Edison made great play of 'alternating current' being dangerous. He aimed to get the business of supplying cities with his direct current system.How and why would his, or any,direct current system be safer than the alternating current system proposed by Westinghouse?

In the USA you use 110 volts in homes. In Europe we use 240/250 volts. The US lines were said here to be safer but more expensive to install and run than ours. Is that true? Surely you can kill yourself with 110 volts at home: it must depend on the amperes you get, mustn't it?

Yes 120 VAC in the US is definitely lethal, depending on the current density in the heart, which depends on (1) the available pathways for current to flow; and (2) the voltage.

By the way, it's standard in the US to speak of 120/240 volts rather than the older 110/220. I think rms is 117 v. Maybe Frankvan could confirm this point?

The legendary radio-next-to-the-bathtub scenario involves a large contact area, a good ground, and wet skin having high conductance. And guess what frequency of alternating current is most likely to induce ventricular fibrillation of the heart?: 50-60 Hz.

In practical terms either you get shocked or you don't. The factor of 2 in voltage (which becomes a factor of 2 in current density) between US/Canada vs. UK/continent is not all that predictive of outcome from an electrical shock. It's a risk that should not be taken. I still remember the days of "hot-chassis" radios and tvs that had a 50-50 chance of bare metal hardware on the back of the device causing electrical shock, depending on how the unpolarized, ungrounded plug was situated.

It's much worse to grab a conductor with each hand, because then current will flow from arm to arm, traversing the heart along the way. Whereas if you touch both a hot and a ground/earth wire with, say, thumb and index finger of one hand, then the current will only flow through the hand. The safety rule for electricians is: keep one hand in your pocket. Better yet, make sure the power is turned off.

In the Edison vs. Westinghouse contest, I think at the time it was much easier to transmit AC than DC over long distances, and it made voltage conversion trivial using simple transformers.

In the modern era power is transmitted at much higher voltages, over much longer distances, than in the late 19th C. Thus I've heard that DC transmission is actually more efficient or cost-effective than AC -- sorry no source for this. (Maybe because dc is not subject to the reactive losses of ac?)

High voltage direct current ( Wikip. ):

quote:

High voltage direct current (HVDC) is used to transmit large amounts of power over long distances or for interconnections between asynchronous grids. When electrical energy is required to be transmitted over very long distances, it can be more economical to transmit using direct current instead of alternating current. For a long transmission line, the value of the smaller losses, and reduced construction cost of a DC line, can offset the additional cost of converter stations at each end of the line. Also, at high AC voltages significant (although economically acceptable) amounts of energy are lost due to corona discharge, the capacitance between phases or, in the case of buried cables, between phases and the soil or water in which the cable is buried.

HVDC links are sometimes used to stabilize against control problems with the AC electricity flow. In other words, to transmit AC power as AC when needed in either direction between Seattle and Boston would require the (highly challenging) continuous real-time adjustment of the relative phase of the two electrical grids. With HVDC instead the interconnection would: (1) Convert AC in Seattle into HVDC. (2) Use HVDC for the three thousand miles of cross country transmission. Then (3) convert the HVDC to locally synchronized AC in Boston, and optionally in other cooperating cities along the transmission route. One prominent example of such a transmission line is the Pacific DC Intertie located in the Western United States.

Of course Edison had his own prejudices but there are valid arguments on both sides of the DC vs AC question. The Russians were the early pioneers in the long distance transmission of DC power, because the vast distances involved between points of generation and towns and factories in the old Soviet Union.

As far as electrocution is concerned a current as low as 200mA is sufficient to kill most people, so that the risk is probably no different in the US/Canada or the EU. It is useful to remember that an RMS value of 120 Volts has a maximum value of 170V, and the resistance of the human body varies over a wide range, and is mostly in the skin. Once the skin is punctured the blood vessels are very good conductors. One thing I noticed in my teaching days is that my students were much more sensitive to electric shock than I was. Apparently older skin is drier and a better insulator. But I also used to have students stand on a plastic milk crate and hold onto the top of a VanDeGraaf generator. A harmless 200,000 volts would make their hair stand straight off their heads with no perceptible shock. They would get the type of small static shock one gets when shuffling across a carpet and touching the door knob, when they stepped off the crate. Voltage, or Current alone is harmless. Ohm's law prevails, but a small shock can knock you off the ladder and the fall can kill you, and a short circuited car battery can blow up in your face. I'd be curious about the incidence of accidental electrocution in US vs EU?

quote:

Originally posted by Professor:
And guess what frequency of alternating current is most likely to induce ventricular fibrillation of the heart?: 50-60 Hz.

