Taking a Look at Ground-fault Path Scenarios: Positive Ground Fault on a Transformer-Based Inverter

When it comes to solar PV systems, understanding ground fault scenarios is crucial for both safety and proper system operation. This video excerpt from our comprehensive “Solar PV Ground-fault Troubleshooting: Theory, Tools, and Field Application” course demonstrates why mastering the fundamentals of fault current paths can make the difference between a quick diagnosis and hours of frustrating troubleshooting in the field.

In this particular segment, instructor Brian Mehalic explores what happens when a positive ground fault occurs in a transformer-based inverter system – walking through how electrons behave when they suddenly have an alternative path back to their source, and why this creates potentially dangerous conditions that every solar technician needs to understand. 

If you want to elevate your ground fault troubleshooting skills, this course provides theoretical insights and practical approaches to help you work through these complex diagnostic challenges more effectively.

Transcript below. 

If you really want to be good at ground fault troubleshooting, you need to have a solid understanding of the path that fault current takes. We’re going to divide this up into a look at transformer-based inverters, transformerless inverters, and then take a little deeper dive into scenarios where we have two faults at the same time, which really it doesn’t matter what kind of inverter or ground fall protection you have in that kind of situation.

Let’s start off with a look at fault scenarios in transformer-based inverters. These are either going to be our larger central inverters or some of our older string inverters. We’ll start off taking a look at the normal current path. Now remember, these transformer-based inverters are typically going to have an overcurrent protection device-based ground fault detection and interruption system.

In this case, you can see we have an inverter with negative connected to ground through that ground fault fuse. What’s our normal current path? It’s going to be out of the positive end of the string into the inverter. It’s going to do the work in the inverter, and then it’s going to go ahead and return home.

You got to keep that in mind. Where do the electrons want to go? They want to take the path of least resistance back to the source. In this case, it would be from one side of the PV cell back to the other side of the PV cell in that same module. Now this particular example is what you might call a negatively-grounded system with that negative connected to ground through that ground fault fuse.

What happens when things go wrong? Here we’ve got that same system, and now we’re going to have a positive ground fault in the system. In this case, it’s on the positive home run, looks like it’s going to the module frame. It doesn’t really matter. It’s a fault from the positive conductor to a grounded metal surface.

So what’s going to happen in this situation? Well, the electrons are still going to start at the same point coming out of the positive end of the home run or the positive home run from that string of modules. Once it gets to that ground fault, though, it has an option. It’s not thinking about this again. It’s just taking the path of least resistance. Going through that inverter and doing work – that’s not it. 

Now we’ve got a different path to get back home. We can get on that grounded metal. We can follow to that grounded conductor. We get over to that connection from ground to negative through that fuse and then back home on that negative home run. 

Remember, the electrons want to return to their source. They don’t want to go to ground. It’s not like, oh, I want to get into the earth. No, the electrons want to return to the source and they will take the shortest, least resistance path to do that. If that means running on grounded metal to get there because there’s a circuit, it’s going to do it.

Now when we have enough current flow, that fuse will blow. When a ground fault current exceeds the fuse rating, it’s going to clear the fuse and stop the flow of current, but it doesn’t do anything about that ground fault. Obviously, that positive fault is still touching ground. We could say in this situation that before the fault – when we had negative connected to ground through that fuse – all of the metal, all of the grounded metal in our system was essentially negative polarity relative to the positive side of the strings.

But with the fault, we essentially flip flop. We removed the connection from negative to ground by clearing the fuse, and now we have a connection from positive to ground through the fault. If there’s a ground fault, normally grounded conductors meaning the negative, may be now ungrounded and energized, meaning they are going to have potential relative to all of the metal in the system, whereas before they did not.

Now that’s a big deal because later on when we get into troubleshooting these things, a lot of times what we’re going to have to do is lift that negative conductor in this scenario in order to be able to perform some of the testing we need to do.