Photovoltaic (PV) modules - Non-uniform snow load testing

IEC 62938:2020 provides a method for determining how well a framed PV module performs mechanically under the influence of inclined non-uniform snow loads. This document is applicable for framed modules with frames protruding beyond the front glass surface on the lower edge after intended installation and as such creates an additional barrier to snow sliding down from modules. For modules with other frame constructions, such as backrails formed in frames, on the side edges, on the top edge and on the lower edge not creating an additional snow slide barrier, this document is not applicable. The test method determines the mechanical non-uniform-load limit of a framed PV module.

Modules photovoltaïques (PV) - Essais de charges de neige non uniformes

l'IEC 62938:2020 fournit une méthode pour déterminer le comportement mécanique d'un module photovoltaïque (PV) encadré sous l'effet de charges de neige non uniformes. Le présent document s'applique aux modules encadrés dont les cadres dépassent de la façade vitrée avant au niveau du rebord inférieur lorsqu'ils se trouvent dans la position de montage prévue, ce qui crée une barrière supplémentaire contre les glissements de neige accumulée sur les modules. Le présent document ne s'applique pas aux modules reposant sur d'autres constructions de cadre, par exemple les rails arrière qui sont mis en œuvre dans les cadres, sur les rebords latéraux, sur le rebord supérieur et sur le rebord inférieur, mais qui n'empêchent pas les glissements de neige. La méthode d'essai détermine la limite mécanique d'un module photovoltaïque encadré aux charges de neige non uniformes.

General Information

Status
Published
Publication Date
13-May-2020
Current Stage
PPUB - Publication issued
Completion Date
14-May-2020
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IEC 62938
Edition 1.0 2020-05
INTERNATIONAL
STANDARD
NORME
INTERNATIONALE
Photovoltaic (PV) modules – Non-uniform snow load testing
Modules photovoltaïques (PV) – Essais de charges de neige non uniformes
IEC 62938:2020-05(en-fr)
---------------------- Page: 1 ----------------------
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---------------------- Page: 2 ----------------------
IEC 62938
Edition 1.0 2020-05
INTERNATIONAL
STANDARD
NORME
INTERNATIONALE
Photovoltaic (PV) modules – Non-uniform snow load testing
Modules photovoltaïques (PV) – Essais de charges de neige non uniformes
INTERNATIONAL
ELECTROTECHNICAL
COMMISSION
COMMISSION
ELECTROTECHNIQUE
INTERNATIONALE
ICS 27.160 ISBN 978-2-8322-8074-4

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

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® Registered trademark of the International Electrotechnical Commission
Marque déposée de la Commission Electrotechnique Internationale
---------------------- Page: 3 ----------------------
– 2 – IEC 62938:2020  IEC 2020
CONTENTS

FOREWORD ........................................................................................................................... 3

1 Scope .............................................................................................................................. 5

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

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

4 Sampling ......................................................................................................................... 6

5 Prerequisites ................................................................................................................... 7

6 Testing ............................................................................................................................ 7

6.1 General ................................................................................................................... 7

6.2 Projections of the test results .................................................................................. 7

6.3 Test plan ................................................................................................................. 7

7 Test procedures .............................................................................................................. 9

7.1 Visual inspection ..................................................................................................... 9

7.2 Maximum power determination ................................................................................ 9

7.3 Insulation test ......................................................................................................... 9

7.4 Wet leakage current test ......................................................................................... 9

7.5 Humidity-freeze test ................................................................................................ 9

7.6 Electroluminescence imaging .................................................................................. 9

7.7 Non-uniform snow load test ..................................................................................... 9

7.7.1 Purpose ........................................................................................................... 9

7.7.2 Load specification ............................................................................................ 9

7.7.3 Apparatus ...................................................................................................... 11

7.7.4 Procedure ...................................................................................................... 13

8 Fail criteria .................................................................................................................... 15

9 Verification of the test results ........................................................................................ 15

10 Statistical analysis ......................................................................................................... 15

10.1 General ................................................................................................................. 15

10.2 5 % fractile value with Student's distribution ......................................................... 16

10.3 Safety factor ......................................................................................................... 16

10.4 Example................................................................................................................ 16

10.5 Quantiles of the t distribution (Student's distribution) ............................................ 16

11 Test report ..................................................................................................................... 17

12 Modifications ................................................................................................................. 18

Annex A (informative) Use of determined values .................................................................. 19

A.1 Estimated snow loads and use of the determined resistance ................................. 19

