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McJuicy posted:On that subject, when you have an ungrounded camping generator, what happens in the case of fault situation? How is the metal of the generator bonded? The electrical grounding system is important in fault clearing; modern grounded electrical systems are designed so that if an energized wire shorts to ground, enough current will flow to trip the breaker, hopefully before someone gets hurt. If it was a simple fault like that in your ungrounded portable generator, like an insulator frays and energizes a metal enclosure, it will create a high-impedance fault. Simply put, electricity needs a complete circuit, and incidental contact between the generator chassis and the earth and creates a very high resistance, which involves only small amounts of current. So, some electricity will flow, but not enough to trip the breaker. Which means it will remain operating under fault conditions, and will stay operating if a person happens to get in the fault path. It may or may not be enough to kill you.
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Jan 18, 2012 02:37
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How does a ground rod ground? Is dirt more conductive than I give it credit for? Does the answer depend on the dirt? I suspect that driving a ground rod into a dry sand dune won't do much.
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Jan 18, 2012 06:30
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Frozen Horse posted:How does a ground rod ground? Is dirt more conductive than I give it credit for? Does the answer depend on the dirt? I suspect that driving a ground rod into a dry sand dune won't do much. Utility companies frequently use the actual earth as the return conductor. Lets them run just one conductor to rural areas.
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Jan 18, 2012 11:51
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Frozen Horse posted:How does a ground rod ground? Is dirt more conductive than I give it credit for? Does the answer depend on the dirt? I suspect that driving a ground rod into a dry sand dune won't do much. Dirt can be conductive, it's got moisture and minerals in it. Now in some parts of the world, you need to do different things to get a good ground, like adding salt to the ground, or installing a "Ufer ground". Concrete also conducts electricity, something I didn't know until I was in college.
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Jan 18, 2012 11:51
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Concrete holds moisture so it makes sense that it conducts (plus I have personal experience being shocked by a faulty appliance while standing on it). Also around here ground rods have to be driven quite deep due the hot summers and for precisely the reasons you'd think. Oh and ground isn't always zero volts which is where ground loop hum comes from in audio systems... Two paths to ground that have slight differences which results in a current flowing between them through the interior wiring which manifests itself as a hum. Or it can be due to voltage drop, etc... Someone more knowledgeable may be able to chime in on that.
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Jan 18, 2012 16:48
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One often overlooked function of a ground is that it sets a reference for all other voltages in the system, making it much easier to analyze and protect. Once you get into ungrounded systems it gets really  , really quickly.
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Jan 18, 2012 18:02
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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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My opinion on the dust collectors? It almost sounds like it's a little too big for LV, and a little too small for MV. Does the plant already have a medium voltage source like 2400 or 4160V? Or does it have a higher MV source like 13.8kV? I also take it that those are induction motors, not synchronous. He's gonna get dinged for power factor unless he installs some form of compensation. What is that at 0.8pf, around 100kVAR? Yeah, 3x350, the bend radius has to be in the multiple foot range. I've seen Okonite 15kV shielded triplex at work and that's even worse, you're talking a 5-10 foot bend radius.
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Jan 19, 2012 11:58
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Medium voltage is expensive, gigantic and a bigger pain in the rear end to work with. It is also potentially more dangerous, which tends to shun people away.
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Jan 19, 2012 21:23
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Cheesemaster200 posted:Medium voltage is expensive, gigantic and a bigger pain in the rear end to work with. It is also potentially more dangerous, which tends to shun people away. Where I work, there are separate electricians that do HV work (over 600V) and LV (less than 600V). That might be one major complication. Has anyone here done electrical stuff in mining? I see these circuit breakers that are for mining applications and they're rated at 1000VAC. (I think that's 1000/690.) What's up with that strange voltage? Three-Phase fucked around with this message at 03:54 on Jan 20, 2012 |
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Jan 20, 2012 01:08
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Criminally stylish posted:Im sorry if this question has come up and feel free just to quote the answer, and I would be a happy man When I was four I stuck a finger in a light socket. That was bad enough that it made me respect electricity very much, and to this day if I am working on anything that can do 48V or higher at more than a milliamp, one hand stays in my pocket at all times. Last year I got just a tiny pop of 120 when I was working on a preamp and it scared me off the workbench for the afternoon. I'd rather be skittish and overly careful than randomly dead with a widow standing over me cursing my hobby. You're not going to meet too many dudes who have taken 480v or higher.
