Here is a thread about how to make sure a microwave oven is grounded in the absence of a three-hole prong or outlet. A lot of people say things in that thread which I do not understand. This is because, though I’ve known since I was a kid (because daddy said so) that electrical devices ought to be grounded, what I haven’t known is:
With this thread, I thee ask.
Ground is to electronics what the ocean is to plumbing.
Grounding isn’t necessary; it is done as a safety feature so that the case of an appliance won’t become live (have voltage with respect to the earth) and thus become a shock hazard if there is a fault which breaches the insulation between the mains and the case. Note that many appliances use two-prong cords, even with metal cases, indicating that grounding isn’t necessary (they usually have to pass some tests though, like applying several kV between mains and case/secondary side and ensuring no current flows, except that through any small noise-filtering capacitors, which must also be certified by the respective safety agencies not to fail in a dangerous way).
The reason for grounding in the first place is because the neutral wire is grounded at the service panel; as for why this became standard practice, I guess it was to avoid a floating circuit, as well as add safety in the event of a short in the distribution transformer (which would be extremely dangerous; not 100-240v but thousands of volts, which would be present on both the hot and neutral wires; distribution lines once used the earth as a conductor, some still do).
ETA: I think Napier is referring not to electrical grounding as I speak of but the common practice to call the return of a circuit “ground” even though it may not actually be grounded in the sense as connected to earth.
In short (no pun intended) the ground makes sure that if a wires comes loose inside the appliance, a fuse blows.
Here’s an example. Let’s say you have a microwave. Over the years, it gets moved and jostled around and starts to age. One day, a live wire inside the unit comes lose and makes contact with the metal casing. The next morning, you go to microwave your leftovers from last night, when you touch the microwave, you get zapped since the outside is energized. Are you with me so far?
Now, if the microwave is ‘grounded’ what that means is that there’s a wire that runs from the case, to the outlet and to the earth. When that hot wire touched the case, the circuit breaker in your basement would pick up on that and cut the power.
If you take the case off your microwave (if it’s metal), you’ll see it’s attached with metal screws to the frame. The frame will have a green wire that attaches to the green power wire coming into the microwave. That green wire goes to the third wire on your outlet, which goes to the breaker box in your basement and attaches to the ground lug. From your breaker box (and you can go check this) you’ll see a copper wire that comes out of it, exits your house and is jammed into the ground, literally. It keeps you safe.
There are three connections on a standard wall outlet - Hot, Neutral and Ground.
Electrically, Neutral and Ground are the same. In fact, they are connected together at the breaker box. Hot is 120V - referenced to Neutral. Since Neutral is connected to earth ground (in many places, but usually at the transformer), you can get shocked by touching a “hot” connection while you are “grounded” (meaning electrically connected to earth ground). To prevent metal-encased machines (like a washing machine) from becoming a deathtrap if the hot wire were to touch the case, which would “energize” the entire case, the case is connected to the ground wire. This means that if the hot wire were to touch the case, it will make a huge bang, and blow the breaker, but at least it won’t kill you if you touch it.
Grounding also reduced nuisance leakage currents, which may cause tingling.
Does this make any sense?
to add to the above; the grounding conductor (often the green insulated wire in cords/cable or in an appliance, sometimes bare wire in cables, third prong in plugs) under normal conditions has no voltage on it and carries no current. the grounding conductor carries current when there is an electrical fault (something broken). this grounding conductor is a safety ground in power circuits.
the exception to this is older 240 VAC circuits supplying electric dryers and ranges with a 3-wire NEMA 10 receptacle where the appliance chassis is connected to the neutral wire. existing installations are grandfathered in but new ones must use the 4-wire grounding NEMA 14 receptacle/plug.
If you live in the U.S, the electricity at your standard receptacle is 120 VAC.
One wire going to your receptacle is a copper wire with black insulation. It is called the “hot” wire. The other wire has white insulation. It is called the “neutral” wire.
There is a voltage on the hot wire relative to the neutral wire. It is not a DC voltage, where the voltage (for example) is exactly 120 V at all times. The voltage on the hot wire goes up to 170 V (relative to the neutral), and then back down to 0 V (relative to the neutral), and then down to -170 V (relative to the neutral), and then back up to 0 V (relative to the neutral), and then back up to 170 V (relative to the neutral), and then back down to 0 V (relative to the neutral), and then down to -170 V (relative to the neutral), etc. etc. etc. etc. Forever. It switches back and forth fairly quickly, like 60 times every second. So at any given instant in time, there is a good chance there is a fairly high (and fairly dangerous) voltage on the hot wire relative to the neutral.
Got that? O.K. Now here’s the kicker: the neutral is electrically connected to the dirt around your home. In other words, the neutral is “grounded.” This is done at your circuit breaker box and through an 8 foot copper rod that was pounded into the soil next to your house many years ago.
