EN IEC 62788-2-1:2023

SLOVENSKI STANDARD
01-januar-2024
Merilni postopki za materiale, uporabljene v fotonapetostnih modulih - 2-1. del:
Polimerni materiali - Prednja in zadnja plast - Varnostne zahteve
Measurement procedures for materials used in photovoltaic modules - Part 2-1:
Polymeric materials - Frontsheet and backsheet - Safety requirements
Messverfahren für Werkstoffe, die in Photovoltaik-Modulen verwendet werden – Teil 2-1:
Polymerwerkstoffe – Frontsheets und Backsheets – Sicherheitsanforderungen
Procédures de mesure des matériaux utilisés dans les modules photovoltaïques - Partie
2-1: Matériaux polymères - Face avant et face arrière - Exigences de sécurité
Ta slovenski standard je istoveten z: EN IEC 62788-2-1:2023
ICS:
27.160 Sončna energija Solar energy engineering
83.080.01 Polimerni materiali na Plastics in general
splošno
2003-01.Slovenski inštitut za standardizacijo. Razmnoževanje celote ali delov tega standarda ni dovoljeno.

EUROPEAN STANDARD EN IEC 62788-2-1

NORME EUROPÉENNE
EUROPÄISCHE NORM October 2023
ICS 27.160
English Version
Measurement procedures for materials used in photovoltaic
modules - Part 2-1: Polymeric materials - Frontsheet and
backsheet - Safety requirements
(IEC 62788-2-1:2023)
Procédures de mesure des matériaux utilisés dans les Messverfahren für Werkstoffe, die in Photovoltaik-Modulen
modules photovoltaïques - Partie 2-1: Matériaux polymères verwendet werden - Teil 2-1: Polymerwerkstoffe -
- Face avant et face arrière - Exigences de sécurité Frontsheets und Backsheets - Sicherheitsanforderungen
(IEC 62788-2-1:2023) (IEC 62788-2-1:2023)
This European Standard was approved by CENELEC on 2023-09-29. 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, France, Germany, Greece, Hungary, Iceland, Ireland, Italy, Latvia, Lithuania, Luxembourg, Malta, the
Netherlands, Norway, Poland, Portugal, Republic of North Macedonia, Romania, Serbia, Slovakia, Slovenia, Spain, Sweden, Switzerland,
Türkiye and the United Kingdom.

European Committee for Electrotechnical Standardization
Comité Européen de Normalisation Electrotechnique
Europäisches Komitee für Elektrotechnische Normung
CEN-CENELEC Management Centre: Rue de la Science 23, B-1040 Brussels
© 2023 CENELEC All rights of exploitation in any form and by any means reserved worldwide for CENELEC Members.

