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Three-Phase posted:Cool video I found: showing the magnetic forces involved in a serious electrical fault. From Ferraz-Shawmut. Bussmann showed us a similar video when they were in our building for SCCR training. They also showed other effects of not properly protecting things. I can't find the videos online, but they basically showed different breakers and fuses blowing with different interrupting ratings. The class J fuses blew, and the circuit breakers blew up. I'm betting that the 2nd video you posted was someone shorting something in the panel on accident and the protective device wasn't rated for the available fault current. On that note, how common is proper coordination? I'm a controls engineer for a panel shop and I handle one of our larger OEM accounts. We almost never talk to the end user, and just about every panel we build for them is rated for 5kA. We've been putting it on every proposal since we got our SCCR training I mentioned above early this summer (we tried it once before the training but the OEM yelled at us because customers were asking questions...). Since then, I think I've had to design 1 panel that met 75kA. Nobody else has mentioned anything. I'm a little worried that something like this video is going to happen to some poor electrician out in the field, but it's drat expensive to meet these ratings when the OEM gives us peanuts for these panels. And I actually still have questions about SCCR. Am I correct in saying that a combination rating must be tested by UL (i.e. class J fuse in front of a contactor) for it to be valid? Or can I just put a 200kA rated class J fuse with a 5kA let-through in front of a 5kA rated contactor and call it 200kA? I ask this because it's drat near impossible to find properly rated combinations without buying a load of unnecessary equipment. I don't want to buy a $2000 molded case switch to protect a 5HP motor starter when I can use a couple J fuses and a 200kA rated disconnect, but a lot of manufacturers have wacky combinations listed for higher SCCRs.
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Sep 24, 2011 16:49
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So I got a strange call from a customer the other day. They were installing a small VFD panel we built for them to control some HVAC equipment and the installing contractor was baffled at the incoming voltage. He says he's been doing this for years and he's never seen anything like it. They claim to have dozens of 3ph 230VAC motors running in the plant, and the electrician swears up and down that it'll work fine. I can see that, but here's what he says is at the main power coming in to the building: code:L1-L2: 240 VAC L1-L3: 240 VAC L2-L3: 240 VAC L1-GND: 240 VAC L2-GND: 0 L3-GND: 240 VAC Their story is that this building was the first to go up in the park, in around 1940 or so. This appears to be a corner grounded delta system, something I've never had to deal with. I've only been doing controls for about 4 years, so I had to do a lot of googling to find that, but at least it explains why L2 isn't fused. I even showed it to our senior engineer and he was just as confused. Based on the voltages I'm sure it'll work, but is it safe? The customer was worried that the VFD would freak out with this. I told him that in my opinion, as long as the ph-ph voltage is OK, then the VFD should be fine...but I also made sure to tell him that I couldn't make any promises (i.e. no warranty) since nobody at my company had ever seen this. Our senior engineer wanted to see a schematic from the power company before making any promises. The rest of the story is that the power company told them that the building was overdue for a service upgrade, so I advised them to talk to them and get the upgrade if it's due to them. Might cost them more now, but it'd probably end up saving them in the long run. This was a week or two ago, so chances are they just wired it up anyway. So what is the purpose of this setup, and how big of a problem is it for them to continue using it? Thinking about it, if L2 is grounded, won't the VFD cause all sorts of noise in their power system? Or would it be the same as any modern setup? edit: Another thought. If L2 is grounded, does that mean that I can't fuse/disconnect it in my panel? I'm using a standard 3ph fused disconnect, so they should all disconnect at the same time, but since it's fused there's no guarantee all 3 fuses would pop the same. I'm not sure how a single phase situation would affect L2. DaveSauce fucked around with this message at 18:02 on Jan 7, 2012 |
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Jan 7, 2012 17:59
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What are some reasons NOT to use medium voltage for motor control? I ask because I'm about to ship 2 control panel systems that each control a 600HP dust collection system. And they run on 460VAC. Each system is 3 panels total, and each has a single panel that houses a 1200A breaker and a 720A soft starter for the blower motor. The wire between the breaker and the soft starter is 3x350MCM per phase, which cost us $12/ft. The way I figure, this will cost $108/ft in copper alone to wire up the motors. Add to that the time it takes to physically run wire of this size and I'm really confused as to why they thought it was a good idea. It took a guy a day to wire up 2 of these things due to the near impossibility of bending wire that size, and there's only about 2.5 feet per wire. So I guess my question is, why didn't the customer use medium voltage for the blower motors? I know for a fact that they're installing a new transformer just to power the panels. In total they're going to need nearly 1500A at 460VAC. An older PE in our company claims that medium voltage equipment is ugly and antiquated, but while knowledgeable, he's a little out of touch with reality.
