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I read that appliance at certain times in the U.S. (for example stoves) were grounded through the neutral wire. Since the neutral wire carries current does this means the case of the stove itself is carrying current. Why didn't homeowners get electrocuted with such a setup? Is it because as long as the neutral is connected/grounded properly the current is going to "chose" to take that path (least resistance) instead of the homeowner (much resistance)?

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Yup. Next question. :) –  BMitch Nov 14 '13 at 0:57
    
And enough people got electrocuted or injured by hot chassis over the years so empirical evidence caused a change in the NEC. Sometimes people like me got lucky and didn't go for a six foot dirt nap. –  Fiasco Labs Nov 15 '13 at 2:15

2 Answers 2

The US distribution system uses a neutral conductor which is grounded at the service entrance of each house/building. The neutral conductor and the ground conductor are physically and electrically bonded at the main service entrance, and tied to actual earth ground via rods and/or metallic water pipes as well.

The devices which were commonly "grounded via neutral" were exclusively 240V primary loads. In the US system those loads are fed by two hot conductors, each 120V away (in opposing directions) from ground/neutral. No current flows in the neutral conductor from one of these loads (generally, they are not even connected to it.) If there was any 120V power used in these items, it was a minor load such as a clock motor. The neutral conductor, however, was full-sized for rated load of the appliance - 30A for a typical dryer, 50 A for a typical stove. So any 120V load that would actually cause current to flow in the neutral conductor would cause a small current to flow in a large wire that was grounded at the far end. As such, no appreciable voltage would ever arise on the neutral conductor.

The move to a 4-wire system with separate ground conductor is, notably, not one where the powers that be decreed that all your old devices needed to be ripped out and replaced since they were scary and hazardous to own; new devices need to meet the new standard. How much "real improvement in safety" there may be is, frankly, debatable; but that debate is somewhat pointless (4-wire is a done deal) and probably also off-topic.

In general, even a current-carrying neutral on a fully-loaded 120V circuit should not have sufficient voltage above ground to cause a shock. Correctly sized wires should limit the total voltage drop in the wiring to 3%, or 3.6 volts, 1/2 of which will lower the hot voltage and 1/2 of which (1.8 volts) will appear as a difference between neutral and ground.

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If you've ever seen a loose neutral connection at the panel, then the safety improvement of a 4 wire system should be obvious. A 40 watt 120V oven bulb could pass 300mA of current to the neutral. If the Neutral is also the ground and bonded to the appliance chassis, a loose neutral at the panel could allow a fatal amount of current through the chassis. "Oh oops, the oven light is out and I just spilled water on the floor... I'll just kneel down here on the web floor and change the bulb...". The ground wire wouldn't be needed at all if everything always worked as it should. –  Johnny Nov 14 '13 at 23:13
    
If the same electrician put in the grounding conductor and the neutral conductor, there's no particular reason to expect that the ground is any better connected than the neutral. As such, the increase in safety is slight, IMHO. –  Ecnerwal Nov 15 '13 at 0:03
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In a 4 wire circuit, a disconnected ground and/or neutral won't cause any voltage on the chassis without a separate electrical failure in the appliance. In a 3 wire circuit, a disconnected neutral will cause a voltage to be present on the chassis in normal operation of the appliance. Maybe such an occurrence is a one in a million event, but there are over 100 million households in the USA. –  Johnny Nov 15 '13 at 0:35

The body of the appliance isn't "carrying" current, because current flows between two points of differing voltage. A grounded object, however, is held to the same voltage potential of whatever it is connected to. In simpler terms, the only way for current to flow OUT of the body of an appliance (say the frame of a stove) is for current to flow IN to the frame of the stove from some other higher potential source. Since (hopefully) the stove isn't wired to anything "hot" (i.e. at high potential relative to ground) then no current will flow through the frame. Think of birds on a power line...although they are connected to a high potential, they aren't connected to anything else, so no current flows. In the stove example, the frame is connected to neutral (which is tied to ground at the panel), and is held at 0V. Unless something touches the stove that is at a higher potential--be it a live wire or a person wearing a sweater--no current flows. In the latter case, only a transient current flows until the person reaches the same potential as the stove.

It's not a matter of "path of least resistance", so much as it is that current flowing into the stove from the neutral wire would have nowhere else to go. The stove is no different from a bird on a wire, and just as no current flows through the birds (regardless of how much is flowing through the wire itself), no current flows through the stove.

The reason the grounding wire is now separate is that--in some cases--the neutral wire can end up at a different potential from ground (i.e. there is a voltage difference between the neutral wire and a person standing on the ground). In that instance, current will flow from high potential to low potential (from the charged neutral line to the person touching the stove) the same way it would if the person had touched any other live conductor at high potential. In modern day wiring, the separate grounding wire will preferentially "divert" any stray currents away from users via the "path of lowest resistance" effect.

It takes some puzzling over to get the hang of it, but hopefully I haven't confused the issue further!

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This is a bad example. No current flows through a bird, because the bird does not provide a path to ground (i.e. something with a lower voltage potential). Also the bird is not sitting in my kitchen, and I don't have the ability to touch the bird and create a low resistance path to ground. –  Tester101 Nov 14 '13 at 10:51
    
The purpose of the bird example was to show that no current flows through an object connected to a wire carrying current, provided the object is at the same potential as the wire. As the neutral wire is (usually) held to ground potential, it is at the same potential as anyone who'd touch anything connected to it. Just as the bird isn't electrocuted because it has no connection to ground, a person isn't electrocuted because they have no connection to "hot"...regardless of how much current is flowing through the neutral wire itself. –  Andrew Bonnell Nov 14 '13 at 14:07
    
Or, to put it another way, current only flows if you connect hot to ground/neutral some way. No current flows through the stove because the frame isn't connected to hot. –  Andrew Bonnell Nov 14 '13 at 14:12

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