If as a child, you go and work on mobile homes that sit on tires and the tongue jack is on a block of wood, you learn this fast and well.
If you do this a long time ago, you also learn to check which trailers come from California because back a ways they used to use green for netural and white for earth ground. (*:: ZAP :: *)
Also, even if the plugs or outlets were designed to make contact on the neutral first, there’s the possibility that an outlet might be wired backwards. Yeah, that wouldn’t pass code either, but you want multiple layers of safety.
This does occasionally happen. Back in high school, I worked as an electrician’s assistant for a few years. Once, we had a case where the homeowner noticed that when she opened the refrigerator, the fridge handle came close to the oven handle, and a spark arced across. Thank heavens that she noticed it by seeing the arc, not by touching both at once. It turned out that the outlet the stove was plugged into wasn’t properly mounted in its (metal) box, and it had tilted over such that one of the screws on the side was touching the box. The appliance ground, meanwhile, wasn’t connected to a proper ground wire, but just to to the box. Oops.
THat’s right and that’s why the electrical code is sadly deficient in not requiring a fuse in the cord of each appliance that is sized to the capacity of the wire in the appliance.
Since the usual outlet is fused for either 15 or 20 Amp. an appliance with #18 wire in the electrical cord, such as a table lamp, can short, catch fire and burn down a house without ever blowing the breaker.
It was only a few code cycles ago that four wire range and dryer cables were mandated. I regularly get phone calls from folks who go to Appliances R Us and can’t plug in their new purchase because the old one only used three conductors. Prior to that change, the neutral also had a bonding strap inside the wiring chamber of the appliance which tied the chassis to neutral.
Random thoughts. I have a hot tub that does use 120 and 240 and my GFI is a 3 wire device i.e. it has a 3 lugs- a lug for the two hots and a neutral. It then has the pig tail neutral that terminates in the neutral bar. Cutler Hammer make them. Home Depot sells 'em.
Someone referenced the neutral and the ground bar at the panel being bonded together. This can only be done when the panel is the main service entrance panel. If you have installed a ‘sub’ panel, the bonding screw between the neutral and ground bars must be removed.
In the UK, where I am not now but used to be, each appliance is fused at the plug and the plug’s gound/earth ‘pin’ is way longer than the live and neutral. The live and neutral ‘holes’ in the receptacle are shuttered and are not opened and accessible until the ground pin is inserted. It has been this way since the 60s. Also you cannot yank a plug out of a UK receptacle simply by pulling it (causes arcing).
US household wiring devices are quite archaic IMO.
That’d be cool if you could dig it up.
I don’t understand how a 240 VAC-only circuit doesn’t need a neutral line. How is the circuit completed? Do hot 1 and hot 2 serve as return lines for each other?
Damn, I’m usually a pretty smart guy with technical thingys, but electricity makes me feel really dumb. :eek: I mean, I’ve gone through several books on the subject, and I still struggle with it…
A 240 volt circuit is just two 120 volt circuits added together. When one side of the circuit ( one of the lines) is going positive the other side (line) is going negative at the same rate. Since the lines are always opposite in polarity, one plus and the other minus, the current goes out the positive side, through the appliance and returns through the negative side, alternating directions every 1/60th (in the US) of a second.
Subway Prophet:
A home’s AC system is powered from the secondary of transformer w/ center tap. Thus there are three taps on the transformer’s secondary winding:
Tap A, which connects to one end of the secondary winding.
Tap B, which connects to the center of the secondary winding.
Tap C, which connects to the other end of the secondary winding.
There’s 240 VAC rms / 60 Hz between Tap A and Tap C.
There’s 120 VAC rms / 60 Hz between Tap A and Tap B.
There’s 120 VAC rms / 60 Hz between Tap B and Tap C.
Electricians call Tap A “Hot Leg 1,” Tap B “Neutral,” and Tap C “Hot Leg 2.”
This is not always true. Two reasons:
An alternating voltage is always present on the system. But the same cannot be said for the current. As an example, if you plug nothing into a receptacle, there will be an alternating voltage at the receptacle, but the current will be zero.
Even if an appliance or branch circuit is pulling current, there’s no guarantee the current is alternating. As an example, the input current waveform for a half-wave rectifier is DC, not AC. By contrast, the voltage is always present and always alternating, regardless of the load.
Because of this, I think it is more correct to call a home’s electrical distribution system “alternating voltage” than “alternating current.”
Whoops, there’s my brain exploding again.
So instead of the current alternating, it’s the voltage? Which one is analagous to water pressure, and which one flow?
If hot 1 and hot 2 alternate between hot and neutral at 60 hertz, then how does that allow an appliance like a dryer or a stove to use both voltages at once? It seems to me that only 120 volts is getting to the device at any given time.
:Subway cracks open his AC Electricity for Extra Stupid Dummies book again:
Sure but it unecessarily complicates a simple question which was simply how is the circuit completed if there is no neutral line. For part of a cycle in a 240 V. half wave rectifier no current flows but during the other half when there is current if goes from the positive line, through the rectifier and returns on the negative line and, of course, the rectifier only allows current in one direction…
I always hated the water pressure analagy because it leads to a lot of confusion. Electricity doesn’t behave like water in a lot of ways. However, the general analagy is that the voltage is the pressure, and the current is the flow.