Why was 50-60hz arrived at as the standard ? Is there some technical reason for it being better than others?

I don't think there is any simple, single answer to the question, is 50 or 60 Hz chosen for electrical distribution? I would say it is a compromise. In lighting, very early installations used 25 cycles, with a noticeable flicker to the incandescent lights. On the other hand, 25 cycles made it possible to produce motors that had sufficiently slow speeds that they could be used in applications where they replaced steam engines. For a given Horsepower motor, the slower speed resulted in higher torque. Oops, I have to leave, dinners on. I'll be back!

In alternating current circuits the speed of motors is directly proportional to the frequency, so a 50 cycle motor would have speeds 5/6 of the speed of 60 Hz ones, consequently they are not interchangeable. Some economic as well as geographical considerations would help determine what frequency is chosen. In order to have interconnected power grids between Canada and the U.S, Canada early on abandoned 25 and 50 cycle systems. I would imagine that the EU has made similar adjustments. I would expect that globalization would favor a common frequency, but the existing infrastructure is so big, that's probably unlikely to happen.

Thinking about the need to compromise, the fact that both inductive and capacitive reactance limit the flow of current in opposite ways: inductive reactance is proportional to frequency and capacitive reactance is inversely proportional would mean that for a given application one or the other (50 or 60 Hz.) would be the better choice. In general for long lines of transmission the higher the frequency, the more the current tends to flow in the skin of the wire, at high enough frequencies it tends to leave the wires entirely and produces radio waves. Lower frequency tends to be less efficient in motor design but DC being the absolute lowest frequency would be most efficient in long transmission lines. But three phase AC is more economical in terms of the number and size of wire used in transmission over long lines. It just occurs to me that I could just as readily advocate for either frequency. It's six of one and half a dozen of the other. IMHO. Prof may have a different opinion, or more "state of the art" information. I belong to the old "slide rule" days.

Very interesting, Frank. By the way, slide rules got me through high school and college -- THEN they invented calculators!

Do you know why US residential single-phase voltage has evolved from 110 to 117 to 120 volts?

I don't think it evolved so much as it grew like Topsy (just growed). When I was teaching apprentices I found that most electricians almost invariably referred to house voltages as "110 volts" so I assumed that they learned that from older electricians and that was the original term. Since early residential electrical use was geared to incandescent lighting I think it was the choice of light bulb manufacturers. I seem to recall that it was a compromise: lower voltages required heavier wire filaments, higher voltages (especially in low wattage bulbs) resulting in too fragile a hot filament. I even remember carbon filament bulbs - God, I'm old! Tungsten filaments and inert gas addition to the bulbs allowed higher temperatures and "whiter light output". I also remember seeing different light bulbs rated at 110v, 115v, 120v, and some 125v. Obviously again in became necessary to compromise between light quality and bulb life. Recognize also, that economically it became smarter to standardize and I'm sure that the NEMA and the NEC had input. House wiring insulation is generally rated at 600 volts, because AC max or peak value is ~170v and switches have to be able to interrupt a circuit without arcing and burning. That, is one of the big disadvantages of DC systems: DC at 120 volts has a tendency to arc like crazy - necessitates arc blow-out features on breakers, fuses, and other devices. Nowadays, we use all sorts of transformer/rectifier combos to run our TVs, DVDs, printers, etc. because we like to be able to control speed in many cases, and DC motors can be varied over a wide range from zero to self-destruction in the larger sizes.

I apologize for getting carried away. I so seldom get a genuine electrical question to discuss these days, I get positively giddy! I suspect I haven't really answered a question so much as having eplained what I don't really know, which could fill volumes.

quote:

lower voltages required heavier wire filaments, higher voltages (especially in low wattage bulbs) resulting in too fragile a hot filament

I remember that P = I²R = V²/R, so power is inversely proportional to resistance. I was reminded of this -- in a different way -- when I recently bought a package of 25-W incandescents. At such low power they use thin filaments, all of which were broken inside the new bulbs! (made in China?)

