Questions for someone electrically inclined

[dave1701]

I've been doing some reading on AC power and how it gets to our outlets. I have a few questions that Google has been unable to answer. Please excuse my ignorance.

#1 If AC travels in both directions, then why does it only come from hot cables? Why does no power come from the ground.

#2 If all appliances with metal cases have a ground prong, why does my metal encased electric fan only have 2?

#3 Why don't downed power lines trip a breaker when they fall on the ground and short circuit?

#4 In the event that a metal cased object (grounded) came into contact with the hot wire, would the case be hot and dangerous for the moments before the breaker did it's job?

 

[DCProductions]

#1 AC current's fluctuation is exactly why it only comes from hot cables. The hot cable has oscillating current and the cold does not. The oscillation is what makes AC appliances work.

#2 Some appliances do not have ground because there is either no way for the hot to contact the cold metal of the appliance, or the manufacturers were too cheap to have the ground installed. Talk about shock hazard!

#3 Sometimes a power line will not ground out when it falls. If it hits an insulator or some other strange occurrence. Also, some power lines are built to handle large discharges of electricity, so a shorted power line may not trip the breaker. The current being pumped into the ground may not be enough to trip some of the larger power lines (this answer is more of a guess- I think most downed power lines trip something).

#4 It is hot and dangerous, Kesha, but for such a short amount of time that the current cannot hurt you significantly.

 

[dave1701]

Thanks! Electricity is interesting stuff.

 

[Doc Sprocket]

A couple points I'd like to make-

Do not confuse "neutral" (what DC called "cold") and ground. They are entirely different. The ground is literally the ground. Usually bonded to the main cold water pipe in your house, and 100% harmless by itself. The Neutral is the other conductor in the circuit.

Do not get lulled into any false sense of security by a breaker. If you find yourself in the current path, you will be dead LONG before the breaker pops. They are not designed to, and are entirely incapable of, protecting human life. They do not trip fast enough to protect you. A breaker is a thermal device, that reacts to overloads and dead shorts by heating up past a pre-set threshold. The amount of current required to kill you is well below the amount required to heat any breaker to that point.

Food for thought- under the right (wrong) circumstances, less than 1/2 Amp can kill you, yet the smallest household breaker is set at 15 amps. That's 30 times dead, folks! More thought-grub- even a car battery can do you in!

Since circuit breakers are useless in protecting you, what do they do? Fire prevention. They (and the accompanying circuit) are engineered to blow before the current rush becomes to great for the other circuit components, which would cause excess heat, and in turn, fire.

So what protects you? We have devices known as GFCI's (Ground Fault Circuit Interrupters) and AF' (Arc Fault). These devices do not react thermally, and they DO react to a low enough threshold to protect human life. You've seen them too- In your bathroom, and probably your outside outlet too- easily identified as a somewhat "normal" looking plug with "test" and "reset" buttons.

No, I am not an electrician, but-

1) I used to paint transformer substations, padmount transformers, and switchgear. I had to take an electrical utility safety course to be cleared to do it.

2) I am on the Joint Health and Safety Committee at work and am working my way towards being certified by the Labour Board. As such, I need to have a pretty good idea what's going on.

3) Years of experience as both a contractor, and a DIY guy.

 

[dave1701]

A couple points I'd like to make-

Do not confuse "neutral" (what DC called "cold") and ground. They are entirely different. The ground is literally the ground. Usually bonded to the main cold water pipe in your house, and 100% harmless by itself. The Neutral is the other conductor in the circuit.

Do not get lulled into any false sense of security by a breaker. If you find yourself in the current path, you will be dead LONG before the breaker pops. They are not designed to, and are entirely incapable of, protecting human life. They do not trip fast enough to protect you. A breaker is a thermal device, that reacts to overloads and dead shorts by heating up past a pre-set threshold. The amount of current required to kill you is well below the amount required to heat any breaker to that point.

Food for thought- under the right (wrong) circumstances, less than 1/2 Amp can kill you, yet the smallest household breaker is set at 15 amps. That's 30 times dead, folks! More thought-grub- even a car battery can do you in!

Since circuit breakers are useless in protecting you, what do they do? Fire prevention. They (and the accompanying circuit) are engineered to blow before the current rush becomes to great for the other circuit components, which would cause excess heat, and in turn, fire.

So what protects you? We have devices known as GFCI's (Ground Fault Circuit Interrupters) and AF' (Arc Fault). These devices do not react thermally, and they DO react to a low enough threshold to protect human life. You've seen them too- In your bathroom, and probably your outside outlet too- easily identified as a somewhat "normal" looking plug with "test" and "reset" buttons.

No, I am not an electrician, but-

1) I used to paint transformer substations, padmount transformers, and switchgear. I had to take an electrical utility safety course to be cleared to do it.

2) I am on the Joint Health and Safety Committee at work and am working my way towards being certified by the Labour Board. As such, I need to have a pretty good idea what's going on.

3) Years of experience as both a contractor, and a DIY guy.

I thought ground and neutral both lead to the same place: the earth.


A car battery? 12 volts can kill?

