Photovoltaic Grounding System: An Engineering Application Paper and Procurement Decision Guide

Problem Boundary

A ground-mounted PV plant began reporting persistent ground-fault alarms on its inverters three years after commissioning. Field inspection found widespread white rust and loosening at bolted connections between the mounting structure and the grounding grid, together with visible corrosion pits at the overlap joints between the ring earth electrode and down-conductors. The plant sits on coastal saline-alkali mudflats. The ground-resistance value recorded at hand-over was, and still is, within the original acceptance limit, yet the metallic path had long ceased to be a low-impedance, continuous whole.

In this real scenario, the ground-resistance reading masked the actual risk. This guide is written for three groups:

• EPC project managers and electrical design engineers, who need to avoid failure traps during equipment selection and construction;
• O&M supervisors, who must read field signals and trigger maintenance before faults occur;
• Procurement and technical bid evaluators, who should purchase grounding materials and connectors as one system, not as separate items.

By the end you will know how to classify site conditions from field-observable signals, derive material and fastener specifications from failure mechanisms, and structure procurement and acceptance records to ensure circuit continuity over a 30-year service life.

This paper focuses on and answers four questions:

1. Why can a system with a “pass” ground-resistance value still fail?
2. How to classify site conditions using field-observable signals related to soil, environment and mechanical stress?
3. How to derive material and specification choices from failure mechanisms instead of guesswork?
4. What key points must be captured in procurement documents and acceptance records to guarantee circuit continuity over a 30-year service life?

Grounding Mechanism: Why Resistance Is Not the Only Metric

Current Path and the Voltage-Dividing Role of Soil

The fundamental purpose of PV grounding is to provide a low-impedance, corrosion-resistant return path for fault current and lightning current. IEC 62548-1:2023 specifies the grounding and equipotential bonding requirements of PV arrays, IEC 60364-7-712 addresses equipment selection within PV electrical installations, and IEEE Std 80-2013 is applied primarily to the safety design of AC step-up substation grounding. The three standards protect different objects, array-level grounding, system insulation coordination, and personnel safety limits. Taking a resistance value from any single standard as a universal target for the entire plant will inevitably miss hazards.

When fault current dissipates into the earth through a grounding electrode, the voltage drop occurs mainly in the soil immediately surrounding the electrode. The higher the soil resistivity, the larger the effective cross-section required for current dispersion. For a given fault current, higher soil resistivity produces a higher ground potential rise near the electrode and a greater risk of dangerous step voltage or transferred potentials. This process has little to do with the material of the electrode or down-conductor, provided the conductor cross-section is adequate, what really limits current release is the soil.

Driving Forces of Loosening: Vibration, Temperature Cycling and Preload Decay

PV mounting structures experience diurnal temperature cycles and wind-induced micro-vibrations daily. As the module surface temperature swings from –10 °C at night to +60 °C during the day, the bolted joint faces undergo repeated shear and tension-compression alternating stress. If the initial preload is not controlled with a torque tool, or if the washer lacks sufficient elasticity, the contact interface develops micron-level relative displacement after hundreds of thermal cycles, and the contact resistance rises progressively.

This loosening is insidious: during a ground-resistance test, the small test current can still find a conduction path through the remaining contact points, so the reading remains “healthy.” However, under a fault or lightning surge, localised high-resistance spots heat instantly, causing arcing and permanent open-circuit of the metallic path. That is why failures often appear suddenly after several years of operation.

Galvanic Corrosion: The Silent Killer of Metallic Connections

In a humid, saline atmosphere or buried environment, metals of different electrochemical potentials form a galvanic cell once connected. For example, at a joint between a copper grounding conductor and a hot-dip galvanised steel support, a thin moisture film acts as an electrolyte, and the zinc layer, being the anode, is consumed preferentially. After the zinc is depleted, the bare steel substrate forms a new corrosion couple with the copper. The corrosion product (rust) expands in volume, wedges open the contact interface and further accelerates loosening.

This failure mode is almost independent of the ground-resistance value; a resistance reading alone cannot detect it. The hazard must be tackled at the design stage, by isolating dissimilar metal pairs or by breaking the electrolyte path through surface treatments on the fasteners.

Soil Corrosivity and Seasonal Fluctuations

Soil is the other half of the grounding “conductor.” Sandy and rocky soils lack cohesion and water retention, exhibit high resistivity and tend to be corrosion-mild. Clay and silty soils retain water well, show lower resistivity but contain more saline organic matter, leading to higher chemical attack on copper or steel. Saline-alkali land combines high salt content with wet-dry cycles, accelerating the corrosion rate of buried electrodes compared with neutral soil.

Before project start, a multi-point soil resistivity profile using the four-electrode (Wenner) method must be performed to build a layered soil model. Soil samples should also be analysed for pH, chloride content, sulfate content and redox potential. Only by understanding the site’s salinity, moisture and acidity can one predict the remaining effective cross-section of the grounding grid after ten years.

Site Condition Classification: Using Field Signals to Identify Risk Levels

“Highly corrosive environment” cannot remain a vague label. The following signals can be observed directly by engineers on site or obtained through routine tests, and serve as the basis for classifying grounding system conditions.

Soil signals

• White crust or salt efflorescence on the surface → saline-alkali soil, highly corrosive to both galvanised steel and copper;
• Long-term waterlogging after rain, dense vegetation roots → high-moisture clay; watch for chemical corrosion and frost heave risk;
• Loose texture with gravel after excavation → high resistivity, poor contact with electrode, prone to forming contact cavities;
• Frost depth exceeding 0.8 m → part of the effective electrode length lies within the seasonal frost layer, shortening the effective leakage length; freeze-thaw cycles also destroy backfill compaction.

