Thank you for the response, and forgive me but I’ve been trying to teach myself electronics and electricity for years, and still can’t grasp some of it. But, is the answer to my previous question “Yes”?
Yes. That extra white wire is there just to connect the chassis to the dryer’s neutral connection in the event that you’re installing the dryer in an old house with no separate safety ground. You don’t have to reconnect that extra white wire to the middle screw with the incoming neutral - the dryer will still work as long as the white wire from the wall is connected to that middle screw. That’s just done to ensure that the extra white wire is not dangling loose inside the machine and nothing gets connected to anything it shouldn’t be connected to.
OK - I have read the above but I do not understand why you need four wires in a single phase supply. In the UK, all single phase appliances/equipment have three or two wires. Small appliances like a clock or a lamp will only have two because there is no chance that the container could become live. Anything more powerful like a dryer, or anything like a computer that need a ground has three wires. There are no four pron plugs or sockets, ever.
There are two carrying the current - These are often labelled hot and return even though we all know that there is no difference. The reason for the label is that most switches only cut one connection and it makes sense if it is always the same one; the ‘hot’ one.
The third wire is what we call an earth and the whole house has a continuous earth circuit which goes back to the main board.
So - what would a fourth wire do?
As I understand it, it is a dedicated ground that serves if the neutral wire fails.
They do seem to end up in the same place.
And you guys have 220v for everything, don’t you?
I’m a little late to the party, but wanted to reiterate this. Asking about wiring and brakes do not require one to turn in his Man Card.
Burning your house down, electrocuting a loved one, or turning your car into a rolling death trap do.
Your wires carry more current. Everything you use operates off 220v. That 220v is carried on 1 wire. That wire is 220v to earth.
In the US we have 120v and 240v. Our 120v is 120v to earth. For us to get 240v we take two 120v to earth wires that are in oposition to each other. Against each other they are 240v.
My 220 volts is on two wires - it alternates; but I guess it’s true that the pd between one live wire and earth is 220 volts.
So the supply to a house in the US is 240 volts and that is divided to make 120? how does that work? Is the house supply on three wire (plus earth?)
See the wikipedia article on the split-phase system.
Residential power in the US and Canada uses a 240v split-phase service with a center-tapped, grounded neutral. Most appliances, lights, etc. are connected between one hot wire and the neutral, yielding 120v. High-current appliances are connected between the two hot legs, yielding 240v.
Modern code also requires a separate safety ground conductor, which is bonded to the neutral at the service panel. This grounding wire is connected to metal equipment chassis for safety. The old practice was to use the neutral wire as both the return path and the ground, but this is potentially dangerous if things are wired with the wrong polarity (now your equipment chassis is hot) or if there are other failure modes.
So the possibilities are:
120v, two wires, hot and neutral - now only allowed for doubly-insulated or plastic things
120v, three wires, hot, neutral, safety ground
240v, two wires, hot, hot. No longer used.
240v, three wires, hot, hot, safety ground.
120v/240v, three wires, hot, hot, neutral. This is no longer considered safe because of the lack of a safety ground.
120v/240v, four wires, hot, hot, neutral, safety ground. This is the current standard for things like electric dryers and ovens.
But that’s a completely different thing - a 240 V circuit will be 180 degrees out of phase. If it’s two legs of a three-phase system, it’s called a 208 V circuit.
Your 220v is 220v to earth. One wire carries current 180 degrees from earth. The other wire is neutral/earth/return. It carries no current when no power is being used.
AC power causes a lot of confusion as it both ‘pushes and pulls’ depending on where it is in the cycle.
Standard US supply is 3 wires. Hot/Hot/Neutral. The ground is made up at the house. On a properly functioning transformer neutral has the same potential as ground.
A standard residential transformer taps 1 leg of a 3 phase grid. The hots are pulled off opposite ends of the transformer.
Great, thanks!
I think I have it now thank you.The UK system does look less complicated though, and we can use thinner wires than you.
Can you explain what the difference between these two are? I don’t mean the voltage, but the difference between “neutral” and “safety ground” in this scenario.
The nuetral all though it connects to ground it is intended to carry current and does. If you grab a nuetral wire on a powered device you will get zapped. A ground wire provides a clear path to ground as a safety measure. It should only carry current if something is wrong
This is turning into a great thread. I’ve learned a lot—and not just about the mechanics of electrical wiring, bob++ and Maus Magill. My wife suggested that I go hunt down a YouTube video. I told her, “no, sweetie—I’m going to The Dope. I’m glad I did.
Thanks for asking that. I wanted to ask the same question, but didn’t because of the remote possibility that friedo would interpret it as a challenge to his knowledge of the subject. This is what make the SDMB great—you can pose a seemingly simple question and a few hours later walk away with not only a definitive answer to the original question, but a broader overall understanding of the subject matter in its entirety.
The difference is that the neutral is supposed to carry return current from a hot wire, giving 120v. If the ground is carrying current, there is a fault.
I believe that the original concept was that if I come into contact with 110v, I will pull less current than if it were 220v. I am not as likely to come into contact with 220v at the dryer or oven as I might with a defective iron or coffee pot.
As you said, thinner wires for 220v, so high current devices use 220v.
I don’t know exactly what your knowledge level is, so you’ll probably understand a lot of this already, but let me explain it from the beginning.
You don’t want the case of an appliance to be electrically hot, as that’s a safety hazard. In the old days, they connected the case to neutral, which was at ground potential (mostly).
This has two basic problems.
The first problem is that the neutral isn’t really at ground potential if there is current going through it (it’s the old V=IR, if you’ve got I and R, then you’ve got a V). The neutral voltage won’t be very big, so this isn’t exactly a huge hazard or anything, to put it in perspective.
The second problem (and this is the biggie) is what happens when something fails. If the hot side of the circuit fails, no biggie. The appliance just stops working, but nobody gets shocked. If the neutral fails, though, now you’ve got a big problem. While the appliance is off, there’s no problem, but once you turn it on, the case floats up to the hot voltage, though the current is limited to the electrical resistance of all of the stuff in the appliance. But still, it’s a major shock hazard.
So what we do now is we use a separate safety ground. This ground carries no current, so there’s no voltage from that and the case is now always at true ground potential. But the big benefit is if something breaks. If the hot wire breaks, again, the appliance just stops working, just like before, no biggie. If the neutral breaks though, all that happens is the appliance stops working. Since the safety ground is a separate wire, it’s still connected and the case is still grounded. You can break any single wire (hot, neutral, ground) and the case remains safe to touch. It’s only when you get multiple faults, like the safety ground breaking while the hot also shorts to the case, that you end up with a shock hazard. So overall it is much safer.
So now let’s look at the two cases that friedo mentioned.
240v, three wires, hot, hot, safety ground.
What you have here is a 240 volt circuit, with no neutral. You’ve got the safety ground, so everything is fine. What you don’t have is a 120 volt circuit. Some appliances like to run the timer/control type circuits off of 120. You can’t do that here. You can’t use the safety ground to carry current.
120v/240v, three wires, hot, hot, neutral.
Here you have both 120 volt and 240 volt circuits available (120 from either line to neutral, and 240 from line to line). So if your appliance is the type that wants to run its control circuits off of 120, that works fine. But you don’t have the safety ground here.
This is not allowed in new construction. Any house built in the last 10 years or so must have the safety ground, so they’ll use a 4 wire system (hot, hot, neutral, ground) instead of a 3 wire like this.
This is probably not true.
Assuming a correctly sized conductor, the voltage drop between Ground and Neutral should be only a fraction of a volt, thus posing no danger.
More voltage, less current.