Assess Roof Load and Drainage
Before you even open the GFDI box, verify the structural capacity of the flat roof. Most single‑ply membranes can support 15 kg/m², while concrete decks typically allow 25 kg/m². If the solar array will exceed this, add steel support rails or a ballasted sub‑frame. Drainage is equally important – a minimum 2 % slope prevents water pooling, and any low‑points should be equipped with crickets or a tapered insulation board.
Select the Appropriate Ground‑Fault Device
Ground‑fault protection for PV systems must comply with IEC 62423 and NEC 690.5. The device is usually called a GFDI (Ground‑Fault Detection and Interruption) or an RCD (Residual‑Current Device) rated for DC. Choose the device based on your system voltage and maximum input current.
| System Voltage (V) | Max PV Input Current (A) | GFDI Type | Trip Current (mA) | Enclosure IP Rating |
|---|---|---|---|---|
| ≤400 | ≤15 | Type A (30 mA) | 30 | IP65 |
| 401–600 | ≤25 | Type A (30 mA) or Type B (300 mA) | 30/300 | IP65 or IP66 |
| 601–1000 | ≤30 | Type B (300 mA) | 300 | IP66 |
For a typical 400 V residential array with 10 A input, a 30 mA Type A GFDI meets the requirement. Commercial systems above 600 V need a 300 mA Type B device that also provides protection against AC leakage currents.
Mount the GFDI Enclosure
The enclosure must be weather‑tight and positioned within 1 m of the PV string entry point to keep wiring runs short. Use a stainless‑steel bracket that can be fastened to the roof deck or the mounting rail. When attaching, follow the torque values in the table below to avoid over‑compression of the sealing gasket.
| Bolt Size | Recommended Torque (Nm) | Seal Material |
|---|---|---|
| M6 | 8–10 | EPDM |
| M8 | 12–14 | Silicone |
| M10 | 18–22 | Neoprene |
For flat‑roof installations that require a low‑profile, tilt‑adjustable rail system, refer to the balkonkraftwerk halterung flachdach solution which provides modular clamps and stainless‑steel straps that maintain IP66 integrity.
Wire the PV Array to the GFDI
- Run the positive (PV+) and negative (PV‑) conductors from each string through a suitable UV‑resistant conduit to the GFDI input terminals.
- Strip 10 mm of insulation, insert the wire, and torque the terminal to 1.5 Nm for a 10 mm² copper conductor (typical for 10–15 A strings). For larger 16 mm² conductors, torque to 2.5 Nm.
- Connect the GFDI’s output terminals to the DC bus bars of the inverter, using the same torque specs.
- Route the equipment grounding conductor (EGC) from the GFDI’s ground terminal to the main building ground bus. The EGC cross‑section must be ≥ 6 mm² for residential and ≥ 10 mm² for commercial.
All connections should be sealed with heat‑shrink tubing and covered with a weather‑proof silicone gland to maintain the enclosure’s IP rating.
Bond Equipment Grounding and Verify Earth Path
Per NEC 250.4, the equipment grounding conductor must be bonded to the building’s grounding electrode system. Use a compression lug with a ¾‑in. bolt and torque to 35 Nm. Install a ground‑rod test point at the main ground bus so you can later perform a ground‑resistance measurement (target ≤ 25 Ω for residential).
Perform Pre‑Energization Tests
- Insulation Resistance Test: Using a 1 kV megohmmeter, measure the resistance between PV+ and PV‑ to ground. A value > 1 MΩ indicates acceptable insulation.
- Polarity Check: Confirm that PV+ is connected to the GFDI’s line side and PV‑ to the neutral side.
- Functional Fault Injection: Use a low‑level fault tester (30 mA for Type A, 300 mA for Type B) to simulate a ground leak. The GFDI must trip within 30 ms (Type A) or 300 ms (Type B) as mandated by IEC 62423.
- Earth Resistance Measurement: Place the ground rod probe 20 m away from the electrode and record the reading. If above 25 Ω, drive an additional ground rod or apply a ground enhancement compound.
“A GFDI that does not trip within the prescribed time window can expose maintenance personnel to lethal DC arcs, especially on high‑voltage strings above 600 V.” — IEC 62423, clause 5.2
Commission and Document
Once all tests pass, label the GFDI with its rating (30 mA or 300 mA) and the date of installation. Keep a record of the torque settings, test results, and the as‑built wiring diagram. For warranty purposes, many manufacturers require a copy of the IEC 62423 compliance certificate and a signed installation checklist.