The first site has low specific yields (that is kWh per kWp per year) and was taking time to get going on a winter morning, a classic sign of an insulation resistance / isolation / Riso fault. I think the low yields were down to a blown fuse in the inverter which I replaced, and marked the date so that anyone else visiting knows that it's had a problem before. The smashed module was located under a tree which is a recipe for trouble, I disconnected it and shortened that string by one module. Not ideal but not the end of the world. At some point when someone is in the area for another reason it'll get replaced. Not worth a visit just to replace a single module.
The second system had three inverters. One with classic Riso fault symptoms of being slow to get going in the morning when it's damp, unfortunately I got there late afternoon so it was generating when I was there but I found a smashed module (I'm guessing by a football) and disconnected it. The other inverter issue wasn't displaying the normal morning issues and was cutting out at various points during the day so didn't initially point to a Riso fault. But that's what it was.
There is another (more scientific!) way to guess where the fault might be in a string is instead of going with your gut feelings. You can measure the voltage from either side of the string to ground. If it's grounded in the middle you'll get something like +300 and -300V. If it's grounded 3/4 of the way along it might be 450/150V etc. Or you can just go "I bet it's there..." and hope for the best.
Both these systems are quite old hence the low voltages. We tend to be designing 2-300V higher now, depending on roof and string layouts.
Neither of these sites are managed by me, and whilst the person that does manage them is rather more efficient than me you can never guarantee that they'll remember to pass on site notes.
I would love to try a cable tracer but they're expensive and I can manage without. I don't want to kill my network cable tracer trying out whether it could work! But if anyone's got a spare one they don't mind me experimenting with...