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Jan 19, 2012 05:12
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Three-Phase posted:Little video from Ferraz Shawmut (BUY OUR FUSES!) that show the effects of magnetic fields generated on cables during a fault. We saw a similar video from Bussmann when they came in to do some SCCR training. The more interesting videos are the ones that show what happens when you put 50kA into a breaker that's only rated for 10kA. That's good fun. I hate Ferraz right now (they're going by Mersen now). Their website lies about competitor fuse cross references. The real problem is nobody double checks this...we spec out a Bussmann fuse, our vendor tells our purchasing people that Mersen has an exact cross for cheaper, so they buy it, and the next thing we know we're getting a variation notice from UL. $600.00 down the drain because of a $1.00 fuse.
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Feb 18, 2012 00:04
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Since the subject is on arc flash, what's protocol for separating 480v power and 120v control in the same panel in order to lower PPE requirements? We have a customer who wants a control panel to do this. Essentially they want to be able to work on the 120v control side while it's live with minimal PPE, i.e. without having to wear the PPE that would be required if it were a 480v power panel. It was originally quoted as 2 separate enclosures bolted together with a pipe nipple to run wires between the two, but I've been told now that they only want a single enclosure "due to time constraints." I have a voice-mail out to the customer to clarify this as this makes no sense to me (it's just as fast for us to get 2 small enclosures or one large box). But for the time being, this is the directive, so my plan is to use a 2-door enclosure and having a custom bracket fabricated that will run the length of the subpanel and will reach as close to the center-post of a 60x60x12 enclosure as possible. So as far as actual arc flash, would this be considered an adequate barrier to lower the PPE requirement needed to work on the control side? There would be air gaps at the top and bottom of the enclosure, as well as an occasional hole punched through to run wires. It's essentially touch safe, but I don't know if that is adequate to lower the PPE. Follow up to that: Unless we ran wire through cord grips, there would be air gaps anyhow even if it were 2 separate boxes joined together via close nipple. Would that even be acceptable? Disclaimer: We are not arc flash experts and we do not guarantee any sort of 70E compliance whatsoever. We do not mark our panels with any sort of arc flash information aside from the generic warning label. We feel this is something that can only be done in the field, since we don't have all the information necessary to determine that.
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Mar 25, 2012 20:26
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So a question on careers. I've been doing industrial controls work for about 5 years. I started at a machine builder doing the controls for that out of college, and now I'm working for a panel shop that does both OEM and end user work. We design, program, and build panels, and also offer turnkey solutions for a wide range of customers/applications, which is really nice because I get to learn how to control all sorts of different equipment. As far as things to do with a Comp Eng/Elec Eng degree, where does controls rank? I'm a huge fan of programming, as that's my background and currently what I like doing. I'd rather program a PLC than design a panel or do power distribution. I'm likely going to be moving in the next year or so, so I'm trying to figure out if I should stick with controls or if I should try to move in to some other career path. For reference, in college I always thought I'd be doing firmware/microprocessor programming. PLC programming is something I rather enjoy, but I always want to be working with hardware of somesort, whether it be electrical or mechanical. One thing I've noticed is that where I am (twin cities), controls is a pretty small world. Everyone knows just about everyone else. I was always under the impression that controls was kind of a default thing to do for sparkies, but I'm kind of learning that it's a much smaller world than I originally thought. I'm kind of curious where this might take me in 10-15 years and if I should stick with it or jump ship at the next convenient opportunity. I don't really know what sort of things I can be doing down the road if I stick with the controls path. I don't know if I want to work at a panel shop forever, but I don't know if I want to go back to a machine builder or try to get a job as a controls engineer for a plant or something.
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Feb 9, 2013 18:34
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What use is it to get a PE when you're a EE? I've only ever met 1 PE that wasn't a Civil or Mechanical engineer, and he works at my company. He's a controls engineer (we all are), but mostly does software. He knows his hardware and is pretty good at NEC/UL interpretation, but I haven't a clue what someone like him would do with his PE. I would understand that as a power engineer, having a PE would be pretty important. Is that about the extent of it? I never took my FE. I might have been able to pass it in college, but I would most certainly fail now that I'm 5.5 years out. I'm more of a software guy, at least that's what I'm trying to focus on, even though right now I'm the 2nd person people go to at my company when they have NEC/UL questions. Hell, just spent nearly 4 hours yesterday flipping around the NEC trying to figure out how to size a specific conductor, and dammit I think I found it after 40+ years of nobody in our company knowing how to do it properly. Is there any use for me to go for my PE? If I weren't doing controls, I'd be doing embedded software or something...hardly an area where people have a PE.