Ref. No. EN IEC 62788-2-1:2023 E

European foreword
The text of document 82/2123/FDIS, future edition 1 of IEC 62788-2-1, prepared by IEC/TC 82 "Solar
photovoltaic energy systems" was submitted to the IEC-CENELEC parallel vote and approved by
CENELEC as EN IEC 62788-2-1:2023.
The following dates are fixed:
• latest date by which the document has to be implemented at national (dop) 2024-06-29
level by publication of an identical national standard or by endorsement
• latest date by which the national standards conflicting with the (dow) 2026-09-29
document have to be withdrawn
Attention is drawn to the possibility that some of the elements of this document may be the subject of
patent rights. CENELEC shall not be held responsible for identifying any or all such patent rights.
Any feedback and questions on this document should be directed to the users’ national committee. A
complete listing of these bodies can be found on the CENELEC website.
Endorsement notice
The text of the International Standard IEC 62788-2-1:2023 was approved by CENELEC as a
European Standard without any modification.
In the official version, for Bibliography, the following notes have to be added for the standard indicated:
IEC 60112:2020 NOTE Approved as EN IEC 60112:2020 (not modified)
IEC 62941:2019 NOTE Approved as EN IEC 62941:2020 (not modified)
Annex ZA
(normative)
Normative references to international publications
with their corresponding European publications
The following documents are referred to in the text in such a way that some or all of their content
constitutes requirements of this document. For dated references, only the edition cited applies. For
undated references, the latest edition of the referenced document (including any amendments)
applies.
NOTE 1 Where an International Publication has been modified by common modifications, indicated by (mod), the
relevant EN/HD applies.
NOTE 2 Up-to-date information on the latest versions of the European Standards listed in this annex is available
here: www.cencenelec.eu.
Publication Year Title EN/HD Year
IEC 60216-1 - Electrical insulating materials - Thermal EN 60216-1 -
endurance properties - Part 1: Ageing
procedures and evaluation of test results
IEC 60216-3 - Electrical insulating materials - Thermal EN IEC 60216-3 -
endurance properties - Part 3: Instructions
for calculating thermal endurance
characteristics
IEC 60216-5 - Electrical insulating materials - Thermal EN IEC 60216-5 -
endurance properties - Part 5:
Determination of relative temperature index
(RTI) of an insulating material
IEC 60664-1 - Insulation coordination for equipment EN IEC 60664-1 -
within low-voltage supply systems - Part 1:
Principles, requirements and tests
IEC 61215-1 - Terrestrial photovoltaic (PV) modules - EN IEC 61215-1 -
Design qualification and type approval -
Part 1: Test requirements
IEC 61215-2 - Terrestrial photovoltaic (PV) modules - EN IEC 61215-2 -
Design qualification and type approval -
Part 2: Test procedures
IEC 61730-1 - Photovoltaic (PV) module safety EN IEC 61730-1 -
qualification - Part 1: Requirements for
construction
IEC 61730-2 - Photovoltaic (PV) module safety EN IEC 61730-2 -
qualification - Part 2: Requirements for
testing
IEC/TS 61836 - Solar photovoltaic energy systems - - -
Terms, definitions and symbols
IEC TS 62788-2 - Measurement procedures for materials - -
used in photovoltaic modules - Part 2:
Polymeric materials - Frontsheets and
backsheets
IEC/TS 62915 - Photovoltaic (PV) modules - Type - -
approval, design and safety qualification -
Retesting
IEC TS 63126 2020 Guidelines for qualifying PV modules, - -
components and materials for operation at
high temperatures
ISO 527-3 - Plastics - Determination of tensile EN ISO 527-3 -
properties - Part 3: Test conditions for films
and sheets
IEC 62788-2-1 ®
Edition 1.0 2023-08
INTERNATIONAL
STANDARD
NORME
INTERNATIONALE
colour
inside
Measurement procedures for materials used in photovoltaic modules –

Part 2-1: Polymeric materials – Frontsheet and backsheet – Safety requirements

Procédures de mesure des matériaux utilisés dans les modules photovoltaïques –

Partie 2-1: Matériaux polymères – Face avant et face arrière – Exigences de

sécurité
INTERNATIONAL
ELECTROTECHNICAL
COMMISSION
COMMISSION
ELECTROTECHNIQUE
INTERNATIONALE
ICS 27.160  ISBN 978-2-8322-6887-2

– 2 – IEC 62788-2-1:2023 © IEC 2023
CONTENTS
FOREWORD . 4
INTRODUCTION . 6
1 Scope . 8
2 Normative references . 8
3 Terms, definitions and abbreviated terms . 9
3.1 General terms and definitions . 9
3.2 Sheet types and orientations . 9
3.3 Electrical insulation . 10
3.4 Temperatures . 11
3.5 Tensile properties . 11
4 Designation and ratings . 12
5 Requirements . 12
5.1 General . 12
5.1.1 Overview . 12
5.1.2 Single-layer constructions . 13
5.1.3 Multilayer constructions . 14
5.2 Insulation coordination . 14
5.2.1 General . 14
5.2.2 Breakdown voltage requirement for complete front- or backsheet . 14
5.2.3 Breakdown voltage requirements for individual layers . 15
5.2.4 Creepage distance requirements . 15
5.2.5 Distance through insulation requirements . 16
5.3 Thermal endurance . 16
5.4 Mechanical requirements . 17
5.5 Model and variant designation . 17
6 Evaluation of test results . 17
6.1 General . 17
6.2 Visual inspection – FBST 01 . 18
6.2.1 General . 18
6.2.2 Reporting . 18
6.3 Tensile properties – FBST 02 . 18
6.3.1 General . 18
6.3.2 Reporting . 18
6.4 Breakdown voltage – FBST 03 . 19
6.4.1 General . 19
6.4.2 Analysis . 20
6.4.3 Reporting . 20
6.5 Distance through insulation – FBST 04 . 21
6.5.1 General . 21
6.5.2 Analysis . 21
6.5.3 Reporting . 22
6.6 Material group – FBST 05 . 22
6.6.1 General . 22
6.6.2 Reporting . 23
6.7 Thermal endurance – FBST 06 . 23
6.7.1 General . 23

