What is the point of the third prong on plugs for electronic equipment?
I know that one of the prongs on a plug goes to the hot side of the outlet, and one goes to the neutral side, but what does the third, grounding prong, accomplish? The neutral side of the receptacle (aka outlet) is connected to the ground back at the main panel, so what does the grounded prong buy you? Is it in case there is some kind of interruption in the neutral wire? Does the grounded prong function all the time to get rid of static charge or something, or is it only there as a back up for the neutral prong? If you trace back the grounded prong, where does it connect inside the computer or other component? Is it the same place as the neutral wire, or does it connect to the body of the component?
The hot and neutral wires carry AC, but there is not necessarily anything to guarantee the equipment using it doesn’t build up a DC (common mode) component. The ground wire is there to guarantee that you don’t get
Also, if there’s a short between the hot wire and the chassis, the ground wire to the chassis guarantees that the current passes straight to ground instead of thru you to ground.
Thanks, bughunter. I understand the second part (about providing a ground connection if there is a short between the hot wire and chassis) but can you tell me more about building up a DC (common mode) component? I haven’t heard that term before? Is this the same thing as saying that the chassis can build up a static charge?
Real wires aren’t superconductors. Because of this, when current flows through your wiring, a potential difference developes between earth ground and the nuetral wire at your outlet. It’s not as much of a shock hazard as touching the hot wire, but it’s still a shock hazard.
The way you get around this is you run a third wire, connected to the nuetral back at the service entrance. This protective ground carries no current, and therefore will always be at ground potential.
ecg: It is true there is a voltage on the neutral when current is flowing. But the voltage is usually quite low for a properly working system. At any rate, this is not the primary reason for the earth ground.
There are five reasons to have an earth ground:
Safety. This is the primary reason. A metal chassis is (usually) grounded in case there is a “hot to chassis” fault in the system. As an example, a wire’s insulation might fail and the copper conductor might make contact with the chassis. Or a transformer’s insulation might breakdown. Since you are (usually) at ground potential, and the chassis is at ground potential, there is little risk of you getting shocked if you happen to touch the chassis. Furthermore, a grounded chassis will hopefully trip the circuit breaker back at the panel should a ground fault to the chassis occur, thus rendering the system safe.
Surge protection. Many surge protectors put transient surge devices (GDTs, MOVs, etc.) across the hot and earth ground lines (and sometimes across the neutral and earth ground lines) to squelch common mode voltage transients.
Shielding. Some sensitive electronic equipment is shielded to attenuate outside EM noise. It is useful in the other direction, also; some electronic equipment is shielded to reduce the amount of EMI radiating from the equipment. A shield connected to earth ground will sometimes improve performance, though this is not a guarantee.
Static discharge path. An earth ground is sometimes used to drain away stored charge in high resistance circuits.
Current path. Sometimes the earth ground conductor is used as return path. The only example I can think of is the reflector on large fluorescent lighting fixtures, where the capacitance between the bulb and the grounded reflector allows a small amount of current to flow to get the bulb started.
Regulations. Some localities require grounded cords on any device not certified as ‘double-insulated’. Such certification may cost more, or require inefficient design aspects.
Cost. On the day they bought them, gounded cords cost $15.00 less than ungrounded cords when purchased in quantities of 1000 or more. Note that in this case, the other end of the cord (inside the case) may or may not be connected.
Some people say, “The neutral and earth ground wires are going back to same spot in the breaker box! That’s stupid… why not just run one wire? If you want to ground the chassis, fine. Just connect the neutral to the chassis.”
This might sound like a reasonable thing to do at first. But upon further reflection we are left to conclude that connecting the neutral to the chassis is a Bad Idea[sup]TM[/sup].
Why?
Let’s say you have a motor with a metal frame. The frame is supposed to be connected to earth ground via the “third prong,” but some genius installed a two conductor power cord and connected the neutral to the chassis. That way, if a hot wire shorts to the chassis, it will blow the breaker.
So you walk over to an outlet with the idea of plugging the motor in. You are grounded (as is often the case). The power switch is in the “on” position. (Or perhaps there is no power switch.) Your right hand is touching the chassis. You left hand is holding the plug.
So far so good, right?
Now when you insert a two-prong plug in an outlet, do you think the hot and neutral prongs make contact at the exact same time? Of course not; that would be impossible. So which makes contact first, the hot or the neutral?
If the neutral prong makes contact first, you won’t feel a thing. If the hot prong makes contact first, you will feel a mighty jolt. This will start when the hot prong first makes contact and end when the neutral prong first makes contact. If this interval is 83 milliseconds (for example), your body would be hit with five complete cycles of 60 Hz AC.
Another scenario is when there is a break anywhere in the neutral line. Still another scenario is when you accidentally pull on the cord while operating the equipment and the plugs comes halfway out of the outlet, with the neutral disconnected while the hot remains connected. In any case, the lesson here is that you can’t connect the chassis to the neutral. And as my previous post stated, it is usually a good thing to ground a chassis. Therefore, you must run two conductors in order to have a grounded chassis.
Ground referencing a circuit. You will sometimes see one conductor of a floating supply connected to earth ground. As an example, you will sometimes see one terminal of a transformer’s secondary winding connected to earth ground. This is for safety reasons, and is done to prevent the common mode voltage of the secondary from “floating up” to the primary voltage due to leakage paths between the two coils.
8b. Ground Loops. In these cases, note that it is important to utilize the same ‘ground reference’ for all components otherwise potentially connected. That is, your amplifier and your sub-woofer should use the same outlet to ensure that the earth ground for each is at the same potential at all times. If they are on different outlets, the ‘common’ ground wires may go all over the house before eventually connecting, and/or one may have a more resistive connection to the common point. This can result in a very bad (for electronics) thing called a ‘ground loop’.