A.2 Calculate the bearable loads for different angles ................................................... 19

Bibliography .......................................................................................................................... 20

Figure 1 – Test plan for inhomogeneous snow load test .......................................................... 8

Figure 2 – Distribution of load on the test specimen at inclination ......................................... 10

Figure 3 – Simplified cross-sectional view of module width along bottom frame .................... 12

Figure 4 – Test procedure for the snow load test .................................................................. 14

Figure 5 – Different deflection graphs under static load ........................................................ 14

Table 1 – Applicable load in relation to angle of pitch of roof ................................................ 10

Table 2 – Quantiles of the t distribution (Student's distribution) ............................................. 17

---------------------- Page: 4 ----------------------
IEC 62938:2020  IEC 2020 – 3 –
INTERNATIONAL ELECTROTECHNICAL COMMISSION
____________
PHOTOVOLTAIC (PV) MODULES –
NON-UNIFORM SNOW LOAD TESTING
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

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indispensable for the correct application of this publication.

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

International Standard IEC 62938 has been prepared by IEC technical committee 82: Solar

photovoltaic energy systems.
The text of this International Standard is based on the following documents:
FDIS Report on voting
82/1670/FDIS 82/1705/RVD

Full information on the voting for the approval of this International Standard can be found in the

report on voting indicated in the above table.

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

---------------------- Page: 5 ----------------------
– 4 – IEC 62938:2020  IEC 2020

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

stability date indicated on the IEC website under "http://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.
---------------------- Page: 6 ----------------------
IEC 62938:2020  IEC 2020 – 5 –
PHOTOVOLTAIC (PV) MODULES –
NON-UNIFORM SNOW LOAD TESTING
1 Scope

This document provides a method for determining how well a framed PV module performs

mechanically under the influence of inclined non-uniform snow loads. This document is

applicable for framed modules with frames protruding beyond the front glass surface on the

lower edge after intended installation and as such creates an additional barrier to snow sliding

down from modules. For modules with other frame constructions, such as backrails formed in

frames, on the side edges, on the top edge and on the lower edge not creating an additional

snow slide barrier, this document is not applicable.

The test method determines the mechanical non-uniform-load limit of a framed PV module.

The loads specified in this document apply exclusively to natural snow load distributions. Any

expected artificial accumulations (e.g. from snow removal or redistribution) are considered

separately.

Methods to eliminate or counteract the occurence of inhomogeneous snow accumulation, such

as a steep installation angle (more than 60°), are not included in this document. This document

assumes a relationship between ground snow-cover and module snow-cover which may not be

applicable in locations where the snow does not completely melt between snow falls. This

document does not consider the effect of snow cover on power generation.

While the test method includes a wait time between load steps, the document does not provide

a complete assessment of the fatigue behaviour of the materials of the module, such as front

glass.

Because typical field failures of PV modules caused by snow load show glass breakage and

frame bending, the test method aims at reproducing the load under which such failures occur.

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 TS 60904-13 :2018, Photovoltaic devices – Part 13: Electroluminescence of photovoltaic

modules

IEC 61215-1:2016, Terrestrial photovoltaic (PV) modules – Design qualification and type

approval – Part 1: Test requirements

IEC 61215-2:2016, Terrestrial photovoltaic (PV) modules – Design qualification and type

approval – Part 2: Test procedures
IEC TS 61836, Solar photovoltaic energy systems – Terms, definitions and symbols

IEC TS 62915, Photovoltaic (PV) modules – Type approval, design and safety qualification –

Retesting
---------------------- Page: 7 ----------------------
– 6 – IEC 62938:2020  IEC 2020
3 Terms and definitions

For the purposes of this document, the terms and definitions given in IEC TS 61836 and the

following apply.

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
characteristic snow load
characteristic value of snow on the ground
Note 1 to entry: S is expressed in kN/m .
Note 2 to entry: The lowest value for S used in this document is 2,4 kN/m .
3.2
characteristic value of snow load
angle dependent snow load

product of the characteristic snow load on the ground and the snow load shape coefficient

Note 1 to entry: The lowest value for S used in this document is 1,47 kN/m .
3.3
snow load shape coefficient

ratio of the snow load on the roof or PV module to the undrifted snow load on the ground

3.4
specific snow weight
weight per unit volume of snow
Note 1 to entry: The specific snow weight γ is considered to be 3 kN/m .
3.5
snow load of the overhang
load vertical to the eaves applied in addition to the uniform load on a roof
3.6
fractile value

lower or upper bounds of a distribution function (Student's distribution, normal distribution, log

normal distribution, etc.) which represents, in construction, strenghts or impacts

