Re (2): UPS with earth leakage device.
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From: Moe Trin on 9 Jan 2010 16:43 On Fri, 08 Jan 2010, in the Usenet newsgroup comp.os.linux.hardware, in article <6p1l17-8ip.ln1(a)lapu-lapu.bildanet.com>, GangGreene wrote: >How many wires from an electical substation to the trasformer on the >pole? >Answer One. Depends on the load, location, and any government requirements. In most cases it's two. >The power is returned through the earth, through a ground system. Maybe in your local conditions - but it's hardly universal. There is this little variable called "ground resistance" sometimes listed in a table as "ground constants". In case you aren't aware, there is well over a 10:1 difference in the conductivity between (for example) pastoral land with a rich moist soil, and rocky soils and sandy deserts. There really are areas of the US (and quite a few other countries) where a 25 foot / 7.6 meter long ground rod will have a ground resistance in excess of 25 Ohms. Cities, where everything is paved have similar problems. Old guy
From: Moe Trin on 9 Jan 2010 16:45 On Sat, 09 Jan 2010, in the Usenet newsgroup comp.os.linux.hardware, in article <7icm17-a4q.ln1(a)lapu-lapu.bildanet.com>, GangGreene wrote: >ArameFarpado wrote: >>> How many wires from an electical substation to the trasformer on >>> the pole? >> 3 life phases, each AC with it's sinosoidal wave lagged 120<C2><BA> to the >> others. every time 2 phases are positive there is one negative and >> vice-versa. >> no need for neutral here. Depends - is it a three phase star or delta? Stars are nearly always grounded at the center, while delta may be grounded through a center tap in one of the legs. >You are mixed up. I am talking residential power distribution you are >talking appliance distribution. You seem to think that the world is all the same as a countryside district - I'm living in a residential district with no industrial loads, yet the distribution is three phase 13.2 KV. 'ArameFarpado' is in Portugal which is in Europe, and it's very common there to distribute 3 phase in residential areas. Single phase _MAY_ only be found out in the countryside well away from towns. >Notice only one leg of the 3 phase is coming down from the overhead 3 >phase lines over head to the xmfr on the pole. > >If it was as you believe then two legs of the 3 phase would be in use. >It's not. Apparently you also live in an area that doesn't get thunderstorms. Most of the North American power companies have a "ground" line above the hot distribution line for lightning protection. This is more prevalent where the distribution voltage is higher. I've seen places where 2300 volt lines lack lightning protection, but the more common 13.2 KV has it. One need only look at the high voltage distribution lines - anywhere from 56 KV (insulators about 18 inches long) on up to 460 KV (insulators 5 feet or more). These are usually three phases, and there are one or two uninsulated conductors on the tops of each pole or tower. >So tell me why is one leg used on the xfmr and one bare copper line >going from the xfmr down the pole to a ground rod beside the power >pole for then? That's copper clad steel, but no matter. This depends on the location, the load, and the local regulations. Here (Arizona), there is ALWAYS a "ground" lead at the top of the pole, and the transformer connects between the hot lead and that "ground". There may or may not be a wire coming down the pole and connecting to a ground rod. >Also when talking polyphase power distribution why then is a grounded >conductor used/required on the service entrance drop from the pole to >the residence? >If it worked as you say it would only require two conductors to be >brought to the residence. I'm going to assume you are really this ignorant, and not trolling. The third wire is used in North America because the standard is two nominal 120 volt lines with respect to that common. The wire is used because the resistance of a ground path between your distribution box and the transformer is at least one and maybe two orders of magnitude (that is "ten" to "one hundred" times) higher than the resistance of the "third" wire - call it a neutral or common as you may. Do you know Ohm's Law? My house has a 300 Amp service (2 120 Volt feeds each "fused" at 300 Amperes). Assuming even a small inbalance in the load on the two sides - say 30 Amps - how much power would be lost by not having a that third wire? Old guy
From: Moe Trin on 9 Jan 2010 17:12 On Sat, 09 Jan 2010n the Usenet newsgroup comp.os.linux.hardware, in article <4b48cda0$0$270$14726298(a)news.sunsite.dk>, ArameFarpado wrote: >>>> How many wires from an electical substation to the trasformer on the >>>> pole? >substation to transformer is residential distribution? since when? The terms used in Europe do not match the terms used in the US. Here, a substation generally means a transformer site where the voltage is stepped down from the 'cross-country' values (which may be 56KV on up) to the levels distributed to the neighborhoods (which may be 2300 to 13200 Volts). Separate transformers - either on the local pole, or on the ground - step the voltage down from this distribution value to the residential values (120/208 or 120/240) and this is distributed to one to as many as ten houses. These transformers are not called sub-stations. Cross-country distribution is almost always 3 phase, but the neighborhood distribution might be single phase as often as three phase - depends on how crowded the neighborhood is. >> So tell me why is one leg used on the xfmr and one bare copper line >> going from the xfmr down the pole to a ground rod beside the power >> pole for then? >aren't you imagining things? It's not common, but it does occur. >tri-phasic residential distribution is done with 4 wires: 3 phases >and 1 neutral, the neutral is necessary