Solar PV Earthing and Bonding: Array Frame Earthing, DC Isolator Placement and Lightning Risk

Summary

Solar PV DC circuits operate at elevated voltages (typically 200–600Vdc for domestic string systems) and present specific earthing and isolation requirements distinct from standard AC domestic wiring. BS 7671 Section 712 is the governing standard; it specifies requirements for DC cable types, earthing connections, fault detection, and isolation.

Understanding these requirements is essential for MCS-compliant installation and for maintaining the safety of the system for 25+ years. This article covers the key earthing and bonding requirements, DC isolation, and the lightning protection question.

Key Facts

• Section 712 (BS 7671) — the IEE Wiring Regulations section specifically for photovoltaic power supply systems; all solar PV electrical design must comply
• DC isolator (array isolator) — an isolator in the DC string circuit, accessible from the roof or at a convenient location, allowing the DC circuit to be de-energised before working on the array; required at the inverter input; also at the array for systems where the open-circuit voltage exceeds safe working levels
• DC circuit earthing — in most domestic TN-S and TN-C-S systems, the DC circuit is unearthed (floating): neither positive nor negative conductor is connected to earth; the inverter includes earth fault monitoring to detect any fault-to-earth on the DC side
• Array frame earthing — the metal mounting frames, rails, and any metal supports are connected to the protective earth (PE) terminal of the installation; this is distinct from the DC circuit conductors, which are not earthed
• BS EN 62446 — the standard for documentation, testing, and commissioning of grid-connected PV systems; specifies the test records required for solar PV installations
• Insulation resistance (DC) — the DC circuit insulation resistance is measured as part of commissioning; typical minimum acceptable: >1MΩ between L+ and earth, L- and earth
• Double-insulated DC cable — solar PV DC cabling (often designated H1Z2Z2-K or similar) is double-insulated and UV-resistant; standard twin-and-earth cable is not suitable for the DC side
• Open circuit voltage (Voc) — the maximum panel voltage when disconnected; for a string of 10 panels at 40Voc each: Voc_string = 400Vdc; must be within the inverter's maximum input voltage rating; typically 600Vdc for residential
• Short circuit current (Isc) — the maximum panel current when terminals are short-circuited; typically 10–12A for current generation panels; relevant for cable and fuse sizing
• BS EN 62305 — lightning protection standard; not mandatory for all buildings, but provides the risk assessment methodology for determining whether a lightning protection system is required
• Surge protection devices (SPDs) — Type 2 SPD on the DC input of the inverter (and on the AC output) protects the inverter from lightning-induced surges; recommended for exposed sites
• Rapid shutdown — for safety during fire or emergency, the DC string voltage should be reduced quickly; UK BS 7671 Section 712 requires isolation means, not automatic shutdown at this stage; some systems (SolarEdge SafeDC, Enphase microinverters) provide this functionality as a feature
• PV string fuse — where multiple strings are in parallel, string fuses protect each string from reverse fault current; required where the array can feed back current into a faulty string

Quick Reference Table: DC Earthing and Isolation Requirements

Detailed Guidance

Array Frame Earthing

All metal components in the solar PV mounting system — aluminium rails, hooks, brackets, and panel frames — must be electrically connected to the protective earth (PE) of the installation.

Why: In the event of a fault where a DC conductor contact is made with the array metalwork (cable damage, insulation failure, panel defect), the fault current must have a path to earth to enable the inverter's earth fault protection to detect the fault and disconnect. Without earthing, the metalwork could become live at DC voltage and present a shock risk to anyone touching the array.

How:

• Most aluminium rail mounting systems incorporate earth bonding via the panel mounting clamps (mid-clamps and end-clamps) — the clamp makes metal-to-metal contact with the panel frame, and the rail is then bonded to the installer's equipotential bonding point
• The bonding must be of low resistance (typically <0.1Ω from array to main earth terminal for the bonding to be effective)
• On coated aluminium rails, the coating under the clamps must be removed (or serrated clamps used) to ensure metal-to-metal contact
• The earth bond from the mounting rail system to the installation PE is typically a green/yellow 4mm² or 6mm² conductor routed with the DC cables to the inverter enclosure

Testing: At commissioning, test the continuity of the earth bond from the array metalwork to the main earth terminal. A low-resistance ohmmeter reading (<1Ω) confirms continuity.

DC Circuit: Why It Is Unearthed

In most domestic solar PV systems, the DC conductors (the positive and negative strings from the array to the inverter) are not connected to earth. This is the standard for grid-connected systems where the inverter includes an isolation transformer or galvanic isolation (transformerless inverters use software-based earth fault monitoring).

Earth fault detection in the inverter: The inverter continuously monitors the insulation resistance between the DC conductors and earth. If either the positive or negative conductor develops a path to earth (through cable damage, water ingress, or panel defect), the insulation resistance drops. When the insulation resistance drops below a threshold (typically 1MΩ), the inverter generates an error (ISO fault or insulation fault) and disconnects. This protects against fire risk and personnel shock.

Transformerless vs transformer-based inverters:

• Transformerless inverters (most modern residential inverters, e.g., Fronius Primo, SMA Sunny Boy): do not provide galvanic isolation between DC and AC; rely entirely on electronic earth fault monitoring for safety; are more efficient and lighter than transformer-based units
• Transformer-based inverters (older designs): provide physical galvanic isolation between DC and AC circuits; some allow grounding of one DC conductor (typically negative); less common in UK residential markets

For most UK domestic installations, transformerless inverters with earth fault monitoring are standard. No connection of DC conductors to earth is required or appropriate.

