My stupid question of the day

Greetings,
Even though I'm an EE, I've never gotten the answer to this question: Why is it that I am staring at 110VAC outlets on my desk and I can
have a normal grounded 3-prong plug going into a monitor, UPS, etc. but then right beside it I can have a 2-prong plug for an electric fan that doesn't use/require the third prong? The reason this came up is that I'm doing some work on electrical interfaces to various "system boxes" and we have all sorts of connections, including AC and DC power hookups. I figured I wasn't worth my salt if I didn't know the answer to this question. :)
Mike
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Zerex71 schrieb:

It is not a stupid question!
The monitor, the UPS etc. may have metal or conducting parts within there housings, the fan does not (should not). When a fault happens and a hot wire touches the housing inside there is no need to gound it, when it does not conduct. If it conducts and it is grounded, you can detect the fault with a GFCI.
Robert
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So is the answer to my question, "Because you don't need it for a fan"? I should have asked it this way - if a three-prong outlet has power 1, power 2 (the voltage relative to prong 1 and prong 2, if I remember correctly, is 110V) and a grounding plug, how does it work if there is no ground(ing) if I'm plugging those prongs and only those prongs into a (apparently) AC electric motor inside a plastic housing? Am I just being dense?
Mike
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Zerex71 wrote:

The electronic/electrical "guts" that do the work get their power from the two prongs. They do not need, or use, the third prong to do whatever it is they were designed to do. The third prong is for safety _only_. As was mentioned, if a device has an exposed metal case, that case is connected to the third prong. If a fault develops in the "guts" such that a live wire touches that metal case, the third prong keeps the case at ground potential, in spite of the internal fault, so that you do not get a shock.
If a device is "double insulated" no third prong is required for safety.
There are some old fans that were nanufactured prior to the present safety rules that are neither double insulated nor equipped with a grounding type plug. They would never make it to the market today. If a fault developed in one of those old two prong fans that would otherwise give you a shock, a GFCI receptacle or breaker would protect you, even without the three prong plug. GFCIs do *not* rely on the ground wire to work.
Ed
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You don't need a ground wire to "detect a fault" with a GFCI. All you need is an alternate path of any kind for the current on either side of the load. That is why you are allowed to use a GFCI on a two wire circuit, (sans ground). You need a ground wire when the appliance is likely to conduct current if it becomes shorted to the hot wire, or if the neutral is open and shorts to the chassis. Some devices, and appliances are constructed in such a way that they are unlikely to present a shock hazard, (plastic housings for instance), and UL will approve a 2 wire cord for them.
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The third prong is ground, one of the other two is neutral. Normal AC current flow is from hot to neutral. Ground *can* provide a return path, but it is not supposed to. It's there for safety. For any appliance connected to the plug that has exposed metal or other conductive chassis parts, the ground is required as a safety. In theory, if a wire insulator becomes damaged inside the appliance a 110V potential might be applied to the chassis. If the chassis is grounded, the ciruit breaker or fuse is going to pop and thus protecting a user who might touch the exposed metal. Some appliances will be labled "double insulated" and built in such a way that the chassis and other metal parts are not exposed to the user. These can have two prong plugs because there would have to be two failures of insulation to expose a user to potential shock hazard.
Some electronic equipment, computers and monitors for example, also use the ground to attach to sheilds which help them comply with FCC radiation regulations. They might otherwise comply with the double insulation exception, but they need the extra ground wire anyway.
I hope that makes sense and helps you understand.
Bill Ranck Blacksburg, Va.
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On Mar 2, 11:47 am, snipped-for-privacy@vt.edu wrote:

Hi Bill,
Thanks so much, I appreciate that. I think I am getting the hang of it now. I also forgot my 3-phase power principles (which I've been rereading again) that one line is the hot, one line is the neutral (center point of a Y-connected power source) and the other ground, although I was certain from my reading that neutral is "supposed" to be ground, thus my question why you'd need a second ground. But I think I get it now, so thanks.
Mike
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Well, with 110 (or 120) you are not talking about 3-phase, usually. For a typical house wiring you have single phase into a center tapped transformer. The center tap is neutral, the 2 ends are 120 with respect to the neutral, and 240 with respect to each other. Code requires that neutral be bonded to an earth ground at the service entrance. From there on, ground and neutral are *not* considered the same, even though from a purely theoretical electrical point of view they are. The neutral wire is supposed to carry current in this situation and the ground is not. The ground should only carry current if there is a fault somewhere.
If you read your 3-phase principles you'll find that neutral in a balanced load should carry no current. This is also true for a balanced single phase 240 volt circuit. The neutral carries the difference in current between the two "ends." For example, in an electric stove the heating elements are all 240 volt loads and if that is all that is using current there should be no current in the neutral wire, but most modern stoves have electric clocks and timers and control circuits that are run on 120 volt from one side of the 240, so there is some small return current in the neutral wire due to that unbalance in the load.
FYI: the terminology is a little imprecise here. A 240 volt circuit is called single phase, but the 120 volt "ends" of that are 180 degrees out of phase with respect to each other, and are often called phases, but the 240 volt is still called single phase.
Bill Ranck Blacksburg, Va.
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On Mar 2, 3:53 pm, snipped-for-privacy@vt.edu wrote:

