PV earthing and bonding: protective, functional, and equipotential

Earthing and bonding rank among the least glamorous parts of a PV design, yet they are the reason a fault current finds a safe path instead of a person or a fire. Installers who treat this as an afterthought - "just clamp the frames together" - routinely fail insulation and continuity tests, and worse, leave systems where a single insulation breakdown can energize an entire array frame.

This article is for designers and installers who understand AC earthing on the grid side but need a rigorous grounding (pun unavoidable) in how PV-specific requirements diverge: floating DC arrays, functional earthing for certain inverter topologies, and the equipotential bonding of large ground-mounted or rooftop metal structures.

We work through the distinction between protective earthing (PE), functional earthing (FE), and equipotential bonding (EB), how IEC 60364-7-712 frames the requirements, and where real installations go wrong.

Why PV earthing differs from standard AC earthing

A conventional AC installation has one clear objective for earthing: provide a low-impedance path so that a fault current trips the overcurrent protection fast enough to prevent dangerous touch voltages. PV arrays complicate this in three ways.

First, the DC side is frequently ungalvanically isolated from the AC side (transformerless inverters), so a DC fault can appear as leakage current on the AC earth path, and vice versa. Second, PV modules and mounting structures are large distributed metal masses spread across a roof or field, often with long conductor runs between frames - this creates real risk of differing potentials during a lightning strike or fault, not just a single point of failure. Third, many arrays operate at DC voltages of 600-1500 V, well above the AC low-voltage threshold, so insulation faults can persist undetected for longer if not actively monitored (residual current monitoring, insulation resistance monitoring).

Protective earthing (PE): exposed conductive parts

Protective earthing connects all exposed conductive parts - module frames, mounting rails, inverter enclosures, combiner box housings - to the main earthing terminal, so that in the event of an insulation fault, the fault current has a defined low-impedance path back to the source and disconnects the circuit or limits touch voltage to a safe value.

Key requirements under IEC 60364-7-712:

• Every module frame must be bonded, not just the ones at the string ends. Daisy-chaining through frame clips without a dedicated bonding conductor is a common shortcut that fails continuity testing once frame clips loosen from thermal cycling.
• The PE conductor must be sized per IEC 60364-5-54, generally following the phase conductor cross-section up to 16 mm2, then per the standard's reduction table above that.
• Continuity resistance between any exposed conductive part and the main earthing terminal should typically be below 1 ohm for a well-bonded array, though this depends on run length and connector count; national standards may specify explicit test criteria.

Frame bonding methods

Three methods dominate: dedicated bonding conductors with lugs at each frame, listed bonding washers/clips that pierce the anodized aluminum coating during module clamping, and bonded mounting rails where the rail itself is the bonding conductor and modules bond to the rail via UL 2703-listed (or equivalent) hardware. Bonding washers are fast to install but degrade if over-torqued or under-torqued; a torque wrench and manufacturer-specified value are not optional.

Functional earthing: when and why

Functional earthing (FE) references one DC pole (or occasionally the array midpoint) to earth for reasons unrelated to shock protection - typically to suppress potential-induced degradation (PID) in certain thin-film or crystalline modules, or because the inverter topology requires a defined reference for correct operation.

Functionally earthed systems are not the default for grid-tied crystalline silicon systems with transformerless inverters, which normally operate with a floating DC array monitored by an insulation resistance monitor. But when a module manufacturer's PID mitigation strategy calls for negative-pole grounding, the installation must accommodate a dedicated FE conductor, sized and protected per the standard, and the earthing must not compromise the residual current protection scheme on the AC side. Mixing functional earthing with a transformerless inverter not designed for it is a common design error that can trip nuisance faults or, in the worst case, mask a real fault.

Equipotential bonding for large arrays

On ground-mounted arrays and large industrial roofs, equipotential bonding addresses a different problem: minimizing potential differences across the array during lightning-induced transients or fault conditions, independent of whether a fault has occurred yet.

Practical measures include:

• Bonding all metallic mounting structure sections together, not relying on bolted joints alone for electrical continuity - use bonding jumpers across expansion joints and pile-driven post sections.
• Meshed earthing conductors buried around the perimeter of ground-mount arrays, bonded to driven earth rods at defined intervals (commonly every 20-50 m depending on soil resistivity and the lightning protection design per IEC 62305).
• Bonding the array earthing system to the building's main earthing terminal on rooftop systems so there is a single reference potential, avoiding separate, unconnected earth electrodes that can float apart during a fault - a known cause of arc-over between systems.