The voltage is a sine wave. It’s not like it’s switching full on and full off every 1/60th of a second. If you are measuring from the neutral, as one “hot” starts swinging towards the positive side, the other “hot” starts swinging towards the negative side. They are always moving in opposite directions. When the first “hot” is positive, the second “hot” is negative. When the first “hot” is negative, the second “hot” is positive. Because they are always working in opposite directions, you get twice the voltage between the two hots as you would between either hot and neutral.
Maybe an example will help. Let’s say you’ve got two light bulbs and a dryer. One light bulb is plugged in between each hot and neutral. The dryer is connected between the two hots. We’ll call the two hots line A and line B. The voltage starts out at zero. Then, the voltage starts increasing on line A and decreasing on line B. At the peak of the sine wave, you’ve got 170 volts on line A and -170 volts on line B. There is an 170 volt difference between each light bulb and neutral (+170 and 0 for line A, and -170 and 0 for line B) but there is a 340 volt difference across the dryter (+170 and -170). Then the voltage starts going back to zero on both lines. Then line A goes negative and line B goes positive, until line A has -170 and line B has +170. Now there is once again 170 volts difference between each light bulb and neutral, and 340 volts difference between each line.
Now you are probably thinking, I’ve got a 120 volt circuit, where did that 170 come from? Well, sometimes the voltage is as high as 170, sometimes it’s as low as zero. It’s a sine wave. If you average the voltage out over time, you are going to get 120 volts. 120 is the average voltage or rms voltage (rms means root mean squared, which describes mathematically how you figure out the average), and 170 is what is called the peak to peak voltage.
On average, you’ll have 120 volts between each hot and neutral, and 240 between the two hots.
Oops. Now you’ve done it. 
Simplification for lay readers is fine but equating average with RMS is going too far. The average value of a sinusoidal wave is zero since it spends exactly the same amount of time below zero as it does above.
The RMS value of a sine wave of current relates to the heating effect of the current in a resistor. That is, if a direct current that has the same magnitude as the RMS value of a sinusoidal current is applied to a resistor it will have the same heating effect. That heating effect is proportional to the power which depends upon the square of the current.
Say we have a sinusoildal current of Asin(x) the square of that is equal to
A[sup]2[/sup]sin[sup]2/sup which equals A[sup]2[/sup]/2 - A[sup]2[/sup]cos(2x)/2
The cos(2x) being a sinsusoid has an average value of zero so the average value of the A[sup]2[/sup]sin[sup]2/sup is A[sup]2[/sup]/2.
Remember, we are looking for the magnitude of a direct current that will have the same heating effect as the AC. The AC has a heating effect proportional to A[sup]2[/sup]/2 and this is the square of the equivalent direct current. So the magnitude of the current itself would be A/√2. That is a direct current will have the same heating effect as a sinusoidal alternating current if the direct current equals the amplitude of the alternating current divided by √2.
Back to the water example … Please… *::: damn Martin Marauder pilots always act smartass. :: * 
Oh pffft. It got the point across. 
(I hope - we’ll have to find out if it made sense to Subway Prophet)
Okay, so if it’s zero 60 times a second (or is it 120… drawing pictures… are we counting only the peaks of the sine wave, or the troughs too?) …anyway, if the difference in voltages is zero so many times per second, does this have an appreciable effect on the appliance? Are we primarily interested in an average over time here? Or does the appliance need to convert to DC at some point to compensate?
Looking at my picture, with two sine waves out of sync by pi radians, the difference is always zero. Or does it not make sense for the voltage to be a negative amount?
Maybe some equations will help…? Something I can graph?
Oh, I just saw David’s post wrt root mean square - lemme play with these numbers a bit…
The difference between two voltages that are 180 out of phase isn’t zero. For example, when one line is as +100 V the other is at - 100 V and the difference is 200 V, either + or -. + 200 - (-200) = 200, or -200 - (+200) = -200.
P.S. Martin B-26 pilots are gooooood as opposed to those clowns who floundered around in thattinker-toy with two tails.
Oh yes. And no matter how convenient, equating RMS and average isn’t the straight dope. I can see such things being done when dealing with the mathematics involved in Quantum Mechanics or General Relativity, but not in the relatively simple math involved in AC circuits. :p[sup]2[/sup]
That’ll depend on the appliance. Some appliances (including light bulbs and ovens) just care about making heat. For that, you can put just about any kind of electrical power through it, and it won’t much care. There will be moments when the element isn’t actually producing any heat, but it’ll still be hot from the last peak (and the peak before that, and…), and it’ll produce heat again. Some things, though, need the current to only go one way, but don’t mind if it goes to zero, so those devices need a simple rectifier (which turns the sine wave into an absolute value of a sine wave), and some things need a constant DC voltage, so they’ll have a more complicated rectifier. Various types of electronics can fall into these two categories. And some things need the current to stay AC, like some designs of motors (other types of motors need DC, and there might be some which can run on either).