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Jan 20, 2012 09:54
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In a recent scientific poll, 100% of electricians interviewed that admitted to having received electric shocks on the job were still alive! Conclusion: electricity is harmless. I've been zapped a few times by 120V, earliest was as a kid, plugging in a weedwhacker to an extension cord, and probably inadvertently touching the prongs in the process. Most recent was a defective table saw. The whole damned case is plastic, including the power switch, so it naturally freaked me out when after 2 years of use I suddenly got a shock turning it off, I just couldn't figure out where I could possibly have been shocked from. When it happened again a few minutes later (wearing leather gloves, even), I dismantled the fucker and discovered one of the screws holding the power switch on had been driven straight through the hot conductor. Must have just been how I was flipping it off that particular day that I brushed it. I have a VERY healthy respect for 480V, and almost always wear proper PPE (goggle, gloves, arc flash jumpsuit, faceshield, etc., as appropriate) when working on anything hot. grover fucked around with this message at 15:35 on Jan 20, 2012 |
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Jan 20, 2012 15:32
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I got shocked with 347 lighting. That poo poo makes you stick.
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Jan 21, 2012 01:12
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McJuicy posted:I got shocked with 347 lighting. That poo poo makes you stick. 347 lighting? You in Canada?
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Jan 21, 2012 23:21
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Cheesemaster200 posted:I found the label making tool in SKM today.... I've actually seen this kind of label, calculated flash was 200+ cal/cm2 and boundary was 50+ feet. "No safe PPE exists" was the last line on the label. It was on a drive cabinet for a MV drive that was installed without an isolation transformer, directly onto a 10kA+ bus network at 4160V. I didn't like being in that room, much less opening that panel (fortunately, I was just there for the LV stuff). quote:Im sorry if this question has come up and feel free just to quote the answer, and I would be a happy man Yeah, I got hit with ~250VDC from a big breaker's trip coil. Hurt like hell, arm tingled for the rest of the day.
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Jan 22, 2012 06:31
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Three-Phase posted:Where I work, there are separate electricians that do HV work (over 600V) and LV (less than 600V). That might be one major complication. I've been doing electrical for mining for almost a year, I haven't seen anything like that...yet. Most sites I've dealt with do their distribution at 13.8kV (4160V for older mines), and step down to 600V (Canada). However, I've only dealt with permanently installed equipment. A lot of the development tools are foreign to me, some of the jumbos drills or kiruna hauls trucks may use 1000VAC to charge or operate.
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Jan 30, 2012 23:07
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How many PDCs can crash into bridges on one project? Also I'm seriously going to bring home some questions from work. I work in materials. Office Bitch Local 616, Pivot Tablers.
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Feb 2, 2012 03:45
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Meow Meow Meow posted:I've been doing electrical for mining for almost a year, I haven't seen anything like that...yet. Most sites I've dealt with do their distribution at 13.8kV (4160V for older mines), and step down to 600V (Canada). I've seen a few 900VAC devices before, and we have a lot of equipment in the 900-1800V class. Several of these drives are installed in mine hoist systems, so I can see 1000V breakers being useful there (though usually we put the breaker on the medium voltage side).
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Feb 2, 2012 07:32
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This is a great thread, I love reading about things like this that most people aren't aware of but are essential to our daily lives. A couple of videos I found interesting: Cleaning 500kv power lines with... a helicopter! http://www.youtube.com/watch?v=lcjhjna9jZE Better yet, let's lower a bloke onto live 500kv power lines to work on them! http://www.youtube.com/watch?v=4SX6Ucbb1l8 Totally bonkers.
