An AFCI or Arc Fault Circuit Interrupter is a device used to detect arcing in an electrical circuit and to interrupt the flow of current. It is installed in many types of electrical circuits to reduce the chances of an electrical fire due to faulty wiring, bad wiring connections or damage sustained during wiring installation.
For PV systems, it is designed to detect series arcing in the DC cabling or junction boxes. In the event of an arc, the AFCI circuit will alert the main control CPU in the inverter and interrupt the power conversion process thereby interrupting the DC current flow and reducing the chances of DC wiring related fires. However, due to the nature of the arc signature coupled with the sensitivity of the arc detection circuit, there can be “false trips” where the arc detector is triggered when no arc is present. This is usually due to rapid changes in current caused by changes in the light from any number of phenomenon. However, as an industry, we cannot afford to become complacent and assume that sites with multiple “false” trips are always “false” and reset, or disable, the arc fault protection. The danger is real. Inadvertently, a faulty system could get overlooked if ALL trips are not taken seriously. This may be a hassle for the short term, but the industry will continue to fine tune this technology and the nuisance trips will subside.
The real issue is how do we tell the difference between a false trip and a real arc? The answer is quite simple; inspect the installation.
Before resetting an arc (even if it has been, or seems to be, a nuisance trip) the site MUST be inspected. There are many conditions where an arc may develop over time, and not all due to a “bad install”. Connections in junction boxes can degrade over time. Terminal connections can come loose due to changes in temperature and cause an arc. Improper crimping of DC terminals can cause an arc. Rodents chewing through wires or wiring that has come loose and rubbed against the array racking can also lead to serial arcing. All of these are common conditions and easily identified with a quick visual inspection.
First of all, arcing leaves some evidence. The may be discoloration of wiring and racking, melted connectors and insulation or even burned junction boxes on the back of panels. A quick visual inspection can locate such issues very reliably. A gentle tug on DC cables whether in the home run connections, panel to panel connections or in the combiner/wiring box can locate wires that have loosened or were not terminated properly. You can also find bad crimp connections using the same tugging method. Do not just concentrate on field-installed connectors, there have been many instances where the factory crimp connection on the PV panel was the cause of arcing issues. The same goes with the PV junction boxes. Damage can happen between the factory and the site or the diodes can fail both of which can cause serial arcing and trigger an arc fault. The visual method is very easy, does not require special tools and is effective at pinpointing the failure to a component.
The second method is somewhat easier to execute but does not pinpoint the issue as in the above method. In some cases, like residential rooftop, it is not easy or even possible to perform the above visual and manual inspection. In this case, it is recommended to use some sort of high voltage tester to test the cabling to locate a failure. A “Megger” or megohm meter is one example of a simple test set that can help locate arcs to a specific string. For more testing functionality, a string tester is preferred because it can detect not only serial arcs but also insulation issues and ground faults in the DC cabling. While it is considerably more expensive, it is far more flexible than the simple “megger” and can be useful in troubleshooting other array issues. Both of these devices usually test at 250V, 500V and 1000V. Neither test at 600V which is what residential and some small commercial systems are rated at for maximum VOC. If the PV and DC wiring is only rated at 600V, use the 500V setting to test. If the system uses panels and cable rated at 1000V, even if it is residential, it is suggested to test at 1000V. The reasoning is simple; a fault may only occur at voltages higher than 500V on cold bright days in a 600V system. The test may pass at 500V, but can fail at 1000V. Once the faulty string is isolated, then the first method can be used to locate the faulty component. You can also use the “megger” or string tester to test individual panels if a junction box is suspected but no visible damage is detected.
If, after all these tests, no defect is found, then it is safe to assume that a “false” or nuisance trip has occurred. In this case, reset the fault and observe the inverter to see if it immediately faults again. If so, the AFCI circuit may be faulty and the inverter needs replaced. If not, try to record the exact conditions when the arc fault occurs. It may not be a sensitivity issue, but a weather related issue. Check the local weather to see if there were any special conditions that may have contributed to the suspected “false” trip. Remember, a simple rain storm can also uncover issues on a system including arc faults that only occurs when the wiring is wet.
Finally, report any findings back to the inverter manufacturer. Your input is very valuable in the continued refinement of equipment. A manufacturer tests to many conditions to cause a failure, but nothing can completely replicate the conditions on your sites in the real world.