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Jul 13, 2013 00:46
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Speaking of Fluke, anyone have any recommendations for a decent all-around model for working in low voltage control panels? One of the sister companies of the place I work for carries Fluke, and I'm told we can buy them at cost for ourselves. All I have at home is a cheap Craftsman POS, so I'd like to upgrade to something decent for personal use. Primarily to take out to the field, because the general use meter for engineers is a POS so we usually have to beg one from a technician when we go out on site. What I'd really love is something that can do clamp-on 4-20mA measurement, but I'm sure that's an expensive accessory or separate unit. I'd need to measure 4-20mA somehow at the least.
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Sep 15, 2013 05:38
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M_Gargantua posted:I have a personal love of the 289. Every feature I've ever needed. Welp, just did some research and I'm minutes away from calling my distributor and dropping a wad of cash on one of those. That's a seriously impressive meter. I think the logging capability is what grabs me. Seems really useful for the many times where I've needed an extra set of hands to watch a signal while I try to replicate a problem. That said, what do you guys recommend for a cheap back-up meter? Something small and simple for basic measurements for when I don't want to haul my expensive rear end meter to somewhere it might get busted (e.g. top of a silo), but at the same time something that won't explode in my hands in the event of a transient.
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Sep 18, 2013 14:50
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KernelSlanders posted:I've got a piece of equipment (a servo drive similar to this) that didn't come with a plug but just has screw terminals for power supply. I assume it's intended to be put in an electrical cabinet wired directly into the mains. It runs on 120V AC, so I'm inclined to wire in a plug and just plug it into an outlet. My only concern is the exposed terminals. If they can be covered is this still a code violation? How severe? I think I read somewhere that we need to keep people 3' 6" away if there's exposed terminals. Is that right? Yes, that is certainly designed to go inside a cabinet of some sort. You might also need some other stuff like fuses/branch circuit protection before it's considered a proper installation. You'd have to check the installation instructions to see what else you might need. I don't know whether or not it's against code (I would guess it at least violates OSHA), but it's certainly a very bad idea to leave unguarded terminals out in the open. From a lawyer's point of view, you could be held liable if someone hurts themselves because they licked their fingers and touched the terminals. From a moral point of view, you don't want to be the one responsible for sending someone to the ER because they accidentally brushed up against the terminals while working on something nearby (which is how these things usually happen). You should be able to pop it in to an enclosure pretty easily. Hell, the project boxes at Radio Shack might be big enough if you're trying to be cheap, but your local electrical supply should be able to hook you up with any parts you might need. I'd also be concerned about heat if you're going to put it in to a box. Servo amps will put out some heat, and how much depends on how much power is going through it. Again, you'd have to check the installation instructions to see if they warn about this. Typically they'll give you a heat dissipation number in watts which you can use to run calculations to determine if you need additional cooling. Just to be sure, just wiring a plug to something and running it that way isn't usually up to code. Throwing it in a box will make it touch-safe, but that won't necessarily mean it's safe to use. There's all sorts of things you usually have to check, such as fusing requirements, wire sizing, power requirements, grounding requirements, etc. DaveSauce fucked around with this message at 18:28 on Oct 22, 2013 |
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Oct 22, 2013 18:26
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KernelSlanders posted:Electrically, it's installed correctly. I'm an EE so I know how to size fuzes, wires, etc. and cleared all that with the manufacturer's engineers as well. I've just never dealt with the physical installation before. Wiring into the circuit directly would require an electrician, which is why I was trying to hook it up as an appliance. Are you saying I can put it in a box and run a plug out of the box? I'm not very literate on appliances in regards to code, so hopefully someone else here is. I don't have access to the NEC anymore since I started working remote to finish a programming project. Most of what I deal with is control panels for permanent installation, and for permanent installations it's very much frowned upon to use cords and plugs (pretty sure it's against code, but I can't cite anything off the top of my head). If it's intended as portable equipment, though, then I believe you should be OK with the plug (again, I can't cite anything, so take this with a grain of salt). What I am saying, though, is that putting it in a box is the least I would do. I can't say it'll meet code just by doing that, because there's a lot to consider regarding cord connected equipment, but it's certainly better than leaving it out in the open. Personally, I'd do it for my own safety, but if this is going to be used by others or otherwise left unattended and energized, I think it's in your best interest to cover it up. As a point of reference, some bigger companies that we've worked with go so far as to require everything INSIDE a control panel to be touch safe.
