Need help understanding 240 volt circuits.

[Steve Smith]

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

 

[Beachcomber]

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

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.

http://www.tpub.com/content/doe/h1011v4/css/h1011v4_136.htm

Beachcomber

 

[[email protected]]

Do they ever connect each 120
volt wire together making one 240 volt wire?

That is a circuit breaker test. ;-)

 

[Steve Smith]

Thank you Beachcomber and G.Fretwell for the information. I'll read more at
http://www.tpub.com/content/doe/h1011v4/css/h1011v4_136.htm . 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

 

[SQLit]

Thank you Beachcomber and G.Fretwell for the information. I'll read more at
http://www.tpub.com/content/doe/h1011v4/css/h1011v4_136.htm . 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

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.

 

[Steve Smith]

He was kidding about the breaker test...

Thanks, I thought he might be. But, I saw the "Grounding to a Water Pipe" at
the bottom of
http://hyperphysics.phy-astr.gsu.edu/hbase/electric/bregnd.html 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.

A phase to phase fault can create heated plasma...

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

 

[[email protected]]

Thanks, I thought he might be. But, I saw the "Grounding to a Water Pipe" at
the bottom of
http://hyperphysics.phy-astr.gsu.edu/hbase/electric/bregnd.html 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

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.

 

[Beachcomber]

Thank you Beachcomber and G.Fretwell for the information. I'll read more at
http://www.tpub.com/content/doe/h1011v4/css/h1011v4_136.htm . 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

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

Beachcomber

 

[Steve Smith]

"Beachcomber"

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

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

Steve

 

[Wrench]

Thanks, I thought he might be. But, I saw the "Grounding to a Water Pipe"
at the bottom of
http://hyperphysics.phy-astr.gsu.edu/hbase/electric/bregnd.html 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

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

 

[daestrom]

Thanks, I thought he might be. But, I saw the "Grounding to a Water Pipe"
at the bottom of
http://hyperphysics.phy-astr.gsu.edu/hbase/electric/bregnd.html 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

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

 

[Beachcomber]

"Beachcomber"

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

Steve

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.

http://www.answers.com/topic/split-phase

Beachcomber

 

[Bob Watkinson]

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

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.

 

[Ben Miller]

Thank you Beachcomber and G.Fretwell for the information. I'll read more
at http://www.tpub.com/content/doe/h1011v4/css/h1011v4_136.htm

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

http://www.bmillerengineering.com/elecsys.htm

Ben Miller

 

[SQLit]

Nice page Ben.

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

http://www.bmillerengineering.com/elecsys.htm

Ben Miller