Yes, arcing at metal contacts can be a problem, though 'solid state relays' (semiconductor devices that can switch high loads with no moving parts) have hardly taken over the mechanical switch market! As a dabbler in electronics, I think most switches intended for 120 vac service seem to be rated at 600 vac. The same is true of triac light dimmers.

The Van de Graaf generator is an oldie but goodie, still used today at science demos for kids. When I was a teen our class toured Argonne Nat'l Lab (government physics installation outside Chicago), where we saw a gigantic Van de Graaf generator (close to a million volts, I think) used to feed their synchrotron. Scientific American had a construction article in the mid-1960s ("conducted" by CL Stong, The Amateur Scientist) for a VdG that used a motorized rubber belt to pump static charge to its dome. Obtaining the dome was simple: "Visit your local aluminum spinner..." or some such advice. Simpler times.

In the Edison vs. Westinghouse question, I hadn't considered the important issues surrounding electric motors for industry and the transitioning from steam to electricity. Thanks for educating us.

I always heard house voltages referred to as 110 and 220 (there's a line in the movie Mr. Mom where the Michael Keaton character, ignorantly boasting of a planned electrical upgrade, is asked if he's going to install 220. His reply: "Yeah, 220, 221, whatever it takes.") But now the standard reference seems to be 120/240. And 117 volts also seems to remain current (). Do these numbers all refer the same actual voltage, or has the standard actually increased over the past 50 years? A google search has been of no help in clearing this up.

And there's still FredPuli's question about why Westinghouse chose 60 Hz, and (I would add) why Europe's frequency standard is different?

All my hands-on experience with electrical wiring is confined to wood-frame residential in the US. I've wired (or re-wired) entire houses, though usually with a knowledgeable friend. What's ironic is that we own a condo with metal-stud walls, and I don't have the slightest idea of how to alter the wiring, other than using a surface-mount Wiremold system.

Patently,we have 50 Hz and not 60Hz because we are so cultured that we could never accept a tone which is not musical. 50Hz is G sharp.

This fact has great scientific and artistic relevance to us in Europe, naturally . The opening note of Richard Strauss'Also sprach Zarathustra is a G sharp. Britons know this as the 'signature tune' of the Apollo 11 mission and first moon walk, the opening bars being played before each broadcast.

60Hz is a nothing! It's somewhere between B flat and B. We get a better class of hum

I dunno Fred. Here's a chart of piano frequencies, using equal-tempered tuning based on A4=440. You're right about 60 Hz. being between B1 and B♭1, which are 61.74 and 58.27, respectively. But 50 Hz. falls between G♯1 (51.91) and G1 (49.00), which doesn't bear out the British claim to superior hum! But are you saying that every time you hear the 2001 Space Odyssey theme, it sounds like an old alarm clock buzzing at mains frequency?

I think the nameplates on electrical equipment actually should be considered an acceptable range of voltages, rather than a target. I read recently that in order to achieve a greater degree of compatability between the various existing voltages in the UK and the EU a compromise was agreed upon. I think the Brits had something like 240 and the EU had something like an average of 220, so the standard they settled on was 230. As long as there isn't too much variation most of the status quo of the various member states is satisfactory. We shoot for 250 at the transformer on the pole at the end of your block to achieve 240 at the house on the other end of the block, and probably if you took your voltmeter to every house on the block you'd get a range of voltages. Drops are inevitable because Impedance is never zero. In the steel industry where I worked we had a 34.KV line surrounding the plant, at the individual mills we had transformers that stepped it down to 6900 V to bring it in to the mills. There we shot for 480V 3 phase for motors, 6900 for MG sets to generate 250 V and 600V DC. The 6900 Volt 3 phase synchronous motors were popular for MG sets because they can supply leading currrent for power factor correction. Around the time that I retired we were beginning to convert some of the DC systems to rectifier supplies. First we used mercury arc rectifiers, later silicon diodes and SCR's.The development of the solid state stuff made big changes but you don't readily get rid of 5,000 Horsepower motors and the generators they turn. I'm sure they had similar considerations in the rest of the world. To this day there are still cities in this country that have 25 cycle street lights.Obviously if you get 480 distribution, the step-down lighting voltage would more likely be 120, and so on down the line.