IEC 62788-2-1:2023 © IEC 2023 – 3 –
6.7.2 Reporting . 23
6.8 Accelerated ageing tests . 23
6.8.1 Damp heat – FBST 07 . 23
6.8.2 UV weathering – FBST 08 . 23
6.8.3 Reporting . 24
6.9 Overview tables . 25
7 Evaluation report . 26
7.1 Report. 26
8 Documentation and testing for similar materials . 26
8.1 General . 26
8.2 Alternate constituent layers . 27
8.3 Thickness variants . 27
8.4 Color variants . 27
8.5 Reporting for similar materials with different color or thickness . 28
Annex A (informative) Chemical analytical material identification . 30
A.1 General . 30
A.2 Examples of fingerprint techniques . 30
Bibliography . 31

Figure 1 – Schematic diagrams of typical constructions of front- or backsheets . 13
Figure 2 – Examples of calculations for determination of DTI ratio and adjusted
breakdown voltage . 20
Figure 3 – Determining distance through insulation from lamination protrusion test as a
function of temperature rating . 22
Figure 4 – Schematic of lamination protrusion test result for determining which
surfaces will be evaluated for minimum creepage distance in the IEC 61730 module
design review . 23

Table 1 – Minimum breakdown voltage requirements for basic and double/reinforced
insulation before and after accelerated aging . 15
Table 2 – Minimum distance through insulation requirements. 16
Table 3 – UV exposure conditions . 24
Table 4 – Evaluations and requirements overview for individual layers . 25
Table 5 – Evaluations and requirements overview for complete front- and/or
backsheets . 25

– 4 – IEC 62788-2-1:2023 © IEC 2023
INTERNATIONAL ELECTROTECHNICAL COMMISSION
____________
MEASUREMENT PROCEDURES FOR MATERIALS
USED IN PHOTOVOLTAIC MODULES –

Part 2-1: Polymeric materials –
Frontsheet and backsheet – Safety requirements

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,
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preparation is entrusted to technical committees; any IEC National Committee interested in the subject dealt with
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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 because 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
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4) In order to promote international uniformity, IEC National Committees undertake to apply IEC Publications
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6) All users should ensure that they have the latest edition of this publication.
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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.
IEC 62788-2-1 has been prepared by IEC technical committee 82: Solar photovoltaic energy
systems. It is an International Standard.
The text of this International Standard is based on the following documents:
Draft Report on voting
82/2123/FDIS 82/2148/RVD
Full information on the voting for its approval can be found in the report on voting indicated in
the above table.
The language used for the development of this International Standard is English.

IEC 62788-2-1:2023 © IEC 2023 – 5 –
This document was drafted in accordance with ISO/IEC Directives, Part 2, and developed in
accordance with ISO/IEC Directives, Part 1 and ISO/IEC Directives, IEC Supplement, available
at www.iec.ch/members_experts/refdocs. The main document types developed by IEC are
described in greater detail at www.iec.ch/standardsdev/publications.
A list of all parts in the IEC 62788 series, published under the general title Measurement
procedures for materials used in photovoltaic modules, can be found on the IEC website.
The committee has decided that the contents of this document will remain unchanged until the
stability date indicated on the IEC website under webstore.iec.ch in the data related to the
specific document. At this date, the document will be
• reconfirmed,
• withdrawn,
• replaced by a revised edition, or
• amended.
IMPORTANT – The "colour inside" logo on the cover page of this document indicates that it
contains colours which are considered to be useful for the correct understanding of its
contents. Users should therefore print this document using a colour printer.

