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 Post subject: Plug and Play Solar
PostPosted: Tue Jun 28, 2016 7:55 am 

Joined: Thu May 26, 2016 6:55 am
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I had someone contact me asking about PPE for installing solar panel arrays. Specifically as he was adding each array to the circuit they come with manufactured plugs, his question is that since each array is already generating the moment it's mounted, he is plugging in a live circuit. I was told that each array has a local fuse block rated at 15 amps, yet as you add arrays together your voltage steadily increases all the way up to 800 Volts. He wasn't able to give me much more information beyond that. Now I would automatically think that the manufacturer would have designed plugs that had in mind someone was plugging together these live circuits and would have recommendations as to how many arrays you should plug together. Has anyone else had any experience with this type of question? Even though solar is a large industry here in CA, I have not installed any myself and am not familiar with the equipment, but have been told that the plugs are designed to not allow any accidental contact. In fact a couple people have told me that you would have to actually try very hard to come into contact with any live parts of the plugs.


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 Post subject: Re: Plug and Play Solar
PostPosted: Tue Jun 28, 2016 12:20 pm 
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MFelts477 wrote:
I had someone contact me asking about PPE for installing solar panel arrays. Specifically as he was adding each array to the circuit they come with manufactured plugs, his question is that since each array is already generating the moment it's mounted, he is plugging in a live circuit. I was told that each array has a local fuse block rated at 15 amps, yet as you add arrays together your voltage steadily increases all the way up to 800 Volts. He wasn't able to give me much more information beyond that. Now I would automatically think that the manufacturer would have designed plugs that had in mind someone was plugging together these live circuits and would have recommendations as to how many arrays you should plug together. Has anyone else had any experience with this type of question? Even though solar is a large industry here in CA, I have not installed any myself and am not familiar with the equipment, but have been told that the plugs are designed to not allow any accidental contact. In fact a couple people have told me that you would have to actually try very hard to come into contact with any live parts of the plugs.


I was involved in installation of the monitoring system for a solar farm once. Even for doing that part of things, just being near the panels, we were required to wear arc rated shirts and pants, and rubber gloves. Obviously no one there really understood arc flash protection, because we would either have had to wear clothing to a particular incident energy, or no special PPE at all. I think we would have had to wear rubber gloves no matter what.

And I think that's the answer I would give here. You're probably going to be asked to wear rubber gloves rated up to 1000 VDC, regardless of how likely actual contact with an energized part is.


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 Post subject: Re: Plug and Play Solar
PostPosted: Tue Jun 28, 2016 6:10 pm 
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Solar PV arrays are like batteries...its sort of uncharted territory as far as electrical safety is concerned. So just be aware of this in general.

The rule for "safe" voltages is usually 100 VDC or less is generally considered safe as far as shock is concerned. The exception at least in North America is New Brunswick province that uses a limit of 28 V, and some folks haven't gotten the message that the 50 V rule is 50 VAC. With AC the concern is that fibrillation (and death) can occur. With DC this doesn't happen. Instead it is essentially based on a pain threshold. At least that's how Charles Dalziel defined it.

This by the way is generally the voltage that a worker could be exposed to, so it is usually taken as line-to-ground, not line-to-line. So if the whole string is 800 V, obviously at some point even though the "bump" in voltage is small from panel to panel, it gets well above Earth potential. So just like all the "strange" rules for batteries where each individual cell/battery is relatively safe, the connectors themselves become shock hazards once a string gets built.

Second concern is...is anything "exposed" which means not protected against inadvertent contact. It generally means everything is insulated, guarded, or isolated and with solar PV generally this is definitely the case. So shock hazards are really only a problem at the combiner or inverter panels where there are exposed conductors.

Then we get to arc flash and this is where things get tricky. First question is...can we get it to arc. Any time that you are connecting or disconnecting something and the motive force is human hands the rate that the contacts are opening or closing can be slow enough to generate a significant arc, if an arc can be formed. So the connectors have the potential to be an arc flash hazard. Seconds question is...can a self-sustaining arc form and if so, is the incident energy significant. Hertha Ayrton's equation can predict the threshold voltage, gap, and current necessary to even achieve a self-sustaining arc. At up to 29 V or so, the answer is no at ANY current. As the voltage goes up, the required minimum current drops pretty quickly. and at 800 VDC, it definitely can happen.

