Thread: About the AC main power plug

Hello experts,
What is the main difference between two pin plug and three pin plug? Is the extra pin in three pin plug necessary? What if my oscilloscope uses only two pin of it? Will any bad ingredient be induced in my test result?

Thanks in advance!

The third pin is a direct link to earth ground and is required by the government safety regulations in most countries. Equipment that is powered from the AC mains is allowed to not have the connection to the third pin if this equipment is adequately isolated by a transformer and has a double insulated case so that all parts of the device inside circuitry cannot be touched by human operators. I cannot imagine a modern piece of test equipment that does not have this third connection.

Although info on 3-rd pin was given in the previous post I encourage you to read this section from free WIKIPEDIA:

http://en.wikipedia.org/wiki/Domesti...lugs_&_sockets


And BTW:
Type A (American 2-pin)
This class II unearthed plug with two flat parallel pins is standard in most of North America (including Central America and the Caribbean) on devices not requiring an ground connection, such as lamps and "double-insulated" small appliances

Also two pins (without ground) are used for TV, VCR, house small equipments like hairdrier, radio, etc. in Europe

Hello flatulent and IanP,
What is so called "double insulated"? Does it mean the inner and outer surface of the case are both insulated?
Why the neutral line in the two pin plug is not tied with ground by default definition?
If my oscilloscope is lack of the third ground pin in its AC power entry, does the wave appearing on the screen still make sense?

Three questions waiting for your kindly explanation.

Thanks in advance!

Dear bittware,

Neutral is the Return Ground of the power line running from the Live (where it is usually fused). Earth is the lowest potential point such as those water pipes, lightning arrester conductor, underground cable shield and lamppost running down to the underground.

You cannot connect Neutral and Earth together. Doing so in the plug, no current will ever flow into the electric appliances. If the appliance has an internal cell or battery, you are discharging it as well.

Double Insulation - The top plastic case and bottom chassis, or sometimes the outermost plastic-moulded cast, is first insulation. The second insulation is that the circuit or any current-carrying electrical circuits (such as PCBs and mezzanines) and devices are insulated from a small distance by air, and metal shields are terminally grounded. All power electronics with ratings greater than 100mA and transformers are usually earthed.

Whether it is the North America Type-A 2-Pin, the Continental Europe Type-C 2-Pin or other 2-Pin Plugs, 2-Pin plugs are used when insulations are officially certified by authorities and properly electrically terminated and isolated by a transformer (usually a centre-tapped).

3-Pin Plugs are necessary when there are protruding metal parts such as the backplace of the laptop or notebook where you connect USB, parallel and serial ports and S-Video and etc.

The German Type-F Schoku 2-Pin plug has two ground pins, if you examine carefully. It's not a typical 2-Pin.

Originally Posted by bittware

What is the main difference between two pin plug and three pin plug? Is the extra pin in three pin plug necessary? What if my oscilloscope uses only two pin of it? Will any bad ingredient be induced in my test result?

If your oscilloscope and test circuit are grounded, then the signal you see in the scope is referenced to ground.

If your oscilloscope is grounded, but test circuit is NOT grounded (e.g. isolated circuit), in some cases the probe will burn if you connect the ground lead of the probe to the isolated circuit (current flows from the isolated circuit through the ground lead of the probe). In this case, you need to use two probes (let the ground lead of each probe open), or differential probe to measure the test circuit.

If your oscilloscope is NOT grounded, but the test circuit is grounded, you can measure the test circuit using two probes or differential probe.

Originally Posted by SkyHigh

You cannot connect Neutral and Earth together. Doing so in the plug, no current will ever flow into the electric appliances. If the appliance has an internal cell or battery, you are discharging it as well.

If one end of a bulb is connected to Live, the other end is connected to Ground, do you think the bulb will light up?

In this first place, who would want to connect Neutral to Earth? Shorting these two terminals serves no useful purpose. Doing so would direct all the current from Live to Earth. This explains why Live and Neutral are usually sufficient, if proper electrical isolation and insulation are provided.

Instrument and Test units should always be subjected to the same signal ground wherever and whenever possible to be referenced on the same balanced ground to provide wider dynamic range sensitive enough to obtain absolute signal change in circuits where DC levels are measured.

