On Jan 20, 7:02*am, "RBM" wrote:
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On Jan 19, 8:02 pm, "RBM" wrote:
"Nate Nagel" wrote in message
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RBM wrote:
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On Jan 17, 4:05 pm, "RBM" wrote:
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On Jan 17, 12:32 pm, "Buerste" wrote:
"Gary H" wrote in message
m...
On Thu, 15 Jan 2009 23:12:14 -0500, "Buerste"
wrote:
"Tman" wrote in message
...
Took a look at the schematic today for the dryer (240v). I've
always
wondered how they serve up 240 V with 120V components (e.g. timer
motor)
on only three wires.
You see, I always thought that the grounded conductor should never
be
current-carrying, in the theory that if the ground feed should
break,
then
the metal chassis of the appliance does not get energized.
Well looks like that "3rd" wire, is a neutral, and judging by the
dryer
schematic, is both used as a chassis ground and a current-carrying
conductor for the 120V items on the dryer -- such as timer and
drum
motor.
OK -- am I totally wrong about the code and theory, or is this
unsafe?
Seems to me that dryer would get real "hot" should their be a
fault
in
the
neutral conductor to the breaker box.
I know I'm missing something here as this is SOP as far as I am
aware.
Someone please inform me?
T
Google: "Split Phase"
In a nutshell, the two hots are 180 degrees out of phase with each
other.
The potential between either phase and ground/neutral is 120v. The
potential between the two hots is 240v. A single phase of
Alternating
current looks like a sine wave on an oscilloscope with the wave
swinging
up
and down. With split phase, it looks like 2 waves on the scope. As
one
wave is bottoming out the other is topping out. They are like
mirror
images
of each other. If you measure the peak of either wave to ground, it
is
120v
Actually, the peak is 170V (RMS * 1.414) but you don't need reality
to
get in the way here :-)
if you measure the top peak of one wave to the bottom of the other,
it
is 240v.
340V. Shut up, reality :-)
There, clear as mud! Any 240v device will run on just the two
hots, any 120v device will run on either hot and ground/neutral.
I understand, however to make it a bit clearer I used numbers that
more
easily translated to the OP's ability to understand. Sorry I wasn't
more
technical, do you think he would have understood your numbers better
in
relation to his dryer?- Hide quoted text -
- Show quoted text -
To the OP, you are exactly correct, the 3 wire system can be dangerous
if the "ground neutral" wire opens becasue most dryers have a 120 volt
motor so that current flows through the neutral ground. NEC code as
picky as it is used to allow this. I think it is now not allowed in
new homes. I have a 3 wire dryer and I added a ground wire connected
from the frame of the dryer to the frame of the washer. This way,
whatever else happens there can be no voltage across those two
appliances and the washer has a 3 prong plug so it is grounded. You
can also connect it to a cold water pipe. I would suggest you do that
as an extra saftey feature.
Good question
Mark
I would recommend using a 3 wire dryer circuit just as it was intended
to
be used or replace it with a proper 4 wire system. Installing a Rube
Goldberg grounding addition isn't helping the situation, and could
potentially cause more harm running line voltage fault current through
your
plumbing or other appliances- Hide quoted text -
- Show quoted text -
an additional ground wire connected to the dryer frame helps the
situation becasue it prevents the possiblity of line voltage appearing
on the dryer frame which is far more dangerous..
Using the 3 wire system "as it was intended" can create this dangerous
condition, that's why they changed it.
I agree, changing over to a 4 wire system is a better option, but if
you have a 3 wire cable it is not easy to do..
To the op re being an EE and calling it 220 VAC two phases, it is
common usgage but technically wrong. It is more technically correct
to say two POLARITIES of one phase.