So the final question is whether or not the incident energy is enough to worry about, keeping in mind that the standard is based on survival, not walking away unharmed. First, there is pathetically little DC arc flash testing that has been done and we are nowhere near the sophistication of the IEEE 1584 empirical equation for 3 phase AC arcs or even the ArcPro model for single phase arcs. We have Doan's equation which is analogous to Lee's equation and published in NFPA 70E, and Ammerman's much more sophisticated equation which produces a lower result. Testing commissioned by Duke shows that Ammerman's equation is off by a factor of around 1.4 to 4 but right now that's all we have. Even then even time I run Ammerman's equation for an arc flash analysis it almost always comes up under 1.2 cal/cm2. I would suspect that with a PV panel having a high series resistance
(it is after all a semiconductor), it will have a low available fault current and low incident energy even if the voltage is fairly high.

So:
1. Yes solar panels just like batteries are ALWAYS energized work almost by definition. So this falls under the exception where you can't de-energize by design.
2. Once you get above 100 V, the rubber gloves should come out if anything is exposed. It sounds like this is not the case for the connectors but I'm not there so I can't comment further on how well guarded they are.
3. Find out your series resistance to determine maximum available fault current...which is basically going to be the same as the maximum output current and it's not going to be very high. Then apply Ammerman's equation. Use your fuse curves and/or assume 2 seconds arcing time to determine what the incident energy could be. Again it probably won't be very high. With this number since this is a generation system, OSHA 1910.269 applies and if you can show that you do not exceed 2.0 cal/cm2 using a working distance of 15" (essentially holding the connectors, as per OSHA 1910.269), no arc flash PPE is necessary. Otherwise, arc flash PPE is necessary dressing up to the incident energy as calculated. Again, this is going to result in overkill because Ammerman's equation overpredicts the hazard but unless you have the money to spend on doing site specific testing, this is as good as it gets.

Note that many utilities basically require rubber gloves for every job, even when their own standards (IEEE 516) specifically prohibit them (for above 40 kV work). Similarly OSHA 1910.269 requires arc flash PPE for everything (starting at 4 cal/cm2) unless the 2.0 cal/cm2 exception applies (read it carefully) because there is a concern for splash of molten metal from nearby components but gives no boundaries on this limit. So rather than try to parse/argue it, most utilities also require arc flash PPE on all jobs no matter whether there is an actual arc flash hazard or not. It seems silly and ridiculous when there are no hazards present but that's the regulatory burden they have been saddled with.

Note also that industrial sites try to argue that only Subchapter S applies to them and not the 1910.269 rules. That's simply not true if you are running a generator (or solar panels) for any purpose other than emergency backup or portable applications, and since the rules also apply to "distribution" (which is not defined) it is questionable whether or not the rules apply to switchgear in an industrial plant or whether it only applies to the utility that the switchgear is connected to.


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 Post subject: Re: Plug and Play Solar
PostPosted: Wed Jun 29, 2016 8:24 am 

Joined: Thu May 26, 2016 6:55 am
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Voltreal and Paul, thank you both for your responses. This gives me something to go back with and see if I can gather more information from the person asking the question. Paul without getting into the equations your examples and information get me thinking of when I have opened and closed fuses on a pole, if there is no load then it's pretty boring to watch, but if there is something ready to pull some current then you get a fun arcing show. Am I correct in that correlation?


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 Post subject: Re: Plug and Play Solar
PostPosted: Wed Jun 29, 2016 3:35 pm 
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MFelts477 wrote:
Voltreal and Paul, thank you both for your responses. This gives me something to go back with and see if I can gather more information from the person asking the question. Paul without getting into the equations your examples and information get me thinking of when I have opened and closed fuses on a pole, if there is no load then it's pretty boring to watch, but if there is something ready to pull some current then you get a fun arcing show. Am I correct in that correlation?


Yes, that's exactly the situation. But lets take this a step further. Compare the incident energy using the hot stick work method to open those cutouts compared to say the rubber glove method used to alter jumpers located on the same pole. If the hot stick is 8 feet long (96 inches), it is 6.4 times further away compared to the rubber glove method. But because the incident energy dissipates at a rate equal to the inverse square of the distance, the incident energy with the hot stick method would be 41 times lower than the rubber glove method.

However going back to OSHA's concern about being splashed with molten materials, someone standing under a pole with cutouts on it has a chance of getting splashed with molten borax if the fuses trip. So for this task if the cutouts are operated from the bucket truck off to the side, no arc flash PPE would be needed but if they are operated while standing directly underneath, arc flash PPE would be required especially when closing them in. In reality though once you understand the potential danger even though it stings and I've only gotten a few burns that look like welding burns from it, who would want to put themselves in harm's way?


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