International Electrotechnical Commission
Marque déposée de la Commission Electrotechnique Internationale
---------------------- Page: 7 ----------------------
SIST EN 60243-1:2013
– 2 – 60243-1 © IEC:2013
CONTENTS

FOREWORD ........................................................................................................................... 4

1 Scope ............................................................................................................................... 6

2 Normative references ....................................................................................................... 6

3 Terms and definitions ....................................................................................................... 7

4 Significance of the test ..................................................................................................... 7

5 Electrodes and specimens ................................................................................................ 8

5.1 General ................................................................................................................... 8

5.2 Tests perpendicular to the surface of non-laminated materials and normal to

laminate of laminated materials ............................................................................... 8

5.2.1 Boards and sheet materials, including pressboards, papers, fabrics

and films ...................................................................................................... 8

5.2.2 Tapes, films and narrow strips ..................................................................... 9

5.2.3 Flexible tubing and sleeving ........................................................................ 9

5.2.4 Rigid tubes (having an internal diameter up to and including 100 mm) ......... 9

5.2.5 Tubes and hollow cylinders (having an internal diameter greater than

100 mm) .................................................................................................... 10

5.2.6 Cast and moulded materials ...................................................................... 10

5.2.7 Shaped solid pieces .................................................................................. 11

5.2.8 Varnishes .................................................................................................. 11

5.2.9 Filling compounds ...................................................................................... 11

5.3 Tests parallel to the surface of non-laminated materials and parallel to the

laminate of laminated materials ............................................................................. 11

5.3.1 General ..................................................................................................... 11

5.3.2 Parallel plate electrodes ............................................................................ 11

5.3.3 Taper pin electrodes .................................................................................. 12

5.3.4 Parallel cylindrical electrodes .................................................................... 12

5.4 Test specimens ..................................................................................................... 12

5.5 Distance between electrodes ................................................................................. 12

6 Conditioning before tests ................................................................................................ 13

7 Surrounding medium ...................................................................................................... 13

7.1 General ................................................................................................................. 13

7.2 Tests in air at elevated temperature ...................................................................... 13

7.3 Tests in liquids ...................................................................................................... 13

7.4 Tests in solid materials .......................................................................................... 14

8 Electrical apparatus ........................................................................................................ 14

8.1 Voltage source ...................................................................................................... 14

8.2 Voltage measurement ............................................................................................ 14

9 Procedure....................................................................................................................... 15

10 Mode of increase of voltage ............................................................................................ 15

10.1 Short-time (rapid-rise) test ..................................................................................... 15

10.2 20 s step-by-step test ............................................................................................ 16

10.3 Slow rate-of-rise test (120 s... 240 s) ..................................................................... 16

10.4 60 s step-by-step test ............................................................................................ 17

10.5 Very slow rate-of-rise test (300 s... 600 s) ............................................................. 17

10.6 Proof tests ............................................................................................................. 17

11 Criterion of breakdown ................................................................................................... 17

12 Number of tests .............................................................................................................. 18

---------------------- Page: 8 ----------------------
SIST EN 60243-1:2013
60243-1 © IEC:2013 – 3 –

13 Report ............................................................................................................................ 18

Annex A (informative) Treatment of experimental data ......................................................... 25

Bibliography .......................................................................................................................... 26

Figure 1 – Electrode arrangements for tests on boards and sheets perpendicular to the

surface ................................................................................................................................. 19

Figure 2 – Typical example of electrode arrangement for tests on tapes perpendicular

to the surface (see 5.2.2) ...................................................................................................... 20

Figure 3 – Electrode arrangement for tests perpendicular to the surface on tubes and

cylinders with internal diameter greater than 100 mm............................................................ 20

Figure 4 – Electrode arrangement for tests on casting and moulding materials

(diameter of the spherical electrodes: d = (20 ± 0,1) mm) ..................................................... 21

Figure 5 – Electrode arrangement for test on shaped insulating parts (see 5.2.7) ................. 21

Figure 6 – Electrode arrangement for tests parallel to the surface (and along the

laminae, if present) ............................................................................................................... 22

Figure 7 – Electrode arrangement for tests parallel to the surface (and along the

laminae if present) ................................................................................................................ 23

Figure 8 – Arrangement for tests parallel to the laminae for boards more than 15 mm

thick with parallel cylindrical electrodes (see 5.3.4) ............................................................... 24

Table 1 – Increments of voltage increase (kilovolts, peak / ) ............................................. 16

---------------------- Page: 9 ----------------------
SIST EN 60243-1:2013
– 4 – 60243-1 © IEC:2013
INTERNATIONAL ELECTROTECHNICAL COMMISSION
____________
ELECTRIC STRENGTH OF INSULATING MATERIALS –
TEST METHODS –
Part 1: Tests at power frequencies
FOREWORD

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

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

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

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

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

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

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

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

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

agreement between the two organizations.