"we own a condo with metal-stud walls, and I don't have the slightest idea of how to alter the wiring,"

I'm not sure but I think there is provision for wiring channels between studs, so it should be easy enough to fish Romex through a few holes in the dry-wall. Of course the local codes may not allow anything but EMT. I have heard that that's the only thing that passes code in New York City?? No non-metallic-sheathed cable - the National Electrical Code is usually only adopted by the counties and towns with added codes of their own I'm sure politics plays a role also.

As to Fred's original question about why choose 50 or 60 Hz; I doubt there is a single answer or a superior/inferior one. There are several considerations. One: 60Hz has higher efficiency in rotating machinery, higher inductive reactance, more radio interference, lower capacitive reactance, more skin effect. 50 Hz is slightly less efficient as far as motors are concerned, higher capacitive reactance, less skin effect. I would imagine that there were people who would have preferred something other than what they had to accept, and people who would have decided differently if they had known what the future technology held in store. The number of poles on a magnet are two, so an alternator has multiples of two. So the least number of field poles in an alternator being two the highest synchronous speed of the rotating magnetic field would be 3600 (120f/poles) for the 60Hz choice and 3000 for the 50 Hz machine. I have a hunch that the early Westinghouse generating systems were hydroelectric. These water-driven generator are huge in diameter and the speed of the coils on the circumference is high. So the numer of field coils would be high also. In order to get a high enough voltage from the slow rotational speed and still get a frequency of 60 cycles per second would necessitate many field coils. One cycle means that the coil must pass a North pole and then a South pole in one direction, and then a North and then a South in the opposite direction. So 60 cycles means that a two pole machine must rotate at 3200 RPM, a 16 pole machine at 400 RPM, etc..
Hydroelectric generators (alternators) at 187.5 RPM and 12,000 volts, 3 phase originally 25 cycle were later converted to 60HZ. Source:http://www.iaw.com/~falls/power.html
Surprisingly, to me, the early AC generating at Niagara Falls was either DC for Electric Railway trains, and AC @ 25 cycles. Only in recent years was the unit Hertz substituted for cycles per second
From the same source:
The Niagara Falls Power Company offered a $100,000 prize for anyone who could develop a method to transmit electricity long distance. No one responded to this offer. A world wide search began. A think tank of the worlds most brightest minds met in London, England.
Younger geniuses favoured alternating current while the elders favored direct current.
Against the advice of Thomas Edison and William Kelvin, alternating electrical current was selected as the standard to be used.
Nikola Tesla was born in Smiljan, Croatia in 1856.
Tesla created an effective alternating current (AC) transmission system that would be adapted worldwide. George Westinghouse, a American inventor and manufacturer began development of Tesla's system.
In 1883, Westinghouse created an illumination system for Niagara Falls using AC current.
On May 6th 1893, the Cataract Construction Company decided to use alternating current (AC) for power generation and transmission.

quote:

Originally posted by Professor:
And there's still FredPuli's question about why Westinghouse chose 60 Hz, and (I would add) why Europe's frequency standard is different?

found this, hope it helps

George Westinghouse did his original engineering using 133 1/3 Hz. Westinghouse had an steam engine driven alternator set running at 2000 rpm (By 1886 mechanical engineers liked to have steam engines in integral numbers of rpm) and with 8 poles the set produced 8000 cycles per minute or 133 1/3 Hz. This was good for lighting as there was no flicker but it turned out it was too high for motors later developed.

The earliest experiments (1886 and 1887) used belt driven generators and tended toward high frequencies like 133 1/3 Hz. This suited illumination, which was practically all that alternating current was used for at that time. By 1889 and 1890 direct driven generators were coming on line. They were more robust but with lower rotation speeds they encouraged lower frequencies.

In the early years of ac there were many frequencies: each engineering team seemed to pick their own. Early frequencies in the US were 133 1/3, 125, 83 1/3, 66 2/3, 60, 50, 40, 30, 25 Hz. When Tesla joined Westinghouse, it was using 133 1/3 Hz. Tesla insisted upon 60 Hz because his ac induction motor was designed for 60 Hz and apparently wouldn’t work at 133 1/3 Hz.

