If you have ever been responsible for specifying or installing photovoltaic modules, you already know the stakes. A module that looks fine on the outside can be quietly losing 30% or more of its power output, and you might not even notice until the performance data comes in months later. That is the reality of Potential Induced Degradation, or PID.
I have seen this happen more times than I care to count. And the conversation always starts the same way: "We used the same frames we always use. What went wrong?"
The answer, more often than not, lies in the interaction between two seemingly unrelated design elements: the anodized aluminum frame and the electrical grounding system. Get either one wrong, and you are setting yourself up for power loss, safety hazards, or both.
Let me walk you through what PID actually is, why anodized frames create grounding challenges, and how to design for both corrosion resistance and electrical integrity.
What Is Potential Induced Degradation and Why Does It Matter?
Potential Induced Degradation is a phenomenon where high voltage differentials between the solar cells and the module frame cause leakage currents that degrade cell performance over time. Under high humidity, high temperature, and high voltage conditions, modules may experience PID.
The degradation mechanism works like this. The voltage potential between the cells and the grounded frame creates an electric field that drives ion migration through the encapsulation materials. This leakage current causes the cell's power output to drop—often by 30% or more. The effect is cumulative and can render a module uneconomical within a few years of operation.
PID is particularly problematic in utility-scale systems operating at 1,500 V DC, where the voltage stress on modules is significantly higher than in residential systems. Premium frames with Anti-PID features are expected to account for 40–50% of market value by 2035, up from 25–30% in 2026.
To reduce the risk of PID, industry practice recommends connecting the negative terminal to ground at the module's DC connection site.
The Anodized Frame: Corrosion Protection with a Catch
Anodized aluminum alloy frames have become the industry standard for solar modules for good reason. The anodizing process creates a protective oxide layer—typically 10 to 25 microns thick—that resists corrosion, weathering, and environmental degradation.
The anodized corrosion resistant aluminum alloy frame provides structural support and durability, making modules suitable for harsh outdoor environments. The anodizing process effectively coats the outer surface of the aluminum electrolytically with a protective oxide that protects the material from environmental effects.
But here is the catch. The same protective oxide layer that prevents corrosion also creates a high electrical resistance. Anodized aluminum has always been thought of as insulative in nature, and anodic finishes are insulative relative to ground.
This means the frame that provides structural integrity and corrosion resistance is also a barrier to electrical conductivity. And that barrier has direct implications for both PID mitigation and electrical safety.
Why Grounding Integrity Is Critical for PID Mitigation
The relationship between grounding and PID is often misunderstood. Grounding is not just about lightning protection—it is an integral part of PID prevention.
To protect modules from lightning and static-electricity damage, the module frame must be grounded. But grounding also serves another purpose: it establishes a reference potential that reduces the voltage stress on the cells, thereby mitigating PID.
The grounding device must be in full contact with the inner side of the aluminum alloy and penetrate the surface's oxide film on the frame. This is not optional—it is a requirement for both safety and performance.
Trina Solar recommends using grounding wires with resistances that are less than 1Ω. The electrical contact is made by penetrating the anodized coating of the aluminum frame and tightening the mounting screw—together with a star washer—to the proper torque of 3–7 N·m. For equipotential bonding across PV array structures, contact resistance must be maintained below 0.1Ω throughout a 25-year operational lifecycle under thermal cycling from -40°C to +90°C.
The Grounding Challenge: Penetrating the Oxide Layer
Here is where design choices have real consequences. The anodized layer insulates the frame, making it difficult to readily form an electrical bonding or grounding connection. If the grounding device does not penetrate this layer, the frame remains electrically isolated—and both safety and PID mitigation are compromised.
How Proper Grounding Is Achieved
The industry standard approach is to use mechanical penetration. The grounding device must be in full contact with the inner side of the aluminum alloy and penetrate the surface oxide film.
Common methods include:
Stainless steel thread-forming screws. A thread-forming stainless steel screw cuts into the aluminum, creating an airtight connection that maintains good electrical connection over time. The screw is typically used with a star washer, which has teeth that dig into the anodized surface.
Grounding clips with sharp nibs. Grounding clips with stainless steel SUS304 teeth use multi-point sharp nibs to penetrate the 10–25 micron anodization layer, bonding the PV module frame directly to the mounting rail via clamp pressure.
Star washers. Star washers have internal or external teeth that are twisted to extend above and below the plane of the washer body, digging into adjacent surfaces when tightened.
Critical Installation Requirements
Do not drill additional grounding holes on the module frame. Holes marked with a grounding symbol on the frame can only be used for grounding—never for mounting.
Avoid direct contact of dissimilar metals. The grounding method should not result in the direct contact of dissimilar metals with the aluminum frame.
Use stainless steel hardware. General grounding hardware—including the grounding screw, flat washer, star washer, and wire—should be made of stainless steel.
Industry Standards and Best Practices
Several standards govern grounding and PID mitigation for photovoltaic modules.
NEC 690.43 specifies grounding requirements for PV systems. Grounding clips must safely divert fault currents up to 10 kA to the grounding electrode system per this standard.
UL 2703 covers mounting systems, mounting devices, clamping/retention devices, and grounding lugs for use with flat-plate photovoltaic modules and panels.