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Oct 22, 2013 20:17
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Pvt Dancer posted:You can view the NEC for free if you make an account at NFPA, you only need to pay if you want a PDF. They also use some silly software protection that locks it to you PC but that's quite easily removed. I didn't know they offered free access, that's really good to know. I was tempted to yank the "FREE PDF" access key from the hard copy they gave us at work and take it with me when I left, but I couldn't bring myself to do it. But as much as I prefer online documentation (or at least the concept of it), nothing beats a paper catalog with tabs. Both my NEC book and UL508A book were stuffed with tabs for all the sections I frequently referenced. The hardcover "handbook" edition is what all the cool kids have, though. It has tons of explanations and examples to help you figure out what the hell the code is trying to say. Only our hardware engineering manager had a copy of that, the rest of us got the standard paperback. There's also a surprisingly large number of websites that are horrible to use. Siemens, GE, and ABB are the worst. Schneider/Square D is pretty bad as well, but the search function at least works relatively well. Allen-Bradley is pretty good if you know what you're looking for, but I really wish they'd all take cues from McMaster-Carr. I've never seen a website that makes it so easy to find the exact product you're looking for. Almost makes me wish I was a mechanical engineer just so I could use it more often. DaveSauce fucked around with this message at 22:32 on Oct 22, 2013 |
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Oct 22, 2013 22:29
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Yeonik posted:Because we Americans have to do things differently. I also never understood why everything has HP ratings, contactors, breakers, etc. Aren't we beyond that by now? Code says everything must be HP rated, but it seems like there are instances where that's not sufficient to size things properly. I usually size things based on nameplate FLA, and even that's flawed since your inrush curve is different for different motors. Seems to me that the HP or kW rating are missing gobs of important information on the electrical side.
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Dec 21, 2013 16:29
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AmbassadorTaxicab posted:All that talk about using PPE a few pages ago is making me wonder if it's safe to flip the house breaker at all. Depends on a lot of things, and the problem is most things aren't labeled properly right now. Calculations and labeling for arc flash hazards are relatively new, and very few places label equipment properly. I've only been to one job site that bothered to do a proper study and label things, and that was in Canada at a site that was for a huge multi-national oil company. I don't have a copy of NFPA 70E handy, but I'm 90% sure it's perfectly safe to flip anything in your house. Don't quote me on this, but I think that anything 240VAC single phase or less doesn't require PPE as long as there are no exposed terminals (which is a shock hazard, not an arc flash hazard). I think one of the requirements is that it's on the isolated secondary of a transformer, which is what your house is probably supplied by. Transformers do a pretty good job of limiting fault current, so if a fault happens it's not going to provide enough energy for things to explode in your face, especially at lower voltages. The 600VAC was probably not entirely safe to switch without some degree of PPE, the minimum being safety glasses, hearing protection, long-sleeve cotton shirt (non-melting material), long pants (non-melting material), cotton underwear/socks (non-melting material), and leather gloves. But that's the minimum required. It all depends on the amount of energy that the circuit can deliver in the event of a fault (e.g. short circuit). A typical household circuit breaker can withstand 10,000 amps before failing destructively (i.e. exploding in your face). This may sound like a lot, but in an industrial or commercial setting it isn't (though in a home it's more than enough). Most power sources can supply a lot more than that in the event of a short circuit. One thing to remember is that the PPE is only calculated to ensure that you survive an arc flash with nothing worse than 2nd degree burns. You're still going to get hurt, but you'll live without too much permanent damage.