4 Sampling

At least seven PV modules are used for testing. Five or more modules are used to determine

the maximum load bearing; one is used for determination of electrical degradation at a load

below the determined maximum load bearing and one is used as a control module.
---------------------- Page: 8 ----------------------
IEC 62938:2020  IEC 2020 – 7 –
5 Prerequisites

The PV module type shall have passed the static mechanical load test (MQT 16) according to

IEC 61215-2 with a minimum positive test load of 5 400 Pa.
6 Testing
6.1 General

These test specifications describe a test method for determining the direct load-bearing

capability of framed PV modules under the effects of inhomogeneous snow loads.
6.2 Projections of the test results

Failure of the (adhesive) bond between module frame and glass/laminate can lead to

• deformation of the module rail or frame,
• glass breakage,
• displacement of rail or frame parts,
• fracture of rail or frame parts,
• loss of adhesion strength in adhesive bonds , and
• breakage or displacement of mounting parts.
In addition, this can impact electrical performance due to:
• interruption of the module’s electrical insulation,
• cell breakage and junction box failure, and
• power degradation.
6.3 Test plan

Figure 1 shows the test flow where the numbers in the boxes represent the test references in

IEC 61215-2. Five modules undergo the mechanical testing until failure as defined in Clause 8

occurs. A sixth module shall be used to determine the highest load bearing at which no power

degradation > 5,0 % occurs.
---------------------- Page: 9 ----------------------
– 8 – IEC 62938:2020  IEC 2020
NOTE The numbers in Figure 1 relate to the test references in IEC 61215-2:2016.
Figure 1 – Test plan for inhomogeneous snow load test
---------------------- Page: 10 ----------------------
IEC 62938:2020  IEC 2020 – 9 –
7 Test procedures
7.1 Visual inspection
This test is performed according to IEC 61215-2 MQT 01.
7.2 Maximum power determination

This test is performed according to IEC 61215-2 MQT 02 after initial stabilization according to

IEC 61215-2 MQT 19. For intermediate and final control measurements, further stabilization

steps might be required dependent on the module technology. The maximum power

determination is a relative measurement only; the measurements do not need to be performed

at standard test conditions (STC).
7.3 Insulation test
This test is performed according to IEC 61215-2 MQT 03.
7.4 Wet leakage current test
This test is performed according to IEC 61215-2 MQT 15.
7.5 Humidity-freeze test
This test is performed according to IEC 61215-2 MQT 12.
7.6 Electroluminescence imaging

Accompanying the visual inspection, electroluminescence according to IEC 60904-13 imaging

could be performed optionally on the electrical verification module to visualize cell cracking.

7.7 Non-uniform snow load test
7.7.1 Purpose

This test specification describes a method for determining the direct load-bearing capability of

inclined, framed PV modules under the effects of inhomogeneous snow loads.
7.7.2 Load specification

The inhomogeneous load distribution of the weights is determined by the diagram showed in

Figure 2.
---------------------- Page: 11 ----------------------
– 10 – IEC 62938:2020  IEC 2020
Figure 2 – Distribution of load on the test specimen at inclination

The load to be applied to the PV module and its distribution by means of separate weight

elements is determined as a function of the characteristic snow load S , the module angle of

inclination α = 37° ± 1°, the shape coefficient µ as a substitute value for pitch roofs, and the

linear load generated from S as a function of an assumed specific snow weight of
γ = 3 kN/m .

Here, it is assumed that the snow can slide off unhindered. For mono pitch roofs or PV modules,

where the snow is not prevented from sliding off the roof, the values showed in Table 1 can be

used (see also Annex A).
Table 1 – Applicable load in relation to angle of pitch of roof
Angle of pitch of roof α 0°< α ≤ 30° 30°< α < 60° α ≥ 60°
µ 0,8 0,8 · (60° – α) / 30° 0,0

At a test angle of 37°, µ = 0,61 applies (this is considered as the most critical angle for snow

slides).

The lower edge of the PV modules represent the eaves of a roof and hence this case needs to

be considered in this document. The minimum of S is 0,72 kN/m.
S = S / γ
E A
where
S is the snow load of the overhang depending on eaves, in kN/m;
S is the snow load on the roof, in kN/m (S = µ · S );
A A i K
γ is the specific snow weight, in kN/m .

