Best Practices in PV System Ground Fault Testing

Ground faults are common faults in PV systems. They manifest as an electrical connection between ground and one or more current-carrying conductors. Ground faults have many potential causes, but most result from improper installation or damaged components. 

Ground-fault detection and interruption typically occur within the PV inverter, alerting the site owner to the fault’s presence. Locating the fault, however, is frequently challenging. This article will overview the field tests technicians can perform and tools for finding a ground fault in a PV array. 

Ground faults are common in PV systems and present an electrical safety hazard. Technicians should have the tools, training, and protective equipment required to perform ground-fault tests in the field

Safety considerations

When a ground-fault detector indicates a ground fault, unless extensive field testing shows otherwise, it is safe to assume that damage has occurred, repair is required, and there is an existing electrical hazard to the system. Technicians should proceed with utmost caution until they resolve the ground fault. 

While the ground fault is active, it can energize the bonded metallic components that comprise the PV system. The risk of severe shock from direct contact with conductive components increases for high-voltage DC systems, such as utility-scale PV arrays, which operate at 1,500 Vdc. Technicians must use the proper personal protective equipment (PPE) based on the hazards present. At a minimum, this will include high-voltage gloves, eye protection, and, potentially, arc-flash clothing. 

The presence of a ground fault not only creates a safety hazard for technicians but also for the equipment within the array. High-energy fault currents can spark fires and melt conductors, which can cascade throughout the rest of the system. When the fault current exceeds what the equipment is rated to handle during normal operation, even small fault currents may be sufficient to damage the equipment. 

Lights or a display screen on the PV inverter will alert technicians when a ground fault is detected

Locating the problem

To find the ground fault location, technicians must be comfortable performing various tests. In order to isolate the fault, best practice is generally to start at the inverter level, then segment the system to narrow the tests to the combiner and eventually the string level. Performing the following tests helps to identify fault locations:

1. Visual Inspection. Damaged components causing a ground fault may be evident through a visual inspection. Taking the time to walk the site and visually inspect the system may provide a technician with a relatively quick identification of the problem. 
    
2. Voltage Measurements. Once the string conductors are safely isolated, voltage measurements can be recorded and compared. Using a digital multimeter (DMM), technicians should measure voltage from positive to negative, positive to ground, and negative to ground. The readings will return different values, which the technician can use in conjunction with the open-circuit voltage of each module to locate the ground fault. This process is well documented and outlined in a field testing guide published by NREL.   

3. Insulation Resistance Tests. If voltage measurements are inconclusive, insulation resistance tests can help locate a faulted circuit. A damaged conductor—from a weather event, long-term environmental exposure, or a hungry rodent—is among the most common causes of ground faults.  

PV technician using a DMM to measure voltage in a combiner box

An insulation resistance tester or high-range ohmmeter applies a DC voltage across the conductor under test and injects a small current through it. The insulation resistance tester will then use Ohm’s Law to calculate the insulation resistance from the known voltage and current. 

Insulation resistance testers apply a known voltage and current to a conductor to measure its insulation resistance

A good conductor will have very high resistance. Conversely, any conductor that is damaged and in contact with a grounded object will return a low resistance value. Below is a table of test voltages and minimum acceptable resistance values based on the system voltage as described by IEC 62446. 

Minimum insulation resistance values as defined by IEC 62446 (source Mayfield Renewables)

Among the most frustrating faults technicians may experience are intermittent ground faults. This type of fault commonly occurs after rainfall, when the inverter may detect a ground fault. When the technician visits the site and performs the above tests, however, no conclusive fault is located, and when the inverter is reset, it operates without error. 

A “wet insulation resistance test” is one way to track down the fault. As the name implies, the equipment is wetted with a water and surfactant (typically a detergent) solution. This process is outlined in IEC 62446 as one method used when necessary. The test requires careful segmentation of the array and methodical testing processes. 

Ground fault testing is not as simple as running a test and checking off results. Tracking down ground faults can be a tedious task. It involves an understanding of the output of several tests, along with some sleuthing and troubleshooting. Technicians must understand the safety risks, and use DMMs and insulation resistance testers to locate faults accurately and safely. The process must be approached methodically to avoid mistakes and time-consuming re-work. Ground fault testing is a critical and worthwhile process to ensure the ongoing safety and reliability of PV systems.

Ryan Mayfield is the Founder of Mayfield Renewables, which brings together accomplished system designers, engineers, installers, and trainers to provide strategic support for new projects and products.

Mayfield Renewables | www.mayfield.energy