Introduction
PV connectors are integral to every solar project: they are
the links through which DC solar power is transmitted from
PV modules through cables into inverters. For a system to
produce AC power safely and reliably, connectors must:
1. Provide low-resistance connections that minimize
resistive losses as electricity flows through the array.
2. Withstand 25+ years of environmental exposure with
minimal corrosion, degradation or current leakage.
However, the industry lacks a universal standard for
PV connector design. While the design details of these
electromechanical devices vary, they usually have a male
part, which is an internal plug that encloses a contact, and
a female part, which is a socket with an extended contact.
Tightly locking these two parts creates an electrical circuit.
Hot connectors, or connectors that have high operating
temperatures, are the most important indicator of latent
failure. Any other warning signs are usually hidden from
view because field-made connectors cannot be internally
inspected after installation without destroying them.
Inside a PV Connector
The inside of a PV connector is rarely seen. Many PV
connectors are field-made, which means their two parts are
pushed together in the field during installation. Once locked,
opening a field-made connector permanently destroys it.
PVEL's field testing team obtained the image shown to the
right using an advanced lab testing technique called X-ray
Computed Tomography, which yields a 3D image of the
inside of the connector.
Connectors and Fires
Connectors are a leading cause of fires instigated by PV
systems in many global solar markets. These rare events
pose severe threats to safety, property and even the public
image of solar power. While many are confidential, there are
documented cases of PV system fires and connector failures:
• In January 2022, SunPower initiated a >$30MM USD PV
connector replacement initiative due to a cracking issue
in third-party products it supplied.1
• In the U.K., 27% of 58 fires instigated by PV systems from
2010 to 2017 were caused by connectors.2
• In Germany, connectors were blamed for 24% of 180
fires caused by PV systems from 1995 to 2012.3
• Japan’s Consumer Safety Investigation Commission
recommended rooftop PV system inspections in a report
citing 127 fires from 2008 to 2017.4
A Growing Concern
As of December 2021, there were approximately 375M PV
connections in the U.S. and an estimated 3.5B PV connections
worldwide. Each of these individual connectors represents
a potential point of failure, but only a tiny fraction of them
are regularly monitored. Fires in operating assets with faulty
connectors are preventable, but only with the right inspection
and testing techniques.
Inside a PV Connector
The inside of a PV connector is rarely seen. Many PV
connectors are field-made, which means their two parts are
pushed together in the field during installation. Once locked,
opening a field-made connector permanently destroys it.
PVEL's field testing team obtained the image shown to the
right using an advanced lab testing technique called X-ray
Computed Tomography, which yields a 3D image of the
inside of the connector.
In order to operate properly, the male and female contacts of
connectors must be correctly matched, fully inserted and correctly
crimped as shown above.
Modern Connectors and Field Failures
Push-fit connectors accelerated the growth of the solar industry in the early 2000s by
simplifying PV system installation. The design allowed construction workers to make
electrical connections in the field instead of licensed electricians
Unintended Consequences
As technology improved, the slow pace of development for
connector standards and regulations led to confusion in the
market and failures in the field.
As a result, many solar installation professionals have
received incorrect training that specifies:
1. Uncertified, generic tools can be used for installation.
2. MC4-compatible connector parts produced by any two
different manufacturers can be paired.
These are myths that cause field failures.
The Signs of Connector Failure
The obvious signs of failure are: loose or disconnected
connectors; high temperatures; melted, discolored or cracked
casings; arc faults and ground faults; fires. But these field
observations are only the symptoms of deeper challenges:
• Uneven, insufficient or improper surface contact on
metal contacts.
• High resistance due to soiling, corrosion or foreign
particles.
• Moisture or water ingress that creates alternate
electrically conductive paths, typically resulting from a
broken seal and/or separated connectors.
• Material degradation due to environmental factors.
The conditions described above arise when fundamental
mistakes are made at the time of connector manufacturing,
procurement or installation, or due to a natural catastrophe
or force majeure event.
A mismatched MC4 and MC4-compatible connector in the
field at a site with extensive connector failures.
