Need help understanding 240 volt circuits.

I recently added a 240 volt dryer circuit. I'm not an electrician or electrical engineer. I read a do it your self book and had no problems what so ever. However, it sparked my curiosity(no pun intended) and I'd like to get a better understanding of the two phases used in US household electric systems. I understand that each hot bus in the panel is 120 volts and they are out of phase with each other, so together there is a potential difference which equals 240 volts. Do all 240 volt appliances require two separate 120 volt out of phase power sources? Do they ever connect each 120 volt wire together making one 240 volt wire? If not, what would happen if this was done? Would the two phases cancel each other out and result in zero volts? Or, would you have a major problem with your electrical system? I'm not thinking about doing this. I just want to get a better understanding of how the system works. I'm also wondering if it's common practice to split your circuits between the two hot buses. Any enlightenment would be appreciated.

Thank you, Steve Smith

Reply to
Steve Smith
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The US System is known as the Edison 3-wire or "split phase system" and as the name indicates, it was invented by Thomas Edsion as a means of saving copper by allowing higher distribution voltages and sharing a common return (neutral wire).

During the 1930's, the U.S. REA (Rural Electrification Agency) charged with bringing electricity to rural areas of the country, selected this system as being the most efficient, as it would provide two voltages (110 or 220 volts) and it saved on copper when compared with the alternative European systems that brought 3-phase power to homes and farms. Note the terminology (3 phase power vs. a 3 wire (Edison) system). They are different even though 3 phase power uses 3 wires (or sometimes 4 wires).

Three phase power has the inherent advantage of utilizing simpler, inexpensive motors for higher power applications (greater than 10 HP) but requires more complex service entrence equipment (3 main breakers instead of two, 3 buss bars, an extra insulated wire to the pole transformer, etc.). Modern 3 phase systems also require special sensors to detect a loss of phase to prevent the motor from locking and burning up, a condition known as single phasing.

Back in the 1930's as America electrified itself, one of the main concerns was the need for farmers to utilize large motors for a variety of needs. The fact that there were efficient motors above 10 hp that could be run on single phase circuits, such as the repulsion start - induction run type, influenced the decision that wiring every farm and home in America with 3 phase power was wasteful and inefficient. The Edison system was a better deal.

This site gives a good technical explanation of the 3 wire Edison Scheme.

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Beachcomber

Reply to
Beachcomber

That is a circuit breaker test. ;-)

Reply to
gfretwell

Thank you Beachcomber and G.Fretwell for the information. I'll read more at

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. Beachcomber said connecting the two 120 volt buses is a circuit breaker test. Is this really a test that an electrician would routinely perform? If so, will it trip all of circuit breakers or just the main?

Thanks, Steve Smith

Reply to
Steve Smith

He was kidding about the breaker test. Connecting 2 phases together is not a test in any fashion. It is a "FAULT" one of the most dangerous conditions that can occur in electrical equipment. A phase to phase fault can create heated plasma which can approach temperature of the sun. Ya know that big ball in the sky. Do not try this anywhere.

There are (EXPENSIVE) pieces of equipment to test circuit breakers. Yes it is done regularly in industry, not for residences. To test a breaker you must have the breaker curve which is published by the manufacture.

I suggest some more library time.

Reply to
SQLit

Thanks, I thought he might be. But, I saw the "Grounding to a Water Pipe" at the bottom of

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and I did wonder if such a tests might be done. I guess it is the equilavent of a gas line installer checking for leaks with a lighter.

Wouldn't the breakers trip and protect against this? Not, that I would ever want to do it, I'm just trying to understand the dangers involved in working on a home electical system

Steve Smith

Reply to
Steve Smith

They call this a "line to line, bolted fault" The only people who test breakers like this on purpose are labs like the U/L. under strictly controlled conditions. It certainly does happen in the real world but so do flash burns on the hands and eyes of the guy who closes that breaker. It is not supposed to happen (the reson for U/L testing) but it still might.

Reply to
gfretwell

Whoa! Beachcomber didn't say that... It was the other guy!

Beachcomber

Reply to
Beachcomber

"Beachcomber"

Sorry about that. I meant to show the quote was from Greg Fretwell, not Beachcomber.

Steve

Reply to
Steve Smith

Try it and see. nice yellow brown burns on the fingers & a touch of arc eye.

Reply to
Wrench

Certainly in a well installed system, yes the breaker would quickly open. But the arc flash can still cause burns. Some of the more serious injuries come from relatively low voltage (600V) but high current systems. The arc can vaporize some of the copper. It will rapidly expand and burn the first thing it touches. If you're lucky, that would be a arc-flash rated face sheild and/or protective clothing/gloves. If you're not so well prepared, your skin could be the receiver of quite a few watt-seconds of energy. All this can happen in the scant few msec it takes for the breaker to open. Please don't try this.

In a poor installation, a 'bolted fault' will draw such a large amount of current, that when the breaker tries to open and stop the current flow, the breaker is damaged and the contacts 'weld'. Then the current just keeps on flowing until the next weakest part of the system fails. And that's a bad thing.

daestrom

Reply to
daestrom

Surfing around a bit, I found that Wikipedia has probably one of the better and complete explanations of the split phase system used in North America.

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Beachcomber

Reply to
Beachcomber

Years ago on of the sparks I worked with at Rovers car plant managed to drop an 8mm spanner across the phases in a 415v bus chamber. The result was 'interesting'. He looked strange with no eyebrows and lobster skin. He was a lucky guy.

Reply to
Bob Watkinson

The first diagram on this page shows the system that they have described to you.

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Ben Miller

Reply to
Ben Miller

Nice page Ben.

Reply to
SQLit

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