Ground faults are among the most dangerous electrical failures in photovoltaic systems. A PV array energised by sunlight behaves as a current source — unlike a conventional electrical circuit, opening a breaker does not de-energise the DC side. When a ground fault occurs, fault current can flow continuously, igniting fires that have destroyed rooftops, buildings and utility-scale arrays. Correct design and testing of ground fault detection interruption (GFDI) and arc fault circuit interruption (AFCI) equipment is not optional.
What is a PV ground fault?
A ground fault in a PV system occurs when a current-carrying conductor (typically a DC string conductor) comes into contact with the grounded metal enclosure, conduit or structure — creating an unintended current path to earth. Ground faults occur due to:
- Damaged cable insulation (UV degradation, rodent damage, mechanical abrasion in conduit)
- Water ingress into junction boxes or conduit runs
- Loose or improperly made terminations that allow conductor-to-enclosure contact
- Physical damage to modules — cracked backsheets exposing conductors
A ground fault on the negative conductor of a grounded-negative PV system (the traditional design) creates a second ground — known as a "bolted fault." The dangerous characteristic of PV ground faults is that fault current magnitude depends on the number of strings in parallel: a large array can sustain 100+ amperes of fault current indefinitely as long as sunlight is present.
NEC 690.5: Ground fault protection requirements
Article 690.5 of the National Electrical Code (US) requires ground fault protection for PV systems installed on or near buildings. The requirements differ by system type:
| System type | NEC 690.5 requirement |
|---|---|
| Grounded PV systems on buildings | GFDI device required; must detect fault current ≥1 A |
| Ungrounded (floating) PV systems | Ground fault detection AND isolation required (both poles) |
| Ground-mount PV (not on building) | GFDI required if conductors are accessible to unqualified persons |
| Building-integrated PV (BIPV) | GFDI required; additional fire isolation requirements apply |
The GFDI device monitors for current imbalance between the current-carrying conductors and the equipment grounding conductor. When a ground fault is detected, the device must: (1) indicate the fault condition; and (2) disable the array (open the faulted circuit or reduce voltage below the threshold that would sustain arcing).
The "nuisance tripping" and fire risk paradox
Early PV systems (pre-2010) used GFDI fuses rated at 1 A. A 2012 Sandia National Laboratories analysis found a disturbing failure mode: under certain ground fault conditions in large arrays, the GFDI fuse could blow without interrupting the fault current — and the blown fuse signal was interpreted as "no fault detected," leaving the system operating with an active fire-risk fault. This led to the 2014 NEC revisions requiring that GFDI devices not only detect but positively interrupt the fault.
Modern string inverters and central inverters integrate GFDI detection within the inverter; the inverter shuts down on fault detection. This is more reliable than external GFDI fuses for residential and small commercial systems. For large utility-scale systems with multiple combiner boxes, additional ground fault monitoring at the combiner level provides faster fault location.
NEC 690.11: Arc fault circuit interrupter requirements
Arc faults — unintended arcing between conductors or between a conductor and ground — generate temperatures exceeding 5,000°C and are the leading cause of PV system fires. NEC 690.11 requires AFCI protection for PV systems with DC source or output circuit conductors operating at 80 V or more and inside or on buildings:
- The AFCI device must detect arcing as low as 500 mA
- Must interrupt within 30 cycles of arc initiation
- Must be listed to UL 1699B (PV AFCI standard)
- Many modern string inverters (Sungrow SG series, SMA Sunny Boy) integrate AFCI as standard; standalone AFCI devices are also available for retrofits
Testing ground fault protection
Ground fault protection must be tested at commissioning and periodically during O&M:
- Commissioning test: with system at low irradiance (or using a shading device), introduce a known low-resistance fault using a test resistor between a string conductor and the equipment grounding conductor; verify GFDI device trips and indicates fault
- Annual functional test: trigger the GFDI test function (built into modern inverters) and verify alarm and shutdown
- Insulation resistance (IR) test: annual Megger test (500 V DC, minimum 1 MΩ between conductors and ground) to detect insulation degradation before it causes a ground fault
- Thermal imaging: annual IR scan of combiner boxes, inverters and junction boxes — hot spots indicate high-resistance connections that can progress to arcs