[RE-wrenches] 1.56 ISC Minimum OCP is STUPID! (Was: ground fault troubleshooting)

Tue Sep 7 20:40:22 PDT 2010

Matt, I regularly size my string fusing at 125% of short circuit, if I'm using a 100% duty rated fuse and assembly. The exception under 690.8 B1 allows this size reduction of the OCP, and I agree it will clear a fault a bit sooner. I wouldn't size any closer than 125%, I've seen irradiance levels hit 1350 w/M2. As Kent pointed out, and I have seen many times, a short circuited array can do a lot of burning damage and never exceed short circuit current.
I watched in horror once as a relay turned into a molten uniblob of metal, an Emeter was in the system, and the current never exceeded 7 amps (120 vdc), and so it never blew the 15 amp fuse.
The solution there was to turn the relay "on" to stop the arc, and then disconnect the circuit with a cable cutter at the battery.

What we need is a more sophisticated combiner box that has a main disconnect incorporated in it, and has not only GFI but arc fault protection too.
(Matt if somebody makes it, I'll install it, whether its NEC required or not)
As William mentioned, GFIs at the inverter that open the bond to ground don't do very much, it needs to be at the source.
The PV industry has protection backwards, all other wiring methods disconnect power BEFORE the fault, not after.

After seeing the arc flash videos, I could see that happening at a large recombiner box, and nothing in the system would stop it.
It would just burn and burn until it burned a wire in two, or the sun went down. (Target fire?....)

Not that I'm a huge fan of micro inverters, but they definitely don't have these problems, IMHO.

R. Walters
ray at solarray.com
Solar Engineer

On Sep 7, 2010, at 8:25 PM, Matt Lafferty wrote:

