AC or DC....that is the question

[Schniapereli]

(fyi, it's E=IR where E is voltage)
There was also something on history channel that I saw about AC. It showed lineman wokring on live wires with just gloves (which is a big deal). I don't remeber the exact voltages, but it was a lot. Enough to carry the wire like accross a state. They lift the workers in a helicopter on a platform. The lineman hold out a clamp, and energize themselves to match the power line. They wear some suits that takes care of the magnetic field. (or some kind of field. The field doesn't hurt them, but is uncomfortable.)
Also when somebody gets hurt, they have to carefully lower the victim while keeping a certain distance from the ground and lines or the current will jump to the victim, then to everybody else and kill them all. I don't remember all it said, and what it called everything, but it was really interesting. I wish they'd show it again. So, there's a small exaple of your body's electrical properties changing. (don't know if it was resistence, or some other property in AC I have never heard of...)

 

[Van]

When working on High voltage lines the guys will clamp onto the line as a way of actually enegizing themselves. The idea is that since there is no path to ground there is no circuit. Linesman in these situations are carring a charge of 7200 volts or more, but they are not hurt because there is no path to ground. It's the same as a squirrel running across a power line and not frying, he's only on one of the hot wires, and there is no nuetral running from the pole. from what I understand the worst side effect is a skin crawling sensation like really bad static electric charge from dragging your feet while wearing a polyester sweater. As far as the distance required for an arc to jump, you'll find most ladders and lift equipment; genies,condors,scissors etc. will have a diagram or chart of minimum safe distances to maintain from high voltage lines. Most people don't realize that even those lines runningn through your neighborhood are carrying 640 + volts.

 

[Chris15]

Ship is an expert in this stuff and can generate tons of great detail.

I ain't Ship, but I hope that what I say will be useful

Here are a few other points to consider.
If you really measure AC voltage, you will find that the 120v etc is really an average, and so it has peaks and dips that are around 40 volts either side of the 120, so in some cases you can take a 120 volt rated lamp and run it at around 100 volts dc and get the same level output.

If anyone knows otherwise, correct me. The 240 volts RMS (Root Mean Squared) that is normal mains voltage down here (Should work with appropriate subsitutions for other countries) means that it can do the same amount of work as 240 volts DC. The actual voltage oscillates between about +340 volts and -340 volts. (I think it is some sort of maths applied to this that explains the 415 volts between phases.)

from what I understand the worst side effect is a skin crawling sensation like really bad static electric charge from dragging your feet while wearing a polyester sweater.

For what it may or may not be worth, static electricity is frequently in the hundreds of kilovolts, though with only a miniscure amperage, hence static rarely does any damage.

As far as the distance required for an arc to jump, you'll find most ladders and lift equipment; genies,condors,scissors etc. will have a diagram or chart of minimum safe distances to maintain from high voltage lines. Most people don't realize that even those lines runningn through your neighborhood are carrying 640 + volts.

Electricity will jump 1cm for every 10 000 volts in DRY air, further in humid air. I thought that some of the lines running through streets were more than say 640 volts, but have no real idea. I should find out.

 

[Van]

again a 240 vs. 120 basic difference . I would never claim to know a thing about electric power supply in a foreign country. As you are in Austrailia I have no Idea what standard power transmission voltages are for your situation.

 

[Chris15]

again a 240 vs. 120 basic difference . I would never claim to know a thing about electric power supply in a foreign country. As you are in Austrailia I have no Idea what standard power transmission voltages are for your situation.

You are absolutely correct there. I have a feeling we use a couple of kilovolts and just assumed halve it for the US, but thinking more about it, I am no longer willing to take that stab in the dark.

 

[Van]

You are absolutely correct there. I have a feeling we use a couple of kilovolts and just assumed halve it for the US, but thinking more about it, I am no longer willing to take that stab in the dark.

