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helno posted:It can keep going without the grid but cant start. PHT pumps take more power than the standby generators can deliver. We need around 40 Mw to run up.
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Dec 22, 2013 03:36
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You only need a handful that can darkstart to get the grid going again. That and it would be extremely rare to have a complete failure of the grid with all generators offline. Even in 2003 there were parts of the grid that islanded and were able to bootstrap the rest once they figured out what was wrong. This is where protection schemes need to be very well orchestrated.
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Dec 22, 2013 05:30
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afen posted:Why do you guys talk about motor power in HP? I've always used kW when talking about electric motors. The kilo-watt (KW) is a unit of electrical energy in the US.
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Dec 23, 2013 17:39
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Groda posted:The kilo-watt ( Watts are a measure of power. Energy is power over a certain amount of time. Energy is measured in kWh (as in, one kilowatt for one hour), J, or BTU depending on the context.
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Dec 23, 2013 18:59
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Enilev posted:Watts are a measure of power. Energy is power over a certain amount of time. Energy is measured in kWh (as in, one kilowatt for one hour), J, or BTU depending on the context. No, I'm pretty sure you're mistaken.
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Dec 23, 2013 19:19
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Groda posted:No, I'm pretty sure you're mistaken.
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Dec 23, 2013 19:34
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edit: beaten
FrozenVent fucked around with this message at 19:39 on Dec 23, 2013 |
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Dec 23, 2013 19:37
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edit: nm, I want to see how this turns out
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Dec 23, 2013 19:38
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Noctone posted:Do tell. It's a U.S. Customary unit: 1 kilowatt x 1 hour = 1 kilo-watt Groda fucked around with this message at 20:34 on Dec 23, 2013 |
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Dec 23, 2013 20:32
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Groda posted:It's a U.S. Customary unit: 1 kilowatt x 1 hour = 1 kilo-watt So you're saying a kilowatt is a kilowatt / hour?
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Dec 23, 2013 20:38
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Even if what you're saying is true (which I highly doubt and can't find any reference to), it doesn't invalidate enilev's post.
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Dec 23, 2013 21:19
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Groda posted:It's a U.S. Customary unit: 1 kilowatt x 1 hour = 1 kilo-watt Groda, I'd love to see a source for this. I need to call my electric company and tell them they're billing me in the wrong unit. 
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Dec 24, 2013 02:00
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Kilowatt‐hours in general are kind of dumb when joules exist, but at least I’ve never seen someone misuse the “kilo‐watt” like that before.
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Dec 24, 2013 02:31
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I prefer calories.
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Dec 24, 2013 10:15
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BTU is not in the SI and therefore unholy and shall never be used.
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Dec 24, 2013 11:21
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I think Tons of Cooling has BTU beat as far as archaic units go.
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Dec 24, 2013 13:43
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NihilismNow posted:BTU is not in the SI and therefore unholy and shall never be used. Also, having six definitions and being used as a unit of power in the US...
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Dec 24, 2013 13:51
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Guy Axlerod posted:I think Tons of Cooling has BTU beat as far as archaic units go. You need to tell me more about this amazing unit NOW.
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Dec 24, 2013 15:13
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Guy Axlerod posted:I think Tons of Cooling has BTU beat as far as archaic units go. Two Finger: it represents cooling power associated with freezing (or melting) a ton of ice per day. BTU was created so that 12000 BTU/hr = 1 ton. grover fucked around with this message at 15:35 on Dec 24, 2013 |
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Dec 24, 2013 15:28
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Two Finger posted:You need to tell me more about this amazing unit NOW. In ye olden days, if you wanted something cooled significantly below room temperature, your only option was ice. For millennia, only kings could afford to have ice hauled from distant locales to chill their beverages, but in the nineteenth century harvesting, storing, and delivering ice year‐round became a major industry. If you wanted your building cooled (generally because it was a warehouse storing perishable food), you could contract with the ice company to deliver x tons of ice to you every day. You just let the blocks of ice sit around and melt, cooling your stuff in the process. That’s where ton comes from in refrigeration context: it’s the amount of cooling provided by one short ton of ice, delivered daily. When mechanical refrigeration came along and you decided to modernise your warehouse, you replaced daily deliveries of x tons of ice with an x‐ton refrigeration unit. This makes it a unit of power. I have a 0.024‐ton computer. If you think about it, it’s no weirder than bomb megatons. Only a few hundred tons of TNT have ever been gathered in the same place and detonated, but that doesn’t stop us from extrapolating to rate atomic bombs. Platystemon fucked around with this message at 16:49 on Dec 28, 2013 |
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Dec 24, 2013 16:15
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Groda posted:No, I'm pretty sure you're mistaken. Groda don't take this the wrong way, but you are absolutely wrong here. Like red flag illuminated with flashing lights level wrong. As mentioned before: power is measured in watts, watts integrated over time is energy (in joules, kwh, etceteras,) Like on the power meter I use at work, there are charts for power and energy. The bus might show a constant load around 2MW, and over an eight hour period the energy chart is a upward line (y=2x+c) and starting at zero, after eight hours gets up to 16MWh. Three-Phase fucked around with this message at 04:35 on Dec 25, 2013 |
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Dec 25, 2013 04:32
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DaveSauce posted:I also never understood why everything has HP ratings, contactors, breakers, etc. HP is voltage-independent, which allows mechanical engineers to just specify a HP and then feign ignorance to this crazy concept called "voltage" and wipe their hands clean.