> Hi Kent,
>  
> Thanks for the 98% vote. Now I'm gonna try to get the other 2% out of you.... You're a smart guy, so it shouldn't be too difficult ;)
>  
> These aren't the days where we were lucky to have a customer with barely enough money to afford a 300W system. We are commonly dealing with >2500W strings of 200+ watt modules. It's a new paradigm and the risks associated with faults continue to grow. You are 100% right... OCP in a current-limited DC application ain't simple.
>  
> To be clear, I am completely in favor of 1.56 ISC as a minimum for conductors. I want that inherent protection and any case I describe hereafter assumes this condition. Also to be clear, I am not proposing that our current-limited power sources should be able to trip the OCP from the source based on amperage alone.
>  
> To your contention that "1.56 ISC isn't in any way responsible for the danger"....
>  
> Compared to a lower fuse rating.... Something like ISC for instance.... 1.56 ISC as a MINIMUM OCP rating does indeed increase the hazards. To both persons and property. By at least 40% in terms of raw amperage. By more than that in terms of kCal/cm2. The biggest difference comes in terms of time to blow and the amount of damage or injury caused during that period. They call it Incident Energy.
>  
> For those who have not studied Time:Current curves of commonly used fuses, you should. KLKD is a typical fuse I hope we are all familiar with. http://www.littelfuse.com/data/en/Data_Sheets/KLKD.pdf  I'm using KLKD as an example because Littelfuse put a little table right on the front of the datasheet to make it simple. Other commonly used fuses have similar characteristics.
>  
> Note that this class of fuse will take 135% of it's rating for up to 1 hour. 200% of its rating for up to 2 minutes. Be sure to check out the fuse curves. These things will take ~125% of their ratings for pretty much indefinitely. How much damage is caused waiting for it to blow? What about when irradiance is <800 W/M2? How long then? Will your conductor hold up during this time?
>  
> If you fault on the array side of a combiner fuse in a 3-string system, you might NEVER BLOW that fuse. (Example: 8.4A ISC module with 15A fuse). Especially if there is an arcing DC fault. Temperatures of arcs are much higher than 90C. Is your conductor up to that? I have been called to troubleshoot a lot of low-performing and broke-down systems. One resi rooftop had a spot where an arcing fault burned through the side of a NEMA 3R steel pull-can on the roof and never blew a CODE COMPLIANT combiner fuse. This system had 3 strings and was down about 1/3 on power output after the original installer replaced the GFP fuse following a "ground-fault". He had checked the combiner fuses and they were good so he called me to troubleshoot it. He was convinced that there must be a bad module and wanted a third party to verify it for warranty claim. Hated to show him what he missed and that it was his fault. (The fault had burned clear so the GFP didn't blow again.)
>  
> Another situation that I am sadly familiar with  has been burning holes in a steel roof since 2007 (I told them not to do it over and over)... At least one spot is about the size of a softball. The others vary in size. Their common characteristic is that you can see the sky thru them from inside the building. The first time it happened was during an ice storm. This POS peel-n-stick system typically burns thru the roof during low-irradiance periods. Most of the time it will EVENTUALLY blow a combiner fuse, reducing the current feeding the fault, and either weld to a short or burn clear (open). This is ~500kW on central inverters with marginal GFP protection. I hear the Tefcel front sheet is a nice insulator.... You want to walk out on that thing in the daylight? I don't. This case happens to be a classic installer-was-either-drunk-or-on-drugs-because-nobody-can-be-that-stupid situation. Rolling UniSolar right over loose screws on the pan? WTF!!!!
>  
> I can list a lot of similar examples where damage has been caused and fuses have either not blown or taken longer than they should have to blow. The key here is that the wiring and OCP in ALL OF THESE CASES ARE CODE COMPLIANT!
>  
> I bring these cases up because, if these things had fuses with smaller current ratings, the fuses would blow before this much damage is done. It takes a lot of heat to burn thru steel. You wouldn't think that a circuit with a 10 or 15A fuse could do this much damage, but they do! Amperage = Heat. The more Amps, the more Heat. When you use an Arc or Wire-feed welder, you adjust Amps to get the heat you want. I want to minimize the amount of potential Amps back-flowing into a fault to a lower level. A level that allows safe and reliable NORMAL OPERATION, yet limits the catastrophic effects in an ABNORMAL CONDITION.
>  
> My contention is that the NEC is flat out wrong requiring 1.56 ISC as a MIMIMUM OCP rating. It creates undue hazard. It is in direct conflict with the spirit, intent, and other long-standing precedents in the Code.... With the exception of Emergency Fire Pumps and other mission-critical equipment that is specifically intended to stay alive until it completely burns to the ground, all other minimum OCP ratings are based on 125% CONTINUOUS OPERATING CURRENT of the equipment. In our case, that equates to 125% Ipmax, as opposed to ISC. An example of an AC equivalent to this asinine "minimum 1.56 ISC" OCP requirement would be requiring motor circuits to be OCP at 156% of the Locked Rotor Amps. I'm sure you can imagine what this would do to the size and cost of starters, etc.
>  
> Since the last time my favorite 6kW system put out 7.5kW was... NEVER.... I'm gonna have to guess that a 1.25 Ipmax fuse would hold just fine. This would be 100% consistent with the rest of the Code. It's also not gonna blow with cloud-edge effect or other irradiance enhancing events. Especially when you consider that it's gonna automatically withstand an extra 20-25%% for an indefinite, possibly forever, period due to the Time:Current curve relationship.