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

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

interested IEC National Committees.

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

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

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

misinterpretation by any end user.

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

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

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

the latter.

5) IEC itself does not provide any attestation of conformity. Independent certification bodies provide conformity

assessment services and, in some areas, access to IEC marks of conformity. IEC is not responsible for any

services carried out by independent certification bodies.

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

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

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

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

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

Publications.

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

indispensable for the correct application of this publication.

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

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

International Standard IEC 60243-1 has been prepared by technical committee 112:
Evaluation and qualification of electrical insulating materials and systems.

This third edition cancels and replaces the second edition, published in 1998, and constitutes

a technical revision.

The significant technical change with respect to the previous edition is that the current version

now includes an option for testing elastomeric materials.
The text of this standard is based on the following documents:
FDIS Report on voting
112/237/FDIS 112/248/RVD

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

voting indicated in the above table.
---------------------- Page: 10 ----------------------
SIST EN 60243-1:2013
60243-1 © IEC:2013 – 5 –

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

A list of all the parts in the IEC 60243 series, published under the general title Electric

strength of insulating materials – Test methods, can be found on the IEC website.

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

the stability date indicated on the IEC web site under "http://webstore.iec.ch" in the data

related to the specific publication. At this date, the publication will be
• reconfirmed,
• withdrawn,
• replaced by a revised edition, or
• amended.
---------------------- Page: 11 ----------------------
SIST EN 60243-1:2013
– 6 – 60243-1 © IEC:2013
ELECTRIC STRENGTH OF INSULATING MATERIALS –
TEST METHODS –
Part 1: Tests at power frequencies
1 Scope

This part of IEC 60243 provides test methods for the determination of short-time electric

strength of solid insulating materials at power frequencies between 48 Hz and 62 Hz.

This standard does not cover the testing of liquids and gases, although these are specified

and used as impregnates or surrounding media for the solid insulating materials being tested.

NOTE Methods for the determination of breakdown voltages along the surfaces of solid insulating materials are

included.
2 Normative references

The following documents, in whole or in part, are normatively referenced in this document and

are indispensable for its application. For dated references, only the edition cited applies. For

undated references, the latest edition of the referenced document (including any
amendments) applies.

IEC 60212, Standard conditions for use prior to and during the testing of solid electical

insulating materials

IEC 60296, Fluids for electrotechnical applications – Unused mineral insulating oils for

transformers and switchgear

IEC 60455-2, Specification for solventless polymerizable resinous compounds used for

electrical insulation – Part 2: Methods of test
IEC 60464-2, Varnishes used for electrical insulation – Part 2: Methods of test
IEC 60684-2, Flexible insulating sleeving – Part 2: Methods of test

IEC 60836, Specifications for unused silicone insulating liquids for electrotechnical purposes

IEC 61099, Insulating liquids – Specifications for unused synthetic organic esters for electrical

purposes

ISO 293, Plastics – Compression moulding of test specimens of thermoplastic materials

ISO 294-1, Plastics – Injection moulding of test specimens of thermoplastic materials – Part 1:

General principles, and moulding of multipurpose and bar test specimens

ISO 294-3, Plastics – Injection moulding of test specimens of thermoplastic materials – Part 3:

Small plates

ISO 295, Plastics – Compression moulding of test specimens of thermosetting materials

---------------------- Page: 12 ----------------------
SIST EN 60243-1:2013
60243-1 © IEC:2013 – 7 –

ISO 10724 (all parts), Plastics – Injection moulding of test specimens of thermosetting powder

moulding compounds (PMCs)
3 Terms and definitions
For the purposes of this document, the following terms and definitions apply.
3.1
electric breakdown

severe loss of the insulating properties of test specimens while exposed to electric stress,

which causes the current in the test circuit to operate an appropriate circuit-breaker

Note 1 to entry: Breakdown is often caused by partial discharges in the gas or liquid medium surrounding the test

specimen and the electrodes which puncture the specimen beyond the periphery of the smaller electrode (or of

both electrodes, if of equal diameter).
3.2
flashover

loss of the insulating properties of the gas or liquid medium surrounding a test specimen and

electrodes while exposed to electric stress, which causes the current in the test circuit to

operate an appropriate circuit-breaker

Note 1 to entry: The presence of carbonized channels or punctures through the specimen distinguishes tests

where breakdown occurred, from others where flashover occurred.
3.3
breakdown voltage
3.3.1

< tests with continuously rising voltage > voltage at which a specimen suffers breakdown

under the prescribed test conditions
3.3.2

< step-by-step tests > highest voltage which a specimen withstands without breakdown for the

duration of the time at that voltage level
3.4
electric strength

quotient of the breakdown voltage and the distance between the electrodes between which

the voltage is applied under the prescribed test conditions

Note 1 to entry: The distance between the test electrodes is determined as specified in 5.5, unless otherwise

specified.
4 Significance of the test

Electric strength test results obtained in accordance with this standard are useful for detecting

changes or deviations from normal characteristics resulting from processing variables, ageing

conditions or other manufacturing or environmental situations. However, they are not intended for

use in evaluating the behaviour of insulating materials in an actual application.