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Feb 2, 2012 10:46
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ruro posted:This is a great thread, I love reading about things like this that most people aren't aware of but are essential to our daily lives. A couple of videos I found interesting: 800kvDC They test some of the apparatus at 1,000,000V AC for a minute. Three-Phase fucked around with this message at 00:52 on Feb 3, 2012 |
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Feb 3, 2012 00:50
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When you're demoing old duct bank or conduit, how do you tell if a line is live without putting someone in danger by accessing it? This assumes you have no drawings and no idea what's in the ground.
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Feb 3, 2012 03:14
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Stonelegs posted:When you're demoing old duct bank or conduit, how do you tell if a line is live without putting someone in danger by accessing it? This assumes you have no drawings and no idea what's in the ground. If there's current flowing, it should have a detectable magnetic field. If it's energized DC at open circuit, this becomes an interesting problem. Then, you might want to consider tracing the conduit to build up your own "as-built" drawings. The bad-movie option: sever the conduit with a linear shaped charge. If the explosion is followed by arcing, it contained an energized circuit. On the other hand, you're at a safe distance provided that you didn't use wires to trigger the charge.
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Feb 3, 2012 08:09
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Stonelegs posted:When you're demoing old duct bank or conduit, how do you tell if a line is live without putting someone in danger by accessing it? This assumes you have no drawings and no idea what's in the ground. You'd visually trace the line or bus back to where it originates. It takes skill and understanding of the installation to do this, particularly in crowded conduit or cable tray. Bigger equipment (including HV) is much easier to trace because at higher voltages special cables/bus are used, smaller stuff can be more of a hazard/pain in the rear end. In a badly crowded cabinet, you can tug or wiggle on a single wire and use that to trace where it goes, or use a fox/hound tracer. As far as safety, there are items like the Fluke VoltAlert pens that glow and beep if there is a hazardous AC voltage present. The problem is when you have 125VDC that the VoltAlert cannot detect. 4160VAC in an insulated but unshielded cable makes the VoltAlert beep from about four inches away. (Yes, I cringe at that a little too.) Frozen Horse posted:The bad-movie option: sever the conduit with a linear shaped charge. If the explosion is followed by arcing, it contained an energized circuit. On the other hand, you're at a safe distance provided that you didn't use wires to trigger the charge. We need a "BAD ELECTRICITY" feature here- like "Bad Astronomy" except where movies and video games get electrical stuff right and wrong. Three-Phase fucked around with this message at 01:08 on Feb 4, 2012 |
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Feb 4, 2012 01:02
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Bad Electricity: Jurassic Park 1 http://www.youtube.com/watch?v=vWmIA5GGeFQ GOOD: You can't just throw the breaker. This is correct for larger low and medium voltage breakers. You have to charge internal springs, this is usually done by an electric motor (sometimes it's a modified electric drill motor!), but if you have no control power available, you can get a breaker with a manual charging device, like a lever or a crank. GOOD: The circuit breaker looks very accurate, I cannot see what make/model it actually is but I'm suspecting a Siemens low voltage breaker. I'm not sure if they make medium-voltage breakers (2400V or 4160V) that have a charging handle like that. GOOD: The operation of the breaker is accurate. BAD: The panel with all the different sections and the controls for that is a bit hokey. GOOD: The color coding for the different subsections panel where red indicates ON and green indicates OFF. Some industries use red (danger) for ON, and green (safe) for OFF. BAD: The raptor pushing through a curtain of large cables. Not neat or workmanlike for an installation- those would likely be inside a cable tray/riser. Overall that scene had surprisingly good attention to detail I wouldn't expect in a movie nowadays.
