A small rooftop solar system commonly uses three separate earthings - one for the DC side, one for the AC side and one for lightning protection - each a dedicated earth pit bonded into a common grid. Larger plants scale the count by the number of inverters, transformers and the earth-resistance target, not by a fixed rule.
The three-earthing rule (small systems)
- DC earth - array frames, combiner and inverter DC side.
- AC earth - inverter AC output, meter and distribution board.
- Lightning earth - for the air terminal or arrestor, kept separate then bonded.
Scaling up for a plant
On an MW plant the number of earth pits is set by: the number of inverter and transformer stations, the structure rows to be bonded, the soil resistivity, and the target earth resistance (usually 1 ohm or less). Rocky or sandy soil needs more pits, deeper rods or earth-enhancing compound to hit the target.
