NREL is a national laboratory of the U.S. Department of Energy, Office of Energy Efficiency and Renewable Energy, operated by the Alliance for Sustainable Energy, LLC.

How Standards Control Module Design for 

Better or Worse

2011 PV Module 

Reliability Workshop

John Wohlgemuth, 

February 16, 2011

NREL/PR-5200-51007
NATIONAL RENEWABLE ENERGY LABORATORY

Introduction

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• PV Modules should be designed and constructed to 

perform in a safe manner outdoor for at least 20 or 25 

years.

• However, it appears that many modules are designed 

and their components are selected primarily to meet 

the requirements of the Performance Standards (IEC 

61215 and IEC 61646) and Safety Standards (IEC 61730 

and UL 1703). 

• This presentation will explore the issues related to 

designing and building modules to meet standards 

rather than to survive in the field.
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Why I believe Modules are Designed for Standards 

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• Too many cases where modules pass one of the tests 

from the qualification sequence but fail when the test is 

extended even a small amount.

• One case where new manufacturer’s cells in standard 

package passed 1000 hours of damp heat with little 

power loss, but just extending to 1250 hours resulted 

in significant power loss > 20%.

• While at BP Solar we tested 10 crystalline silicon 

module types from 9 different manufacturers from all 

over the world, all qualified to IEC 61215. 

• Only 3 passed BP Solar’s extended test protocol. 

• Most failed damp heat at 1250 hours.
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Extending Test Duration for Cr-Si Modules Qualified to IEC 61215 

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Manufacturer DMLC/TC/HF 1250 Hr DH 500 TC

A Pass Fail Fail

B Pass Fail Pass

C Pass Fail Fail

D Pass Fail Pass

E Pass Fail Pass

F Pass Pass Pass

G Pass Fail Pass

H Pass Pass Pass

J Pass Fail Pass

K Pass Pass Pass
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What are the ramifications

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• Qualification test is not a 25 year lifetime test. 

• Qualification test does an excellent job of identifying 

design, material and process flaws that lead to 

premature failures (Infant mortality).

• Qualification test does not address wear-out 

mechanisms that limit lifetime.

• We do not have an accelerated test sequence that can 

assure 25 year lifetime in all terrestrial environment.

• Relying solely on the Qualification test sequence can 

mean no assessment or evaluation of how the module 

will actually perform long term in the field.
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Over Design

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• The safety standards put requirements on the 

materials within the module.

• These requirements typically come from other fields.

• They may or may not be appropriate for PV 

applications.

• They often add unnecessary cost to the PV module.

o Extra testing cost

o Use of more expensive materials/components just to meet 

the requirements of the standard not to increase safety.

• Going to discuss 2 examples
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RTI

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• The UL RTI test has been included in both UL 1703 

and IEC 61730.

• Both standards require any polymeric materials 

within the module to have an RTI rating equal to the 

temperature measured during the “Temperature Test -

T40 ” plus 20 °C.

• RTI is an exceeding long test (more than 1 year). So 

slows down and impedes the use of new materials.

• There is little scientific basis for the RTI test. It 

determines when a property has fallen to 50% of initial 

value no matter whether that loss of property has 

anything to do with its performance in the application.
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RTI

o RTI usually used where the 

operating temperature is well 

known and fairly constant when 

an appliance is operating. This 

is not the case for PV.

o Modules rarely operate at 

even the temperature measured 

in the temperature test. 

o Typical Module T40 = 90 °C

o See figure for a roof mount 

module in 6 locations including 

Saudi Arabia, one of the hottest 

places in the world. 

o A module only spends a few 

hours a year above the T40

temperature and never reaches 

T40 + 20.

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“EVALUATION OF HIGH-TEMPERATURE EXPOSURE 

OF PHOTOVOLTAIC MODULES” By Kurtz etal, TBP in 

Progress in PV
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Partial Discharge Test

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• IEC 61730 requires the back sheet material to have a 

partial discharge rating for the highest system voltage the 

module will be rated for (Typically 600 volts in the US and 

1000 volts in Europe).

• The partial discharge test comes from the insulation 

industry and was typically used to evaluate the insulation 

on windings in transformers.

• Partial discharge is suppose to help predict wear out of 

the insulation material after long term exposure to electric 

fields.

• The test uses 

o 2 conductive planes with the backsheet sandwiched in between.

o AC to excite the field and cause the discharges
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Issues with Partial Discharge Test

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• First issue is that there very few cases where the 

geometry in PV module resembles the test (backsheet 

sandwiched between 2 conductive planes).

In most cases there are no conductive materials 

touching the backsheet except maybe at the edges 

where the internal circuitry (cells and bus bars) must 

meet the clearance distance for the proposed voltage. 

• Second issue is the use of ac rather than the dc that 

PV modules see.

Very little information is available to indicate that the 

AC test does anything to predict DC performance.
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Why is Partial Discharge Test Important?

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• Partial discharge test drives the construction 

requirements of the backsheet.

• Because of partial discharge test backsheets are 

much thicker (and therefore more expensive) than 

would be required to meet all other requirements in IEC 

61730 or IEC 61215/61646.

• One module manufacturer estimated that this 

requirement adds $5 to $6 to the cost of every ~200 

watt module.
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Termination Requirements

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• Both UL 1703 and IEC 61730 have a similar requirement for 

module termination.

“Strain relief shall be provided so that stress on a lead intended 

for field connection, or otherwise likely to be handled in the field, 

including a flexible cord, is not transmitted to the connection 

inside the module or panel.” from UL1703

• Good idea but implemented incorrectly.

• NRTL’s have allowed this criteria to be met using a potting 

compound (such as silicone or epoxy).

• If the solder bond on the lead fails the pottant holds the 

lead in place allowing an arc to develop and possibly 

continue for an extended time period.

• This has been one of biggest PV safety issues. 
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Solutions 

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• For Design to Qualification Test: 

Develop a set of accelerated stress tests that predict module 

performance over 25 years in various terrestrial climates. 

QA workshop In Tokyo in May, 2011

• For RTI

Replace RTI with requirements for materials that are developed 

specifically for the environments in which PV modules operate. 

Underway in WG2 - Participate in this effort 

• For Partial Discharge

Only require partial discharge test when module is designed to 

have conductive material in contact with the back of the 

backsheet. (Part of mounting system or metal foil for humdiity

barrier)

Support this change in your national committee when voting on 

IEC 61730 edition 2
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Solutions (Continued)

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• Termination Issue

Latest draft of 61730 edition 2 changes wording on 

termination to require redundant electrical connections. 

Support this change in your national committee when 

voting on IEC 61730 edition 2.

• Redundancy Issue

Replace UL 1703 with IEC 61730 so US industry isn’t 

hampered by having to carry double safety certifications 

nor have to correct the problems in 2 standards.

Let UL, Solar ABCs and DOE know that this is a priority for 

US PV industry.