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Feb 4, 2012 01:19
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I get that conductors have expanding and collapsing magnetic fields but I don't truly understand how a hot and a neutral can "cancel each other out" magnetically as to not heat up a conduit and not being able to hook an ammeter around both conductors. Can you explain this concept?
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Feb 7, 2012 01:35
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McJuicy posted:I get that conductors have expanding and collapsing magnetic fields but I don't truly understand how a hot and a neutral can "cancel each other out" magnetically as to not heat up a conduit and not being able to hook an ammeter around both conductors. Can you explain this concept? Magnetic field lines exist in a cylinder around each wire. An easy way to picture this is the "right hand rule" where you give a thumbs up: your thumb is the direction of the current, and your fingers represent the magnetic field lines that encircle the wire. As the direction is reversed in the neutral and hot wire, the magnetic fields cancel each other out.
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Feb 7, 2012 02:45
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McJuicy posted:I get that conductors have expanding and collapsing magnetic fields but I don't truly understand how a hot and a neutral can "cancel each other out" magnetically as to not heat up a conduit and not being able to hook an ammeter around both conductors. Can you explain this concept? Another way of explaining this is that the flux is the total current crossing through a surface. In this case the surface is the cross section of the conduit or the loop of the ammeter. Your hot and neutral are carrying equal currents through this surface in opposite directions so they cancel out.
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Feb 7, 2012 04:16
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Little video from Ferraz Shawmut (BUY OUR FUSES!) that show the effects of magnetic fields generated on cables during a fault. Fuses have a lot of drawbacks, but to my knowledge they offer the best current-limiting protection in a power system. Bigger fuses aren't cheap, I saw where three fuses blew out that cost over $1000 each. However, them blowing protected a massive (multi-million dollar) >10,000HP drive. Three-Phase fucked around with this message at 17:28 on Feb 11, 2012 |
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Feb 11, 2012 17:25
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Simply Perfect!
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Feb 11, 2012 17:41
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Guy Axlerod posted:Simply Perfect! The only thing I could think of, in the future, if we are able to create solid-state circuit breakers that have a high interruption rating. Then you have the near-instantaneous action of a fuse, with the adjustability and reusability of a circuit breaker.
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Feb 11, 2012 18:46
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You guys talk about opening panels, and most of those fatal videos show a guy opening a panel or whatnot - it seems like there's some problem with the idea of the panel itself. There's no better way of closing that off while still having it accessible? I'm dumb, I know.
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Feb 16, 2012 22:30
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Ibsen posted:You guys talk about opening panels, and most of those fatal videos show a guy opening a panel or whatnot - it seems like there's some problem with the idea of the panel itself. There's no better way of closing that off while still having it accessible? I'm dumb, I know. grover fucked around with this message at 17:01 on Feb 17, 2012 |
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Feb 17, 2012 02:39
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This thread has really driven home how little I know about electricity.
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Feb 17, 2012 06:02
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Just wanted to say I really appreciate this thread. Being from Västerås, Sweden where the Asea part of ABB got its start it's kind of cool to hear about the sort things they do (although obviously this thread isn't ABB exclusive in the slightest). ABB was a huge part of the town as I as growing up and my and several friends dads worked in various departments there. I'm currently at a consulting firm that has contracts with ABB (in both HVDC and Robotics). Hopefully I'll end up working with some of their HVDC stuff in the future, seems like really interesting work.
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Feb 17, 2012 16:39
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Three-Phase posted:347 lighting? You in Canada? All of the big buildings here seen to have 347V lighting, is this only a thing in canada?
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Feb 17, 2012 20:45
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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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The spring pumping thing on that Jurassic Park breaker, that's real, eh? I guess these things need a good bit of force/pressure to close/open the contacts fast and far enough, that's why it's assisted? Makes sense, never thought about it that way.