The weight elements for S are distributed over the bottom area of the inclined module over a

length of approximately, but not greater than, 2/3 of the vertical length of the module (l).

---------------------- Page: 12 ----------------------
IEC 62938:2020  IEC 2020 – 11 –

The weight elements for S are distributed over the bottom area of the inclined module over a

length of approximately, but not greater than, 1/2 of the vertical length of the module (l).

Quasi- triangle PV for the hipped and/or broach roof, and roof shingle PV which has very short

vertical length are out of scope, because it is assumed less impact of snow slide.

The subsequent load increases shall be applied as angle-dependent loads per area. Each load

corresponds to the angle-dependent pressure given the shape coefficient (example: in the first

2 2

step, 2,4 kN/m corresponds to an angle-dependent pressure of 1,47 kN/m at 37° (±1°)

inclination angle, as defined in Formula (1).
S = S · µ (1)
A K i
The linear load S is then calculated and increased according Formula (2):
S = (S /γ) (2)
E A
To calculate the force which can then distributed inhomogeneously according

to Figure 2, the result needs to be multiplied with the factors out of Figure 2 and the bottom

length of the module L .
Example: Additional force for the bottom segment = 0,4⋅⋅ L

The initial load with which all tests begin is derived from the minimum design qualification of

PV modules according to the static mechanical load test (MQT 16) of IEC 61215-2.

The initial load corresponds to the combination of characteristic snow load S of 2 400 Pa and

linear load S . In this example:
S = 2,4 kN/m (3)
S = angle-dependent load at 37° = 1,47 kN/m (4)
S = 0,72 kN/m (5)

The weight elements used shall be able to slide on the surface of the module with as little

friction as achievable. For example, a polytetrafluoroethylene, PTFE surface on the weight

elements is suitable.

For each total load, it shall be ensured that the individual weight elements be placed according

to the distribution shown in Figure 2. Further weight elements (e.g. weight disks) are placed on

the bottom half of the module to form the linear load S per Figure 2, in order to ensure a

simulated "bulging" snow accumulation. A deviation of the distribution of up to ±10 % can be

tolerated.
7.7.3 Apparatus

The test bench has a substructure on which PV modules can be mounted at 37° ± 1° as specified

by the manufacturer.
---------------------- Page: 13 ----------------------
– 12 – IEC 62938:2020  IEC 2020

As seen in Figure 3, the effective length of a weight element acting on the frame is designated

by the symbol L .
The width of the PV module is designated by the symobl L .

To achieve a sufficiently high homogeneity of the surface load (weight element/module area)

and a sufficiently high number of contact points (weight element/frame), the following conditions

shall be fulfilled:
∑ a
Homogeneity of the surface load: ≥ 90 % (6)
Number of contact points: ≥ 5 (7)
Figure 3 – Simplified cross-sectional view of module width along bottom frame

The weight elements shall be designed such that the load is introduced in planar form,

homogeneously and without torque to the modules surface.

The contact between the module frame and the weight element shall be form-fitted.

It shall be ensured that the downhill-slope force is transmitted to the module bottom frame

member.

The contact of the bottom of the weight element and the module surface shall be realised with

as little friction as possible.

Two neighbouring weight elements shall be placed with sufficient space in-between them to

avoid them getting stuck to each other when the frame bends and the module surface deflects.

Abrasion or scratching of the module surface due to weight elements sliding shall be avoided.

For safety reasons, the weight elements shall be secured against falling down. The fixation of

the loads shall not hinder the loads to fully lie on the PV module.

The test apparatus shall be equipped with means to monitor movements in the joint between

frame and laminate (bending of the frame).
---------------------- Page: 14 ----------------------
IEC 62938:2020  IEC 2020 – 13 –
7.7.4 Procedure

The module to be loaded is mounted on the test bench as specified in the instruction manual of

the supplier. Installations with clamping along the long frame side usually use cross bars as the

substructure. Bending of the module under load may cause the backsheet to touch the

substructure. This real condition shall be represented in the test set-up (no free bending on the

test bench, if in reality such bending is prevented by the sub-structure).
The environmental conditions for performing the tests are 25 °C ± 5 °C.

As most adhesives will perform worse under elevated temperatures, room temperature is

considered to be a worst case condition for testing. If the adhesive used for a particular module

type is known to perform better at room temperature, the test shall be performed at 0 °C ± 5 °C.

The test procedure shall be as followed (see also Figure 4).
Apply an initial load of S = 2 400 Pa for at least 24 h. After the
...

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