Hi Bill,
The short answer is: I'll have to get back to you on this with more questions. I sort-of understand what you are saying but I want to get it straight in my mind. Thanks for writing. And I actually did mean to say "the single phase of a 3-phase system" - I didn't mean to imply I had actual 3-phase power coming to my outlets here.
Mike
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Easy way to understand starts with 'wire is an electronic component'. Electriicity is not same at both ends of a wire because wire is an electronic device.. More electricity through a wire means greater difference between both wire ends.
We need the appliance case to be electrically connected to mains breaker box. Only then is it grounded. Any wire that is not conducting current is therefore more connected to that breaker box. Again, current means difference between both ends of a wire is greater. White neutral wire is not really grounded because it is carrying current. But green equipment (safety) ground wire is grounded - when carrying no current.
Just another perspective to the problem.
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On Mar 2, 3:53 pm, snipped-for-privacy@vt.edu wrote:

Hi Bill,
Okay, terminology time - what is meant again by "balanced" as opposed to "unbalanced"? Is it anything like "matched/unmatched"? What does it mean to match or balance a load to a power supply? Are we talking impedance of the load?
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wrote:

If EE in your original pst means Electrical Engineer then your question is STUPID because what you have asked is so fundenetal you should never be allowed near electricity. -- John G
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And you were allowed to live why?
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wrote:

I have lived a long time (more than the Bible allowed. Three score years and ten.) because very early in my training I listened to the teachers and tried to UNDERSTAD what they were teaching about electricity which you have obviously missed altogther.
You should have learned all this stuff in your Training?? Where was that???
Obviously you did NOT, so do not get obnoxious at me.
If you asked a question from a position of a newby then I would try to help. You claim to be an EE so why ask very elemental questions then object when it is pointed out you know none of what your claimed qualifications imply
-- John G.
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Yeah, but in all that time did you ever go to English class?
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wrote:

This is and electrical discussion and being picky about some simple english aberation only proves you have not got enough knowledge to argue against my criticism of your abysmal knowledge of electricity when you claim to be an Electrical Engineer.
-- John G.
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And in all this time that you have spent trying to admonish me, you have yet to answer my question. So your answer to me should just simply be, "I don't know." That'll suffice, thanks.
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wrote:

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On 3/2/07 6:44 AM, in article snipped-for-privacy@j27g2000cwj.googlegroups.com, "Zerex71"

First of all, is your 110VAC really 120VAC. It is amazing to me that the term "110VAC" is still used. If my supply voltage were that low, I would complain bitterly to the power company.
As to your question, the wiring is that way because someone wired it that way. Whether it meets code or is otherwise safe is a question an EE should be able to track down.
Get a VOM and track down the wiring. Know what the connections are. My son has a home with an electrical system that was "upgraded" by replacing two-prong outlets in a bathroom with three-prong plugs. The third grounding socket was not connected! It turned out to be very difficult to run a ground connection to it.
Although not good practice, having a bad or no ground connection on a three-prong outlet can be mitigated if the outlet is GFCI protected.
Bill
-- Fermez le Bush--about two years to go.
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There is a serious anomaly in the National Electrical code that allows lamps including metal lamps to be supplied by 2-wire cords without a ground using a polarized type plug. The Code says that the (dumb) consumer is responsible for checking for possible faults (REF: Section 410.45.) We had a 2 year old baby die in Alaska after a metal floor lamp sat in the living room for almost a year before the two year old touched the lamp while in contact with the metal baseboard heater. This did not change things because when the proposal was made to change the code to require a grounding conductor for lamps citing this particular electrocution, the reply was that it would "cost the industry too much."
I suppose the same problem exists for other two wired devices and appliances.
REF: 410.42 Portable Lamps.
(A) General. Portable lamps shall be wired with flexible
cord recognized by 400.4 and an attachment plug of the
polarized or grounding type. Where used with Edison-base
lampholders, the grounded conductor shall be identified and
attached to the screw shell and the identified blade of the
attachment plug.
410.45 Tests. All wiring shall be free from short circuits
and grounds and shall be tested for these defects prior to
being connected to the circuit.
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