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Feb 18, 2012 11:22
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Jonny 290 posted:The spring pumping thing on that Jurassic Park breaker, that's real, eh? That's right. Small hand-operated molded case circuit breakers (low voltage, usually 600V or less) have handles that can be thrown up or down, and when you push the handle into the CLOSED position, it automatically should tension the mechanism for tripping the breaker. Those you don't pump, and I think those range from a fraction of an ampere up to 1200 amperes. On a molded case circuit breaker, there are three positions for the operating mechanism: -CLOSED (usually up, circuit is completed and electricity can flow) -TRIPPED (middle, breaker detected or told there's a problem and opened the circuit) -OPEN (down, circuit is broken on purpose) When you reset a tripped breaker, you need to typically move the lever from TRIPPED to OPEN and then to CLOSED so you prime the spring that can trip the breaker. --- Usually in a larger circuit breaker it's done with internal electric motors that prime the close and open springs. If you don't have any control power (like in Jurassic Park, the park only had some emergency power), you may have an override lever to pump the springs (like in the movie) or a crank to crank the springs. (You need to be careful, sometimes there are also cranks that rack the breaker out of the cubicle, and that can be extremely dangerous to do.) It's important that (if they're separate) BOTH the close and open springs are primed so that if you close the breaker into a fault, it can immediately trip open. Most molded-case breakers have a trip-free design, so even if the handle is pushed (or held/locked) into the CLOSED position, the breaker can still trip. For large circuit breakers, you have a TRIP and CLOSE coil. You energize (or de-energize for failsafe operation) the TRIP coil to trip the breakers, and you energize the CLOSE coil to close the breaker. The breaker has no way to tell if there's a problem in the electrical system, you need to install a protective relay to sense a problem and command the circuit breaker to open. In a lot of installations, the circuit breaker and relay control power runs off 120VDC. This is so you can have a bank of batteries to run the breakers even if there's a power outage, and if need be, you can get car batteries out of ten people's cars and operate the breakers in a pinch. In real life, the circuit breakers and critical power systems in Jurassic Park would probably have been run off 120VDC, so that whole priming thing would have been unnecessary. The 120VDC system allows a facility to "pull itself up by its bootstraps" in the event of a power failure. Three-Phase fucked around with this message at 14:15 on Feb 18, 2012 |
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Feb 18, 2012 14:12
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One other interesting tidbit about big breakers: Where I work, the large medium-voltage breakers are racked into and out of cubicles, this is so the breakers can be maintained, and additional isolation can be provided for safety. But you can also rack the breakers into a test position. This allows the breaker to be controlled, closed and opened, but it's not actually plugged into the bus - so no electricity flows. Pretty Cool Name posted:Just wanted to say I really appreciate this thread. Being from Västerås, Sweden where the Asea part of ABB got its start it's kind of cool to hear about the sort things they do (although obviously this thread isn't ABB exclusive in the slightest). ABB was a huge part of the town as I as growing up and my and several friends dads worked in various departments there. As an aside: I was joking around with co-workers about how cool it would be to have an ordering option to include an ABB engineer with the larger drives.  "So there's the new Megadrive... wait, who the hell's that guy?" "Oh, he's Paul. You add an '-ENG' to the end of the part number, and they also ship a engineer in that other crate." "Hallo! Ich kam mit dem Hochspannungs-Laufwerk!" "I can't understand a word he says, but he showed me how to replace the IGBTs."
Three-Phase fucked around with this message at 14:44 on Feb 18, 2012 |
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Feb 18, 2012 14:19
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Jonny 290 posted:The spring pumping thing on that Jurassic Park breaker, that's real, eh? All breakers have springs that trip open the breaker during a fault that you "charge" when you close a breaker (EG turn it on). On small household breakers, it's small/weak enough you can still just flip it pretty easy with a couple fingers. Get bigger (400-800A) and you have to use your palm and put some force into it. A lot have breaker-bar style levers you can put on the handle to help. Much bigger than that, and they start using internal ratchets to charge the spring. grover fucked around with this message at 14:42 on Feb 18, 2012 |
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Feb 18, 2012 14:25
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