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. :)
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.
So is the answer to my question, "Because you don't need it for a
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
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
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.
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.
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
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.
On Mar 2, 11:47 am, email@example.com wrote:
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,
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.
On Mar 2, 3:53 pm, firstname.lastname@example.org wrote:
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
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.
On Mar 2, 3:53 pm, email@example.com wrote:
Okay, terminology time - what is meant again by "balanced" as opposed
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?
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
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
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
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.
On 3/2/07 6:44 AM, in article
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.
-- Fermez le Bush--about two years to go.
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
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
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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