– 6 – IEC 62788-2-1:2023 © IEC 2023
INTRODUCTION
This document provides test procedures and specifications for polymeric front- and backsheet
constructions employed in a PV module for safety qualification on a component level. Test
methods have been compiled to match the general requirements for polymeric materials used
as relied-upon insulation defined in the IEC 61730 standard series in consideration of test
methods in IEC TS 62788-2 (characterization of front- and backsheets), IEC TS 62788-7-2
[4] (UV weathering test) and the retesting guidelines IEC TS 62915. This document provides
clarifications on definitions of front- and backsheet construction types and related test
requirements, and additional environmental stress testing, to which IEC 61730-1 refers.
Separating out the component level testing into this document was considered to limit the
complexity of the IEC 61730 standard series, also in view of the implementation of the test
methods in the frame of IEC System of Conformity Assessment Schemes for Electrotechnical
Equipment and Components (IECEE).
Test methods on a component level and PV module level are different for practical reasons. On
a component level, the daylight filtered xenon test (IEC TS 62788-7-2) is applied for UV
weathering, which is regarded as more representative to assess the durability of polymeric
materials under outdoor weather conditions than the UVA test of IEC 61215-2. The latter has
been developed in view of practicality of applying UV exposure to larger scale PV modules.
This document focusses on the safety relevant properties of front- and backsheets as required
by IEC 61730.
The lamination protrusion test (aka DTI test) is required to measure the thickness of relied-upon
insulation on the component level. The thickness of the RUI layer(s) is verified by MST 04 of
IEC 61730-2 on PV module level. The test provides additional information needed for evaluation
of the Comparative Tracking Index (CTI) and dielectric strength / breakdown voltage.
– The lamination protrusion test applies default lamination conditions, that are representative
for a typical PV module manufacturing process. Using a 800 µm diameter solder wire that
mimics severe solder peaks and/or slanted ribbons, the test serves as worst case scenario
for measurement of potential displacement of material under lamination conditions. Even
more harsh lamination process conditions can be selected as recommended by the
manufacturer of the front- or backsheet.
– The lamination protrusion test is also used to identify additional inner layers of the front- or
backsheet that potentially may be in contact with live parts and for which CTI shall be
determined. Additional layers may require CTI depending on the construction of the PV
module, e.g., due to specific sheet openings and through wiring for junction box connections
with background provided in IEC 61730-1.
– The ratio of the measured distance through insulation (t ) to the total thickness is used to
DTI
calculate the effective dielectric strength or required withstand voltage when measured on
final products that contain inner layers, which can be displaced under lamination conditions
(see breakdown voltage test in 6.4).
– Because of the relationship with thermal endurance, t can be listed as a function of the
DTI
module operating temperature rating.
___________
Numbers in square brackets refer to the Bibliography.

IEC 62788-2-1:2023 © IEC 2023 – 7 –
This document specifies a suite of environmental stress tests to characterize the durability of
the relied-upon insulation. In the evaluation of tensile testing a minimum elongation at break is
considered in addition to retention of tensile strength, as this allows to differentiate known-bad
and known-good materials. The thermal endurance performance, which is historically evaluated
by tensile strength and dielectric strength in terms of TI or RTE (RTI), is therefore
complemented by a thermal failsafe test to also evaluate elongation at break. The combination
of these elements, tests covering thermal, damp heat and UV weathering stresses and
evaluation of elongation at break, represents a step forward in safety testing of polymeric front-
and backsheets that is still balanced in terms or practicality (duration) of testing.
The requirements in this document for model or variant designations and (re)testing of similar
materials have been aligned with developments for the IEC TS 62915 module retesting
guidelines. The current requirements provide a first step towards more detailed requirements
which may be developed in a future revision of this document or a dedicated component
retesting standard.
A future revision of this document may consider sequential testing on engineering coupons with
(solder wire) bumps to better mimic the combination of UV and cyclic stress fatigue, that is
currently discussed as the next level in endurance testing of polymeric front- and backsheets
in IEC TS 62788-2 and IEC TS 63209-2[6]. However, method consolidation requires more time
than available for this project.
In view of requirements for material identification in the context of the retesting guidelines
(IEC TS 62915), approaches for “finger-printing” are provided in Annex A.
The requirements in this document may be used in the context of Manufacturing Quality
Assurance of polymeric front- and backsheets as explained in the guideline IEC 62941[5].

