Matt, I agree with you on about 98% of this. You are 200% correct that a faulted high-voltage or high-current PV array is a serious and dangerous situation and that the person looking for the trouble in a faulted PV array needs the proper tools and knowledge of how all the components work. But the 156% rule for fuse sizing per NEC 690 is not in any way responsible for the danger. The danger is a result of the nature of the PV module: a power source with the current nearly proportional to the illumination and a short circuit current that is only 10% greater than the normal operating current. If one were to select a fuse that could blow when the array was shorted, occasional edge of cloud irradiance enhancement would cause nuisance trips and it still wouldn't clear a fault when the irradiance is 900 watts per square meter. There will never be a simple fuse that can provide the protection that is needed. The existing ground fault protection in the inverters is inadequate and current plans for arc fault protection may not be a satisfactory either. These have been slow incremental improvements; much more is needed. -- Kent Osterberg Blue Mountain Solar 541-568-4882 www.bluemountainsolar.com Wrenches all, I 100% second Bill B's comment Correct that... I 200% second it. It should be the law.... "Don't begin to troubleshoot a faulted PV circuit without a reliable DC clamp meter." The MOST DANGEROUS PV system is a wounded PV system. This includes danger to persons and property. Safely and efficiently troubleshooting a faulted PV circuit requires a voltmeter AND an ammeter. And PPE. And adequate knowledge and understanding of operational and non-operational characteristics of PV systems. The simple reason for this is that, when one or more circuit conductors are faulted to a short condition, the voltage between the faulted elements is zero. Relying on just a voltage reading to determine whether or not to open a circuit under this condition will result in an arc. The amount of energy in that arc will depend on the amount of available sunlight and the amount of PV that is feeding into it. The amount of potential hazard will correspond to these factors as well. Using a clamp style ammeter will allow you to understand where and how much current is flowing in a circuit before you decide to open it. It is one thing to know you have a 45 amp load in a circuit with a potential of ~450V because you clamp it before you break it. With this knowledge you can assess the situation. You can do something to mitigate or remove the potential hazards... Cover the array, open a disconnect somewhere, put your PPE on and go for it, select a different location to open the circuit, use insulated cable cutters, wait 'til dark.... You have choices. It is quite another to be surprised by the resulting arc in tight quarters because you measured the voltage and figured it was a dead circuit! When you react to the startlement (word?) by dropping your screwdriver and yanking your hand back... Assuming you don't receive a shock, flash injury, or fall off the roof in the process, of course.... The result just may be that the now-dislodged conductor is arcing and zapping and spitting. Now you're gonna have to stick something back into that box to deal with it. In the meantime, a number of possible things can happen, most of which are not favorable.... Melting insulation and conductor material are the most common. The degree (not just a pun) of damage and remaining hazard will be determined by the amount of sunshine and amount of PV feeding into the arc. The MOST DANGEROUS single point on the DC side of a PV system is ANYPLACE on the Inverter side of a fuse(s). This is a simple function of the assinine "1.56 ISC minimum fuse" rule in the NEC. The source cannot create enough current to blow the fuse(s). If you have a fault between a combiner and the inverter, you WILL have current flowing into the fault as long as the sun is up! If you are relying on just a voltmeter in a central-inverter plant, you could very well be in for a 15-20kW surprise, or greater! The combination of shi##y wire, sloppy conduit installation, and crappy wire-pulling methods have resulted in too many DC feeder faults to count. It boggles my mind every time I hear of yet another guy nearly joining the dead because he touched or opened up a connection somewhere in a faulted circuit without de-energizing it. Time and time again I hear that they tested it for voltage and it was "dead". Sometimes they even opened up the service disconnect at the string combiner, "just to make sure". Time and again it's a "journeyman electrician". I like it best when it's the same card-carrying jackass who "built" the thing. I consider THWN-2 to be on the list of shi##y wire types for DC, by the way. I'm an XHHW-2 guy, personally. Why would anybody select an insulation that is easy to nick/slice/tear when you can have a super-tough insulation for a couple pennies more? Why would anybody select an insulation that only has about 5% of the dielectric resistance of one that is a couple pennies more? Why? Oh, I know... It's that race to the bottom on BOS costs... Which leads to the next step in stupidity... Designing and building LARGE PV plants without sufficient DC SERVICE disconnects... This is what's going on out there.... PV plants with 500kW Central-inverters being installed without string-combiner disconnects. Without any DC service disconnects. The NEC considers the fuseholder in the combiner &/or the connector on the module to be a "disconnect" and does not require a "service disconnect" in the circuit. So these smart-ass engineers and project developers are out there building this shi#. Some of these projects are being built by PV module manufacturers masquerading as developers. "Vertically integrated..." Others are being designed & built by formerly respected integrators who have either sold out or lost their conscience altogether. The trend is to build them to sell to PPA companies who ostensibly own and "operate" them. These solar timebombs are being built on both sides of the fence. Frosty ain't the only one with a solar flamethrower! All in the race to the bottom of the $/Watt pile that they are now calling LCOE. Har Dee Har Har! I hate to say this, but I hope somebody gets really hurt out there, and soon. I hope it's the same smart-ass engineer (or his boss) who thought it was alright to design this way after some field technician walks away from it because it's dangerous. And then I hope his family sues the crap out of the company and companies involved with designing, supplying, building, and owning it so they STOP DOING THIS SHI#! And then I hope he takes his cooked carcass on the road doing safety awareness training so others don't repeat these stupid, avoidable catastrophes! And then I hope these cheap-ass developers go out to every site that doesn't have sufficient disconnects and re-fits the systems with them to avoid further injuries and $$$$ settlements. What is the levelized cost of energy for that system now, Mr. CFO? Unfortunately it isn't likely to be that smart-ass engineer. Or his boss. It is far more likely to be a Wrench. A Wrench without a DC clamp and the knowledge that he needs one. A Wrench without the proper PPE because he "tested it and it was dead" so, even if he had his gear on to "test it", he took his gloves and face-shield off to work on it. A Wrench who doesn't fully understand the operation of GFP circuits. A Wrench who doesn't understand that not all faults are ground faults and the characteristics of a fault change in terms of potential and magnitude with varying environmental conditions. A Wrench that doesn't fully understand that power can be coming from both directions. A Wrench who figures he doesn't have the time to completely isolate a section of a circuit because there AIN'T NO REAL DISCONNECTS. I hope it's not your Wrench. As the size of the inverter grows, so does the hazard. To a point. The idiotic 1.56 ISC rule only increases the potential hazards. Central-inverter plants should not be serviced by anybody who doesn't have an extremely comprehensive understanding of these systems, and the tools and PPE to safely work on it. For systems with inverter-integral re-combiners, the most dangerous spot in these systems are the feeders between string combiners and re-combiners. Anything between the output of a string combiner and the input of a re-combiner. For systems with standalone re-combiners, a fault between the re-combiner output and the line side of the next disconnect is the most dangerous point, but certainly not the only dangerous point. If either of these systems are built without load-break disconnects at the string-combiner level, the cost to service goes thru the roof. It either goes thru the roof to do it safely or it goes thru the roof in terms of risk to do it not safely. Pick one. There is an interesting dynamic between the potential hazard on a faulted DC homerun feeder and the kW of the inverter. The less re-combiner inputs you use, the greater the potential hazard on faulted input feeders. Again, this is because of the UNSAFE AND STUPID 1.56 ISC rule. In systems with a relatively low number of re-combiner inputs, there are large portions of time when there isn't enough combined amperage in the non-faulted feeders to blow the re-combiner fuse of the faulted feeder. If your system only has 4 or 5 re-combiner inputs and it's winter-time, it is quite likely that a faulted feeder is being fed from both ends. (Commonly 100A fuses in the re-combiner with ~60A ISC feeding a string-combiner) That feeder can be fed from the re-combiner end, by anything up to about 105% of the fuse rating, for pretty much ever without blowing the fuse. The more parrallel inputs there are, the more likely there will be sufficient current generated by the other feeders to blow the fuse. Since the vast majority of systems out there don't have load-break disconnects at the re-combiner inputs, the technician needs to be able to open disconnects at each string combiner in order to isolate this feeder. But what about systems without DC service disconnects? Repair at night? My hope is that anybody on this list will refuse... Say it with me now... R-E-F-U-S-E to install PV systems without adequate disconnect provisions to isolate faulted feeders. And only allow technicians with proper knowledge and equipment to work on a busted PV system. "Journeyman electrician" does NOT automatically mean that person has the proper knowledge to do it safely. Safely working on a faulted PV DC circuit requires ALWAYS clamping the thing for starters. It might also mean "not working" on it until the sun goes down. A technician with the proper knowledge and equipment should be able to determine the proper course of repair. In the case of the faulted lightning arrestor, it was "only" a small circuit, but it got the guy's attention and apparently nobody got hurt. The bigger these systems get, the bigger the potential hazard. To answer Tom's question about jumping around a fault: Maybe, maybe not, depending on the nature of the fault (+/-, +/G, -/G) and the location of the jumper relative the fault and the power source. Even if jumping to ground eliminates the arcing when you are working with the terminal, you will still have arcing when you land/un-land the jumper &/or remove the fault. If the sun is shining and you have a DC fault, you will have arcing at some point when you make/break the circuit. Hopefully it's safely contained and localized to the contacts of a service disconnect! Pray for Sun! Matt Lafferty |
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