Why Failures Happen: The Most Common Root Causes are Simple Mistakes
-Improper installation
Contacts that are not inserted fully into housing is a
common issue.
-Improper installation tools
Uncertified tools may not meet product specifications and/
or damage components.
-Lack of training
Proper torquing technique and the use of end caps are often
overlooked.
-Mismatched connectors
Design differences may prevent complete, watertight
locking.
-Counterfeit connectors
Untested, uncertified products may be unsafe and degrade
quickly.
-Faulty Faulty materials
Some polymeric materials degrade rapidly after contact
with oil or sunscreen.materials
Some polymeric mater
What Happens When Connectors Fail?
Some connector failures are worse than others. Catastrophic incidents that jeopardize
safety and property are more likely to occur when faulty connectors continue
operating in the field over time.
-Compromised Connectors Can Be Time Bombs
Connectors that are left unplugged and uncapped during
construction are often exposed to moisture and foreign
particulates, such as dirt or other organic matter. Without
connector caps, the metal pins of the connectors will be
exposed to moisture and begin to corrode.
Then, once the connector is closed, the corrosion continues to
propagate, increasing connector resistance until the connection
eventually fails. These connectors can become time bombs
waiting to ignite to the field.
-Poor Sealing Causes Nuisance Tripping
When connectors are not properly sealed against the
environment, current can leak to ground. Inverters have
sensitive circuits to detect the isolation of the system from
ground during start up. In PVEL and HelioVolta's experience,
inverter nuisance tripping during periods of high humidity (e.g.
from morning dew) is frequently traced back to the combined
ground leakage current from many poorly sealed connectors.
Connectors and Personal Safety
The safety of personnel and consumers cannot be overlooked in any discussion of connector failure. Silent,
non-detected or non-flagged ground faults can be lethal for technicians and other personnel, as well as the
customers of solar-powered businesses and homeowners with PV systems.
The Spectrum of Connector Failures
Initial symptoms of failure may go unnoticed, such as minor reductions in energy yield caused by resistive losses. The chart below
describes the spectrum of failure modes and range of potential impacts for asset owners
| Field Observations of Failure | Performance Impacts
| Commercial Impacts
|
Power Loss | Visible signs of failure may or
may not be apparent
• Disconnected, loose or
connectors
| 0.3% to 1% power loss
• Resistive losses may go
unnoticed
| Up to a few thousand dollars
per year in financial losses
|
Ground/
Arc
Faults
| Compromised enclosures
• Melted connectors
• Sparks
• Frequent inverter tripping
| Periodic downtime and/or
inactive strings are possible
• Major underperformance and
power losses can occur
| Up to a few thousand dollars
per day in financial losses
• Safety risks to personnel
• Equipment damage
|
| Fires | Extremely hot connectors that
ignite system components
• Explosions
• Destroyed equipment
| System failure
• Total power loss
• Repairs may require extended
system downtime
| • Severe safety risks to staff and
general public, for which asset
owners may be legally liable
• Total or near-total financial
losses possible due to property
and equipment damage
• Business closures for repairs
and restoration
|
Analysis assumes the following initial conditions: 5 MW commercial project; $0.15 PPA rate; 1400 kWp/kWh production; 425W modules; 588 strings; 10 A
module current; 42.5V module voltage; 1000 V string voltage. Connectors with resistance of 150mΩ affect 10% of the project. 1% of field made connectors
are open, rendering six strings inactive
Diagnosing Connector Failure
There are many ways to make mistakes when creating a PV connector, but few ways
to validate their quality after installation.
4
Assessing connector quality in the field is challenging. While these devices can safely be unplugged when they are not under
load, they cannot be disassembled without compromising their integrity: assembly is irreversible. Visual inspection of the
connector’s exterior can reveal some errors but it does not validate the quality of the internal connection
Failing Connectors are Hot
Compromised connectors have a higher resistance than
properly made connectors. That resistance decreases energy
yield and produces heat. As a result, high temperature is the
primary warning sign of looming connector failure:
• If major issues are present at the time of installation,
thermal identifiers may be present when the system is
commissioned.