Table: earthing conductor sizing guidance

Worked example: bonding conductor sizing

A 500 kWp ground-mount array uses DC home-run cables of 6 mm2 copper between combiner boxes and a central inverter station, 180 m away. Per IEC 60364-5-54's adiabatic method (simplified for illustration): for phase conductors up to 16 mm2, the PE conductor cross-section equals the phase conductor cross-section, so a 6 mm2 PE conductor accompanies the 6 mm2 DC cable. Voltage drop and fault-loop impedance both improve with this direct 1:1 sizing rather than an undersized shared earth bus. If ten combiner boxes each carry a 6 mm2 PE conductor back to a central earth bar, the total earthing infrastructure adds roughly 1,800 m of 6 mm2 copper (10 boxes x 180 m) - a material cost worth including explicitly in the BoQ rather than assuming "a bit of earth wire" covers it.

Testing and verification

Continuity testing (per IEC 62446-1) confirms every exposed conductive part is bonded before energization. A typical test injects a known current (commonly 200 mA to a few amps depending on the tester) between the main earthing terminal and each frame, verifying resistance is low and consistent across the array - large deviations flag a broken bonding path. Insulation resistance testing on the DC side, done between each polarity and earth, verifies the array has no unintended path to ground; typical acceptance thresholds scale with system voltage and array size per the standard's tables, commonly in the range of 1 Mohm or higher for utility-scale strings, though exact minimums are tabulated for different voltage/power bands.

Documentation and handover

A complete bonding record should include a single-line earthing diagram showing every bonded component, conductor sizes, and earth electrode locations, plus the measured continuity resistance for each string or frame group. Handover packages that omit this diagram make future fault-finding slow and expensive, since technicians must reverse-engineer the bonding topology from photographs and guesswork rather than a drawing. For larger sites, tagging each bonding jumper termination with a unique identifier that matches the as-built drawing pays for itself the first time a fault needs isolating under time pressure.

Common mistakes

• Bonding module frames in a daisy chain through clamps only, without a dedicated jumper, so continuity fails after the first thermal cycle loosens a clip.
• Treating the mounting rail as automatically continuous across expansion joints without a bonding jumper, leaving a gap invisible until a continuity test is run.
• Installing functional earthing to satisfy a module PID datasheet note without checking the inverter is designed for grounded operation.
• Under-sizing PE conductors relative to DC cable cross-section to save copper, violating the 1:1 (or tabulated) sizing rule.
• Running separate, unconnected earth electrodes for the PV structure and the building, creating a step/touch voltage hazard during lightning events.
• Skipping documented continuity and insulation resistance test records, leaving no evidence of compliance at commissioning.

Frequently asked questions

Does every module frame need its own bonding conductor?

Yes, in principle every exposed conductive part must have verified continuity to the earthing system; listed bonding hardware between adjacent frames can satisfy this if tested, but relying purely on mechanical clamping without continuity verification is not acceptable practice.

Is functional earthing required for all PV arrays?

No. Most transformerless-inverter, crystalline-silicon systems use a floating DC array with insulation monitoring rather than functional earthing; functional earthing is applied selectively for PID mitigation or specific inverter topologies.

What is the difference between equipotential bonding and protective earthing?

Protective earthing provides a path for fault current to enable disconnection and limit touch voltage; equipotential bonding minimizes potential differences across conductive parts during transients such as lightning strikes, independent of a fault occurring.

How often should earthing continuity be retested after commissioning?

Many maintenance regimes retest at each scheduled O&M visit (commonly annually) and after any structural or module replacement work, since bonding hardware can loosen with thermal cycling.

Can aluminum mounting rails be used as the sole bonding path?

Only if the rail system and its joints are listed and tested for bonding continuity (for example under UL 2703 or equivalent); unlisted rail joints should not be assumed continuous without direct measurement.

Standards and references

IEC 60364-7-712: Low-voltage electrical installations - Requirements for special installations or locations - Solar photovoltaic (PV) power supply systems. IEC 60364-5-54: Earthing arrangements and protective conductors. IEC 62446-1: Photovoltaic systems - Requirements for testing, documentation and maintenance. IEC 62305: Protection against lightning. Sources: NREL, "PV System Grounding and Bonding" technical reports; Fraunhofer ISE, PV reliability and PID mitigation studies; IEA PVPS Task 13 reports on PV system performance and reliability.

See also /blog/mppt-explained, /blog/cable-sizing-and-voltage-drop, /calculators/system-sizing, /calculators/voltage-drop, and the /glossary for definitions of PE, FE, and equipotential bonding terms.