David, here's a little clarification on a couple of points.
The potential (voltage) across the two 120 VAC wires or contacts does not vary between +120V and -120V as you state. It actually varies between -170V and +170V, the voltage peaks. 120V is the root-mean-square (RMS) voltage. It is the AC voltage that is equivalent to a DC voltage.
- 120 VDC at 5 amps is 600 watts
- 120 VAC at 5 amps (AC) is 600 watts, but the peaks are 170V and 7.07 amps
The peaks are higher to compensate for the points where the voltage or amperage are small or zero. The RMS voltage is sort of similar to an average, but not exactly the same.
The above applies directly to pure resistive loads: incandescent lamps and radiant heaters, toasters, and water heaters. Capacitive loads and inductive loads (motors, converter, air conditioner) are a bit trickier.
The white wire is very close to ground. They are wired together at the panel. If there are loads on the circuit, there will be some voltage loss on the white wire so it might be a couple of volts above ground, but very unlikely to shock you. (The black wire swings both above and below the white wire.)
Here's an example:
AWG 14 wire (15 amp circuit) has a resistance of 2.525 ohms per 1000 feet.
Suppose you have an appliance that is drawing 1500 watts--very common for a space heater.
Suppose you are 100 feet from the panel.
Resistance of white wire (and black wire)=.2525 ohms
1500 watts/120V=12.5 amps
Voltage drop in the white wire: V=IR V=12.5 x .2525=3.15625
So the difference in potential (voltage) between the white wire and ground wire is around 3 volts.
Try this simple experiment, Part 1.
- Plug a toaster into a duplex outlet.
- Turn it on.
- Set your voltmeter to a low scale.
- On the other outlet in the duplex pair, put one probe in ground (the big round hole) and the other probe in the wider slot.
- You won't get a full three volts because you are likely less than 100 feet from the panel and probably on AWG 12 wire, but you will see some difference.
- Now turn the toaster off and take the reading. The difference should be much less.
Okay, now here is Part 2
Guess what? The black wire also has a voltage drop, equal to that of the white wire!
Set the voltmeter to at least a 120VAC scale.
Turn the toaster back on.
On the other outlet of the duplex pair, measure the voltage between the two slots.
Turn the toaster off.
Measure the voltage between the two slots again. It will be a few volts higher. The difference between the two measurements represents the voltage drop in both black and white wires. It should be double the value in part 1.
There's a hidden simplification in the above calculation. I started by calculating the current of the heater as 1500 watts/120V=12.5 amps. That's a proxy for the resistance. In this example situation, the heater won't have 120V across it, only 114V. So the calculation will be off by about 5%. But you get the idea.
But that's not what people were saying. What they were saying is that often the voltage is at zero, near zero, or small, so the switch should last longer on AC than when operated on DC where the make is always at full voltage and the break is always at full current.
And one poster stated that there are solid-state relays that sample the voltage and always make and break right at the zero-voltage crossing.
Others have mentioned that inductive loads under DC make the problem worse, but that excess can be eliminated with flyback diodes. That's true (although Chrysler never learned it), but I am restricting this discussion to resistive loads.
And I stated earlier that the failure mechanism of switches (and I've taken apart my share of failed ones) is current transfer from one contact to another. On an AC switch, 50% of the time the transfer goes one way and 50% of the time it goes the other way. On a DC-rated switch, it always goes one way; these switches have thicker contacts or contact material less susceptible to transfer. Here are images of a pair of new contacts and after they have been operated for nearly 100,000 cycles on DC. Note the asymmetric wear. Material has transferred from the right contact to the left contact.
Hope this is educational.
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