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Hello to everyone. I am new to this forum and would like to solicit your opinion on what I am planning to do. I am a registered engineer and have been doing power system studies for more than 10 years. Less than two years ago, I have started doing DC arc flash studies for some clients through my former company and this includes printing of their labels. now that I have moved to a new company, I plan to start a small company that will cater that will do DC arc flash. While it is true that I have done significant studies with my previous company because they do PV design but also performed performance evaluation of PV facilities, I am not sure if there is a strong demand of for this type of service. My instinct tells me to go for it but my heart tells me to wait....any advise that you can provide will be appreciated.
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Hi and welcome
What sort of voltages and current availability did you work with on the DC arc flash ?
PV arrays are generally less than 600V and <20A The panels current limit themselves, a 20A array cannot produce even 30A when shorted. And when shorted, the array voltage drops close to zero.
Batteries are different, generally under 50V, but with thousands of amps for 10's of seconds possible.Powerfab top of pole PV mount (2) | Listeroid 6/1 w/st5 gen head | XW6048 inverter/chgr | Iota 48V/15A charger | Morningstar 60A MPPT | 48V, 800A NiFe Battery (in series)| 15, Evergreen 205w "12V" PV array on pole | Midnight ePanel | Grundfos 10 SO5-9 with 3 wire Franklin Electric motor (1/2hp 240V 1ph ) on a timer for 3 hr noontime run - Runs off PV ||
|| Midnight Classic 200 | 10, Evergreen 200w in a 160VOC array ||
|| VEC1093 12V Charger | Maha C401 aa/aaa Charger | SureSine | Sunsaver MPPT 15A
solar: http://tinyurl.com/LMR-Solar
gen: http://tinyurl.com/LMR-Lister -
What I have learned in ARC Flash training is that "low" voltage does not mean it is safe. In a past life we had a 200amp 240v 3 phase panel that was listed as Class 5 for ARC Flash. Meaning there wasn't any type of PPE that would keep someone within 5 feet in front of the open panel from getting roasted alive should an ARC Flash occur.
The reason was there wasn't any Main OC in the panel. The only fuses in the circuit were on the Primary side (4160V) of 3 pole mounted oil transformers. It would take "ages" for those fuses to open and stop the build up of the ARC flash at the panel. The level of voltage is not the main factor for ARC flash energy. It depends more on the type of OC and the clearing time to open the circuit.
I have not been trained on DC ARC flash but would imagine that without the proper OC device a lot of energy could be present should a bolted short occur in the DC input wiring to the Inverter.Comment
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Low voltage circuits (but still high enough, generally greater than 120V at 60Hz to be able to sustain an arc for very long) can indeed generate a higher incident energy over a period of time. But that is because the current can still be very high (in the thousands to tens of thousands of amps) and yet the ratio of short circuit/arc current to rated current may be low enough that the OCPD opening time is high compared to that used in a higher voltage situation.
The OCPD will not "protect" you, but it will limit the total arc energy and so will determine what protection, if any, will save your life in the event of a flash.
With DC you have the serious problem that there are no zero crossings in the current waveform to help extinguish the arc, so if there is no DC OCPD the arc may continue for seconds or even minutes until enough structure has burned or melted away that the arc gap is too long for the available voltage. Beyond the first 5 seconds, the energy does not matter as much since it is assumed that you will have voluntarily or involuntarily exited the flash zone (which even for 600V DC the arc itself does not extend more than a few inches from the conductor ends and the energy per unit area falls off according to the inverse square law.)
With a PV source, even the instantaneous current is limited to no more than Isc, since there is no energy storage in the panels. The capacitance of the wiring is negligible for this purpose.
The energy for any arc source will be proportional to the current and, to the extent that it allows a longer arc and greater power dissipation within the arc, to the voltage.
For a fixed length spark path the available voltage does not make a significant difference. The voltage across the arc can be calculated by a formula involving primarily the available current, but to a power somewhat greater than one.SunnyBoy 3000 US, 18 BP Solar 175B panels.Comment
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I would guess that the only effective OCPD in their particular case was on the primary side where the power available until the OCPD operated was really high.
OCPD operating at a low let-through time/power product will help some. But I would never rely on it unless I had protection corresponding to that calculated value.
In the case of a transformer bank an arc on the secondary terminals could lead to an arc on the primary side too, increasing the available power well beyond what the transformer impedance itself would allow in the secondary side.SunnyBoy 3000 US, 18 BP Solar 175B panels.Comment
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Hello to all and thank you for responding. The studies I have done before were of different voltages and current levels. For utility scale projects, the currents involved were significant and there were high values of incident energy calculated. For commercial projects such as school, current levels are not so high. One particular school project I recall consisted of the following : (2) Interconnect switchboard, (2) Distribution switchboards, (7) 480/277V panel boards, (45) 1000V string inverters & (45) 1000V string inverters. DC voltage was around 700 VDC and current per array was only 40 Amperes. For this project, we have calculated the DC incident energy on the combiner boxes to be somewhat .40 cal/cm2 for an arc duration of 2 seconds. Our calculations were based on a "Arc Flash Calculations for Exposures to DC Systems ", Doan, D.R., IEEE IAS Electrical Safety Workshop, 2007, Record of Conference Papers, March 2007. Keep in mind that DC arc flash studies is not as matured as its AC counterpart and therefore the paper referred was used due to its conservative approach.
in any case, if you are installing PVs, do you see any need to do arc flash studies/labeling in your projects? Thanks for your feedback.Comment
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True. The OCPD does not protect you, the proper PPE can if worn correctly.
What I was trying to explain (and I guess failed) is that a properly engineered OCPD can reduce the energy generated in the ARC Flash (and blast) low enough to be Class 0 which only requires PPE consisting of cotton clothing instead of Class 4 which requires a Moon Suit rated for 40 cal.Comment
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I don't think they felt a thing, they feel forward on their faces toward the pole and killed instantly. That fine looking gentleman kissing the pole was lying on the cable cutters still in his hands. Or what was left of his hands. They were the arc flash. I got another picture further out where you can see bottom of their feet blown off through there shoes. They cut a live 13.2 underground dip.MSEE, PEComment
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