Diagnosing a Rooftop Solar Fault

Diagnosing a Rooftop Solar Fault 

One of our customers operating two sites near Newark contacted us with concerns about a rooftop solar system. They had already brought in other engineers due to repeated faults on one of their inverters, but without success. That’s when they called NerG. We worked through a telephone consultation before heading over to the site. 

The Situation 

The customer had experienced intermittent issues with one inverter, which had been previously replaced. However, they were still encountering problems; strings appeared to be disconnected or non-functional, and the system was no longer producing results as expected. At this point, they asked us to take a look. 

When we arrived on-site, we were expecting multiple string faults across the roof. But once we disconnected and tested each string independently, we found all four roof-mounted strings were functional. The confusion stemmed from how the original installation had been wired. 

What We Found 

The inverter had six sets of DC inputs, but only four actual strings were connected. Instead of fitting blanking plugs into the two spare connectors, the original installers had used MC4 plugs with short cable runs into a junction box. This created the appearance of six active strings and caused ongoing confusion. 

Over time, previous engineers had disconnected working strings, mistaking them for faults, and left inactive connections in place. By working through the connections methodically, we confirmed: 

• All four active strings on the roof were in working order. 

• The inverter itself had a startup fault caused by a failed main relay. 

The Outcome 

With the source of the issue now clear, we advised the customer that the inverter was no longer viable and would need replacing. A new unit has been ordered and is being scheduled for installation in line with the customer’s production schedule. 

From the initial phone call to our diagnostic visit, everything was turned around within a day thanks to the site’s proximity and accessible layout. Once we fit the new inverter, we expect the rooftop system to return to full production.