• When latent defects are present, thermal identifiers may
not emerge until the system has operated for some time.
Thermal imaging on the ground can identify latent connector
reliability issues before they become catastrophic failures.
However, drone thermal imaging does not adequately
identify hot connectors because they are typically positioned
underneath modules that hide their heat signatures.
Identifying Failures with Thermal Imaging
There are no specific standards for on-site thermal imaging
of PV connectors, but HelioVolta’s data-driven approach has
proven successful in the field. Their process establishes pass/
fail criteria that includes two categories:
• Absolute temperature failure, which is defined according
to manufacturer specifications, typically 85°C to 95°C. All
connectors that display a temperature above this limit are
considered failures.
• Differential temperature failure, which is project-specific.
Some connectors that operate below the threshold for
absolute temperature failure will measure at higher
temperatures than others at the site. These connectors
still present performance and safety risks.
The criteria for differential temperature failure is established
using baseline average connectors temperatures at specific
temperature and irradiance conditions. The threshold for
failure is based on the deviation from that baseline average.
The images above show differential temperature failures.
One of the connectors in the bundle (left) has an operating
temperature that is 15°C higher than the others, as indicated
by the thermal image (right).
The sample of failed connectors above melted due to high
temperature operation in the field. PVEL removed the devices
from the field for testing and root cause failure analysis.
Advanced Lab Testing for Root Cause Analysis
Determining the root cause of connector failure typically
requires sophisticated destructive testing conducted in
a laboratory setting. Neither thermal imaging nor visual
inspection reveal the inner workings of connectors. PVEL
recommends the following techniques for advanced analysis:
• X-ray computed tomography (XCT) to create a 3D
image of the connector, including metal and plastic
components.
• Scanning electron microscopy (SEM) and energydispersive x-ray spectroscopy (EDX) to identify metal
corrosion and arcing biproducts.
• Cross-sectioning the connector at the position of the
crimp to evaluate crimp quality.
• Subjecting connectors to accelerated stress testing in
high temperature/humidity conditions and/or thermally
cycling in a laboratory environment to accelerate
failure of suspected bad connectors.
The above XCT cross-section images show the difference
between a correct (left) and poor (right) connector crimp
found by PVEL in an active PV installation.
Inside a Root Cause Failure Analysis
Connectors regularly melted and caught in fire in a >150 MW
project in California.
The owner contracted PVEL to conduct a root cause analysis
that determined why the failures occurred and identified the
EPC as the party at fault. PVEL evaluated 1,500 of the nearly
100,000 field-made connectors at the site with a combination
of visual inspection, infrared thermography and XCT.
Installation error was identified as the overarching root cause
of connector failure:
• Critical connector installation errors were extensive,
including under- and overtorqued nuts, poor placement,
improper crimping, and improperly seated contacts.
• 40% of the connectors evaluated with XCT had
installation issues, and 20% had incomplete insertion
of the contact, which suggests issues are widespread
throughout the site.
• Persistent inverter nuisance tripping based on insulation
resistance faults could be due to the widespread poor
sealing of connectors due to under-torqued back nuts.
PVEL recommended further inspection and/or total
replacement of all field-made connectors in the project due
to significant installation errors
Inspecting Rooftop PV Systems
More than 70% of the commercial and industrial projects
inspected by HelioVolta have serious connector issues.
HelioVolta is usually contracted by the asset owner after a
safety event occurs. Several trends have emerged:
• Connectors can be difficult to locate and poor wire
management is common. Inspectors often crawl on their
knees with hand-held thermal cameras.
• Cross-mated connectors are frequently found on
homerun cables. This classic problem usually occurs
because modules come with a connector type that the
EPC does not have readily on-hand.
• Poor installation practices are often to blame for
failures. Connectors may be exposed to the elements,
resting on roof membranes, have a tight bend radius, or
poorly torqued backnuts, among other issues.
• Hot connectors are the most obvious indicator of
underlying issues. These connectors have higher
operating temperatures than other connectors installed
on-site, which indicates a higher electrical resistance.
