Exploring Electrical Bonding in PV Structural Joints

This report, authored by Karle Wigginton and Jon Ness, PE, of Matrix Engineering Consultants for Lawrence Berkeley National Laboratory, examines a largely overlooked reliability issue in solar PV mounting systems: the long-term electrical bonding performance of structural joints. In modern PV systems, value engineering has pushed module attachment and mounting system joints to serve double duty — carrying structural loads while simultaneously providing the low-resistance electrical bond required for equipment grounding. These dual-purpose “structural-bonded joints” are now standard across residential, commercial, and utility-scale installations.

The problem is that structural-bonded joints are subject to loosening, corrosion, and environmental degradation throughout the life of the mounting system — and when they loosen or corrode, the electrical bond degrades with them. Current standards test bonding resistance in new, properly tightened assemblies but do not comprehensively address how that bond behaves over decades of dynamic loading, thermal cycling, and environmental exposure. The result is a potential gap between the electrical safety assumed at commissioning and the electrical safety actually present in aging systems.

What the report covers:

• The physics of electrical resistance in bonded joints — Total resistance across a structural-bonded joint is the sum of four components: bulk resistance (inherent to the material), constriction resistance (driven by the microscopic contact area between surfaces), spread resistance (where current transitions from contact points into bulk material), and film resistance (caused by oxide layers, corrosion products, or contaminants). Bulk and spread resistance are set by material selection and joint geometry. Constriction and film resistance are dynamic — they change as joints loosen, surfaces corrode, or coatings degrade. The report provides resistivity data for common PV materials (structural steel, galvanized coatings, 6000-series aluminum, 300-series stainless) and their oxide films, showing that aluminum oxide in particular has an electrical resistivity of approximately 1×10¹⁴ Ω·m, making anodized coatings effectively insulating.
  
• Why clamp load governs bonding resistance — At the microscopic level, joint surfaces are rough, and electrical current passes only through tiny asperity contact points (“a-spots”). When a joint is tightened, those asperities plastically deform and flatten, increasing effective contact area and reducing constriction resistance. When a joint loosens, elastic springback reduces contact area and resistance climbs. A loose joint also allows environmental exposure at the interface, enabling oxide and corrosion films to form on previously sealed surfaces. The report illustrates this progression through three states: passing bonding (tight joints, continuous low-resistance path to ground), intermittent bonding (partially loose joints with unstable resistance), and failed bonding (resistance too high for current to flow).
  
• How common joint loosening actually is — DOE-SETO-funded research involving structured interviews across 17,000 PV systems representing over 94 GW of capacity found that nearly 44% of reported joint failures were attributed to loosening. Only 13% were traced to installer error, while 37% stemmed from design-related issues. The report distinguishes between relaxation loosening (non-rotational loss of pretension through embedment, differential thermal expansion, joint yield, or fretting wear) and self-loosening (rotational loosening caused by repeated shear overload and joint slip). PV mounting systems are particularly susceptible because they are lightweight, flexible, dynamically loaded by wind, and subject to daily and seasonal thermal cycling that can produce repeated joint slip.
  
• Aluminum oxide as an electrical barrier — Anodized aluminum module frames and mounting components carry an oxide layer typically 10–15 μm thick that is effectively non-conductive. This coating must be pierced by a listed bonding device to establish an electrical path. When properly assembled, the bonding device embeds into the aluminum and the joint self-seals, resisting further oxidation. When the joint is loose, that seal is broken, exposing bare aluminum to accelerated oxidation and galvanic corrosion between the stainless steel bonding device and the aluminum substrate.
  
• The star washer problem — Contractors have widely used external tooth (star) washers as improvised bonding devices, and some electrical inspectors accept them. The report documents a failure investigation at a 5 MW site where star washers caused chronic joint loosening and module detachments. Root cause analysis showed that bolt pretension and applied loads concentrated at the small contact area of the star washer protrusions, causing compressive yield of the aluminum module frame and progressive relaxation that could not be resolved by retightening. Comparative laboratory testing by Burndy — including short-time current testing per UL 467 and 500-hour salt fog testing per ASTM B117 — showed that purpose-built bonding washers (WEEB®) outperformed star washers in both electrical stress resilience and corrosion resistance, with star washer samples exhibiting erratic resistance readings and, in some cases, complete loss of continuity.
  
• UL bonding resistance tests and their limitations — UL 1703, UL 61730, and UL 2703 all require bonding path resistance not to exceed 0.1 Ω, verified by passing twice the fuse ampere rating through the joint and measuring voltage drop. The report walks through the test procedure in detail but notes that it is performed on new, properly tightened assemblies — it does not evaluate bonding performance under field conditions where joints may loosen, corrode, or degrade over time. Utility-scale installations typically lack continuous monitoring of bonding resistance, and bonding resistance testing is not commonly part of routine O&M.
  
• Design, assembly, and O&M best practices — The report provides practical guidance across all three phases: use only listed bonding devices positioned between the module frame and structure (not under bolt heads), consider Belleville washers to maintain clamp load through relaxation, select galvanically compatible materials appropriate for the site’s corrosivity category, clean and dry joint surfaces before assembly, adhere strictly to manufacturer torque specifications with calibrated tools, and conduct periodic inspections that include retightening and bonding resistance verification where feasible.
  
• Research gaps and standards improvement opportunities — Priority areas include field measurement of equipment ground path resistance across operating PV systems to confirm the link between loosening and bonding degradation, refined laboratory testing that accounts for dynamic loading and humidity cycling together with normal relaxation, development of real-time resistance monitoring and non-destructive testing methods, and clarification of UL 2703 to acknowledge that its bonding resistance test applies to properly tightened joints and does not characterize field-aged performance.
  

The central finding is that a structurally sound joint is very likely a reliable electrical bond — and a loose one is not. As PV deployment scales and installed systems age, the industry cannot rely solely on commissioning-stage quality to ensure decades of safe grounding performance. Advancing the understanding of how structural-bonded joints degrade, closing gaps in standards and testing, and integrating bonding verification into routine O&M are necessary steps to ensure these joints remain both structurally and electrically dependable from installation through end of life.