DC Cable Requirements

DC wiring between panels and inverter must use:

• Double-insulated cable — two separate insulation layers; typically designated H1Z2Z2-K (or TÜV 2Pfg 1169 equivalent); cross-linked polyethylene (XLPE) insulation
• UV-resistant outer sheath — for external cable runs; standard PVC cable degrades rapidly in UV exposure; PV-specific cable is UV-stabilised
• Halogen-free — recommended for installations in buildings; reduces toxic smoke in the event of fire
• Minimum conductor size — 4mm² for standard domestic string circuits; 6mm² for longer runs (>20m)

Cable routing: DC cables on the roof must be routed in conduit or cable management that protects them from mechanical damage, UV exposure, and vermin access. Cables should not be draped loosely across roof surfaces. Where cables enter the building, they must pass through fireproof or sealed entry points (fire-stopping putty or sealed conduit glands).

Bundling: DC positive and negative cables from the same string should be routed together (clipped together or in the same conduit). This reduces the magnetic loop area of the circuit and minimises inductive effects from lightning-induced surges.

DC Isolation

At the inverter: A DC isolator (sometimes called the "array isolator" or "generator isolator") is fitted at the inverter, in the DC string circuit. This allows:

• Safe isolation of the DC circuit before working on the inverter
• Compliance with BS 7671 requirements for means of isolation
• Many modern inverters include this isolator integrally; otherwise it is a discrete switch unit

The isolator must be:

• Rated for DC use (DC-rated isolator; AC switches are not suitable for DC isolation due to arc-quenching differences)
• Rated for the string voltage (typically 600Vdc for domestic installations)
• Rated for the string current
• Accessible and clearly labelled

At the array: For strings with open-circuit voltage above 120V (virtually all modern domestic strings — a 3-panel string of modern panels already exceeds this), an isolator near the array is recommended by MCS guidance and IET Guidance Note 7. This allows first responders (fire brigade) to de-energise the DC circuit at the array level without going into the building.

Lightning Risk

Is a lightning protection system required? For most UK domestic solar PV installations, a formal lightning protection system (BS EN 62305) is not required. However, a risk assessment should be performed. The BS EN 62305 Part 2 risk assessment method considers:

• Ground flash density for the location (lightning frequency)
• Building dimensions and height
• Whether the building is isolated or in a built-up area
• The consequences of a lightning strike (fire, structural damage, loss of life)

For most suburban domestic properties in the UK (moderate ground flash density, ordinary building, existing structure), the risk assessment typically indicates that lightning protection is not required.

Where SPDs are recommended: Even where a formal lightning protection system is not required, surge protection devices (SPDs) on the DC input of the inverter are recommended for:

• Sites with high exposure (isolated rural buildings, hilltop properties, coastal locations)
• Expensive inverter systems where replacement cost is high
• Historically high lightning-strike areas

Type 2 SPDs (BS EN 61643-11) at the inverter DC input and AC output divert lightning-induced surge currents to earth before they reach the inverter electronics. Cost is approximately £50–£150 per SPD; recommended as standard for higher-risk sites.

Frequently Asked Questions

My inverter shows an "ISO fault" error. What does this mean?

An ISO fault (insulation fault) indicates that the inverter has detected a reduced insulation resistance between one of the DC conductors and earth. The most common causes are: water ingress into a DC connector (MC4 connectors on the roof can fail if not fully clicked together or if they are aged); a damaged panel junction box; DC cable damage; or a failing panel. Check all DC connectors on the roof for moisture or damage; test insulation resistance on each string separately to isolate the fault string.

Can I use standard grey armoured cable (SWA) for the DC circuit?

Standard SWA (steel wire armoured cable) is not rated for DC PV circuits and should not be used for DC string cabling. DC PV cable (H1Z2Z2-K) has the required double insulation, UV resistance, and temperature rating. Where mechanical protection is needed for the DC cable route on or off the roof, use stainless steel or plastic conduit over appropriate DC PV cable.

Does the earthing bond need to cover every panel frame, or just the rails?

The rails must be earthed. If the panel frames make reliable metal-to-metal contact with the rails (via mid-clamps and end-clamps with proper metal-to-metal contact), the panel frames are earthed through the rail. If any panel frame is isolated from the rail (e.g., by a non-conductive coating), it should be separately bonded. Check the mounting system's installation guide for bonding requirements.

Regulations & Standards

• BS 7671:2018+A2:2022 Section 712 — Photovoltaic power supply systems; all DC circuit earthing, isolation, and cable requirements

• BS EN 62446 — Grid connected photovoltaic systems; testing, commissioning, and documentation

• BS EN 62305 — Protection against lightning; risk assessment and protection measures

• IEC 60364-7-712 — International standard (equivalent to BS 7671 Section 712)

• MCS 012 — MCS Solar PV Product Standard; installer requirements for earthing and isolation

• BS 7671:2018+A2:2022 — IET — 18th Edition Wiring Regulations; Section 712

• IET Guidance Note 7 — Solar PV — BS 7671 Section 712 guidance for installers

• BS EN 62446 — BSI — commissioning and test documentation

• bs 7671 ev wiring requirements — parallel reading for another BS 7671 specialist section

• solar pv commissioning handover — testing requirements per BS EN 62446

• solar pv fault finding — earth fault and ISO fault diagnosis

• string inverter vs microinverter — microinverter architecture avoids high-voltage DC on the roof