Recently I saw a power line with 2 wires (hold on). Only one AC power line and one ground wire on top. If I understand power correctly you need at least 2 wires to make a circuit and power stuff. Also another question, which wires on the power line do what?
I’m in the UK here, so my answers are biased towards our system. We have 3 pin mains plugs here, in a triangle. Bottom left is live, bottom right is neutral(also called return), and top is ground. The purpose of these three wires is as such:
Live and neutral are the current path for a circuit. There’s a potential difference (voltage) between them, and when you connect a load across them, current flows.
Ground serves as an emergency backup. If your device has a metal chassis, one potential issue arises if the case is ever accidentally shorted to the live wire. In this instance, the path of least resistance will be the body of the human touching it. To this end, the neutral wire is connected to the metal case, and if the live wire ever shorts to the case, current will flow from the live wire to the ground wire, away from the human.
The ground wire therefore serves as a convenient out-path for any stray electricity. your house will also detect an issue using an RCCB, which measures the difference between live and neutral current to detect stray current.
In the case of the cable you saw, I suggest it was probably not the live and ground cable, but the live and neutral cables. This is because the engineer designing the system decided it didn’t need ground protection.
I’m not sure what kind of cable you saw, if it’s an overhead cable, it didn’t need grounding because it’s difficult for a human to touch.
If it was an appliance cable, it didn’t have ground because the casing of the device is plastic and there was no way for a short to affect a human.
Hope this helps!
Three or 4 wires power lines are 3 phase transmission lines.
There is also a thick line that runs under the 3 ones that carry low voltage data like telephone or cable tv.
Update to anyone reading this in the future.
If you see a pole with only 2 wires, the one on top is neutral and the one lower is 7200 AC main. Before the power hits a house, it’s converted into 3 phase by the transformer so it can be used by your washing and drying machines and anything else using a large electric motor in your home. The neutral wire is sent to your home.
Your home will convert the 3 phase into single phase and a ground wire from your home will be added into a standard outlet.
However, most 3 phase poles carry more power and can’t be carried by a single wire. The thin wire on top doubles as a static wire (balancing out charges) and a neutral which is also sent into your home
If you think of it like water, ground is like the ocean and the “hot” wire is a straw. The ocean has virtually limitless supply of water, but there is no force pushing it into the straw. When you make the straw “hot”, you can both suck water out of the ocean and push water into the ocean without making any significant change in the amount of water that is available there.
This a very simplistic comparison to how AC works. It keeps alternating if it is sucking or pushing the current and the force of going back and forth is what does the work.