Rooftop PV earthing and bonding on an existing office
A 300 kWp rooftop PV system is being added to an existing eight-storey office with a TN-S supply from a 1000 kVA transformer. The main earthing terminal is bonded to the foundation piles and measures 6.2 Ω by fall-of-potential at 62% distance; soil resistivity is 180 Ω·m. The PV installer proposes to run a separate 10 mm² earth from the inverter to a new 3 m rod in the planter box, and to leave the array frames unbonded because the anodised aluminium rails are 'already earthed through the mounting brackets'. The DC strings are 1000 V, 6 mm² double-insulated, run in cable tray on the roof. Two Type 2 surge protective devices are proposed, at the inverter AC side only. The installer argues that 6.2 Ω fails an 'earth resistance below 1 ohm' rule and wants to sell the client a 12-rod grid. The client asks you to decide both issues before the roof is closed up.
1. Is 6.2 Ω a compliance failure for this TN-S installation? Explain what actually governs and what you would verify.
Read the worked answer
In a TN-S system, fault disconnection depends on the metallic earth-fault loop (phase conductor, fault, protective conductor, transformer star point), not on the resistance of the consumer's earth electrode. There is no general 1 Ω rule for LV consumer electrodes in MS IEC 60364, so the installer's premise is wrong.
What actually governs:
- Zs at each board must satisfy the disconnection time: 0.4 s for final circuits up to 32 A, 5 s for distribution circuits.
- Protective conductors must be continuous and correctly sized.
- Main protective equipotential bonding must connect all extraneous conductive parts: incoming water and gas, structural steel, lift rails, AC risers and the PV mounting structure.
- Touch voltage under fault conditions must remain below 50 V AC.
The electrode resistance does matter for potential rise during LV earth faults and lightning, and for the effectiveness of equipotential bonding. For the lightning protection system, MS IEC 62305-3 commonly calls for an earth termination resistance not exceeding about 10 Ω; 6.2 Ω already satisfies that, so a 12-rod grid is not justified on those grounds.
Verify: measured Zs at the furthest boards, continuity and size of main bonding conductors, and a potential-difference measurement between the main earthing terminal and remote earth. Only add electrodes if a touch-voltage or lightning risk assessment shows a need.
2. Is the installer right that the array frames and rails need no bonding? State the requirement and a compliant method.
Read the worked answer
No. Anodised aluminium is an insulator at the contact surfaces, and clamps or brackets are not a reliable low-resistance path. The array frames, rails and mounting structure are exposed conductive parts and must be bonded to the protective earthing system under MS IEC 60364-7-712 and IEC 60364-5-54.
Compliant method:
- Use listed bonding hardware - toothed or star washers, or dedicated bonding clips - that penetrates the anodising at each joint.
- Bond with 6 mm² copper (or equivalent) and UV-rated, corrosion-resistant connectors; use bimetallic copper/aluminium connectors to avoid galvanic corrosion at aluminium interfaces.
- Bond each array section to a common roof earthing bar, then take a single earthing conductor to the main earthing terminal, sized at least 6 mm² and larger if it shares a route with lightning down conductors.
- Do not create a separate isolated earth for the inverter; a second earth path invites potential differences and circulating currents.
- If the inverter requires DC functional earthing, arrange it as a single-point connection to the same system.
Verify with a low-resistance ohmmeter, not a multimeter: record continuity between each bonded metallic part and the main earthing terminal, typically ≤ 0.1 Ω, and photograph the bonding points before the roof is closed.
3. Before certifying, what do you require for surge protection and for closing out the roof works?
Read the worked answer
Surge protection
- A Type 2 DC SPD at the array junction or inverter DC side, rated for 1000 V DC and the string short-circuit current; rooftop arrays with long DC runs and a credible lightning risk need it, and the AC-side device alone does not protect the DC side.
- Short connecting leads, ideally under 0.5 m, and a DC-rated isolating device ahead of the SPD.
- Type 2 AC SPD at the inverter, coordinated with the main board SPD; if the building has a lightning protection system, check whether Type 1 is required at the main board and confirm the energy coordination per MS IEC 62305-4.
Closing out
- As-built single-line diagram and earthing/bonding layout.
- DC string test results: open-circuit voltage, short-circuit current and insulation resistance with correct polarity.
- AC insulation resistance and Zs at the inverter board, and the recorded bonding continuity values.
- Inverter anti-islanding test to MS IEC 62116 and the utility interconnection approval.
- Written confirmation that the installer's proposed isolated rod has not been installed and that there is one common earthing system.
I would certify only the electrical scope and note any structural mounting items as outside my scope.