– 8 – IEC 62788-2-1:2023 © IEC 2023
MEASUREMENT PROCEDURES FOR MATERIALS
USED IN PHOTOVOLTAIC MODULES –

Part 2-1: Polymeric materials –
Frontsheet and backsheet – Safety requirements

1 Scope
This document specifies the safety requirements for flexible polymeric front- and backsheet
constructions, which are intended for use as relied-upon insulation in photovoltaic (PV)
modules. The specifications in this document define the specific requirements of polymeric
front- or backsheet constructions on the component level and cover mechanical, electrical,
visual and thermal characterization in an unexposed state and/or after ageing.
This document covers class II and class 0 modules, as defined in IEC 61730-1. Class III
modules are out of scope.
For qualification to IEC 61730-1 of a PV module using a polymeric front- or backsheet, the sheet
must pass the requirements in this document for the specified module’s safety class, rated
system voltage, and module temperature rating.
Compliance with the safety requirements for a front- or backsheet on the component level does
not replace the need for a safety qualification of the complete PV module, in which the front- or
backsheet is integrated. The appropriate requirements for testing and qualification of PV
modules, are defined in IEC 61730-1 and IEC 61215-1 (or IEC TS 62915 in case of retesting),
with test methods provided by IEC 61730-2 and IEC 61215-2, respectively.
This document provides the requirements for qualification of front- and backsheets to be used
in module safety qualification according to IEC 61730-1. Test method descriptions are provided
in IEC TS 62788-2, along with additional characterization methods useful for performance or
quality assurance.
2 Normative references
The following documents are referred to in the text in such a way that some or all of their content
constitutes requirements of this document. For dated references, only the edition cited applies.
For undated references, the latest edition of the referenced document (including any
amendments) applies.
IEC 60216-1, Electrical insulating materials – Thermal endurance properties – Part 1: Ageing
procedures and evaluation of test results
IEC 60216-3, Electrical insulating materials – Thermal endurance properties – Part 3:
Instructions for calculating thermal endurance characteristics
IEC 60216-5, Electrical insulating materials – Thermal endurance properties – Part 5:
Determination of relative temperature index (RTI) of an insulating material
IEC 60664-1, Insulation coordination for equipment within low-voltage supply systems – Part 1:
Principles, requirements and tests
IEC 61215-1, Terrestrial photovoltaic (PV) modules – Design qualification and type approval –
Part 1: Test requirements
IEC 62788-2-1:2023 © IEC 2023 – 9 –
IEC 61215-2, Terrestrial photovoltaic (PV) modules – Design qualification and type approval –
Part 2: Test procedures
IEC 61730-1, Photovoltaic (PV) module safety qualification – Part 1: Requirements for
construction
IEC 61730-2, Photovoltaic (PV) module safety qualification – Part 2: Requirements for testing
IEC TS 61836, Solar photovoltaic energy systems – Terms, definitions and symbols
IEC TS 62788-2, Measurement procedures for materials used in photovoltaic modules – Part 2:
Polymeric materials – Frontsheets and backsheets
IEC TS 62915, Photovoltaic (PV) modules – Type approval, design and safety qualification –
Retesting
IEC TS 63126:2020, Guidelines for qualifying PV modules, components and materials for
operation at high temperatures
ISO 527-3, Plastics – Determination of tensile properties – Part 3: Test conditions for films and
sheets
3 Terms, definitions and abbreviated terms
For the purposes of this document, the following terms and definitions apply, in addition to those
in IEC TS 61836 and IEC TS 62788-2.
ISO and IEC maintain terminological databases for use in standardization at the following
addresses:
• IEC Electropedia: available at http://www.electropedia.org/
• ISO Online browsing platform: available at http://www.iso.org/obp
3.1 General terms and definitions
3.1.1
FBST
front- or backsheet safety test in accordance with IEC 62788-2-1
3.1.2
tolerance
permitted deviation between the declared value of a quantity and the measured value
[SOURCE: IEC 60050-411[2]:1996,411-36-19]
3.2 Sheet types and orientations
3.2.1
air-side
side of the front- or backsheet oriented towards the outside of the PV module, i.e., away from
the cells
3.2.2
backsheet
(combination of) outer layer(s) of the PV module, located as substrate on the back of the PV
module and providing protection of the inner components of the PV module from external
stresses and weather elements, as well as providing electrical insulation

