So I’ve had builders/electricians and all other sorts of installers on my roof, in my roof and around my roof for the last 9 months.
I’d like to know how and if I can run some tests on my installation to ensure there’s no ground faults on the PV strings, or that every string is working as it should.
Can anyone help me with some tests that I could do myself (so how to do it, and what to measure)? I have a multimeter… but I should be able to borrow more specialised equipment from my father-in-law if needed.
I can pull a fuse on each PV string independently, but my knowledge of electricity is dangerous and I don’t know what would happen on a live system if I simply pull on string’s fuse, for example, and whether the MPPT would get angry at me.
A simple way to test current on each string with a multimeter is to put the multimeter in current mode (Probably the 10A scale) and put the probes on either end of one fuse at a time then pull the fuse while holding the probes across the fuse holder. This way the multimeter conducts the current of the string and you can see what each string is doing. Once you have a current reading, close the fuse before removing the multimeter. Not sure if there is a simple way to test for PV ground faults though, other than check for voltage between PV and ground (i.e. PV+ to ground and PV- to ground) and if there is nothing significant then you could maybe check resistance to ground as well.
Simply check resistance to ground on all PV cables. The resistance should be very high…(and all the same)
Also your MPPT might have earth fault detection. Read the fine print…
I would check for voltage to ground first, because an earth fault at some point in the string would cause potentially high voltages between the PV terminals and ground which may damage the multimeter if you test it in resistance mode.
Edit: If you do measure voltages to ground, you could also then test with your meter in current mode. The current that can flow between PV and ground will tell you what the impedance of the fault is.
Seeing as he has access to more specialized equipment, rather get a clamp meter that can do DC voltage.
So I can pull the fuse while the unit is in operation, nothing will shout at me?
Maybe a follow up question then: Where would I find a reference to ground? Will I need to sommer use the MPII’s casing?
No it will be fine
I’v got dc breakers and to check if the string is working I checked the output on VRM while flipping the switch
You should be able to pull the PV fuse while in operation but it may arc a bit. My suggestion was to bridge out the fuse with the multimeter in current mode which means that when you open the fuse it will not break the circuit, instead the current will flow through the multimeter.
I’m sure there should be a ground available somewhere near your installation. The PV frames and rails are supposed to be grounded and your inverter should be grounded.
Yes there is such a pole, but it is a bit farther than my multimeter’s probes would go. I’ll have to rig something up with some of the spare 4mm I have laying around. Thanks!
Presumably my multimeter should be happy with conducting the current for about 10 second if it has a 10A for 10 seconds max rating?
Yes, I am assuming that your PV strings can produce less than 10A
Maybe the afternoon is safer. I don’t want the cloud edge to burn up my multimeter if it can be avoided.
Just be careful! This is DC you’re dealing with. The safer way is to use a CT meter. Unfortunately DC meters like this are not so common. If you do use your multimeter use clips on your probes rather than holding the probes.
DC arcs are no fun!
No no no no, NEVER pull the fuse under load! If you do, even as suggested that you use a multimeter to have alternative path, you will draw an arc (perhaps smaller). This causes your connectors to become “worn” which causes hot spots which (best case) will cause sporadic fuse failures, worst case is burning your house down. I would bet on the worst case scenario.
The correct procedure to see if their is a ground fault:
- Read the MPPT manual, if the system is NRS compliant or VDE standard or sold in Aus, it should have isolation measurements and the inverter or mppt will error or refuse to start up if it detects anything.
- Switch everything off, use a insulation resistance tester (Megger) and test the PV circuit
- Use a I-V curve tracer (preferably with pyranometer).
Point 1, I think is very common and if your inverter isn’t shouting at you, you are most likely safe.
Point 2, is the correct way of finding if there is a problem with your cable insulation to earth
Point 3, this will determine if your panels are still doing what they are supposed to do.
Thermal scanning of all connectors and panels.
Individual panel IV curve testing
At night, multi meter to ground (will most likely not give you much info, really you need the correct tools)
Jacques. If, as you say, you are not trained in this type of work (as this is now becoming a bit specialized) DO NOT DO IT. I cannot stress this enough. There is a point that you need to step away. Even as an electrical engineer, I would step away to let electricians handle certain things. No shame in knowing where your limits are…
Thanks, appreciate the input. I’m comfortable measuring some voltages with my multimeter. That is about where it stops. I have two Victron MPPTs, I know they have a fault finding code, but not sure if mine has the ability to measure it (Bluesolar 150/45).
What Rautenk helped me design, to check panel production, was to bring down 5 strings to the combiner box, yeah more cable, so what.
At the combiner box, I have 5 x NoArk breakers, also serving as fuses.
So now I can take one string, switch it on, check on the watts on the Venus … next string … this way I can see if a string has “issues”.
If I do this “test” midday, I can see the max I could expect per string - or at least see that all 5 strings are at the same value.
I have the same (6 strings in my case), so I can just check production that way (adding the strings one after the other).
For me the more important thing was to do a fault to ground test, just because I’m concerned that the wires could have been damaged with so many people working here.
Test them separately. Ensure your load is bigger than the potential production of a single string. Close one Fuse, test, open, close the next one, reopen after testing. Continue till all 6 strings has been tested. That should give you a pretty good indication of what each string is producing.
I use this test often with the help of the client. Should anyone question the production, i monitor the app while the client open and closes the fuses. On the 150 and 250 volt Mppt’s the arc is pretty small and if you open them fast enough you hardly if ever draw an arc, but on PV inverters with string voltages of 700VDC and above, I will personally not pull the fuse, let alone ask the client to do so.