Isolated Ground Rod at a Detached-Garage Array
A 9.6 kW array is mounted on the roof of a detached garage about 90 ft from the house. The garage is fed from the house by a 60 A feeder with a 6 AWG equipment grounding conductor. The PV inverter is on the garage's exterior wall, and the PV feeder to the house main panel is 8 AWG copper with a 10 AWG equipment grounding conductor protected by a 40 A breaker. The installer bonded the rails with listed bonding washers at every other module and ran a 10 AWG equipment grounding conductor from the array to the inverter. At the inspection you find: the equipment grounding conductor lug at the end of the rail is fastened with a self-tapping screw driven through powder coat; and a new 8 ft ground rod at the garage corner is connected to the rail with 6 AWG copper, but no conductor connects that rod to the garage feeder equipment grounding conductor or to the house grounding electrode system. The installer says, "The rod gives us a low-impedance path - we measured 22 ohms."
1. Is the isolated ground rod acceptable as a grounding means for the array? Explain what a fault current actually does.
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No. The rod is an electrode, not a fault-current path.
What happens during a ground fault:
- Fault current leaves the array frame, enters the earth through the rod, and must return to the source - the utility transformer neutral - through the earth and back up the service grounding electrode.
- The impedance of that earth loop is dominated by the two electrode-to-earth resistances, not by the 22 ohms measured at the single rod, and it is far too high to open a 40 A breaker.
- The result is a persistent touch voltage on the array frames and rails instead of a cleared fault.
The effective path is metallic: module frames and rails to the equipment grounding conductor, along the PV feeder to the house, and to the service grounded conductor and grounding electrode system. That is why bonding is required at the separate structure and why a grounding electrode at the PV array must be connected to the building's grounding electrode system rather than left isolated.
Correction: install a bonding conductor from the array electrode and rack to the grounding electrode system at the garage, sized per the grounding electrode conductor table (a connection to a rod electrode need not exceed 6 AWG copper), so the rod becomes part of one system instead of a separate one.
22 ohms at a rod is not evidence of anything useful.
2. Evaluate the equipment grounding conductor termination and the every-other-module bonding. What tests support your conclusion?
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Termination: a self-tapping screw through powder coat is not a listed bonding means. The coating is an insulator at the contact point, the screw is not evaluated for the fault current, and the connection will loosen and corrode. The code requires equipment to be bonded with a listed means and the equipment grounding conductor connected by listed fittings to bare metal or to surfaces the fitting is listed to penetrate. Correction: remove the coating at the contact area and use a listed lay-in or compression lug, or a listed bonding device rated for the rail.
Every-other-module bonding: acceptable only if the rail is a listed bonding path and the washers are listed for that module and rail combination. Confirm from the manufacturer's instructions rather than assuming that a star washer equals a bond. The unwashed modules depend entirely on the clamped connection through the rail.
Tests:
- Low-resistance continuity from the farthest module frame on each rail, and from each rail, to the equipment grounding conductor at the inverter and to the ground bus at the service. A calibrated meter, clean probe points, and a recorded value per point. A commonly accepted pass is on the order of 0.5 ohm or less; anything higher means a bad joint.
- Visual inspection of every clamp and every frame-to-rail interface, since one unbonded module is the failure that matters.
- Confirm the 10 AWG equipment grounding conductor is correct for the 40 A overcurrent device protecting the PV circuit.
Record each reading with its location so a re-inspection can be compared.
3. How do you write this up, and what is in scope for the re-inspection?
Read the worked answer
Report it as a correction-required nonconformance with two distinct items, because the fixes and the responsible trades differ.
Item 1 - isolated electrode: state that a rod and 6 AWG conductor were installed with no bond to the building grounding electrode system, cite the applicable grounding and bonding requirements, and require a bonding conductor to the garage grounding electrode system. Photograph the rod, the conductor, and the absence of any connection to the feeder equipment grounding conductor.
Item 2 - unlisted bonding termination: state the location, that a self-tapping screw through powder coat was used, and require a listed lug on prepared bare metal. Photograph the connection with enough context to find it again.
Also note in the report:
- The measured continuity values and the meter used.
- That the installer's 22 ohm earth reading does not demonstrate a fault path.
- Any related deficiency at the separate structure, such as the grounding electrode requirement for the garage feeder itself, referred to the electrical contractor.
Re-inspection scope: visual verification of both corrections, a full continuity sweep from the farthest module on each rail to the service ground bus, and a check that no other module was left unbonded during the repair.