And to further confound things we have major differences here in whether a supply is running into an industrail area or a residential. While standard in some residential areas might be 640, 720, 1040,etc. when you get into an industrial area all bets are off. then you've got that whole 220 vs. 208, and don't even get me started about Delta ! You know sometimes I think I could have saved myself a lot of headaches if I had listened to my first college stagecraft instructor whos said and I quote," Electricity is easy. You turn on the switch here, the light comes on up there. Got it ? " I had to go out and get further eji-mucated, and go and confuse myself.

 

[Chris15]

You know sometimes I think I could have saved myself a lot of headaches if I had listened to my first college stagecraft instructor whos said and I quote," Electricity is easy. You turn on the switch here, the light comes on up there. Got it ? " I had to go out and get further eji-mucated, and go and confuse myself.

Take it a smidgen further, The electricity travels through a whole load of wires and other things, then you turn the switch on and it works (or it should)

 

[SHARYNF]

Reminds me of the electronics theory that all electronics work on self contained smoke, if the smoke comes out, it doesn't work.;-)

Sharyn

 

[koncept]

someone else was just telling me about magic smoke that's inside all those little components and when it comes out they stop...it definately simplifies the explanation of things.

outlet + magic smoke = working something....

i think there are advantages to both systems. if you think about it all electronics equipment has transformers in it that step it down and convert it to dc...

 

[SteveB]

You are absolutely correct there. I have a feeling we use a couple of kilovolts and just assumed halve it for the US, but thinking more about it, I am no longer willing to take that stab in the dark.[/QUOTE
Long distance electrical transmission in the US can be as high as 765,000 volts A.C. There's a transmission line with this capacity that runs from Messina, NY - from the Robert Moses dam on the St. Lawrence river at the Canadian border, down the west side of the Adirondak Mt.s to Marcy, NY where it hooks up to the rest of the NY State grid. The power mostly comes in from Canada as there's a lot of Hydro generation up in the east Hudson Bay area of Quebec as well as the Labrador area.
In general, most Hi-Vo lines are 365,000 or so for long distances. Local street local distro is often around 13,800v which the local transformers ramp down to 120/208. Obviously there are specific industrial applications which use 277v, 480v, etc...
The Long Island area of NY recently got a boost in local capacity with the construction of a 660 megawatt DC transmission line, from Sayerville, NJ, under the Atlantic Ocean across the mouth of NY Harbor, to a point on land in western Long Island. They are building a new conversion station in Hicksville, NY to transform to AC and then into the local power grid. It adds about 10 percent capacity to the system. This is all European technology, where DC for long distance is common. One of the advantages to DC in this instance, is there's no need to sync separate generating systems to the 60 cycle frequency that all US and Canadian generating systems operate on. I'm still a bit baffled as to how it is that DC works in this instance as my understanding of the initial days of electrical system start-ups, was that DC had very poor long distance capability. I suspect that they are running this at very high voltages to avoid line losses.
Here's a great link to a site that shows the power of electricity
http://205.243.100.155/frames/longarc.htm
SB

 

[SteveB]

Never mind.

Wikipedia has a decent description of the reason we went to AC from DC

http://en.wikipedia.org/wiki/Electricity_distribution

SB

 

[fosstech]

I know for a fact that the single phase line above my house back home is 7200 volts. How do I know? I read the specs on a new transformer before they lifted it up the pole

 

[Van]

Reminds me of the electronics theory that all electronics work on self contained smoke, if the smoke comes out, it doesn't work.;-)

Sharyn

Oh I'm going to steal that one. I like it a lot.

 

[SHARYNF]

Van
Have fun with it, usually gets some grins
Sharyn

 

[SteveB]

Never mind.
Wikipedia has a decent description of the reason we went to AC from DC
http://en.wikipedia.org/wiki/Electricity_distribution
SB

Oi !.

I meant to say "DC to AC". In any case, an in depth read of the Wikipedia article has good links to recent developments in DC power systems, so my post wasn't too far off.

SB