Measured values of the electric strength of a material may be affected by many factors,

including:
a) Condition of test specimens

1) the thickness and homogeneity of the specimen and the presence of mechanical strain;

2) previous conditioning of the specimens, in particular drying and impregnation
procedures;
---------------------- Page: 13 ----------------------
SIST EN 60243-1:2013
– 8 – 60243-1 © IEC:2013
3) the presence of gaseous inclusions, moisture or other contamination.
b) Test conditions

1) the frequency, waveform and rate of rise or time of application of the voltage;

2) the ambient temperature, pressure and humidity;

3) the configuration, the dimensions, and thermal conductivity of the test electrodes;

4) the electrical and thermal characteristics of the surrounding medium.

The effects of all these factors shall be considered when investigating materials for which no

experience exists. This standard defines particular conditions which give rapid discrimination

between materials and which can be used for quality control and similar purposes.

The results given by different methods are not directly comparable but each may provide

information on relative electric strengths of materials. The electric strength of most materials

decreases as the thickness of the specimen between the electrodes increases and as the time

of voltage application increases.

The measured electric strength of most materials is significantly affected by the intensity and

the duration of surface discharges prior to breakdown. For designs which are free from partial

discharges up to the test voltage, it is very important to know the electric strength without

discharges prior to breakdown. However, the methods in this standard are generally not

suitable for providing this information.

Materials with high electric strength will not necessarily resist long-term degradation

processes such as heat, erosion or chemical deterioration by partial discharges, or

electrochemical deterioration in the presence of moisture, all of which may cause failure in

service at much lower stress.
5 Electrodes and specimens
5.1 General

The metal electrodes shall be maintained smooth, clean and free from defects at all times.

Electrode arrangements for tests on boards and sheets perpendicular to the surface are

shown in Figure 1.

NOTE This maintenance becomes more important when thin specimens are being tested. Stainless steel

electrodes e.g. minimize electrode damage at breakdown.

The leads to the electrodes shall not tilt or otherwise move the electrodes, nor affect the

pressure on the specimen, nor appreciably affect the electric field configuration in the

neighbourhood of the specimen.

When very thin films (for example <5 μm thick) are to be tested, the standards for those

materials shall specify the electrodes and special procedures for handling and specimen

preparation.
5.2 Tests perpendicular to the surface of non-laminated materials and normal to
laminate of laminated materials

5.2.1 Boards and sheet materials, including pressboards, papers, fabrics and films

5.2.1.1 Unequal electrodes

The electrodes shall consist of two metal cylinders with the edges rounded to give a radius of

(3 ± 0,2) mm. One electrode shall be (25 ± 1) mm in diameter and approximately 25 mm high.

The other electrode shall be (75 ± 1) mm in diameter and approximately 15 mm high. These

two electrodes shall be arranged coaxially within 2 mm as in Figure 1a.
---------------------- Page: 14 ----------------------
SIST EN 60243-1:2013
60243-1 © IEC:2013 – 9 –

NOTE Radii for surface not in contact with the electrode are not critical with respect to test results but should

avoid partial discharges in the surrounding medium.
5.2.1.2 Equal diameter electrodes

If a fixture is employed, which accurately aligns upper and lower electrodes within 1,0 mm,

the diameter of the lower electrode may be reduced to (25 ± 1) mm, the diameters of the two

electrodes differing by no more than 0,2 mm. The results obtained will not necessarily be the

same as those obtained with the unequal electrodes of 5.2.1.1.
5.2.1.3 Sphere and plate electrodes

The electrodes shall consist of a metal sphere and a metal plate (see Figure 1c). The upper

electrode shall be a sphere of (20 ± 1) mm in diameter and the lower one is a metal plate of

(25 ± 1) mm in diameter with the edge rounded to give a radius of 2.5 mm. The discrepancy of

the central axes between upper and lower electrodes shall be within 1 mm.
5.2.1.4 Tests on thick sample

When specified, boards and sheets over 3 mm thick shall be reduced by machining on one

side to (3 ± 0,2) mm and then tested with the high-potential electrode on the non-machined

surface.
When it is necessary in order to avoid flashover or because of limitations of
...

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