Mark
What you are proposing to do, routes dangerous line voltage fault
current
from the dryer to another circuits grounding conductor, or worse, a
water
pipe, which was not designed for that purpose, and is not only
dangerous, but illegal as well. I suggest you get an NEC code book and
research "grounding"
Correct me if I'm mistaken, but doesn't the NEC allow you, in a retrofit
type situation, to replace an ungrounded receptacle with a grounded one
so
long as you ground the receptacle and box to any convenient point "in
the
grounding system" which would include a copper water pipe? I agree,
would
be far better to drag it back to the panel... I honestly don't know what
it says running a ground wire from a plug-connected appliance to a
water pipe. I'd sooner replace the recep. with a 4-wire and use the
exception there; then replace the dryer cord with a 4-wire and remove
the
bonding jumper in the dryer.
Disclaimer: IANAE, I'm posting this for feedback and comment, not as
advice.
nate
--
replace "roosters" with "cox" to reply.
http://members.cox.net/njnagel
First of all, the code doesn't allow you to ground a receptacle to a water
pipe. It allows you to ground it to any point along the "grounding
electrode
system", which will include the metallic water pipe feeding the building,
but only the first 5 feet of it. A properly installed 3 wire dryer circuit
must meet 4 conditions to be acceptable. The whole idea is to have a
better
quality circuit than just some old piece of Romex making the connection..
Most importantly, electricity takes the path of least resistance. What
happens if this guy's auxiliary ground, which he believes is to protect
someone, is a better path for the normal motor circuit currents to run
through? Now current that should be running through a properly sized
conductor is running through what? And if it's running through some
section
of plumbing, and some plumbing work requires cutting that particular pipe,
what happens to the plumber?- Hide quoted text -
- Show quoted text -
Nothing...
You have the normal neutral/ground *wire in parallel with *the aux
ground wire. * "Electricity takes the path of least resistance" is an
oversimplfication. *In fact the current divides inversly
proportionally to the resistance in the path. *Yes some of the motor
current may flow through the auxiallry ground. *But if that ground
should be opened by the plumber then all the current will flow through
the original ground. *The only voltage across the water pipe will be
the small drop due to the resistance of the wire.
I agree this is not an ideal situation, but the problem started
becasue the NEC made a mistake in the first place and allowed 3 wire
220V circuits which are inherently dangerous for all the reasons you
are pointing out. *The BEST way to fix this is to replace the 3 wire
system with a 4 wire system , which could requires *pulling a new 4
wire cable to replace the existing 3 wire cable. Thats a big job. *A
second best altternative is to connect an auxiallry ground so that in
case the ground/neutral in the orignal 3 wire circuit should fail OPEN
which is unusal but can happen, and if it does it hapen it places a
LETHAL voltage on the case of the dryer. *This is unacceptable to me!
But I'm not going to pull a new 4 wire cable. *An auxialry ground will
prevent electrocution due to a single open failure in the original 3
wire NEC approved system. In my case the auxially ground is the case
of the washer right next to the dryer and the washer is plugged in via
a 3 prong 120V plug. *I didn't modify the outlet or cords in any way,
I simply connected a wire between the metal frame of the washer and
dryer. *This is legal and makes it safer.
Bottom line for me is.... the big green wire I added connecting the
washer case *and dryer case ensure that whatever else might happen,
there can be no dangerous voltage across those two applicances. *The
orignal NEC approved 3 wire system did not ensure that.
Answer these two questions and then I am done ..
1) Does adding a big green wire between a washer and dryer case make
the system more or less safe? *I say more.
2) Does adding a big green wire between a washer and dryer case
violate any code? *I say no.
Do you disagree?
thanks
Mark
I disagree on both counts. The first for the reasons already stated, and
no,the NEC does not approve your method- Hide quoted text -
- Show quoted text -
Out of curiousity, what exactly in the NEC says you can't bond the
metal cases of two appliances together? In some cases, SPAs for
example, code specifically requires that all metal within a certain
distance be bonded together. So, if that washer or dryer happened to
be within the specified distance from a spa, it would have to be
bonded to the spa pump, heater, etc.
I see both sides to this discussion. I'm generally not in favor of
rube goldberg adaptions. But on the other hand, not sure what says
you can't bond two metal objects together. Also, in the case of the
washer/dryer, bonding would seem to be a good idea, because the washer
is a very likely object to come in contact with simultaneously with
the dryer, so having them at equi potential seems like a good idea.