Earthing, DC Protection and Bonding for a Coastal 10 kWp Rooftop System
A 10 kWp grid-tied rooftop system on a coastal building is earthed through one 3 m long, 50 mm diameter GI pipe electrode. The measured earth resistance is 18 Ω. Module frames and the mounting structure are bonded with 6 mm² green-yellow copper to the same electrode; the inverter AC earth is also connected there. The array is 25 m from the inverter, and the DC cables run in PVC conduit with no DC isolator at the array, no DC SPD and no insulation monitoring. On the AC side a 32 A MCB protects a 6 mm² copper cable to the main distribution board, 20 m away. The local soil resistivity is 120 Ω·m and thunderstorms are frequent. The owner has added a satellite dish and a steel water tank on the roof, both unearthed. The utility requires the installation to be inspected before energisation.
1. Evaluate the earth electrode resistance. Estimate how many additional rods would be needed to reach 5 Ω, and state the practical installation requirements.
Read the worked answer
For a single vertical rod, use the approximate formula:
R = ρ/(2πL) × [ln(4L/d) - 1]
With ρ = 120 Ω·m, L = 3 m, d = 0.05 m:
4L/d = 12/0.05 = 240; ln(240) = 5.48.
R = 120/(2π × 3) × (5.48 - 1) = 6.37 × 4.48 = 28.5 Ω.
The measured 18 Ω is better than the theoretical single-rod value, probably because of local moisture and soil contact, but it is still well above a typical 5 Ω target for this installation.
For n rods in parallel, use R_n ≈ R1/(n × η), where η accounts for overlap. With η ≈ 0.7-0.75 for closely spaced rods:
n = 28.5/(5 × 0.7) ≈ 8.1, so at least 9 rods are needed, or a buried ring/plate of equivalent length.
Rods must be spaced at least twice their length, about 6 m apart, otherwise the parallel resistance is much worse. Use copper-bonded or GI rods of adequate length, connect them with properly clamped conductors, and measure resistance after rain settling. A 25-30 m buried ring conductor may be more practical than many rods. Bond the electrode to the main earthing terminal and keep the connection accessible for testing.
2. Identify the DC-side protection deficiencies and the AC breaker/cable problem. Give the corrective actions with approximate current and voltage-drop checks.
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DC side deficiencies
- No array-side DC isolator: a lockable DC switch is needed near the array so the DC cable can be isolated for service and firefighting.
- No DC SPD: fit Type 2 DC surge protective devices at the array and at the inverter, coordinated with the earthing system.
- No insulation monitoring or DC residual-current protection: the inverter may provide insulation monitoring, but the installation must be checked and the function verified.
- String fuses: if the array has more than two parallel strings, each string needs a fuse or equivalent protection. With two strings, check the module series fuse rating and reverse-current risk.
- DC cables in PVC conduit should be double-insulated, UV-rated, and clearly labelled as DC; check polarity and connector torque.
AC side
A 10 kW single-phase inverter at 230 V draws about 10,000/230 = 43.5 A, so a 32 A MCB is undersized. Use a breaker above the inverter maximum output current, typically 50 A or 63 A, with a cable whose derated ampacity matches.
For 6 mm² Cu, 20 m one way, loop length 40 m:
R = 0.0175 × 40/6 = 0.117 Ω.
At 43.5 A, ΔV = 43.5 × 0.117 = 5.1 V, or 2.2% of 230 V. This may exceed a 1-2% AC drop target. Consider 10 mm², especially if the cable is grouped or in a hot route. Verify the main breaker, RCD type, and inverter maximum fault current.
3. Explain how the satellite dish and steel water tank should be treated, and why separate earths are unacceptable.
Read the worked answer
The satellite dish and steel water tank are metallic parts on the roof that can be touched and can carry lightning or fault current. They must be bonded to the same equipotential earthing system as the module frames, mounting structure, inverter, and AC earth. If they are isolated from the PV earthing, a lightning strike or an insulation fault can create a dangerous potential difference between the tank and the array frame.
Separate earth electrodes are unacceptable because they sit at different potentials during a fault or surge. Current will flow through interconnecting cables, signal cables, or a person touching two parts. A single common earth bar with short, direct bonding conductors is required. The water tank and dish should be bonded with a conductor sized for the expected fault and lightning current, with bolted or clamped connections to bare metal.
Because thunderstorms are frequent, carry out a lightning risk assessment. If the building has a lightning protection system, the PV array and bonding must be coordinated with it, maintaining separation distance or bonding according to the lightning protection design. If there is no air termination system, the risk of direct strike remains and SPDs plus good bonding are essential. The 18 Ω electrode is not adequate; improve it to the target resistance and re-test all bonding continuity.