Three-wire dryer outlet -- how can it be safe?

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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
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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, if you measure the top peak of one wave to the bottom of the other, it is 240v. 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.
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Actually, the peak is 170V (RMS * 1.414) but you don't need reality to get in the way here :-)

340V. Shut up, reality :-)

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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?
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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
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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
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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
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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"
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RBM wrote:

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 re: 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
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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?
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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
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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
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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.
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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.
With a hot tub, you want anything within reach, likely to become energized, to be bonded to the very substantial grounding system of the tub
Three wire electric dryers are more of an isolated application, so typical laws or rules that normally make sense, don't apply. The body of the machine is part of the live circuit, not just fault currents. It requires a special cable (SE) or insulated ground/neutral, originating only from a main service panel. Under no circumstances would you want the grounding system of another circuit to become or act as a live neutral conductor from a clothes dryer, or potentially more dangerous would be using an internal plumbing system. In the event you lose your neutral/ ground on a three wire dryer circuit, the motor won't turn and although the chassis of the machine would be live, you'd still know that a problem existed, and would have it fixed. If you add a supplemental ground to the neighboring washer, you'd have no way of even knowing that dangerous neutral currents were running through the washer outlet's grounding system
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RBM wrote:

Applies if there is not a grounding conductor at the receptacle. In this case there is.
Don't think there is a prohibition against a redundant ground, except in this case it becomes a parallel neutral conductor.

The NEC allowed this "dangerous" method for about 30 years. Apparently the NEC didn't think it was particularly "dangerous". And since the best way to get the code changed is to show dead bodies, apparently it was not a "dangerous" method.

If I had an electric dryer with a 3-wire circuit I might replace the wiring if it was easy. If not I would use the existing 3-wire circuit. As Roy pointed out, there were major limitations on how these circuits were wired. Like for instance you couldn't use Romex.

The argument that can be made in this case is that the alternate ground path also becomes an alternate neutral return. That puts some normal neutral current on the grounding conductor which is verbotten. Just like bonding neutral and ground at a subpanel.
In some cases, SPAs for

Without looking it up, I believe it is isolated metal that might become energized.

I believe the specified distance is 5 feet (so you can be in contact with the spa and the other metal). You can't have receptacles in that area. You wouldn't want a hypothetical dryer either.

One can certainly argue both sides. I agree with Roy.
--
bud--

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wrote:

Damn, where you been Bud, I'm exhausted already. What's really interesting is that the Consumer products safety commission has the data on electrocutions from household appliances, and there is no difference between deaths from clothes dryers and any other appliances, in fact airconditioners cause more deaths than dryers. go figure
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RBM wrote:

I can see why. By the time I get to this newsgroup you've answered all the questions. (NOT a complaint.) I don't know how you get anything else done.

Why do you want to go and ruin a good argument with facts???
--
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No, it's not. It's perfectly and absolutely true. Electricity will NEVER seek any other path but that of the lowest resistanct/impedance/whatever.
In fact the

Uhh, yeah, to the lower resistance path.
Yes some of the motor current may flow

Depends.
240Vac (or 220 if that's what you like to call it) in a Class II constructed appliance is exactly as safe as 120V in a Class II constructed appliance. They will not be inherently dangerous. Even under the scenario being attempted to be described here, without a fault occurring, there can be no danger. The world changes and the NEC, being the first level of protection, changes with it. Also note that depending on the NEC can still very easily result in a non-compliant installation in almost every North American jurisdiction. Loca code enforcement always has the last say.
The

Without knowing what you actually have there, it's hard to say. You may feel safer, but you aren't to code whenever you daisy chain an earth connection like that. Plus, you didn't necessarily get ALL exposed metal with such a wire - many metal parts could still be ungrounded, besides not being arranged in the spedified start connection required of earthing methods. Earth has to be the middle of the star, and all components star connected into it. A dryer cannot be a star component; all stars must be internal.

Other than missing information having neglected any local codes, you're very likely out of complaince. Additionally, how do you know that ALL exposed metal has been earthed by the addition of that one wire? I suspect you don't; so you may well still have the possibility of further points of access to the theorized voltage.

More. But not completely.

It may well violate local codes. Disconnection, or damage to, the wire can, witout notice, return you to the previous same situation you had (like, someone besides you moves one of them, maybe a service man, next owner, whatever). Codes don't allow relying on such a thing. UL, CSA and the EC safety codes are all the same at that point too, in addition to local codes. Since UL/CSA/EC/NOMs are required to sell such a product in NA, you have disturbed the design of both systems and they are no longer certified. UL1459 is the only spec that comes to mind right now, and it's going to have a superceded by ... note. It's been quite awhile. Remember, the NEC is MINIMUM requirements, NOT the BEST! NEC is always specced as the first requirement, then code offices add their own additional requirements to them where it's deemed necessary.

Sort of. I think the thread has become disengaged from reality at some point and too many factors are missing to try to go back and recapture them.

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So you think if you put a 20 Ohm resistor in parallel with a 1 Ohm resistor then 0 current will flow through the 20 Ohm resistor and ALL the current will flow in the 1 Ohm resistor?
If that's what you think, you are ABSOLUTLY WRONG. I'm not going to debate this with you, its a fact.
Mark
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snipped-for-privacy@yahoo.com wrote:

This whole thread and that exchagne above are exhibiting the difference between and electrician and an EE. EE's are right. Electricians are practical.
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