PV Array RISO issues. Can they be dangerous?

Question

Good evening. 
 We have found a series of issues whilst performing solar maintenance and testing on a metal framed and cladded building (including the roof). 
 The System: 
 installed in 2015 
 442 Panels installed over 22 Strings. Panels installed JA Solar - JAM6-R-60/275 JA Modules 
 The system is 3 phase and comprises of four 20kW three phase inverters (AC coupled) 
 No Batteries on site. No earth spikes found. The PV array frames and mounting are directly attached to the metal structure 
 The DC string voltage ranges from 700-800V DC on the healthy strings. No string surge protection. No string over current protection. Just panels directly connected to inverters via 4mm solar PVC cables 
 System Condition: 
 Many panels are damaged/broken. Some with holes tennis ball size in. Heavy seagull damage. Lots of fauna growing between panels. 
 Site weather conditions: 
 Foggy and Raining 
 The Fault: 
 Of the 22 Strings 17 have faults to earth. Insulation Resistance tests (1000V DC test voltage) were attempted from both Positive and Negative MC4 connectors (taken out of the inverters) to earth (the structure). The IR test was not possible as we were getting 600-800V from both positive to earth and negative to earth. We do find voltage during these test normally, but it generally dissipates down to a level were we can test within 2-10 seconds. This is commonly known as a RISO issue, i'm aware. On the healthy strings we were getting 0.1 - 3 Mohms. 
 I'm aware we can test which panel is likely causing an issue by taking the volts present between P-E and dividing this by the individual panel Voc. 
 My question is: 
 Even with the inverter DC isolators in the open position (Off), on all 22 strings, is there a risk to life or a chance of fire with the system in its current condition. 
 Does the building structure pose a risk of electrocution? (nearest building 35m). Trucks and vehicles are operating within the building 12 hours a day. 
 Fault example: If we had a P-E fault on panel 1 of 20, and a negative to earth fault on panel 20 of 20 you could get a short circuit. 
 
 Action taken: 
 We have issued a danger notice whilst we investigate this further. This was suggested by our accrediting body as well. They are currently looking at this with us to help find a solution. 
 
 Any thoughts or solutions welcome.

mapj1 · Accepted Answer

RISO (Resistance Isolation warning - 'translated from the foreign') is what the better inverters report when there is a connection to earth in the middle of the string of photodiodes on a panel ) In a panel with puncture damage, this is quite believable. 
 Panels are inherently current limited - you will never get more electrons per second than photons that land on the thing - in practice quite a lot less. But faulty panels are a bad thing, and the partial earthing means that things that should float wrt earth will not, and may well be capable of if not electrocuting someone, at least throwing them to the extent they drop off the roof, which is just as serious. And the inverter will not be able to make use of their output. 
 Lay off the meggar - it will not take sensible readings and may damage good panels which are essentially strings of low voltage diodes each one generating half a volt or so open circuit. A more useful test is to look for a voltage between wires and frame, with some sensible load - in the old days a test lamp, maybe now a resistor. Anything voltage line to frame that does not fall away to be just a few volts once a load has been connected in parallel with the meter is a sure sign of a mid-string fault. 
 The only sensible thing to do is to all-pole isolate or unplug the obviously damaged panels, and if you have to remove them in daylight, then you should reduce the output current available by covering the active surface, and treat the wire tails and any exposed damaged panel regions where the seagulls have landed (*), as if they might be live relative to the panel frame. 
 Tricky one, I do not envy you. 
 