 

[devino246]

I've been doing some reading on AC power and how it gets to our outlets. I have a few questions that Google has been unable to answer. Please excuse my ignorance.

#1 If AC travels in both directions, then why does it only come from hot cables? Why does no power come from the ground.

#2 If all appliances with metal cases have a ground prong, why does my metal encased electric fan only have 2?

#3 Why don't downed power lines trip a breaker when they fall on the ground and short circuit?

#4 In the event that a metal cased object (grounded) came into contact with the hot wire, would the case be hot and dangerous for the moments before the breaker did it's job?

1) Think of ac as a person pulling on two ropes, one in each hand. With 120v, one rope is being pulled back and fourth 60 times a second, while the other rope is still. Basically, the hot wire is positive half the time and negative the other half. The negative wire is negative all the time.

Now on a larger 220 volt appliance, both ropes are being pulled back and fourth, one forward, one backward. When one wire is negative, the other positive and vice versa. Both wires are hot, thats why clothes driers and ovens often have 4 prongs.

3) Power grids arent that simple, they dont have circuit breakers like your house. Supply is regulated at the power station.

 

[machinist@large]

I thought ground and neutral both lead to the same place: the earth.


A car battery? 12 volts can kill?

When you look inside a breaker box for residential use, you find 3 main cables coming in and connected to the input side of the breaker panel; 2 hots and a neutral. This is what is needed to cause current flow when you close a switch on anything. The ground is a separate circuit entirely.

Car batteries are used for welding; they have more than enough amperage capacity to stop your heart. It's not the voltage that kills you, it's the amperage.

 

[Doc Sprocket]

I thought ground and neutral both lead to the same place: the earth.

Why then have them as two seperate conductors? No- they're not the same.

A car battery? 12 volts can kill?

Voltage doesn't kill- current (Amps) kills. And a car battery goes into the hundreds of amps of current potential.

 

[dave1701]

When you look inside a breaker box for residential use, you find 3 main cables coming in and connected to the input side of the breaker panel; 2 hots and a neutral. This is what is needed to cause current flow when you close a switch on anything. The ground is a separate circuit entirely.

Car batteries are used for welding; they have more than enough amperage capacity to stop your heart. It's not the voltage that kills you, it's the amperage.

Why two hots?

 

[CaptCanadia]

That's why civilian tasers have about 50k volts and very low amps. This makes the taser non lethal but has so many volts it causes your muscles to spasm uncontrollably. Cool stuff.

 

[machinist@large]

Why two hots?

Devino summed it up pretty well in his description of 220; each hot leg coming into a home is only 110 volts (approximately).

 

[dave1701]

So both the Neutral and the Ground end up in the same place- the Earth. Ground is for safety so I don't get electrocuted grabbing a metal case on a appliance who's hot wire has come in contact with the case. Ground allows the circuit to be completed, drawing more power than the breaker will allow, cutting power, and saving my life.

Right?

 

[Orange Krate]

#1 If AC travels in both directions, then why does it only come from hot cables? Why does no power come from the ground.

#4 In the event that a metal cased object (grounded) came into contact with the hot wire, would the case be hot and dangerous for the moments before the breaker did it's job?

#1 It still has to have a circuit completed. AC oscillates because of magnets turning in the generator (simplified explanation) Think of the one on your flywheel. At TDC it's in full push, BDC full pull. If you only have the hot wire, you are pushing and pulling but not going anywhere.

#4 the ground wire being an excellent conductor is the path of least resistance, your body, a poor conductor is not.

 

[dave1701]

So the power lines (the ones by the road) only carry hot?

 

[devino246]

So the power lines (the ones by the road) only carry hot?

No, they carry multiple hots (3 phase power, more complicated) and neutral.

 

[dave1701]

Why do they carry neutral if the earth functions as the return?

 

[devino246]

Why do they carry neutral if the earth functions as the return?

EARTH DOESNT FUNCTION AS THE RETURN!! Neutral runs all the way back to the power station. At your house, you have two, 6 foot coper rods running into the ground, 2 feet apart. Those function as ground. Remember that ground as it relates to automobiles as totally different than your home electrical system. Go ahead, separate the two in your mind, Ill wait:toetap05:... Ok now, got it?

 

[Doc Sprocket]

That, right there, is the confusion. In a low-voltage automotive style electrical system, "Ground" and "negative" are synonymous (in most cases- one of my tractors has a positive ground). In AC, the ground is the ground, and only the ground. An overflow circuit if you will. The neutral is the other active conductor.

 

[dave1701]

HowStuffWorks- http://science.howstuffworks.com/environmental/energy/power3.htm

And what about this "ground," as mentioned above? The power company essentially uses the earth as one of the wires in the power system. The earth is a pretty good conductor and it is huge, so it makes a good return path for electrons. (Car manufacturers do something similar; they use the metal body of the car as one of the wires in the car's electrical system and attach the negative pole of the battery to the car's body.) "Ground" in the power distribution grid is literally "the ground" that's all around you when you are walking outside. It is the dirt, rocks, groundwater, etc., of the earth.

There is the source of my confusion.

 

[dave1701]

I was thinking it worked like a car or tractor.