– 10 – IEC 62788-2-1:2023 © IEC 2023
3.2.3
frontsheet
(combination of) outer layer(s) of the PV module designed for prolonged exposure to direct
sunlight (> 300 W/m ) and providing protection of the inner components of the PV module from
external stresses and weather elements, as well as providing electrical insulation
3.2.4
inner side
side of the front- or backsheet that is oriented to the solar cells, typically laminated to the
encapsulant
3.2.5
machine direction
MD
direction along which the material layer was extruded or produced, extending out of a die or
other manufacturing equipment in a production line
3.2.6
transverse direction
TD
direction perpendicular to which the material layer was extruded or produced
3.2.7
sun-facing side
side of the front- or backsheet that is oriented in direction of the sun-facing front side of the PV
module
Note 1 to entry: Sun-facing sides are the air-side of a frontsheet and inner side of a backsheet.
3.3 Electrical insulation
3.3.1
breakdown voltage
V
BD
DC voltage at which electric breakdown occurs under prescribed test conditions, or in use
Note 1 to entry: Breakdown voltage testing in context of PV modules and components materials applies direct
current (DC).
[SOURCE: IEC 60050-212[1]:2010, 212-11-34, modified – added symbol, and added Note 1 to
entry.]
3.3.2
comparative tracking index
CTI
numerical index value related to the maximum voltage that a material can withstand without
formation of a permanent and electrically conductive carbon (tracking) path and without a
persistent flame occurring, when evaluated under specified test conditions defined in IEC 60112
Note 1 to entry: The mentioned maximum test voltage is not in conjunction with any system or operational voltage,
but it is used for evaluation of material groups.
[SOURCE: IEC 60050-212:2010, 212-11-59, modified – it has been rephrased by also clarifying
that CTI is an index value to evaluate material groups according to IEC 60112. Note 1 to entry
added.]
IEC 62788-2-1:2023 © IEC 2023 – 11 –
3.3.3
distance through insulation
t
DTI
thickness of relied-upon insulation (RUI) after the lamination protrusion test, with the minimum
allowable value defined by the rated system voltage
3.3.4
material group
category of insulation materials according to IEC 60664-1 as defined by the results of the CTI
test
3.3.5
rated system voltage
�𝑉𝑉 �
sys
max
maximum system voltage for which a module is rated
3.3.6
relied-upon insulation
RUI
solid insulation system providing protection against electric shock in the final application, with
material’s requirements for thermal endurance and resistance against environmental stress
factors
Note 1 to entry: Thin films used as polymeric front- or backsheet can consist of RUI plus additional layers that have
other functions, e.g., they protect the polymeric materials from UV radiation (see Figure 1).
3.3.7
withstand voltage
V
w
DC voltage which the material under test can withstand for a defined minimum amount of time
without occurrence of an electric breakdown
3.4 Temperatures
3.4.1
th
percentile module operating temperature
T
temperature at which a module is operating below more than 98 % of the time when deployed
in a PV system
th
Note 1 to entry: The 98 -percentile temperature is calculated by ranking measured or calculated module
temperature data taken at hourly (or more frequent) time intervals for a typical calendar year.
th
Note 2 to entry: For a standard year, the 98 percentile temperature is met or exceeded for 175,2 h.
3.4.2
rated module operating temperature
[T ]
98 max
th
maximum allowed 98 percentile operating temperature (T ) of the PV module
3.5 Tensile properties
3.5.1
elongation at break
ε
B
strain at which the specimen under test breaks
Note 1 to entry: Break defines the point in the tensile test, where the tensile stress drops below 50 % of its highest
value.
– 12 – IEC 62788-2-1:2023 © IEC 2023
3.5.2
tensile strength at break
𝜎𝜎
B
maximum engineering stress measured when a specimen is elongated in tension to the point of
breaking
4 Designation and ratings
The front- or backsheet shall be evaluated for its intended use in a PV module, including
designation as front- or backsheet, maximum module operating temperature, maximum system
voltage and module safety class. Due to the coordinating requirements for temperature and
system voltage ratings, a front- or backsheet may be rated for multiple sets of ratings (e.g.
maximum V and T ).
sys 98
Frontsheets are designed for prolonged exposure to direct sunlight (> 300 W/m ). Backsheets
are designed for exposure through glass and encapsulant on the interior side and restricted for
use with indirect or limited direct sunlight (< 300 W/m ) on the exterior side. Because of this,
the test conditions for the UV weathering test (FBST 09) are different for front- and backsheets.
A product can be assigned as a frontsheet, backsheet or both.
A front- or backsheet is rated for a maximum T module operating temperature. The module
operating temperature is related to a combination of environment and mounting conditions as
described in IEC 61730-1 and IEC TS 63126:
th
– IEC 61730-1 (by default) specifies PV modules with 98 percentile operating
temperatures T ≤ 70 °C.
– IEC TS 63126 defines extended test conditions for high temperature operation with T
≤ 80 °C [level 1] or ≤ 90 °C [level 2]. The requirements for the thermal endurance test
(FBST 06) depend on the specified [T ] . IEC 62788-2-1 deviates from
98 max
IEC TS 63126:2020, in that the UV weathering temperature conditions applied in the UV
weathering test (FBST 08) shall be as specified in Table 3.
NOTE IEC TS 63126:2020 is currently under consideration for modification and will likely be changed to conform
with this document.
In addition, a front- or backsheet is rated for a maximum system voltage and a module safety
class. IEC 61730-1 defines PV module safety class and system voltage ratings. Front- and
backsheets can be used in modules of safety class II (individual and/or system level electrical
outputs at hazardous levels of voltage, current and power) and safety class 0 (intended for use
in restricted access areas that are protected from public access by fences or other measures
of the location that prevent general access).
5 Requirements
5.1 General
5.1.1 Overview
Requirements for front- and backsheets are derived from insulation coordination requirements
from IEC 60664 and IEC 61730-1, and additional requirements for long term reliability based
on observations of field failures.
Requirements shall be met for the full front- or backsheet and also for individual layers
designated as RUI (from here referred to as RUI layers). Typical constructions are described in
Figure 1.
IEC 62788-2-1:2023 © IEC 2023 – 13 –
a) Single-layer
a1) homogeneous layer a2) combined layers (e.g. co-extrusion)
a2-i) polymeric composition a2-ii) polymeric composition
differs by ≤ 10 % differs by > 10 %