Environmental signals

• Within 5 km of coastline with prevailing onshore winds → high salt spray deposition; atmospheric chloride accelerates corrosion of unsealed connections;
• Agricultural machinery or heavy vehicles regularly passing through the site → mechanical vibration transmitted to the structure stronger than ordinary wind vibration, increasing bolt loosening risk;
• Low-lying terrain or shallow groundwater table → electrodes remain in moist or alternating wet-dry zones for long periods, with corrosion rates faster than in dry soil.

Structural signals

• Module frame and mounting structure connected by a single bolt without anti-loosening washer → interface prone to fretting wear;
• Aluminium down-conductor bolted directly to copper grounding grid → aluminium-copper galvanic couple, aluminium side will corrode preferentially;
• Bolts tightened and then painted with anti-rust paint instead of anti-corrosion conductive paste → interface forms an insulating oxide film over time, increasing contact resistance.

These signals individually do not determine the ground-resistance value, but together they determine the probability of continuity failure over a 20–30 year operating period. Based on this, the design team can divide the plant into “low-risk,” “medium-risk” and “high-risk” zones, and apply differentiated connection schemes and material grades to each zone.

Selection Derivation: Explicit Paths from Mechanism to Specification

Derivation 1: High Soil Resistivity → Grounding Impedance Exceeds Limit → Deep Well Grounding or Soil Replacement

Mechanism: When the surface soil (0–5 m) resistivity exceeds 500 Ω·m (a common engineering threshold for considering deep grounding; the exact limit should be confirmed with the IEC 62548-1 design curves applicable to the project or local grid codes), simply adding conventional ring or vertical electrodes does not proportionally expand the leakage radius, because the current must pass through a large volume of high-resistivity medium near the surface.

Failure mode: Relying solely on extending electrode length yields diminishing returns. Moreover, in rock formations, horizontal electrodes are difficult to bury at the required depth and coverage; construction defects can introduce discontinuous “false grounding.”

Material/specification derivation:

• If deep soil (>10 m) has lower resistivity, use deep well grounding. Fill the well with low-resistivity material (such as graphite-based conductive backfill) and use copper-clad steel casing to maintain metallic conductive cross-section while improving corrosion resistance.
• If deep resistivity remains high and site space allows, consider local soil replacement: excavate around the electrode and backfill with bentonite-water mixture to reduce near-zone resistance. This scheme must evaluate groundwater level and shrinkage after drying; acceptance cannot rely solely on the initial resistance reading after construction.

Derivation 2: Salt Spray + Humidity → Galvanic Corrosion and Loosening of Bolted Connections → Isolation + Anti-loosening Combination Design

Mechanism: Salt spray provides chloride ions. When an electrolyte film forms on the bolt head and contact surface, the potential difference between a stainless steel bolt (e.g., 304) and galvanised steel plate is about 0.4–0.6 V—enough to drive galvanic corrosion. Meanwhile, humid environment accelerates washer elasticity degradation.

Failure mode: The bolt shank root corrodes and thins, reducing fatigue strength; after washer tension decays, preload drops rapidly and the connection loosens. Eventually, under large current surge, the connection point suffers arc erosion and loses continuity.

Material/specification derivation:

• Change the fastener combination to “stainless steel bolt + spring washer + flat washer” and apply anti-corrosion conductive paste at the interface to isolate the electrolyte.
• Alternatively, use a bimetallic transition piece (e.g., copper-aluminium transition) to separate dissimilar metals and prevent direct contact.
• For buried connections, use exothermic welding or compression connectors instead of bolted joints to eliminate contact interfaces.

Derivation 3: High Mechanical Vibration → Bolt Loosening → Preload Control + Anti-loosening Washer

Mechanism: Wind-induced vibration and thermal cycling cause micro-slip at the joint interface. If the preload is insufficient, the bolt gradually loses clamping force.

Failure mode: Contact resistance rises, and under fault current the joint overheats and arcs.

Material/specification derivation:

• Use torque-controlled tightening to achieve the specified preload (refer to manufacturer’s data or standards such as GB/T 3098.1 for bolt grades).
• Select wedge-locking washers or nylon insert lock nuts to prevent self-loosening.
• For critical connections, consider using thread-locking adhesive as an additional measure.

Procurement and Acceptance: Key Points for 30-Year Continuity

Procurement Document Requirements

When purchasing grounding materials and connectors, the following points must be specified in the technical specification:

Acceptance Checklist

At hand-over, the following records must be complete:

Decision Roadmap

The following decision path helps select the appropriate grounding scheme based on site conditions:

Summary

Two decision points stand out: first, high-frequency vibration conditions call for wedge-locking washers; spring washers are nearly ineffective in Junker tests. Second, in saline or humid environments, galvanic corrosion at dissimilar metal joints is a primary failure mode that ground-resistance readings cannot detect; isolation and surface treatment are mandatory.

Next Steps

To prepare for procurement and design, gather the following:

• Soil resistivity profile and soil analysis report for the site
• Environmental data: distance to coast, prevailing wind, groundwater level, frost depth
• Mechanical vibration assessment: wind load, traffic, equipment operation
• Existing grounding design drawings and material specifications for review

Then use the decision roadmap above to select the appropriate scheme, and ensure the procurement documents include the material, coating, anti-loosening and acceptance requirements listed in the tables.