Installing a Switch in a 3-Wire Power Cord

[Cyberdude]

This isn't a software question ... hope that's OK.
I spent 5 years studying electrical engineering, and I regret to say that I am unsure of how to install a switch in a 3-wire power cord. I'm never sure what the "ground" wire is being used for.

With a 2-wire cord, I can install a switch in either wire, and that will break the circuit. Preferably, if possible to determine, the wire chosen should be the "hot" wire.

In a 3-wire cord, I presume that the "hot" wire is where the switch should go. But what's the electrical status of the other two wires? Will anything ugly flow through them? It almost seems like one should install at least two switches to be electically comfortable that nothing unwanted happens. But that's quite bulky looking and, unless you turn off both switches at exactly the same time, it would seem that you could (at least temporarily) have an electrical circuit in the remaining two wires.

Incidentally, this power wire will go from a house wall socket to my satellite receiver, for what that's worth.

While I'm on the subject, I guess I still am not sure what the function of the ground wire is. I always thought that the "return" wire in a 2-wire cord eventually got connected to the neutral from the power transformer outside. That neutral is solidly conected to a copper rod or such that is buried in the ground near the transformer. If that's true, then what purpose does the third "ground" wire serve? Presumeably it is also connected ultimately to the transformer's neutral.

I always thought that maybe it was a protective device to prevent folks from being electrocuted. The manufacturer of the using equipment that might have a metal case would connect the ground wire to the case, then if something electrical inside the case broke loose and touched the case, the ground wire would provide an electrically convenient path for the electric current to flow through. That path would be very low resistance compared to the human body, so very little current would go through the body. Will someone explain this to me?

[lucas]

With a 2-wire cord, I can install a switch in either wire, and that will break the circuit. Preferably, if possible to determine, the wire chosen should be the "hot" wire.

Hello Cyber,
scary misconception.....you can install a switch in either wire and it will turn of your light, etc. but......if you put the switch on the hot wire you have stopped electricity from flowing past the switch...If you put the switch in the other wire...you still have electricity going all the way to the light and back to the switch where the circuit is broken.

A very dangerous situation if someone thinks that just because the light is off that they can safely work on the light fixture.

[Cyberdude]

True, lucas, and I fully understand what you said.
My uncertainty is in regard to the 3-wire power cord. As I said, putting the switch in the hot wire leaves the other two to float. I'm never sure what the electrical relationship of the second (non-hot) wire is to the ground wire.
I take it that you believe it is sufficient to install the switch in the hot wire.
Thanx for the reply.

[lucas]

Yes as I understand it the third wire or second ground is used as a frame ground in case you get shorted out to the body of a drill etc. supposedly gives it a solid, short path back to ground to follow instead of through you. Guess the same could apply to a light fixture. Looks like I don't know any more about it than you...maybe someone will enlighten us.

[TonyJollans]

In the UK, the third wire is connected to ground - sometimes via a spike in the ground, sometimes via water mains going striaght to ground. It is a safety circuit providing, in simple terms, the shortest route to safety for any leaking electricity. It should not be broken, by a switch or anything else.

As far as putting a switch in a circuit you should always break the live wire. if you know what you're doing with a two-wire cord, just ignore the third wire and do the same thing with the three-wire cord. The actual mechanics of it may depend on the design of the switch.

Caution. Although I am comfortable playing with electricity, I am not an electrician - and American circuits are different from British ones.

[mdmackillop]

Hi Sid,
We occasionaly use here a dipole switch, which is used to control higher power items such as washing machines. In this type, both the Live and Neutral are disconnected. The Ground or Earth wire is always unswitched.

Here's an article with some basic info on why you have a Neutral wire.
http://en.wikipedia.org/wiki/Three-phase_electrical_power

Regards,
Malcolm

PS
Tip 1 Turn off the mains power first
Tip 2 Wearing rubber gloves and wellies
Tip 3 Don't hold the end of the wire in your mouth while you look for your screwdriver

[TonyJollans]

I never follow any of those tips :D

[Cyberdude]

Thanks, Gang! You have reassured me of what I had eventually concluded. I admit that I never take safety precautions when diddling with house wiring. Once in awhile I get bitten, but it never seems life threatening.

For what it's worth, I believe that the "ground" wire is used by manufacturers to connect to radio frequency shielding material that some laws require. (I wear an aluminum hat that protects me from alien radiation, and I never know what to do with its ground wire, but I suppose that's material for another post. :devil2:)

[Cyberdude]

I have a war story that I was just reminded of. In my youth I once had to go aboard a Navy aircraft carrier to work on some electronic equipment. I got to talking to some of the locals about the necessity for good grounding of the equipment. It seems that the ships hull was used for the common ground for EVERYTHING electrical. They told me that at some points of the ship they had measured currents of several thousand amps in the hull. As a consequence they had ground voltages fluctuating several volts (not millivolts), which was causing the electronic equipment all kinds of fits.