Earthing Design for a Rooftop PV Array on a TN-C-S Supply

On a TN-C-S (PME) supply, the protective earth is the PEN conductor, which carries neutral current and can rise in potential relative to true earth, and can be lost entirely if the PEN breaks. A transformerless inverter has no galvanic isolation, so its DC side is referenced to the AC side; a PEN loss or a DC injection fault can raise the array frames to a dangerous potential relative to earth.\n\nIf the frames are bonded to the building steel and hence to the MET/PEN, the frames become part of the PME earthing system. Under a PEN fault the frames could sit at a hazardous touch voltage, and a person touching a frame and true earth (e.g. a wet roof, a metal rainwater pipe that is not bonded) completes a circuit through the body.\n\nThe RCMU at 300 mA is a residual-current monitor for DC and AC leakage, not a substitute for a proper earthing arrangement; it does not protect against a PEN potential rise on the frame. The hazard is therefore touch voltage on the array frames under a PEN fault, not a simple lack of bonding.

Bonding to structural steel that is itself bonded to the MET is electrically the same as bonding to the MET directly - it does not remove the PME hazard. It satisfies the requirement that exposed conductive parts be bonded, but it makes the frames part of the PME earth, which is exactly what the DNO has said must not be relied upon.\n\nThe proposal is therefore incomplete. A compliant approach for a transformerless inverter on PME typically requires one of: (a) an additional independent earth electrode for the array frames, separated from the PME earth, with the array's earthing arrangement designed so that a PEN fault does not create a hazardous touch voltage on the frames; (b) a Type B RCD or an isolation transformer to provide galvanic separation; or (c) a combination of supplementary bonding and an earth electrode with a resistance low enough to limit touch voltage. The 12 ohm existing electrode is a starting point but its adequacy must be calculated against the expected PEN potential rise and the touch-voltage limit, not assumed.\n\nAlso, the 25 mm2 main protective conductor should be checked against the adiabatic requirement and the array's fault current, and the frames must be bonded with a conductor sized for the fault current and mechanically protected on the roof.

(1) Confirm the earthing arrangement in writing with the DNO and record it on the installation documents. (2) Measure the resistance of the independent array electrode and calculate the touch voltage under the design PEN fault current; compare with the applicable touch-voltage limit for the location (wet roof, accessible). (3) Verify continuity and resistance of all frame bonds and the bond to the inverter enclosure, and confirm conductor sizing by adiabatic calculation. (4) Confirm the inverter's RCMU type and trip level and whether a Type B RCD is required by the manufacturer for this earthing arrangement. (5) Test the RCD/RCMU operation and record results. (6) Confirm the array frames are not inadvertently bonded to a separate earthed metal system that would create a parallel path. (7) Issue a certificate recording the earthing arrangement and the basis of design. Without these, energising a transformerless inverter on PME is not safe.