Unfortunately, failure is only a matter of time for connectors
that are time bombs. To prevent future safety issues,
HelioVolta advises asset owners to:
• Ensure that connector quality assurance/control
protocols are in place for construction and
commissioning.
• Mandate proper field-made connector training for
personnel.
• Include connector inspection requirements and pass/fail
criteria in operations and maintenance contracts.
A single overheated connector caused this catastrophic failure.
Per PVEL's testing, installation error is likely the root cause.
These XCT images show two
poorly installed connectors. The
top image reveals incomplete
insertion and the left image zooms
in on a channel for water ingress
due to an undertorqued nut.
Drilling Down on Connector Inspection Techniques
Different types of connector inspections can reveal different issues. The table below describes the types of connector failure
modes that can be identified through visual inspection, thermal imaging and advanced lab testing
Failure Mode
| Root Cause | Failure Mode | Visual Inspection - Connected* | Visual Inspection - Disconnected* | Thermal Imaging | Advanced Lab Testing |
Lack of Trainingor Improper Installation Practices | irty contacts or contamination by
foreign particles | No | Yes | Somewhat likely | Yes |
| Corroded by exposure to water during installation | No | Yes | Somewhat likely | Yes |
| Contact not seated in housing | No | Yes | very likely | Yes |
| Cross-threaded | YEs | Yes | Not likely | Yes |
| Incorrect choice of contact for wire gauge or strand count | No | No | Likely | Yes |
| Incorrect choice of plastic housing for wire insulation outer dimension | No | No | Likely | Yes |
| Connectors exposed to elements | Yes | Yes | Not Likely | No |
| Wire nicks during stripping | No | No | somewhat Likely | Yes |
| Lost wires during stripping | No | No | somewhat Likely | Yes |
| Wires too bent going into connector | Yes | Yes | Not Likely | No |
| Low point location (water dripping) | Yes | Yes | Not Likely | No |
| Not fully seated (plastic pins do not click) | Yes | Yes | Likely | Yes |
| Too much tension | Yes | Yes | Likely | No |
| Counterfeit Products | Many failure modes are possible due to lack of certifications and testing | Maybe | Maybe | Somewhat Likely | Maybe |
| Improper Tools | Improper torque | Maybe | Yes | Not Likely | Yes |
| | Bad crimp | No | No | Likely | Yes |
| | Incorrect stripping length | No | No | Somewhat Likely | Yes |
| Mismatched Parts | Disimilar contacts and couplers specifications | Yes | Yes | Somewhat Likely | Yes |
Faulty Materials / Manufacturing Defects | Fast degradation of polymer materials | Maybe | Maybe | Somewhat Likely | Maybe |
| Propensity for fretting corrosion | No | No | Somewhat Likely | Yes |
*Non-destructive visual inspection can be conducted on connectors that remain connected; i.e., the male and female parts are
locked together. It can also be conducted on connectors that have been disconnected, so long as they are opened safely and are
not under load. Removing the back nut and opening the plastic housing of a connector to inspect its interior will compromise it.
**Thermal imaging is presented by probability of detection because this technique can only identify issues that cause resistance
rise to the point that the connector causes significant resistive losses and is close to failure. While thermal imaging does identify
hot connectors, it typically does not reveal the specific issue or root cause of failure.
Insights from Non-Destructive Visual Inspection
During O&M inspection, thermal imaging is the primary tool for identifying PV connector issues. However, there are also visual
cues that can identify the longer-term connector issues soon after commissioning, before they are apparent in a thermal scan.
These cues are also helpful if irradiance conditions are below a minimum threshold for inspection.
Connectors may be unconnected,
loose, or improperly connected. If the
connector is not pushed in all the way,
the connection is not perfect and can
lead to increased resistance over time.
 |
Signs of overheating ("shine effect"
or thermal deformation). Materials in
connectors change appearance when
they degrade due to operation outside
of specified temperatures.
 |
Cross-threaded back nuts. When
torquing back nuts, threads can bite
in the wrong channels, or crossthread.
