Troubleshooting a String Inverter

Functioning inverters are absolutely key to a successfully operating residential solar system. In the event there’s an issue with the string inverter onsite, the system downtime means the homeowner can’t use any of their own solar energy during that period. Getting your customer’s string inverter operational again as quickly as possible minizines their losses and ensures they are benefiting from the system’s production as much as possible. 

In this excerpt from Residential Field Operations and Maintenance, HeatSpring instructor Keith Rohman walks students through his process troubleshooting an inverter that appears completely dead when he arrives on site. By following a systematic process, he discovers that the issue lies outside of the inverter, and instead a fuse is blown in the AC disconnect. 

Here’s a video excerpt of Keith’s troubleshooting or check out the transcript below. 

Want to see the whole process? Consider enrolling in Residential Field Operations & Maintenance to follow along with Keith as he finishes fixing this challenge and many more residential solar site issues.

 On this inverter troubleshoot, we have an issue on the AC side of the system. 

Let’s start from the beginning. The call comes in. The homeowner says, “hey, my system’s not working. I think the inverter is down.”  They’re correct.  

So when you first get there – visual inspection. Just walking up, what do you see? What do you notice now on this site? 

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You see here, this display screen, it’s off. There’s nothing on there. The DC disconnect switch – on. Toggle switch – on. This AC disconnect was also on. 

You should see lights on that display screen on the Solaredge inverters. What normally happens, even if there is an issue with that inverter, it’ll throw an error code and you’ll be able to see that error code on the display screen. Super smart. They’re basically like little computers. 

When you’re not able to get any display on the screen, it means that there’s generally not AC voltage flowing to that inverter at all. Right then and there. That’s my first impression. Why is the screen not on? Is there not voltage? 

The first thing that you would do here after turning everything off, open all the circuits, kill that, kill that, and pull this lever down. [Now] you can safely access the inverter.  

So you take the screws off and you want to check the AC voltage on the AC terminals in the inverter. You check L1 to L2, you should see 240 volts (in that vicinity). You should also see 120 [volts] from L1 to neutral and 120[volts] from L2 to neutral. I saw nothing. There was zero voltage. 

So that first impression I had ended up being correct. There’s voltages somewhere in this equipment, it’s not passing through.  

So when you go, okay, inverter = no voltage, what’s next? Next is your AC disconnect.  

Let’s talk about AC disconnects for a little bit.  You’re going to see one across the country on almost every single residential solar system. It’s the main disconnecting means. It’s a way for the utility to isolate from the solar system, to isolate the solar system from the utility. It’s a lever, a switchblade disconnect. This is a 60 amp GE fusible disconnect. They come in non-fusible types too, where there’s no fuse holders. They essentially just have lugs on either side.

And just talking terminology here, you have what’s called a line side and a load side. In this situation, our line side in this AC disconnect is the top. Our load side is the bottom. When you see a fusible disconnect in the solar space, that generally means that the interconnection method is a line side tap. 

What does a line side tap mean? A line side tap or supply side tap: what that is, is the point of interconnection is between the main breaker of that main service panel and the utility meter. There’s conductors that go from the load side of the main revenue meter – the customer’s meter that every single house has – to the main breaker in that panel. And you can interconnect on those conductors. There’s different methods to do that, whether it’s Polaris blocks or ILSCO taps. We’re not going to get into that right now. Understanding what it is, you’re basically tapping ahead of the main breaker. 

So when you see a fusible disconnect and those taps are installed properly, you will always have voltage on the line side of your AC disconnect. So from a safety perspective, be aware of that. Understand that you see fuses, even with this lever off – this top side, it’s still going to be hot.

Now, you do want to check that it is hot, right? That’s one of the first things that I did here, after checking in the inverter. And okay, let’s see if that voltage is present on my line side of my AC disconnect. 

So you take your voltmeter, you go L1 to L2, there should be around 240 [volts]. There was in this case. L1 to neutral and L2 to neutral, [should be] around 120. All checks out.  

Now, how do you see if there is voltage flowing through these fuses into the load side of your AC disconnect? Now you have to turn this on. Remember when this is off like this, you’re cutting off the voltage and the current from the load side. 

Understand that, I’m being safe. You have your proper PPE. You have insulated tools. You have your gloves on. You have everything that you need to do this safely.

You’re going to turn that lever on and you’re going to test the same way you did up here at the top on your load side. You should see – if the fuses are not blown – you should see the same readings that you saw up here, down here, L1 to L2. 

In this case, it was not present. My 240 [volts] was not there. When I went L1 to neutra, 120 [volts] was there. L2 to neutral,  no 120 [volts].  

What does that tell us? That says, the fuse on L1 is good. It’s okay. The fuse on L2 is not. It’s likely blown. 

The next step is going to be to actually verify that this fuse is blown.

To hear what happens next, enroll in Residential Field Operations & Maintenance.