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Dec 28, 2013 18:41
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Is there an easy way to calculate available fault current in a place where no study has been done? I ask because at my new job, they're having me re-design some of the controls for a test station. I wanted to go the route of using MPCBs in a group installation to save panel space, but the current panel is "rated" at 100kA SCCR, and the best I can do with MPCBs is 30kA. I say "rated" because the panel has been monkeyed with over the years, so I don't know if that rating is valid anymore, but I don't want to downgrade from that if I can avoid it. To be sure, this is all a moot point anyhow since cost dictates that I stick with spare parts that I can scrounge from the shop, which will be fuses and contactors, so I'm not worried about the SCCR anymore. But, this kind of spurred me to dig deeper and now I'm curious about how to get a rough calculation so I can at least get a worst-case scenario to start from when no other data is available. I've never done this sort of calculation before, so I did some googling and found a document from Bussmann (located here). I trust them in this arena, so I looked it over. I know that the supply transformer is the limiting factor, so I used Bussmann's document to gather the parameters I needed and run the numbers: it's a 500kVA transformer with a 480Y/277V secondary and 2.7% impedance. With this, I calculate that the available fault current at the transformer is about 27,500A (assuming a 10% voltage sag and a 10% lower impedance to account for tolerance). Now without accounting for motors adding fault current (which I could find if necessary), would this be a reasonable worst-case scenario for the entire building? Or are there other factors that might increase the fault current? It seems to me that the length of wires will just add impedance, which would result in a lower fault current at any given point beyond the transformer. What I'm trying to avoid here is doing a full on analysis that includes cable sizes and lengths, since I don't have any of that information. If I can get a worst case estimate from just the supply transformer and any motor loads, I can at least get a quick ballpark that will tell me if I need to beef up my panel's SCCR or not. edit: Just checked the main 800A breaker to the building and it has a 35kA IR. I sure hope my calculations are right, or else we're in trouble... DaveSauce fucked around with this message at 20:07 on Jan 23, 2014 |
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Jan 23, 2014 19:58
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grover posted:Are there any generators at this site? If so, are they open or closed transition? If your napkin calculations are coming out that close to the MPCB rating, you may have to bite the bullet and call in a pro do to a full fault current analysis & coordination study. Honestly, arc flash analysis is code required now anyway. None that I know of. Just the transformer and a few motors. Really, I'm just trying to educate myself on how to get a basic calculation done to get a ballpark for my own reference. I've already scrapped the idea of MPCBs for this panel, but the whole thing just got me thinking is all. Since most places don't have any sort of fault current study done, I just want to be able to run a quick number to see if a potential worst case scenario is worth further investigation. My original question is a perfect example. Since my quick calculation puts me a hair under 30kA, I know that a panel with a 30kA rating is pushing it and would require further study to see if it would work. But, since I know there aren't a significant number of motors or other sources, a 65 or 100 kA rated panel will be more than adequate. So, all that said, how does one get in to doing short circuit and coordination studies? I just do controls so I'm usually given a SCCR number to design towards. I don't ever get in to calculating the available fault current, but recently I've found it interesting. Since it affects my design, I'm interested in what goes in to this calculation so I have a better idea of the hows and whys of it all.
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Jan 24, 2014 15:09
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some texas redneck posted:Lets talk about what it takes to avoid rolling blackouts. Oddly enough, this is exactly what they do. At least, it's what I've seen from an end user perspective. Several years ago I worked in the IBM building in Rochester, MN doing a 2 semester co-op. It's a pretty big campus and probably uses a good deal of power for all the servers and shock/vibration test equipment they have. Several times that summer there were announcements over the PA that told everyone to shut off lights and unneeded computers/equipment because the power company called and asked nicely. I don't know how much good it did, but I'm pretty sure they were the largest power customer in the area, so they were probably first on the "shut off your lights" phone tree.
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Feb 6, 2014 02:21
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Anyone have any experience with conductive grease for corrosion inhibition inside control panels? We're evaluating the use of something like that in our panels on ground bars/lugs/etc. The main problem being that when you scrape the paint away, you open up the subpanel/enclosure to corrosion. One of our senior engineers is recommending we move towards galvanized subpanels for that reason, but I would think that there will still be some remaining issues. Ideally we're looking for something conductive so we can use it on all exposed surfaces and not worry about connection issues. We've come across 2 options: T&B Kopr-Shield and Sanchem NO-OX-ID . Anyone have experience with either of these? Kopr-Shield is UL listed, so we're leaning towards that one. However in researching this I keep seeing NO-OX-ID come up as a more commonly used option. The other question is, is NO-OX-ID actually conductive? The meter says no, but I don't know if that's really a valid test.
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Feb 8, 2016 15:02
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We're a machine builder, and this is going in to a panel for a customer, so our insurance has no bearing on this. We occasionally UL list our panels, but I don't believe UL508A addresses this. I do know that NEC addresses it, but I think the only application NEC cares about is protecting the threads on field threaded conduit. I don't know if its use on bus bars/ground lugs/terminals is considered at all. This came up because another engineer is particularly concerned about an application that will be near the ocean. The enclosures are stainless, but the subpanel is painted steel. Our panel shop always scrapes the paint for ground bars/lugs, but they never seal them with anything. The panel shop at my last company always put some sort of goop on to protect the bare steel, but that was a shop standard so I never knew what they used...it just happened and I took it for granted, so now that I'm at a place that doesn't do that I have to re-invent the wheel. That said, all else equal, I would personally pick the UL listed version over the non-UL version. So that's where we're at: are these in fact equivalent products, or do they each have their place?
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Feb 8, 2016 21:45
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