With me so far? O.K. Let’s say you somehow come in contact with the hot wire. Like you stick a screwdriver into the receptacle’s slot that connects to the hot wire, and you touch the metal part of the screwdriver. Will you get shocked? It depends. If no other body part is making contact with earth ground, you’ll be O.K. But if any other body part (e.g. hand, foot) is making contact with earth ground you will get shocked. Why? Because there is a voltage on the hot wire relative to the neutral wire, the neutral wire is connected to the earth, and thus there is a voltage on the hot wire relative to earth ground. One of your body parts is touching the hot wire (via the screwdriver), the other body part is touching earth ground, and you get zapped.
“Yea, but if I’m in my house, there’s no way I can make contact with the earth. So it’s impossible for me to get electrocuted inside my house.” Nope, not true. There’s lots of stuff in your house that is connected to earth ground. Like a SS sink, faucet, water from a faucet, and even a concrete floor (if you’re barefoot). In other words, at any given time one of your body parts may be grounded when you’re inside your house.
O.K., now let’s talk about grounding an appliance. Your microwave oven has a metal chassis. There’s a hot wire and neutral wire going to your microwave oven (via the electric cord), correct? Neither of these wires is connected to the oven’s metal chassis. So far so good, right? Now what would happen if the insulation on the hot wire inside the oven was compromised and the hot wire somehow made electrical contact with the oven’s metal chassis? The chassis would then be at the same voltage as the hot wire. Which means there would be a dangerous voltage between the metal chassis and earth ground. As mentioned above, one of your body parts is often grounded (especially in a kitchen), and if you touch the metal chassis of the oven you will get zapped.
So what can be done to prevent this from happening? Simple: run a *third *wire from your circuit breaker box to the oven. This wire is called the “ground” wire, and it has green insulation. At the circuit breaker box it will connect to earth ground (via the long copper rod pounded into the ground next to your house). At the oven this wire will connect to the oven’s metal chassis. Now let’s replay the scenario… a hot wire inside the oven (or some part of the circuit) somehow makes contact with the oven’s metal chassis. (Also note that the hot wire doesn’t even have to make “good” contact to the metal chassis – even a resistance of 10,000 ohms would be dangerous.) What will happen? If it is a very low resistance (“hard”) short, the circuit breaker in your panel will probably trip, and you’ll be safe. But even if your circuit breaker *doesn’t *trip because the resistance between the hot wire and metal chassis is 100 ohms or 1000 ohms 10,000 ohms or whatever, you will still be safe, because the ground wire connected to the chassis will keep the chassis at 0 V.
But why not just tie the case to neutral if they are the same? Why have a seperate wire?
Safety. The ground is connected all throughout the house. (In theory) every outlet, every metal box, every piece of conduit, every metal water pipe, all the concrete, the dirt outside…all connected. The neutral wire, OTOH, not so much. While it should have an uninterpreted connection back to the box, it’s not always the case. A homeowner may switch the neutral instead of the hot when installing a light switch. Things get wired backwards. You can have cases of hot neutrals. An outlet can be abandoned and the wires tied off leaving the neutral disconnected from things upstream.
Yes, you can ground to the neutral and it will almost always work, but the actual ground is just an extra safety feature.
There are several reasons, but this is the big one:
Neutral carries current. If you tie the case to neutral, and you break neutral, you get a live case.
This is the main reason. The neutral does carry current. If it is carring a lot of current then because resistance the voltage at the outlet may not be zero in referance to ground. By adding a ground wire that normally does not carry current the referance to ground will be zero.
Everyone is talking about a major sort but more common is leakage currrent. If the insulation has broken down in a part just a little current can flow from the part to the case. If the outlet is ungrounded then without a path for the current voltage of the case will be above zero. If you are grounded, lets say by turning on the water, at the same time you touch the case. You can recieve a shock.
An example from real life when I worked at The Emporium department stores. I wired up two show cases that were sitting side by side. I put on a short 3 wire cord with a three prong plug on each case. Then I pluged the cases into a outlet on the floor. After I left the DSM unpluged one show case used a two wire multiplyer. Used an ungrounded adapter to plug in the show case, then plugged in sunglass lighted stand.
I got the call that customers were getting shocked by the cases I put in and I had better fix it. When I saw what was done before fixing it I got my Fluke meter out. There was a metal ring arounf the glass of the cases. I tested for voltage between the two cases. I got a reading of 91 VAC. After removing and throwing away the two wire cord and multiplyer I had a discussion with the DSM about using any ungrounded devices on the sales floor.
Imagine you have an electrical generator, and it’s connected to something (a light bulb or whatever, doesn’t matter). If the generator is not connected electrically to ground and neither is the load (the light bulb) then you could stand on the ground and touch either wire and you wouldn’t get a shock. This type of system is called an “isolated” system since it is isolated from ground.
Now at first, you might think that this type of system is safer, and in some cases it is. In hospital operating rooms and such they use isolated systems to prevent tiny electrical currents from equipment from flowing through surgery patients and killing them. Isolated systems have other uses as well. They are common on certain types of ships, for example. The problem you run into if you try to run an entire power system isolated is that mother nature is very good at randomly inserting ground connections into your system. While hospitals do a lot of maintenance and yearly testing to insure that their isolated systems stay isolated, that sort of maintenance isn’t practical in a power system that may span several states. Since an isolated system isn’t practical and a randomly grounded system would be dangerous, power companies run an intentionally grounded system.