This can compromise the connector's
watertight seal.
 |
Over-torqued back nuts. Torquing
specifications vary by manufacturer
and must be followed. Over-torquing
compromises the seal and integrity of
the connector body.
 |
Under-torqued back nuts.* This error
is usually caused by skipping a step
or using plastic installation wrenches.
Some manufacturers specify the
number of visible threads.
 |
Inconsistent back nut visible threads.
This can indicate a mistake because
all connectors in a single array are
usually made with the same tools and
conditions. They should match
 |
Exposure to sunlight and water.
Connectors should be protected from
moisture and direct UV rays as they
will degrade the material over time.
This is often manufacturer-specified.
 |
Insufficient bend radius or too much
tension on the leads. These conditions
can compromise the integrity of the
seal at the back nut or even damage
the internal crimp.
 |
Crossmated connectors. The term
"compatible" refers to the shape of the
termination and does not guarantee
safety. Parts from different brands are
not usually tested/certified together
 |
Additional Notes
• Connectors should not be located at a low point in cabling. This allows water to seep along their cables and interact with
the backing nut. If the back nut seal is compromised, moisture ingress may create an alternate path for electron flow.
• Exposure to mechanical interference can also damage connectors and lead to electrical problems. For example, connectors
may be pinched due to tracker movements or affected by vibrations from heating and cooling systems
The Solar Industry's Most Common Torquing Myth
It is a common misconception that the plastic wrenches used for installation slip when there is sufficient
torque on the back nut. This is false. Plastic wrenches may be used to initially make the connection, but
then the nut must immediately be torqued properly with the manufacturer-approved torque wrench.
Torque on the back nut cannot be checked post-commissioning. It is imperative that asset owners are
confident in their installation teams and the quality of the connectors in their projects
Best Practices to Ensure Connector Reliability
Connector failure creates safety risks and causes underperformance in PV assets, but
these negative outcomes are avoidable. Take action throughout the project lifecycle
to prevent connector failure by following these five steps:
1. Specify Connectors
Specify certified connectors by manufacturer and exact product type in both module
supply agreements and EPC contracts. Mandate and validate product authenticity to avoid
counterfeits.
2. Install the Right Connectors
Do not allow cross-mated connectors unless they are tested and certified together as a single
component. Obtain new connectors from module manufacturers if necessary.
3. Install Connectors Properly
Use manufacturer-provided crimping tools. String modules immediately to prevent
environmental exposure and use end caps when necessary. Document the status and location
of all field-made connectors at the time of installation.
4. Validate Connector Installation
Include third-party as-built inspection and verification requirements in EPC contracts. Ensure
connectors are covered under EPC warranties.
5. Regularly Inspect Connectors
Periodically inspect connectors throughout the project lifecycle using SolarGrade software and
an acceptable quality level (AQL) sampling approach. Always conduct inspections after force
majeure events because they can compromise the integrity of well-made connectors
Next Steps
Connectors are not complicated when they are installed correctly and functioning properly – but when they fail, the
ramifications can be enormous. While field inspections can prevent costly fire incidents, the root causes of connector
failure are more fundamental and must be addressed
 |
Standardize Connector Design
The industry lacks a clear, consistent standard for connector design. MC4 has emerged as the de facto
connector standard, but it is a proprietary design. Product marketing that promotes “MC4 compatibility” has
resulted in widespread and dangerous assumptions about connector intermatability. The best way to prevent
compatibility-related failures is to standardize connector design.
|
 |
Improve Installer Training
Solar training programs do not always provide accurate guidance for connectors. In our experience,
problematic practices that lead directly to connector failure are widespread and deeply ingrained. A concerted
industry-wide effort to improve training programs is necessary, especially as the solar installation workforce
grows. The best way to prevent installation-related failures is to require robust, accurate installer training.
|
 |
Conduct More Research
The industry will benefit from new tools and more test methods that enable frequent, low-cost connector
inspections on a regular basis. The humble PV connector should be prioritized by researchers precisely
because it is easy to overlook in the field. With the right technology, connector inspections can become a
standard operating procedure for solar PV systems instead of a reactive response to obvious signs of failure.
We urge those interested in joining PVEL and HelioVolta’s research and development efforts to reach out.
|