IEC 61730-1 establishes safety qualification requirements for PV modules. Grounding holes are identified with a grounding symbol per this standard.
UL 467 covers grounding and bonding equipment.
Recommended Grounding Resistance
General recommendation: Grounding wire resistance should be less than 1Ω.
Equipotential bonding: Contact resistance should be maintained below 0.1Ω.
Ideal for properly bonded array: Below 0.1Ω.
Torque Specifications
Trina Solar recommends tightening mounting screws (with star washer) to 3–7 N·m.
Ulica Solar recommends the same torque range of 3–7 N·m.
For grounding clamps with copper core wire, torque of 2.3 N·m is recommended for 12 AWG wire.
Design Strategies for Anti-PID and Grounding Integrity
Based on what I have seen work in actual installations, here are the key design considerations.
Frame Design
The frame should be designed with pre-drilled grounding holes marked with a grounding symbol. These holes should be located on the inner side of the aluminum alloy frame to facilitate proper grounding device contact.
The anodized layer thickness should be controlled—typically 10 to 25 microns—to balance corrosion protection with the ability to penetrate for grounding.
Grounding Hardware Selection
Use stainless steel hardware for all grounding connections. The star washer or grounding clip must have sufficient hardness and sharpness to penetrate the anodized layer.
For grounding clips, stainless steel SUS304 with hardened teeth is recommended. The clip should be positioned between the PV module frame and the mounting rail, with teeth that pierce both the frame's anodized layer and the rail finish.
System-Level PID Mitigation
At the system level, PID mitigation requires proper grounding strategy:
For isolated PV inverters: The negative of the module's DC connection side can be directly grounded.
For non-isolated PV inverters: An isolated transformer must be installed before applying virtual grounding.
Installation Best Practices
Use only pre-drilled grounding holes—do not drill additional holes.
Tighten grounding connections to specified torque values (3–7 N·m for screw connections).
Ensure the grounding device penetrates the oxide layer and makes full contact with the aluminum.
Use 12 AWG copper wire for grounding connections.
Verify grounding resistance is below 1Ω (and ideally below 0.1Ω for equipotential bonding).
Real-World Application: Utility-Scale Solar Installation
Let me share an example that illustrates why these design considerations matter.
A 100 MW utility-scale solar project was experiencing higher-than-expected degradation rates in the first year of operation. Performance data showed that modules at the end of each string—those experiencing the highest voltage stress—were losing power at nearly twice the rate of modules near the neutral point.
The root cause analysis identified two issues. First, the grounding connections were not consistently penetrating the anodized layer on the module frames. Some connections had been made to the outer surface of the frame, where the oxide layer remained intact. Second, the system grounding strategy had not been optimized for PID mitigation.
The solution involved two interventions. First, all grounding connections were re-torqued using star washers to ensure penetration of the anodized layer. Second, the system was reconfigured to ground the negative terminal at the DC connection site, per industry recommendations.
The results were significant. Degradation rates returned to expected levels, and the project recovered its performance targets within the next quarter. The cost of the intervention was minimal compared to the cost of lost production.
Frequently Asked Questions
What is PID and why does it matter?
Potential Induced Degradation is a phenomenon where high voltage differentials between solar cells and the module frame cause leakage currents that degrade cell performance over time. Under high humidity, high temperature, and high voltage conditions, modules may experience PID. The effect can reduce power output by 30% or more.
Why does an anodized frame need special grounding consideration?
The anodizing process creates a protective oxide layer—typically 10 to 25 microns thick—that is electrically insulative. This layer must be penetrated to establish an electrical connection for grounding.
How do I ensure proper grounding of an anodized frame?
Use a grounding device—such as a stainless steel screw with a star washer or a grounding clip with sharp teeth—that penetrates the oxide layer and makes full contact with the aluminum. Tighten to the specified torque (3–7 N·m for screw connections).
What grounding resistance should I aim for?
Trina Solar recommends grounding wire resistance less than 1Ω. For equipotential bonding, contact resistance should be maintained below 0.1Ω.
Can I drill additional grounding holes in the frame?
No. Do not drill additional grounding holes on the module frame. Holes marked with a grounding symbol can only be used for grounding—never for mounting.
What hardware should I use for grounding?
Use stainless steel hardware for all grounding connections. The star washer or grounding clip must have sufficient hardness and sharpness to penetrate the anodized layer.
Final Thoughts
I have been around enough solar installations to know that grounding and PID mitigation are often treated as separate concerns. "Just ground the frame" is something I hear all the time. And for many installations, that works.
But if you are operating at high voltage, in hot and humid conditions, or in utility-scale systems—the interaction between the anodized frame and the grounding system matters. The anodized layer that protects the frame from corrosion is also a barrier to electrical conductivity. And that barrier must be deliberately penetrated to achieve both safety and PID mitigation.
The data is clear. Premium Anti-PID frames are expected to account for 40–50% of market value by 2035. Grounding resistance must be below 1Ω—and ideally below 0.1Ω for equipotential bonding. The grounding device must penetrate the oxide layer.
The cost of proper grounding hardware and installation is minimal. The cost of PID degradation—lost power, reduced revenue, and early replacement—is substantial.