 Mike 
 
 * Seagull has landed sounds like a line from a Mike Myers movie - a low grade version of " The eagle has landed"

Jam · Answer

Charles Yabsley said: generally dissipates down to a level were we can test within 2-10 seconds. This is commonly known as a RISO issue, i'm aware 
 You will not easily get useable results from a typical Megger. I think what you're seeing here is the capacitance of the array, amongst other things, and if normal and expected behaviour. This specific behaviour is not a fault. 
 As Mike says Riso is a symbol for insulation resistance, and an "RISO error" is generally the inverter reporting a low value. The inverters measure insulation resistance taking this and the operating voltage of the array into account. An Riso alarm does need to be addressed and should not be ignored. Bear in mind though that the inverter's measurement can be spoofed and also that an appropriate threshold varies with the physical size of the installation and environmental/installation conditions. 
 Diagnostic IR tests should be done by someone equipped (and skilled) with a suitable PV test device. Not least these testers are designed to operate with the array voltage on. Personally I find the HT Instruments PV-ISOTEST quite well equipped; it can carry out PI and DAR tests as well as simple IR and in another configuration can help locate faults within a string; I've not used a Seaward device for a while to be fair so can't comment on their more recent offerings. 
 
 On the other hand, it does sound like there are some major faults here that don't need an IR test. 
 mapj1 said: Panels are inherently current limited - 
 This they are. On the other hand it is worth noting that under short circuit conditions, tens of amps arising from a P-N short do still pose a considerable fire risk, particularly if this is via faulty connections etc. 
 Charles Yabsley said: Even with the inverter DC isolators in the open position (Off), on all 22 strings, is there a risk to life or a chance of fire with the system in its current condition. 
 Yes. 
 Given the condition you describe, there may also be current flowing in conductors even if the isolators at the inverter are off, due to parallel paths. These paths are likely high resistance and hence may well get hot. The presence, or absence, of these should be identified urgently. 
 Also, by the sound of it, there's a risk that someone has cheaped out / been done over by cowboys, so it's probably worth verifying that the isolators/switch disconnectors are appropriate for DC on load switching. 
 I agree breaking down the installation and isolating damaged sections is advisable. However, as well as covering up the array do take precautions (which might getting a DC clamp meter on conductors prior to working and using insulated equipment and gloves, tools and arc-rated PPE) when making any disconnections as you might be trying to break substantial DC current. 
 If it's really bad, with many interpole faults that can't be easily found and isolated, I might consider making a deliberate, temporary short circuits that can be broken safely (e.g. linking P-N downstream of a suitable switch disconnector) so that that there's negligible volts across the array and current through (hopefully relevatively high impedance) fault paths is minimal. This would be an emergency procedure to prevent fire while the installation is being broken down, not a long term answer, and could make things worse if there are series faults / resistances (e.g. dodgy connectors) so only to use after your own assessment - I cannot see the installation. That said if the faults are limited and the array (or section thereof) can be successfully open-circuited, that will usually be safer. 
 Charles Yabsley said: Does the building structure pose a risk of electrocution? (nearest building 35m). Trucks and vehicles are operating within the building 12 hours a day. 
 Sounds like a ground mounted array? Without seeing the installation, if the inverters are disconnected (so there's no AC ripple present) I'd suggest this is unlikely for general users of the building unless they're able to get between two parts of the array somehow. As you approach the array the risk assessment will need to be considered carefully. 
 Don't forget that Riso testing only identfies faults where there is a return path. Cables with faulty insulation dangling in free air may not be detected. 
 mapj1 said: Tricky one, I do not envy you. 
 Nor do I. Good luck!

mapj1 · Answer

Do you mean Fauna (animals)- or Flora (plants) ? 
 Neither is good, but if things are nesting under the panels that may bite, peck or sting when disturbed, and/or may have chewed the insulation off the string wires then that is another startle hazard waiting for anyone unwary going near. 
 Losing footing on wet green stuff is a different hazard, but again, at any sort of height, even the apparently comedy risks become far more serious. 
 And you will need a large amount of tarpaulin, or arrange for the sun to be turned down or off .i.e. evening working Once sun is no longer striking panels directly, the output will be single figures % of the 'full sun' value.- there is a secondary reason that solar panel thefts occur at night... 
 Once separated the individual panels are not too bad but still avoid touching exposed copper or any damaged regions - the open circuit voltage will still be present down to very low light levels, even if there is no longer any great current behind it. 
 
 Mike.

PV Array RISO issues. Can they be dangerous?

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