This is going to take a bit of explaining.
When you are talking about lightning, you are talking about a charge differential that has built up between the earth and the clouds. What you have are two big charged areas, one negative and one positive, with a great big air gap in between them. A bug zapper is the exact same thing, on a much smaller scale, so you can really think of lightning as a really really big bug zapper, and you are the bug. Lightning is generally going to take the easiest path it can find to ground, and a tree or a great big metal pole (aka a lightning rod) will generally make a better path than some yutz inside of his house. So, in this case, it is better to have a great big grounded thing nearby, because the lightning is more likely to strike it.
A couple of lightning tips. Don’t stand next to trees. They are big which makes lightning more likely to strike them, and the lightning bolt can jump from the tree trunk, through you, and go into the ground. The lightning bolt just jumped a few miles from the clouds to the ground, so the little jump from the trunk to you is no biggie for a lightning bolt. Even if just a fraction of the bolt jumps out and hits you, it can turn you into toast. The second thing is that lightning contains a lot of energy, and it can heat up the sap in the tree so quickly that the expanding sap gasses blow the tree apart. You can be hit by tree shrapnel, or a big limb (or even the entire tree itself) could fall on you.
Don’t stand on a porch. People think they are safe because they have a roof over their head. Porches, overhangs, open garages, open sided picnic pavilions, and the like are a really bad place to be in a lightning storm.
Cars protect you, but it has nothing to do with the rubber tires. The metal body of the car forms a Faraday cage of sorts, so the lightning will tend to travel around the car body leaving anything inside the car untouched.
If you do have to be in an open field, crouch down in a ball until the lightning passes. Standing up tall makes you the biggest thing around, which isn’t guaranteed to get you killed, but it does make it more likely.
Ok, enough about lightning. Now let’s talk about ground. There are basically two types of electrical systems, grounded and ungrounded. The ungrounded type is safer, which is why we use the grounded type. Confused? Ok, let me explain.
In an electrical system, you’ve got your generator, some wire, and whatever stuff you have connected to it. The electricity flows in a great big circle. It goes from the generator, out through the wire, through the stuff, then back through the return wire to the generator. Break the circle and no electricity flows. If you have an ungrounded system, you can grab either the supply wire or the return wire and there’s no problem. It is only if you touch both that you complete the circle (circuit) and electricity flows.
So why don’t we use ungrounded systems for residential power? Quite simply it’s all because of Mother Nature. She likes to randomly do things like blow tree limbs into power lines and such, which makes the wires then electrically connected to the earth, and the earth is electrically conductive. So, what happens if you try to run an ungrounded system in something large like a residential electrical distribution network, you end up with a randomly grounded system instead, and it is no longer safe. It is much better to intentionally ground one of the wires and run a grounded system. That way you know that you can always safely touch one of the wires, even if what you are standing on or otherwise touching happens to make a good electrical path to ground (like a water pipe, aluminum siding, etc). Sure, we would prefer to run an ungrounded system, but it’s just not practical.
There are ungrounded systems in common use, though. They are referred to as “isolated” systems because they are isolated from earth ground (typically via an isolation transformer). If you happen to go into a hospital, look for all of the red outlets. Those are isolated outlets. Hospitals use isolated systems to stop tiny currents from things like heart monitors from going across your heart and killing you. This occurs much more easily in places like operating rooms when they have your chest cracked open, so any location that is defined as “wet” has to have isolated power. Hospitals have to spend an awful lot of time and effort keeping their isolated systems isolated though. If you look carefully, somewhere near the red outlets you’ll see a little electrical meter on the wall that looks something like this:
http://www.iaei.org/magazine/wp-content/uploads/2009/05/07djohnston_photo12_976015905.jpg
That’s part of a test circuit that the hospital uses to make sure the isolated power stays isolated.
So, being “grounded” isn’t bad. You are grounded all the time. Go touch the knob on your kitchen sink. Chances are it is metal and it makes contact through the water pipes to the earth. Touching an electrically “hot” wire while also being grounded is what is bad. In that case, you complete the circuit between the “hot” wire and the “grounded” wire, because the earth acts like a wire between you and the wire that is grounded from the electrical supply.
Your outlet has 3 prongs in it (assuming you are in the U.S., which some dopers aren’t).In an AC system, the current goes out through one wire and back through the other, then flips around and goes out through the second wire and back through the first wire, and keeps flipping back and forth like that 60 times a second (60 Hz). So, in an AC system there isn’t really any different between the wires, if they are isolated. What we do is we arbitrarily pick one of the wires and ground it. We call this the “return” or “neutral” wire, and it is relatively safe to touch, even if you are standing on electrically conductive ground. The other wire is not safe to touch, and it is called the “hot” wire because of this. Touch the hot wire while you are touching something grounded, and ZAP. There is a third wire, called the “ground” wire, which is used as a safety connection to earth ground. The neutral wire and the safety ground are both connected together at your breaker box, and they are also connected to earth ground from there as well. You may think that it is kinda silly to run a separate wire if you are just going to connect it to the neutral wire anyway, but the important difference is that the safety ground never carries current, so it is always at true earth potential. Also, if you run through the different types of failures you can get, if your electrical device (like your stove, fridge, or whatever) has a fault and the metal case of it is connected to the safety ground, in all of the different scenarios the separate safety ground is more likely to mean you don’t get shocked.