>  
> My NON-CODE-COMPLIANT-SELF prefers to size OCP at "ISC or next larger standard fuse rating not larger than the LISTED Series Fuse Rating of the module". You've got a 6-7% headstart between Ipmax and ISC plus the ~25% indefinite Time:Current characteristic. Ain't no way in hell that a NORMALLY OPERATING SYSTEM will blow fuses rated this way. And it's ~2/3 the amp rating (or less) that is now required as a minimum. Which is exactly what we want. Reliability during normal operation and safety.
>  
> The only time we want an OCP device to trip is during abnormal conditions such as a short circuit. When we have this type of fault, we want that circuit to open up as quickly as possible in order to minimize damage. At least I do. In the case of 1.56 ISC, the NEC is GUARANTEEING GREATER DAMAGE AND INCREASED HAZARDS compared to a lower-amp fuse. I have gone thru this logic with numerous building inspectors over the years. Every single one agrees with it. Some, but not all, have agreed to allow lower-amperage fuses. The only, and I mean ONLY reason given by inspectors that have not allowed lower-amperage fuses is because... "The Code requires 1.56 ISC so I have to require it."
>  
> String inverters only bother me so much in this regard. Central inverter systems are where the real bad ju-ju starts to happen. For the sake of definitions, I consider a string inverter to be one that has one or less combiners and the modules are configured in series strings. A central inverter is one that has string-level combiners and one or more re-combiners.
>  
> I am seeing more and more faults in the DC feeders between string combiners and re-combiners in these central systems. The power levels you are dealing with here are pretty significant. 100% of the recent faults I've been seeing are due to shi##y workmanship, particularly in conduit installation and wire-pulling. Some of these faults would certainly have been avoided if they had selected a tougher insulation such as XHHW-2, instead of Quik-Nick THWN-2/THHN. (THWN stands for "This Heiffer Will Nick". THWN-2 stands for "This Heiffer Will Nick 2wice")
>  
> I have zero tolerance for crappy workmanship and even less sympathy for the people who do it. Just got off ANOTHER call this morning where the installer has re-pulled one DC feeder four times and still can't pass megger testing. They have re-pulled every feeder at least once. The spec only calls for 250 Mohms even though wire and cable engineering formulas say the minimum should be 688 Mohms for that size, length, and insulation type of wire. And they can't even get it to 250. Every set of wires that has been pulled out has obvious physical damage. The sub is crying, wanting more money and to have the work accepted (NOT!). Come to find out, one of the field guys working for the developer has witnessed these guys beating on the 1/0 AWG with a mallet to get it into the LB.
>  
> Says right here in my NECA 1-2006 Standard for Good Workmanship in Electrical Construction... Section 9 Wire and Cable.... "c) Wires and cables shall be installed so as not to damage the insulation or cable sheath." Sounds like this electrician sub wannabe is in violation of his contract.... You know that clause... "Workmanlike manner". (Sub, if you are reading this... I am NOT your friend in this case. You WILL re-pull these feeders correctly, at your own expense. I will repeat the advice already provided: Use pulling condulets. I will add some advice: Fire your electricians.)
>  
> The scary thing is, this practice goes on every day. A LOT! The sad fact is that many (most?) of these systems are not having thorough, comprehensive Insulation Resistance Testing performed. And IRT will only catch SOME of the future faults! I have been involved with post-mortem in two cases where 500kW AC feeders have been properly IRT'd and blew up later. Not good! (Side Note: Each of these cases involved big feeders in standard LB's. Make a note of it.)
>  
> It's only a matter of time before these things go Pop Sizzle Smoke! These failures WILL occur. A lot of them already have and the number is growing. I hope and trust that most of us on this list practice Good Workmanship on every project. That being said, none of us are perfect. What about cases where we miss something or even cases like a tree falling across one of our conduits?
>  
> My contention is that we should do whatever we RESPONSIBLY can to minimize the damage when this happens and the hazards when it's being troubleshot and repaired. It's a simple principle.
>  
> One RESPONSIBLE way we can minimize the damage is to reduce the fuse size by ~40%. (i.e. ISC or next higher standard fuse rating) This method will provide adequate operational reliability. It will also ensure that there is a better chance of the fuse blowing sooner when there is a fault, thereby minimizing the damage caused. It will minimize the hazard to personnel performing troubleshooting and repair because the incident energy at the fault will be reduced in all cases. By ~40%. Whether or not the fuse blew.
>  
> Happy to discuss this issue with all who care and are not on NFPA 70 CMP. (Just kidding. You CMP guys are welcome to discuss it, too... Just be ready to issue a memorandum/addendum to the 2011 NEC allowing OCP with lower than 1.56 ISC...)
>  
> Extra Credit for BOS Mfrs: Make a Combination Device that has DC Arc-Fault Interruption and OCP that fits in a standard fuse configuration. Start with midget-class so we can simply drop it into our string combiner fuseholders.
>  
> Pray for Sun!
>  
> Matt Lafferty
> gilligan06 at gmail.com
> 
> From: re-wrenches-bounces at lists.re-wrenches.org [mailto:re-wrenches-bounces at lists.re-wrenches.org] On Behalf Of Kent Osterberg
> Sent: Tuesday, September 07, 2010 10:59 AM
> To: RE-wrenches at lists.re-wrenches.org
> Subject: [RE-wrenches] ground fault troubleshooting
> 
> 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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