b) Multilayer
b2) laminated multilayer
b1-i) coated multilayer with RUI coating
b3) multilayer with
with non-RUI adhesive
a
conductive layer
b1-ii) coated multilayer with non-RUI coating
layers “x”
NOTE In actual front- or backsheet constructions the numbers of sublayers may differ from the examples shown.
a
The case of a conductive layer as in example case b3) will require additional module level considerations for
insulation coordination as described in IEC 61730-1.

Figure 1 – Schematic diagrams of typical constructions of front- or backsheets
5.1.2 Single-layer constructions
A single-layer construction is a front- or backsheet produced in one production step. It may
consist of a single homogeneous layer (a1) or combined layers (a2) manufactured in one
production process (e.g. co-extrusion) such that they cannot reasonably be separated without
fundamentally changing their properties, e.g., residual strain or crystallinity, in ways that might
affect their performance in a PV module. For combined layers, the thermal endurance
requirements depend on the layer composition (see 5.3). If the polymeric composition differs
by ≤ 10 % (by mass) between sub-layers or gradient zones within the material layer (a2-i) it is
treated equivalent to a single homogeneous layer (a1). If the polymeric composition differs
by > 10 % (a2-ii), the linearity of the Arrhenius plot shall be assessed (see 5.3). The 10 %
criteria refers to differences in blending ratios and/or to copolymer ratios of the polymers.

– 14 – IEC 62788-2-1:2023 © IEC 2023
The complete single-layer construction shall pass the insulation coordination (5.2), thermal
endurance (5.3), and mechanical (5.4) requirements before and after accelerated aging (6.8).
5.1.3 Multilayer constructions
For front- or backsheets designed as multilayer stacks, typically one (monolithic) layer in the
multilayer stack provides mechanical (tensile) strength (often referred to as core layer),
whereas additional layers attached to the core layer in separate production steps may provide
other functions, e.g., UV protection or adhesion to the encapsulant. The presence of sequential
processing steps is a differentiator to a combined single-layer (a2).
Examples of front- or backsheets designed with multiple layers (referring to Figure 1):
• A coated multilayer stack (b1) with coated layer A on a single core layer B, both potential
RUI layers. Typically, the coating is designed for a particular function in the layer stack (e.g.
a UV protective air-side layer or inner layer). Coating processes include slot coating of
functional formulations or extrusion coating of polymer melts onto another material layer
that has been produced in a different process step.
• A laminated multilayer stack (b2), with adhesive layers x between (potential) RUI layers A
and B. Such a construction represents a combination of at least two single-layer materials,
each produced as a film in a separate process step, which are then combined into one
multilayer film using adhesive layers between them in a final lamination process. In this
example, the (non-RUI) layers x do not pass the requi
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