for monophasic devices. Residential use of three phase is rare - it may be found in large apartment buildings, but even that is rare. >in europe we have 400volts between phases and 230volts between any >phase and neutral, if you have 400volts light bolbs then you can >connect them between 2 phases, but all monophasic devices are >230volts over here, so the distribution company must provide the >neutral along with the phases. Residential use is 120 volts (phase to ground) for all except fixed appliances (water heater, electric range, electric dryer, central air conditioners). Those fixed appliances may get 240 volts (an Edison three-wire system supplied by a center tapped transformer) or 208 volts (two phases). >it is also common to see 3 phases and a neutral entering a bilding >only for monophasic service, that's about balance the phases: 1 phase >feeds 1/3 of the apartments, the 2nd fase the other 1/3 and the 3rd >phase feeds the last 1/3 of the apartments. This does occur, but is not very common. Large apartments are not as common here. Old guy
From: ArameFarpado on 9 Jan 2010 17:20 Em Sábado 09 Janeiro 2010 21:42, Moe Trin escreveu: > On Sat, 09 Jan 2010, in the Usenet newsgroup comp.os.linux.hardware, in > article <4b47d15a$0$270$14726298(a)news.sunsite.dk>, ArameFarpado wrote: > >>Moe Trin escreveu: > >>> So by your definition, there can not be a "neutral" in a car or >>> airplane. > >>no neutral, cars and planes have a body and one if it's poles is >>connected to the body, usualy the negative pole. > > The metallic body (more correctly the frame) is being used as a > conductor. Most cars do not have multiple electrical systems. Planes > with more than one engine ALWAYS do - and even single engine aircraft > often have more than one electrical system. Something like an airliner > will have a minimum of three electrical systems (a legal requirement), > often more. Each has it's own generator or alternator (28 VDC or > 115/200 V3Phase400Hz). Some _may_ also have a very limited amount of > 50/60 Hz single phase in the lavatories for passenger use. > >>planes have a kind of stick that touches he ground (and some cars >>also) to put the body at the same potential as the ground they are >>standing; > > I am a commercial airplane pilot, and have had my license for over > 30 years. You are absolutely wrong in this statement. There is no > "stick" and never has been - simple mechanical problems like the > compression of the tires, and wind resistance preclude it, never > mind the considerable maintenance issue of constantly replacing it > when it would be knocked off by the ground contact. > >>planes must have this or people could get electricuted by touching >>the plane body after a landing... > > What is the resistance of the rubber compound used in tires? Assume > an aircraft with a surface area of 350 square meters sitting 2.0 > meters off the ground (equivalent to a capacity of ~1600 picofarad) > and a tire resistance of 250 MegOhms. Do you know how to compute > (electrical) discharge time constants? How many seconds will it > take to dissipate a static charge (math: 1 TC ~ 0.25 seconds)? > > The only time airplanes are intentionally connected to an electrical > ground is during refueling operations - the plane gets grounded, as > does the fuel truck or hydrant, and a separate ground connection is > made between the fuel hose and the aircraft before the host/nozzle is > allowed to touch the aircraft. This is to prevent static discharges > from causing a spark and igniting the fuel vapo[u]rs. > >>some people more sensitive to estatic also place this sticks in >>there cars so they don't get shocked when they touch them or when >>the put the feet on the ground getting out of the car. > > Quite a lively imagination you have - same problem as with the > "stick" you claim is on aircraft: compression of the tires as > well as the movement of the suspension system, and the maintenance > issues. It used to be common for fuel trucks to have a length of > steel chain dragging on the ground to discharge any static build-up. > The end of the chain would eventually wear enough to allow the chain > to break, often leaving sharp pieces of metal on the highways. This > discharge function has been transferred to the tires, which are > slightly conductive due to the carbon black material used in creating > the tire. When the fuel trucks are loaded or unloaded, they have the > same grounding wire technique used with aircraft. Next time you see > petrol/gasoline being delivered by truck at a gas/petrol/fuel station > notice the grounding wires connecting the truck and hoses to the > ground lugs. > > Old guy you never seen one of these? http://www.explainthatstuff.com/car_static_strip.jpg http://www.made-in-china.com/image/3f2j00lMKTGsLgHhoiM/Reflective-Strip- Static-Belt-with-Light.jpg
From: ArameFarpado on 9 Jan 2010 17:32
Em Sábado 09 Janeiro 2010 21:45, Moe Trin escreveu: > >>> no need for neutral here. > > Depends - is it a three phase star or delta? Stars are nearly always > grounded at the center, while delta may be grounded through a center > tap in one of the legs. wait, that's a common error that most people have; i've seen electricians connect the star point to neutral in triphasic motors, but that is unnecessary. any machine that is 100% triphasic (like a commom triphasic motor) don't need neutral at all, regardless if it is connected in star, delta or zigzag (this last ir rarely seen and it requires 6 coils instead of 3). > > That's copper clad steel, but no matter. This depends on the location, > the load, and the local regulations. Here (Arizona), there is ALWAYS > a "ground" lead at the top of the pole, and the transformer connects > between the hot lead and that "ground". There may or may not be a wire > coming down the pole and connecting to a ground rod. same here. that's primary for lightning bolts |