220V question

[Tom Horne]

Let's review what happens when the neutral goes open at any point in the circuit between the panel and the stove. The stoves 120 volt loads will serve as rather low resistance conductors between one of the ungrounded conductors and the frame of the stove. Another poster has alleged that that is not a problem because you will be in series with the load. When two impedances are in series the largest quantity of the voltage will be reflected across the higher impedance. In the circuit in question the higher impedance is the human being. If the voltage applied across that human being is higher than 30 volts there is a likelihood that that person may be injured or killed the only thing missing is contact with another conductive surface that happens to be grounded. The kitchen sink, refrigerator, dishwasher, and even many kitchen floor coverings are sufficiently conductive to provide that conductive pathway.

It has also been alleged that it makes no difference because both conductors terminate on the bonded buss bar of the service equipment enclosure. That position ignores the fact that since the neutral conductor carries current it expands and contracts with each use of the appliance. As a result of that normal thermal cycling it's connections are under far more stress than those of the equipment grounding (bonding) conductor.

In the older three wire configuration any failure of the neutral conductor energizes the frame to the potential of the circuit. In the four wire configuration the neutral opening causes the appliance to stop functioning but the frame will remain at zero volts relative to other grounded surfaces.

I'm only one member of the entire nations fire service and I have attended three accidents that were secondary to failure of neutral terminations on three wire appliances. It is true that I have been in active service for thity plus years. One was an electrocution. the other two were electrical injuries that were short of being fatal. One was a working code who then spent weeks in the hospital and was provided with an implanted defibrillator while her injured heart muscle healed. The other suffered a dislocated shoulder and a fractured arm as a secondary effect of the severe muscle contractions that occur during electric shock. In all three cases the appliance in question was an electric clothes dryer were the frame was bonded to the neutral.

It is your house and your family so which one do you think is better.

-- Tom H

[Tekkie]

Minnie Bannister posted for all of us....

How much is YOUR life or anothers worth?

[zxcvbob]

They need a neutral, but a 3-wire cord doesn't give them one. It dumps the unbalanced return voltage on the ground. A small fixed load, and a very large low-resistance ground. [I didn't design it] Should be OK with copper wires and tight connections; I wouldn't trust it with aluminum wire even with proper terminations. I would use aluminum for a

4-conductor dryer or range circuit. [now that I shot my mouth off, I gotta go doublecheck whether that grounded wire is technically a ground or a neutral]

My dryer has an old 3-wire outlet; it's connected directly to the service panel with a short length of rigid metal conduit, so even if the grounded wire were to somehow come loose I'd have a good equipment ground through the conduit so I haven't bothered to change it to 4-wires.

Best regards, Bob

[Chris Lewis]

According to zxcvbob :

This is really more of a terminology quibble than anything else, for what really matters is what the third wire is being used _for_. The white wire in your house isn't technically a neutral _either_. That's just the term everyone uses. It's official name is "grounded conductor".

It's used as _both_ in the case of a 3-wire stove or dryer. It's being used as frame ground _AND_ neutral return. This starts to become obvious when you start looking into (older) code restrictions where, for example, you (normally) couldn't 3-wire a stove or dryer from a subpanel that has split neutral and ground.

This is slightly better than unconduited 3-wire, in that you have better connections to the ground. However, if the neutral separates in your panel or on the pole, the frame of your stove _still_ goes hot. The problem is that the frame is connected to the device neutral, not how well it's connected to the system ground.

Because the fact of the matter is, if your neutral separates in the panel, AND if you have a neutral-ground interconnect anywhere in your house, _every_ grounded object in your house can potentially go to damn close to line voltage. By NEC rules, ground electrode conductivity is not necessarily high enough even to trip a 15A breaker, let alone the mains.

[zxcvbob]

That happens anyway if the service loses its neutral; the bonding screw at the service disconnect panel will energize all the grounds in the house. I think that's why the power company takes it so seriously if you call them and say your service seems to have lost its neutral. (that plus the risk of fire from one leg operating at too high a voltage.)

-Bob

[Chris Lewis]

According to Tom Horne :

Generally an excellent post, but I think this bears commenting on:

The problem with 240V/120V appliances is that if you lose the neutral, the appliance doesn't necessarily "completely" stop working. Regardless of whether it's 3 or 4 wire attached.

For example, with most stoves, if you lose the neutral, the only likely symptom is that the clock stops working (but it won't necessarily![*]), yet, the cooktop and oven continue working. Because they don't use the neutral.[+]

Similarly, with a dryer, the heating elements will continue to work, but the drum and blower will stop operating.

With a stove, you may not even notice that this has happened - the stove still cooks dinner. With a dryer, if you're not there to notice (or, you think it finished it's cycle), you won't notice.

In and of itself, losing the neutral for a stove is harmless. With a dryer, you may well end up having to rely on the thermal cutout preventing a house fire.

Furthermore, if the stove or dryer is on a three wire connection, the frame now becomes directly part of the circuit - if you touch a better ground (like a faucet), you can get zapped by the unbalanced current flowing thru the clock (or drum rotator motor or timer motor on a dryer).

At least with four wire, you have a vastly better chance of the frame _not_ becoming part of the circuit and you won't get zapped by the clock.

[*] the clock may continue to function but _only_ during the times that the elements are powered up. On a dryer, the timer/motor may "work" (possibly very badly or not at all with black smoke) when the elements are powered up. Depends on where the break is. [+] I encountered a similar situation. In reverse. I helped rewire a friend's house. They had to live in the place during renovations, so, there was always being work done. Got a call several days after I installed (and tested) a new 4-wire stove circuit with the complaint that "the clock's working, but nothing else is". After much head-scratching, I managed to figure out that my friend had removed and reinstalled the duplex breaker for the stove (because he was adding something else). This particular panel (a FPE) alternated legs in _pairs_ (AABBAABB) not individually (ABABAB) and he had reinstalled the breaker such that both breakers were on the same leg. _Both_ ends of the heating elements were attached to the same leg, so the elements didn't work. But the clock was quite happy.