In the old days, they would just take one of the two conductors and ground it. This is literally done by connecting it to ground, i.e. driving a copper rod into the ground and attaching a wire to it. You may not think of dirt as being particularly conductive, but if you drive a copper rod several feet into the ground it actually conducts pretty well. The conductor that is grounded is now called the “neutral” wire since its electrical potential is neutral with the earth. You can safely touch the neutral and not get shocked (well, sorta, more on that in a moment). If you touch the other wire while touching something grounded though you will get shocked, so that wire is called the “hot” wire. There are a lot of things that you can touch that are electrically conductive to the earth, or in other words they are effectively grounded. Your water pipes, for example, make a very good electrical connection to the earth. Other things that you may not think of can also make excellent electrical connections to earth, like the metal siding on your house and your screen windows as well.
So now that you have a “safe” neutral and a dangerous hot wire, you can now design all of your appliances and such so that if it has a metal case (like an oven or a refrigerator) you connect that to the safe neutral and you won’t get shocked. And this is the way they used to do things.
You end up with two problems doing this. The first is that unless your metals are superconductors (which outside of laboratory conditions they won’t be) they have a voltage drop as current passes through them. This (which was already mentioned upthread) means that if current is flowing through the neutral, the neutral will no longer be at earth ground potential, and touching it will be a slight shock risk. The second, and more dangerous problem, is what happens if one of the wires breaks. If the hot wire breaks, no biggie. The appliance just stops working. But, if the neutral breaks and you turn it on, the entire case of the device becomes electrically hot, and if you touch the case and something grounded you face a very severe shock hazard.
To solve this problem, they went to the three conductor system we use now. The neutral is still grounded, but instead of using that for case grounds and such, a separate ground wire is run to all of the outlets and this wire is called your safety ground. Now, if any wire breaks, the device just stops working, and the case doesn’t become electrically hot. And if a wire shorts to the case, it will trigger the fuse or breaker to blow. If you have multiple things fail it is still technically possible to end up with a hot case, but in many more scenarios you don’t end up with a hot case where you would have with the two wire system.
In the U.S. the hot wire is black, the neutral is white, and the safety ground is green. The green wires all connect together and are then connected to the neutral back at the breaker box, and the safety grounds and neutral are then all connected to earth ground at some point close to there. It used to be that you were required to make the ground connection to the cold water pipe, since this was a handy ground connection that was present in pretty much all homes. Then they started using PVC pipe which was fine for water but not so good for electrical connections. So they changed it so that you can no longer use the water pipe as your ground connection. Instead you have to drive a copper rod into the ground nearby and use it instead. But you still have to connect your water pipes to this ground connection because you wouldn’t want something to short to your water pipes and then have all of your water pipes in your house become electrically hot.
Is the third prong the one for the safety wire current?
Yep.
Let’s look at this scenario:
a) You have a portable appliance where the metal chassis is connected to neutral.
b) The appliance’s power switch is in the “on” position (or the appliance does not have a power switch).
c) You are standing barefoot on the wet concrete floor in your basement.
d) Your left hand is touching the appliance’s metal chassis.
e) You plug the appliance in to a 120 VAC receptacle.
No problem, you say? Wrong. There’s a 50/50 chance you will get zapped for a brief period of time. Think about it… every time you plug or unplug an appliance in to a 120 VAC receptacle, there’s a 50/50 chance the plug’s hot prong will make contact *before *the neutral prong. Which means that, 50% of the time, there will be a short amount of time when there’s a break in the neutral.
So for a 120 VAC receptacle, neutral breaks are happening all the time. If the neutral is tied to the appliance’s metal chassis, and if you’re grounded (which is often the case), and if you’re touching the appliance while plugging it in to an outlet (which is often the case), there’s a 50/50 chance you will get zapped for a brief period of time.
See this pic.
It is worth mentioning that, besides being a safety ground, some devices require an independent ground in order to even operate:
Some devices require an EMI shield to be connected to earth ground.
Some (not all!) fluorescent lamp fixtures rely on current in the ground wire in order to start. For these fixtures, if you don’t connect the metal reflector to earth ground, the light will be unreliable, or may not work at all.
yes the 3rd rounded prong (USA) is the safety ground in ordinary convenience 15A or 20A receptacles.
there are other kinds of receptacles with 3 or 4 straight/curved prongs which are different.
Grounding also protects you and your electrical system from lightning strikes. The surge protector you use for your computer and other expensive electronic devices sends that surge current from a lightning strike or power line surge to earth ground rather than frying your computer or whatever else your protecting. Also note that your phone and cable tv lines need to be connected to the surge protector as well.
The way a surge protector protects your equipment from a lightning strike is by exploding. There is no device that can fit under your desk with circuitry capable of isolating some equipment while directing that much current to ground.
