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Contract No. DE-AC36-08GO28308

How Can We Make PV Modules 

Safer?

Preprint

John H. Wohlgemuth and Sarah R. Kurtz

Presented at the 2012 IEEE Photovoltaic Specialists Conference

Austin, Texas

June 3–8, 2012

Conference Paper

NREL/CP-5200-54145

June 2012
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1

How Can We Make PV Modules Safer?

John H. Wohlgemuth and Sarah R. Kurtz

National Renewable Energy Laboratory, Golden, CO, 80401, USA

Abstract — Safety is a prime concern for the photovoltaics 

(PV) industry. As a technology deployed on residential and 

commercial buildings, it is critical that PV not cause damage to 

the buildings nor harm the occupants. Many of the PV systems 

on buildings are of sufficiently high voltage (300 to 600 Volts dc) 

that they may present potential hazards. These PV systems must 

be safe in terms of mechanical damage (nothing falls on 

someone), shock hazard (no risk of electrical shock when 

touching an exposed circuit element), and fire (the modules 

neither cause nor promote a fire). The present safety standards 

(IEC 61730 and UL 1703) do a good job of providing for design 

rules and test requirements for mechanical, shock, and spread of 

flame dangers. However, neither standard addresses the issue of 

electrical arcing within a module that can cause a fire. To make 

PV modules, they must be designed, built, and installed with an 

emphasis on minimizing the potential for open circuits and 

ground faults. This paper provides recommendations on 

redundant connection designs, robust mounting methods, and 

changes to the safety standards to yield safer PV modules.

Index Terms — photovoltaic modules, fire hazards, shock 

hazards

I. INTRODUCTION

Safety of photovoltaic (PV) systems is critical to the 

commercial success of PV. Safety issues could lead to 

rejection of PV by consumers, inspectors, regulatory 

authorities, financiers, and/or insurance companies. These 

issues are most important for PV on buildings or where there 

is potential exposure to the general population. Safety is not as 

critical for behind-the-fence utility-scale PV where exposure is 

significantly limited.

Some of the safety issues related to high voltages can be 

addressed at the systems level by:

• Limiting dc system voltages via the use of modular 

inverters or dc-to-dc converters on each module.

• Requiring dc arc detectors and interrupters on all high 

voltage PV systems as has been proposed for the 

National Electric Code.

• Leaving the array itself floating (not grounded) so the 

first ground fault does not cause a ground loop and can 

easily be detected by the ground fault detector and be 

repaired.

None of these is likely to be the solution for all PV systems. 

Modular inverters and dc-to-dc converters may have their own 

reliability issues, and it is not clear whether they can match the 

lowest cost achievable with central inverters. Today there are 

no commercially available dc arc detectors certified to UL 

1699B [1] nor do the prototypes under development detect and 

interrupt all types of dc arcs.

In order to make all PV systems safer, a redesign of the PV 

module itself is proposed. Developing PV modules that are 

inherently safer can improve the safety of all PV systems. To 

begin the process, this paper will discuss how modules can be 

dangerous and then look at today’s module safety standards to 

better understand the safety gaps that remain to be addressed.

II. HOW MODULES CAN BE DANGEROUS

There are three main areas of concern for PV module safety:

1. Shock hazard: Someone touches exposed high voltage. 

2. Mechanical Safety: The module or parts of the module 

fall on someone, or ice or snow falls off of the module

onto someone.

3. Fire Safety: The module can either spread a fire that 

started somewhere else or start a fire itself.

III. MODULE SAFETY TESTING

Modules are safety tested to either IEC 61730 [2] or UL 

1703 [3] or both. Both have similar requirements, including 

design criteria, testing of materials, and testing of completed 

modules. Both do a good job of requiring designs that 

minimize electric shock, mechanical problems, and the spread 

of flames. Each contains tests that should identify shock 

hazards and mechanical problems and include spread of flame

tests. Neither addresses the issue of modules starting a fire. An 

effort is now underway to modify IEC 61730 to improve the 

way it handles the potential for a module to cause a fire. The 

rest of this paper will discuss proposed solutions to mitigate 

the fire danger and provide technical information to support 

the proposed changes to IEC 61730.