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Dec 27, 2013 04:17
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Three-Phase posted:Groda don't take this the wrong way, but you are absolutely wrong here. Like red flag illuminated with flashing lights level wrong. Papercut posted:HP is voltage-independent, which allows mechanical engineers to just specify a HP and then feign ignorance to this crazy concept called "voltage" and wipe their hands clean.
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Dec 27, 2013 05:22
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Platystemon posted:....Tons of refrigeration...
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Dec 28, 2013 16:31
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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. At a summer job, I got to flip a 600VAC switch for a plug for a pressure washer. I had my doubts about it when I was doing it, but turned out ok. I later learned I should have been using a wooden stick to throw the switch.
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Dec 28, 2013 17:08
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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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Just make sure all panel covers are installed, and figure out why it's tripping. I would like to note however, that all entrance cable which is before your main breaker, is protected only by your utility and is EXTREMELY hazardous. Say you've got a 4/0 service coming out of a 100 kva underground transformer to your house (common in a subdivision). It will take a massive amount of fault current to trip the utility protection. Probably enough to burn the fault in the clear before it trips. In newer houses, they usually install a combination meterbase that has a main breaker. So, this dangerous current potential is limited to this one box outside and everything past the breaker is protected. But in slightly older homes, they would sometimes just have a meterbase outside, feeding a main panel somewhere in the house. The entrance cable from the meterbase to the main is a serious hazard, and in some cases is stretched all across the house depending on where they decided to locate the main panel.
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Dec 28, 2013 19:35
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Groda posted:Okay, no more playing it straight--I'm just joking about how much people say "kilowatt" as shorthand for "kilowatt-hours" in the US, even in the industry. As an EE, you probably don't hear it as much at work, I'd assume... More "megawatt" and "megawatt-hours". Also "megaVAR" and "megaVAR-hours" (with leading/lagging direction). But we also do horsepower but only for motors (tens of horsepower to thousands of horsepower). DaveSauce posted: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. The problem with labels is that they must be accurate. Like if I have a newly installed bus servicing six motors, those motors plus the feed will contribute to the fault current during an electrical faults (the motors can act like generators at the instant of the fault). So you have your labels. In five years you upgrade the transformer going into the bus from 500kVA with 6%Z to 750kVA with 5%Z, and add four new motors. Now the calculations need to be re-run and new labels applied.
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Dec 29, 2013 02:02
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The risk of an arc flash would only be present with a switch under load, would it not? In an industrial/commercial environment, the load could never be assumed. In a residential environment, there's control over the load, especially if you're doing your own work. How much of it is thinking of every eventuality and how much of it is for the sake of protocol?
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Dec 29, 2013 03:00
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What if the switch is faulty?
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Dec 29, 2013 03:50
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Or your work. :P
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Dec 29, 2013 06:22
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One example I saw was a story about this guy who couldn't open a fairly big 600V disconnect after it was installed. He pulled pretty hard on the lever, but no dice. Eventually they decided to have an outage. They kill power feeding the panel, bypass the switch interlock, and remove the cover. It turns out the switch mechanism was in the path of one of the cables, blocking it from operating. It was even starting to dig through the insulation.
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Dec 29, 2013 13:09
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Note to self: don’t pull too hard. 
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Dec 29, 2013 13:45
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I had a trouble call on a sewage lift station one Saturday morning, electrician said one phase is out. It's a big platform bank, 480 corner ground delta. Their switchgear and breaker panel are inside a fence directly under the platform structure. It is a relatively confined space for them, in front of the panel. I check voltage ok up top, but they still have bad phase to phase on one leg, on the load side of one of their breakers. They decide to reset the breaker. I ask them to let me get set up to pull the fuses up top just in case. He throws it off, then back on, and it starts sizzling like bacon in a pan. He throws it off again, and as I say "I don't think that's a go..." he goes on with it and the back of the panel lights up like an arc welder. I ran out and snatched the fuses as quick as I could. Fortunately no one was hurt, which was luck given their proximity, and the cover plates they had already removed which exposed them further. As I recall, they said the breaker had an internal fault.
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Dec 29, 2013 15:41
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I'm surprised the fuses didn't stop the fault...
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Dec 29, 2013 17:25
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If you mean the high side fuses, they're at least 150% of kva, maybe more on that bank, and there is a delay curve on all our fuses. Everything in utility protection assumes a fault is temporary and gives time to clear it. Why their fuses didn't clear it, well that's a good question. I didn't get to look at it afterwards, maybe the fault path limited the current somehow.
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Dec 29, 2013 18:36
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If it was an arc fault, that would have less current than a bolted fault. Which would result in longer clearing times.
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Dec 29, 2013 18:42
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Groda posted:At the US coal plant I interned at while an undergraduate student in Sweden, the only power unit on our GE turbine's nameplate was "134 100 hp." The generator was labeled in metric, at least. You know, those could have been metric horsepower. I wish I was kidding, but I'm not. Per google unit calculator: 1 metric horsepower = 735.49875 watts This. This is why we cannot have nice things.
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Dec 29, 2013 18:43
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angryrobots posted:He throws it off again, and as I say "I don't think that's a go..." he goes on with it and the back of the panel lights up like an arc welder. I ran out and snatched the fuses as quick as I could. Fortunately no one was hurt, which was luck given their proximity, and the cover plates they had already removed which exposed them further. As I recall, they said the breaker had an internal fault. Stupid question! Why didn't pulling fuses on a faulted circuit cause another arc fault in your face?
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Dec 30, 2013 04:55
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It does, unless you break the circuit faster than the arc can catch up with the retreating contact. At least that's how I understand it. How fast you need to move depends entirely on the current flowing and the reactivity of the load.
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Dec 30, 2013 06:06
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