Should either instrument or test unit be subjected to independent signal ground, differential probing techniques are used, however, this is usually done if only AC signal change are observed and DC levels are not the concerned.

And this is not true.
Here, all instalations have NEUTRAL connected to EARTH at 1 point (obviously before RCD device).
And, besides, electrical current has better things to do than "walking around".
This thing is to go back to where it comes from trough specially design path which is called a wire.

When you only use a 2-Pin (Live and Neutral), the return path has to be Neutral. Earth is never a signal return path. In any building, Earth is connected to the earth reservoir, usually a underground plane, where the lowest potential is found. Neutral is connected such that the current returns to the power main, via intermediate substation and eventually back to the power station, a complete closed circuit.

When you only use a 3-Pin, check if the Earth pin is even or actually connected. I have seen 3-pin used in such a way where only the Live and Neutral are used and the Earth pin is not even used at all. It's as good as a 2-Pin.

I am not talking about 2-pin, 3-pin et cetera .. and earth connected with N inside a plug. It is never connected there, or at least it should not be..
My point was about installation in general and just to repeat what I said before, these intallations have Neutral line connected with EARTH line at the entry to the building, before RCD devices.

This is serious! Earth and Neutral together? Oh my ***!!! Let's hope nobody get a shock somewhere in the building shoud there be any non-uniformity in the Neutral.

Sky High,

Where I live earth and neutral are connected in all fuse panels and junction boxes so what could go wrong here?

Earth and Neutral should be separate. At least this is how it should be technically and in any official authority, it is certified this way.

Neutral is the power return path, just that. Earth is exclusively and solely used for electroshock protection, just that.

Whether it is in the American Standard, the British Standard, the European Standard, it is well-recognised this way. No argument. It is a good practise and protocol to follow.

Stick with good practise. If there is a Neutral for you to use, then use it. Do mix with Earth. It's not as if a miracle will happen if you do otherwise.

The question was “what could go wrong here?” and I asked it based on “Let's hope nobody get a shock somewhere in the building shoud there be any non-uniformity in the Neutral.” because I do not see the danger here!

Obviously this is wrong! It's already against the general practice.

The same analogy goes without saying when you are allowed to drive on the right-hand side of the road, why do you want to drive on the left-hand side against the traffic?

Nothing happen doesn't mean nothing is wrong. People like you will give many people unnecessary problems.

Undergraduate students would have already known that Neutral is noisy due to cross-coupling in power cabling and IR-drop, and Earth is noisy due to subterrainal ambience.

Earth and Neutral are the lowest potentials but prior is meant as a power and signal return ground and the other is solely for terminal return. Shorting them together jeapardise the systems connected closest to the short-circuited Earth and Neutral. Such systems are subjected to additional noises from Earth.

Isn't is silly to make life difficult and systems more susceptible to unwanted noises?

Added after 4 minutes:

Back to basic:

Earth does not offer a return path back to the transformer at the closest substation or to mobile power generator. It is the neutral.

This discussion is going to nowhere...
And this is my last suggestion: open your bloody meter box and check. In my, and all the other houses around local E is connected with N ... and this is not just a practice, it is a rule required by the Energy Commission.

That's the way done for your house.

As far as the European standard already certified, this is not so. Earth is connected down to the Earth plate of buildings or to the distributed and isolated Earth links.

What's at your place could be a shortcut done by contractors, and possibly your area may not have a dedicated Earth link to you and your neighbourhood.

Hope this will help the discussion.

It depends from the type of distribution used by the power company.
Here in Italy the distribution is TT, this means that the ground of the mains transformer is separated from the ground of my hopuse installations.
At the mains transformer ( property of the power company ) the neutral and the Ground ( PE ) are coinnected togheter, but the power company distributes only Phase and neutral.
By rules the neutral should at ground potential, but the power company warns you to consider always the neutral wire as a live conductor.
In Germany the power company distributes the power as TN, this means that the power company distributes ALSO the PE ( ground ) conductor or the ground and neutral conductor are connected togheter ( in pratically is the same wire ).
This adds a big responsability for the power company, being its responsability to guarantee the PE conductor, on the other side this assures that is alwais existing a ground connection.

Mandi

I don't think there is something like European Standard and I agree that Earthing standard may vary from coutry to country or, as FANT said, from power supplier to supplier...