IV. PV MODULE FIRE HAZARDS

There are three typical ways that a module can overheat. 

Each is discussed along with the probability that it can cause a 

sustainable fire within the module. 

A. Hot Spots

A hot spot can occur when a cell (or cells) are forced into 

reverse bias because it (they) cannot carry the peak power 

current being produced by the other cells in series. Hot spots 

can be caused by poor cell matching, cracks, localized soiling 

(bird droppings), or shadowing. Cells are supposed to be 

protected by the bypass diodes that limit the reverse voltage 

across a cell to less than ~ 10 volts. Problems can occur when 
2

the bypass diodes fail or are never installed correctly or when 

the cells have very low shunt resistances and therefore 

overheat at 10 volts reverse bias before the diodes activate.

Figure 1 shows an example of reverse bias leakage current 

measured on 50 commercial cells at -10 volts, the approximate 

voltage level that occurs when 20 cells are protected by one 

bypass diode. Figure 2 shows an IR picture of six cells with 

high leakage currents from the set in Figure 1 [4]. The highest 

measured temperature was less than 100°C. So, for these cells,

using adequate bypass diode protection (no more than 20 cells 

per bypass diode) and screening for low shunt cells should 

limit the temperature increase and not cause a sustained fire.

Fig. 1. Reverse leakage current for 50 commercial cells biased at 

-10 volts.

On the other hand, if the shunt path is localized enough and 

the cells are not adequately protected with bypass diodes, hot 

spots can melt silicon. While such events are hot enough to 

melt/decompose encapsulants and backsheets, in most cases 

they quickly result in total shunting of the junction and 

elimination of the hot spot. Cunningham claimed that neither 

hot spots nor resistive heating are likely to lead to sustained 

fires in PV modules [5].

Recent results, however, indicate that as cells and modules 

get larger, this may no longer be the case. Figure 3 shows the 

result of a hot spot occurring within a large module with 72 

15.6 cm by 15.6 cm solar cells. The cell shown in the picture 

has suffered a hot spot that was so severe that it burned a 

significant area of the cell and broke the glass. In this case the 

bypass diode protecting the string containing the damaged cell 

was still functional. So this failure was caused by the power 

produced in the 24-cell string protected by the diode. For large 

cells, module manufacturers may have to either use fewer cells 

per diode or improve their screening of cells with low shunt 

regions as described in the section on Improving Module 

Design and Construction.

Fig. 2. IR pictures of the six cells with highest reverse leakage 

current and temperature increase when biased at -10 volts.

Fig. 3. Burned area in module caused by hot spot.

B. High Series Resistance

Failure of solder bonds within the module can lead to 

overheating at the failing solder bond in addition to the

remaining bonds that are left to carry the additional current as 

shown in Fig. 4. Such high resistance bonds do lead to 

significant output power loss. However, the temperatures 

reached at these weak solder bonds are typically not high 

enough to cause fires. The danger comes when the resistive 

heating results in total failure of all of the bonds connecting an 

individual cell, resulting in an open circuit, which can lead to 

an arc.

C. Arcing

Two types of arcing can occur within a PV module:

1. Series arcs are caused by an open circuit in a highvoltage dc array. In a PV module a series arc can occur 

whenever the current path is disrupted. Examples 

include when one of the output leads loses electrical 

conductivity to the cell circuit as shown in Fig. 5, or 

when the second interconnect on a cell fails and the 

bypass diode is not operational as shown in Fig. 6. 

Series arcs are reasonably easy to detect and can be 
3

stopped by opening the series circuit in which they are 

occurring.