On the Westinghouse Museum website it says that G. Westinghouse assigned his engineers Stillwell, Shallenberger, Schmid, and Scott to find a good frequency. Practical considerations of connecting alternating generators to reciprocating engines then in use demanded a lower frequency than 133 Hz.
Before the end of 1892 they chose 2 frequencies: 60 Hz for lighting and 30 Hz where power was to be converted to DC.

Why did Tesla/ Westinghouse engineering team choose 60 Hz? If it was Tesla that was the driving force, various biographies of Tesla declare different theories ranging from Tesla “thought it was the fundamental frequency of the universe” to “… considered the natural earth had a frequency of 10 Hz and doing experiments with 8 to 20 Hz and 20 to 40 Hz and finally 40 to 100 Hz; he decided that 60 Hz was safe.” It doesn’t seem to have been a desire to do accurate clocks because Henry Warren didn’t patent the synchronized clock until 1916 long after the frequency was chosen. Although Warren was diligent in getting utilities to have tight specs on frequency this didn’t happen until into the 1920’s.

Back in the early 1890’s Westinghouse was involved in bidding electrical equipment for the Niagara Falls power project. However the Cataract Company (in charge of the Niagara Falls project) had already selected hydraulic turbines running at 250 rpm. So if a 16-pole generator were chosen the frequency would be 33 1/3 Hz and if a 12-pole machine were chosen then the frequency would be 25 Hz. The project consultant proposed an 8-pole generator or 16 2/3 Hz. The compromise was 25 Hz. At the time lower frequencies were easier to handle on transmission lines. Another reason is that the Steel industry liked 25 Hz because of huge slow speed induction rollers, which had a low power factor for 60 Hz and worked better at 25 Hz. Niagara Falls generated 25 Hz way into the 20th century. The website says that the Westinghouse Company later wished it had forced through 30 Hz.

By 1910 it looked there would be two frequencies in North America, 25Hz for transmission and heavy industry that needed dc or slow moving heavy machinery and 60 Hz for lighting (less flicker) and general use.

There was an effort by GE to introduce 40 Hz as a compromise between 25 Hz and 60 Hz in the 1890’s but it was too late to overtake the 60 Hz and 25 Hz infrastructures already in place although there were some 40 Hz installations. Even so most installations in the US were done in 60 Hz after Westinghouse and GE cross licensed their patents.

Development of high-speed turbines instead of slow reciprocating machinery and later developments of the rotary converter that worked well at 60 Hz made it easy to shift everything to 60 Hz. By 1920 most of the problems associated with 60 Hz transmission had been solved so that there was no longer any advantage of transmitting 25 Hz over 60 Hz. That seems to be why the US is 60 Hz.

Germany took the lead in Europe of developing electrical power (primarily Emil Rathenau of AEG) and AEG seems to have used 50 Hz from day one. In 1891 AEG had demonstrated power delivery over long distances using 50 Hz. I don’t know why AEG chose 50 Hz. Did the penchant for integer rpm help influence AEG for 3000 rpm and 50 Hz as opposed to 3600 rpm and 60 Hz? Did the preference for preferred numbers influence the choice of 50 Hz over 60 Hz? Did Tesla’s influence pull Westinghouse to choose 60 Hz and resultant 3600 rpm over 50 Hz and 3000 rpm? Europe was even more fragmented in the early days than the US. In 1918 in London alone there were 70 electric authorities with 50 different types of systems and 10 different frequencies and 24 different voltages. But by the 1920’s and 1930’s more and more of Europe was changing to or working with 50 Hz.

As for voltages both Europe and the US seemed to have begun with about 100 to 110 Volts DC because of Edison’s success with replacing gas lights with electric lamps. Although many inventors worked on electric lights, generators and electrical systems, Edison was one of the first and was successful in putting together whole systems not just the pieces. Edison picked 110 VDC because that was the voltage he needed to get enough light out of his bulbs to compete with common gas lamps of the time and yet not blow the filaments in his bulbs too soon.

The Berlin Electric Works (utility owned by AEG) changed from 110 V to 220 V starting in about 1899 to enlarge the capacity of their distribution system since the city (Berlin) was already wired 2 wires. They were probably changing from dc to ac at the time also. They paid for their customers to change their lighting and motors to 220 V and saved on the cost of copper by avoiding having to add more wiring. This spread throughout Germany and later Europe but didn’t take hold in the US.

