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Suggestions on how to troubleshoot a ground fault

Issue:
Customers contact us requesting support for troubleshooting ground faults on our grid-tie inverters and solar charge controllers

Product Line:
Solar charge controllers / 1PH grid-tie solar inverters

Environment:
System with ground fault

Resolution:
Troubleshooting should proceed as follows:

Visually inspect the PV system.
Check GFDI fuse continuity.
Measure current to ground.
Measure voltages.
Segment the array wiring.
Interpret PV voltage to ground measurements.

Visually inspect the PV system. A visual inspection, starting at the inverter and moving to the array, is a good way to start.
Are there signs of a fire, or melted or scorched components? Are any of the modules visibly damaged, front or back?

Check GFDI fuse continuity. Grid-tied inverters for grounded PV arrays bond one of the DC conductors to ground. If a GFDI fuse is blown, a ground fault significant enough to allow 1 A or more of fault current is or was present. Most digital multimeters indicate continuity with an audible tone when they are set to measure resistance. In this mode, when the multimeter leads are placed at opposite ends of the fuse, no tone indicates no continuity; resistance in the fuse is effectively infinite.

Measure current to ground. Use a digital multimeter with a DC current clamp to take this measurement in the inverter or at a service disconnect. 

If the GFDI fuse is blown, you should not expect fault current to continue flowing, except in worst case scenarios. Fault current from a single grounded to grounding or ungrounded to grounding fault will cease when the fuse blows. If there is still current present after the GFDI fuse blows, either the fault is from grounded to ungrounded directly or there is a double fault, one from positive to ground and one from negative to ground.

Measure voltages: open circuit, PV positive to ground and PV negative to ground. The next steps involve measuring the DC voltage of the PV array. These measurements can be taken at multiple locations: inverter, DC disconnect or combiner box. First, record the array Voc, which is measured between PV positive and PV negative. Next, record the voltages between PV positive to ground and PV negative to ground. If the system consists of a single series string, this is easily accomplished. If the system includes multiple series strings in parallel, additional steps are required. To identify a faulted string and locate the ground fault, you will need to isolate individual strings.

Segment the array wiring. Most residential grid direct PV systems consist of one to five series strings of PV modules. These series strings may be wired in parallel at the inverter or at an external combiner box. It is important to know whether series strings are paralleled and, if so, where. If strings are paralleled prior to or at the inverter, any DC voltage measurements taken at the inverter records the combined output of the paralleled strings. You will need to isolate each string to figure out which one has the ground fault.

To isolate a single string or a combiner box, disconnect both the grounded and ungrounded current-carrying PV circuit conductors. Once the string or individual combiner box is electrically isolated from others, you can use the fuses in the ungrounded current-carrying conductors to isolate the string. Opening the fuses in the combiner box isolates each series string electrically, provided that you take the voltage measurements on the line side of the fuse. If the ground fault is on a source-circuit grounded conductor, you need to disconnect these conductors from the common grounded bus in the combiner. By following these procedures, it is possible to measure voltage for an individual, electrically isolated PV source circuit.

Keep in mind that a ground fault may occur anywhere in the source circuit and array wiring. If a ground fault is present in the PV output circuit—for example, between the PV source circuits and the inverter—a DC voltage measurement taken in a combiner box that is electrically isolated from the inverter will miss the fault. The ground fault is present electrically “after” the combiner box.

Interpret PV positive voltage to ground and PV negative voltage to ground measurements. What DC voltages should you see? If the GFDI fuse is intact and in the circuit, the grounded conductor is still bonded to the EGC and the grounding electrode through this fuse. This means that the grounded conductor and the EGC should be at the same potential. Voltage measurements taken under these circumstances should be 0 VDC.

If the GFDI fuse is intact, it is possible that a ground fault is not present. It is also possible, however, that a ground fault is present, but with ground currents above the detector rating and below the trip rating of the GFDI fuse. Either way, you must isolate the grounded conductor from ground to make additional voltage measurements. So the next step is to remove the GFDI fuse or open the GFDI breaker. Now take comparative voltage readings between PV positive and ground and PV negative and ground.

In an array without a ground fault and the GFDI fuse removed, the PV positive-to-ground measurement and the PV negative-to-ground measurement both should be roughly equal to one-half of the array Voc. The voltage measurements should slowly decay towards zero as the energy stored in the capacitive coupling between the array and ground is discharged. With the GFDI fuse removed or GFDI breaker open, the grounded current-carrying conductor is no longer connected to ground. It is ungrounded and "floating."

If the array Voc is 480 VDC, for example, the PV positive to ground measurement should be within 5% of 240 Vdc, and the PV negative-to-ground measurement should be within 5% of -240 Vdc. The difference between the two readings is 480 Vdc, which equals the potential between PV positive and PV negative. Comparable measurements in the field may indicate that a ground fault is not present, and you can restart the inverter.

Cause: 
The National Electrical Code and UL require that grid-tied photovoltaic inverters shut down in the event of a ground fault; shutting down the inverter does not normally clear the fault. 
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