2. Parallel arcs occur when two different dc polarities 

come in close proximity. In a PV module, a parallel arc 

can occur due to a ground fault. Parallel arcs are more 

difficult to detect and much more difficult to stop, as 

the current flow is either directly from plus to minus or 

through a ground loop.

No material selection or module design is going to prevent a 

module from catching fire once an arc is sustained because of 

the extremely high temperatures within an arc.

Fig. 4. Resistive heating of solder bonds connecting two ribbons 

from the same cell.

Fig. 5. An arc caused by failure of the solder bond that attached an 

output lead to the module circuitry.

IV. IMPROVING MODULE DESIGN AND CONSTRUCTION

A. Stopping Open Circuits from Occurring

The first step in reducing the potential for open circuits 

within the module is to design modules with redundant 

electrical connections so that it takes multiple failures to cause 

an open circuit. For crystalline silicon modules, most cells are 

tabbed with multiple (two, three, four, or even five) tabbing 

ribbons each soldered at multiple locations or otherwise 

electrically bonded to the cell over a large area. This provides

redundancy and protection against open circuiting.

Bypass diodes provide an additional level of protection, as 

they should carry current around any failed connections within 

the cell matrix. To ensure that this safety feature of the bypass 

diodes is functioning, the diode function should be checked on 

each new module before it is shipped from the factory. In 

addition, bypass diodes are recommended on all modules,

including thin films regardless of whether or not they are 

necessary to protect against hot spots.

Fig. 6. An arc across the broken interconnect on a cell.

The most likely place for an open circuit to occur in a 

module is at the output leads because:

• These connections are usually not protected by bypass 

diodes;

• Often they are not redundant;

• Typically the process for these connections is manual; 

and

• They can be the connections with the most stress as 

they interface with the rest of the PV system.

To improve the design, the next edition of IEC 61730 has 

been written to require redundant types of soldered 

interconnects at the output leads. For example, the module 

could be designed to have both a mechanical clip and a solder 

bond to meet the requirement. Other types of module 

termination will be subjected to design criteria (such as 

pressure from a polymeric material cannot be used as the 

means for holding a connection together) and testing as 

detailed in EN 50548 [6].

Finally, with the output connection and all electrical 

connections, design for manufacturing, process control, and 

personnel training are very important. The manufacturing 

process should be easy to perform and easy to inspect to 

ensure that the electrical connections have been made to 

specification.

B. Stopping Ground Faults from Occurring

The majority of ground faults are installation related. 

Improved installation safety requires improved installer 

training, improved installation documentation, and publication 

of installation safety standards. Module mounting systems 

should follow specific design rules that forbid the attachment 

of conductive mounting hardware directly onto polymeric 

backsheets behind solar cells and/or other components of the 

electric circuit. Module framing should be mounted outside of 
4

the active area, meeting the creepage and clearance distances 

for the rated system voltage.

Module manufacturers (for both crystalline silicon and thin 

film modules) should pay particular attention to adhesion 

between encapsulant and glass. Electrical leakage from active 

circuit to the ground plane along a delamination between 

encapsulant and glass is a failure mode observed in the field.

Such leakage is a shock hazard if the mounting system is not 

grounded and a ground fault hazard if it is grounded. The 

solution to this problem is a robust manufacturing process 

with good process control and QA system.

C. Bypass Diode Protection and Cell Screening

The number of cells per bypass diode establishes the 

maximum reverse voltage that a shadowed cell will see. Each 

cell must be capable of surviving a reverse voltage equal to the 

sum of the forward voltages of all of the other cells protected 

by that diode while passing approximately peak power current. 

In a module with 20 cells per diode, this means each cell must 

be capable of surviving the voltage provided by the other 19. 

However, if there are more cells per bypass diode, say 24 like 

in Fig. 3, then the reverse voltage will be higher, equal to the 

voltage provided by the other 23. Especially in high-current 

modules, this can represent a significant amount of power 

dissipated in a small area. For the module in Fig. 3 it would 

not be unusual to have a shadowed cell dissipating 100 watts.