I wonder if the residue from the bitter conflict between Edison and Westinghouse about the safety of AC vs. DC spilled over into not going above 110 volts for residential users even after Edison’s forces conceded the need for AC.

A lot of this information comes from Thomas Hughes Networks of Power : Electrification in Western Society, 1880-1930 and Benjamin Lamme Technical Story of Frequencies IEEE transactions 37 (1918) 60. Benjamin Lamme was chief engineer for Westinghouse in the early 1900’s.

Thank you, j2f, for the informative posting.

Welcome to the pool. J2F, and thanks. I think your very informative post pretty much confirms what I had suspected; 50Hz or 60 Hz - a toss up! Just thankl God 25 Hz has almost disappeared.

I can't even imagine 25 Hz -- there would probably be noticeable flicker even in incandescants, and fluorescent lights would probably induce headaches and maybe even seizures in a lot of people!

Plus, at 25 Hz. there are fewer zero-crossings, reducing the bandwidth of small signals that "ride" on the AC waveform at those points (pretty much limited to X-10 lighting controllers now -- but just wait a few years for wireless internet to be delivered through your power wiring...)

quote:

Originally posted by Professor:
I can't even imagine 25 Hz -- there would probably be noticeable flicker even in incandescants, and fluorescent lights would probably induce headaches and maybe even seizures in a lot of people!

Plus, at 25 Hz. there are fewer zero-crossings, reducing the bandwidth of small signals that "ride" on the AC waveform at those points (pretty much limited to X-10 lighting controllers now -- but just wait a few years for wireless internet to be delivered through your power wiring...)

just a personal observation in trying to understand power line carrier (PLC) Wi-Fi, broadband or PON. A quick search comes up with PLC networks have been used exstensively for a few years with some local utilitiy companies, cable TV operators appear to be using WAN Wi-Fi within concentrated rural area's where signal is a problem or when laying of land line cables becomes cost prohibitive.

Cell networks for this discussion (internet and broadband) in rural, small outlying regions, density and terrain can be a problem also for internet users or when the traffic for the internet provider sees it cost prohibitive to service those areas. So why not set-up PLC and have the transceiver (like a cell repeater) strapped to the electricity (hydro) poles on the outskirts of a village, town where the power to the area comes in, and have a two way communication along the power line...technology is available ain't it!

In house PLC adaptors are already available for those that have modems installed. On a web search, I found those products.

Technology is moving fast and with the new DOCSIS 3.0 standard and PON, things can only get better

The wikipedia article on PLC (power line communication) talks about BPL = broadband over powerlines:

quote:

BPL seems, at first glance, to offer benefits relative to regular cable or DSL connections: the extensive infrastructure already available would appear to allow people in remote locations to have access to the Internet with relatively little equipment investment by the utility...Some industry observers believe the prospect of BPL will motivate DSL and cable operators to more quickly serve rural communities.

Of course the article points out the numerous pitfalls, such as high levels of noise and attenutation. It goes on to say,

quote:

Broadband over powerlines has developed faster in Europe than in the United States due to a historical difference in power system design philosophies. Nearly all large power grids transmit power at high voltages in order to reduce transmission losses, then near the customer use step-down transformers to reduce the voltage. Since BPL signals cannot readily pass through transformers — their high inductance makes them act as low-pass filters, blocking high-frequency signals — repeaters must be attached to the transformers.

In the U.S., it is common for a small transformer hung from a utility pole to service a single house or a small number of houses. In Europe, it is more common for a somewhat larger transformer to service 10 or 100 houses. For delivering power to customers, this difference in design makes little difference with power distribution, but it means delivering BPL over the power grid of a typical U.S. city will require an order of magnitude more repeaters than would be required in a comparable European city. However, since bandwidth to the transformer is limited, this can increase the speed at which each household can connect, due to fewer people sharing the same line.

(boldface mine)
DOCSIS, I have learned since your post , is Data Over Cable Service Interface Specifications, "...employed by many cable television operators to provide Internet access over their existing hybrid fibre coaxial (HFC) infrastructure." Sample of an actual spec.

Sorry, janus2faced, but I don't know what PON stands for.

By the way, in my previous post I meant to say "internet delivered thru your power wiring" -- which is definitely not wireless! There is, however, ongoing research into using magnetic induction inside every room (since people are relatively immune to high magnetic fields) to provide wireless, battery-less electrical power to portable devices.