This article contains background information to supplement these articles:
Applicable Inverter Error Codes: 201 and 203
DC ground faults are the most common type of fault in PV systems, and half go undetected. A DC ground fault is the inadvertent condition of current flowing through the equipment grounding conductor in the circuits carrying DC power (commonly referred to as source circuits). Ground faults can lead to safety issues, such as arc faults and arc flashes. Ground faults also create a fire hazard when short-circuited current heats bare metal. Therefore, it is essential to exercise caution when troubleshooting a PV array ground fault since you will become the path to ground for any stray current flowing in the metal PV module frames, PV racking, conduit, junction boxes, and other metallic equipment.
To better understand a DC ground fault, let’s review some National Electric Code terminology and look inside a PV system.
- Equipment Grounding Conductor (EGC) = The conductive path(s) that provides a ground-fault current path and connects normally non-current-carrying metal parts of equipment together and to the system grounded conductor or to the grounding electrode conductor, or both.
- Grounding Electrode Conductor (GEC) = "A conductor used to connect the system grounded conductor or the equipment to a grounding electrode or to a point on the grounding electrode system.”
- Grounded Conductor = “A system or circuit conductor that is intentionally grounded.”
The EGC is used to bond all conductive metallic parts (modules, racking) and provide a path for the fault current to flow to the GEC. The GEC connects the entire system to the grounding electrode. The grounding electrode is a large metal rod driven into the earth at least 8 feet in depth. It is usually copper, aluminum, or copper-clad aluminum. Notice in the image that there are two ground rods. Sometimes this is required by Code, and there are spacing requirements, usually 6 feet apart, for dual GEC’s.
In a DC ground fault, current flows through the EGC or any piece of grounded metal that comes into unintended contact with the grounded conductor. Examples of unintentional contact are damaged conductor insulation, improper installation, pinched wires, and water, creating an electrical connection between the conductor and EGC.
Why are DC ground faults hazardous?
DC ground faults are hazardous in large PV systems because they can go easily undetected. Ground fault protection (GFP) devices do not sense the small (< 1 amp) current leaking in a ground fault, hence why it is called a “blind spot.”
In the event of a second fault with larger current flow in which the GFP would trip the circuit, the initial DC ground fault becomes a parallel path for massive current flow. This is precisely what happened in the 2009 Bakersfield, California fire in a 383 kW PV array that led to a major fire. An initial 2.5-amp ground fault on a 12 AWG conductor became the path for a second 311-amp ground fault. To make matters worse, an unnoticed conduit expansion joint had separated on a large 500 MCM (7.7 AWG) output cable. While the GFP cleared the second ground fault, the high currents returned through the first undetected ground fault, quickly melting the insulation on the conductor and starting a fire. This famous PV fire forever changed NEC 690 and PV inverter manufacturing. As a result, article 690.11 was added to the Code requiring all PV inverters to monitor AFCI faults.
How are DC ground faults detected, diagnosed, and mitigated?
As mentioned, detecting a DC ground fault is difficult, particularly in large PV systems, because DC ground fault currents are often less than the minimum sensitivity of the GFP device. Techniques for detecting DC ground faults include insulation resistance monitoring and residual current detectors (RCDs). Most transformerless PV inverters perform an isolation grounding test (R-Iso) every morning to measure the resistance of the current carrying conductors to ground. The test is performed while the array is in open circuit condition, meaning it has not started producing power. The test reveals two possibilities.
- The insulation resistance is above the minimum threshold (typically 600 MΩ), and the system can start.
- The insulation resistance is below the minimum threshold, which indicates damaged insulation and the potential for a ground fault. By Code, the inverter may not automatically restart. A qualified technician must go to the site, find and correct the ground fault and restart the inverter.
Even when the ground fault detection interrupter (GFDI) in the inverter successfully trips the circuit, it can be challenging to locate the source of a ground fault. First, technicians should check for blown string fuses using a continuity test. A continuity test is performed by placing the leads of a multimeter on the metal ends of a fuse and turning the dial to resistance. If the resistance is high (>5 Ohms), or the multimeter does not beep, the fuse is blown and must be replaced.
Next, technicians should perform an insulation resistance test on the conductors using an insulation tester. The test equipment applies a voltage on the conductors, generating a current on the wire that is measured (and compared against a baseline for insulation in good condition) to determine the state of the insulation resistance.
In practice, identifying the source of a ground fault can be challenging since a ground fault can occur between the grounded conductor and the EGC or a metallic component at any point in the circuit. To determine the source of a ground fault:
- Ensure the inverter is isolated from the array by removing the positive and negative conductors;
- Close the DC disconnect to put a live voltage on the conductors;
- Measure the voltage between the positive and negative conductors to determine the open circuit voltage of the array; and
- Measure positive to ground and negative to ground.
A healthy array reading should be 0 volts to ground from either conductor. If voltage to ground exists from either conductor, check each connection point (DC disconnect, combiner box) back to the array. Once the fault is discovered, replace the wire(s) and record tests and replacements.
The voltage readings provide a general clue to the location of the ground fault. First, let’s look at this example array of 5 modules, each with an open circuit voltage (Voc) of 100 volts. The total array Voc is 500V. In a healthy array, the home run to ground measurement should read 0V. However, the positive-to-ground measurement is 200V, and the negative-to-ground reading is 300V. Our meter reveals a ground fault between the second and third modules from the positive home run or between the third and fourth modules from the negative home run. If the home run to ground reading is 500V, the ground fault is on the home run itself.