This level of power dissipation could easily result in 

overheating of a localized shunt. In order to prevent 

overheating, cells should be screened to eliminate hot spots. 

This can be done by sorting out low shunt resistance cells by 

measuring the leakage current under reverse bias conditions or 

by sorting out cells with hot spots as measured by a fast

infrared camera under reverse bias conditions. Shunt screening 

is probably easier but not as effective because cells with fairly 

high shunt resistance (> 10 Ohms) can still have localized hot 

spots that may overheat. Use of manufacturing screening tools 

to sort by hot spots is the most effective approach.

Historically, modules used 24 cells per bypass diode. 

However, as the cells have gotten larger they must dissipate 

more power, so it is very important to test the design using a 

Hot Spot Test. The Hot Spot Test in the present IEC 

Qualification Standard – IEC 61215 Edition 2 [7] is not a good 

test for determining if a particular cell/diode combination is 

adequate because the cell selection does not test the most 

susceptible cells [4]. The Hot Spot Test in the draft of IEC 

61215 Edition 3 or the test in ASTM E2481 will do a much 

better job of determining whether the selected bypass diode 

design is adequate. To ensure the best performance, the Hot 

Spot Test should be performed using cells that represent the 

borderline of screening; that is, use cells with the lowest shunt 

resistance or the highest hot spot temperature allowed for the 

product under test.

V. CONCLUSIONS

Making modules inherently safer with minimum additional 

cost is the preferred approach for PV. Safety starts with:

• Proper screening to remove cells with hot spots or low 

shunt resistances;

• Module design to ensure adequate bypass diode 

protection so that cells within specification with the 

worst case allowable hot spots or lowest acceptable

shunt resistances can survive the IEC 61215 Edition 3 

or ASTM E2481 Hot Spot Tests;

• Module design to ensure redundancy within the 

electrical interconnection to minimize open circuits;

• Electrical termination meeting the requirements of the 

EN 50548 J-box standard; and 

• Proper installation protocol to prevent installation 

related ground faults.

Module manufacturers must control the raw materials and 

processes to ensure that that every module is built like those 

qualified through the safety tests, the reason behind the QA 

task force effort to develop a “Guideline for PV Module 

Manufacturing QA.” Periodic accelerated stress testing of 

production products is critical to validate the continued safety 

of the products being produced.

Combining safer PV modules with better system designs

and the use of lower voltage circuits and arc fault detectors/

interrupters can solve many of the safety problems observed 

with today’s PV systems.

ACKNOWLEDGMENTS

This work was supported by the U.S. Department of Energy 

under Contract No. DE-AC36-08-GO28308 with the National 

Renewable Energy Laboratory. The authors wish to thank 

Dave Miller of NREL for helpful suggestions.

REFERENCES

[1] UL 1699B - Outline Photovoltaic (PV) DC Arc-Fault Circuit 

Protection.

[2] IEC 61730 – Photovoltaic (PV) Module Safety Qualification –

Part 1: Requirements for Construction and Part 2: Requirements 

for Testing.

[3] UL 1703 – Standard for Safety: Flat-Plate Photovoltaic Modules 

and Panels 

[4] Wohlgemuth and Herrmann, “Hot Spot Tests for Crystalline 

Silicon Modules,” Proceedings of 31st IEEE Photovoltaic 

Specialist Conference, 2005, p. 1062.

[5] Cunningham, “Analysis of hot spots in crystalline silicon 

modules and their impact on roof structures,” NREL PVMRW, 

2011.

[6] EN 50548 - Junction Boxes for Photovoltaic Modules.

[7] IEC 61215 – Crystalline Silicon Terrestrial Photovoltaic (PV) 

Modules- Design Qualification and Type Approval.

[8] ASTM E2481 – Standard Method for Hot Spot Protection 

Testing of Photovoltaic Modules.