Title, really. I recently had to reverse the polarity of a badly installed receptacle. I read that the current will work either way but I could shock myself on the light bulb socket, for instance, if the receptacle has its hot and neutral reversed. But why any of this matters when the current is constantly changing direction anyway? Why is the neutral “safe” ?
In: 3
The neutral side is bonded to ground. This provides safety. Devices are made so that the neutral is connected to ground. If a part fails and shorts, the current flow will go to ground, blowing a fuse or breaker. If there was no ground and the device became energized, then the person touching it would complete the circuit. Think of it the way, would you want to get into a pool that was ungrounded, with a pump hooked up to an ungrounded power source. The pump could fail and energize the entire pool. The failure could just be the insulation failing causing the frame to be energized. Since there is no bonding or grounding (no neutral), the breaker may not trip. Somebody getting in or out could complete the circuit.
Simple: There isn’t much difference between “neutral” and ground. Your neutral line is connected to ground at the point where it comes into your house. So if current can go from your to neutral, it’ll go from hot to ground about as easily.
So, touching the neutral wire while also touching, say, a metal faucet handle will not shock you (except in really unusual circumstances where something else has already gone very wrong). But touching the hot wire while touching that metal faucet handle WILL shock you.
This is handy for a few reasons. One reason is that you can use hot+neutral for 120V for most things, but power appliances that need more power with hot+hot (using two of the three phases) to get 240V.
It also simplifies the design of some small appliances: because you have a “hot” and a “not hot” wire, you can switch it on and off by disconnecting the “hot” wire where it enters the appliance. There’s no need to disconnect the neutral wire also. If you used “hot/hot” then your switch would be to disconnect both wires.
AC is more properly called an alternating voltage source. So voltage is almost always referenced – ie it alternates with respect to something else (it measures a difference in voltage not an absolute voltage)
So the neutral provides several functions.
1) The neutral line serves as the reference level for voltage. The voltage is alternating with respect to the neutral.
2) The neutral line provides a return path for the current. “Return” here being relative again since it alternates sourcing and sinking current BUT always at the reference “0” volts. Without a neutral, a circuit will not be complete and therefore will not function.
3) For most 110V or 220V (or similar) in a household context, the neutral is also typically tied to ground. But we differentiate neutral and ground for SAFETY reasons. A badly wired or misfunctioning device may cause parts of the “neutral” line to be at “live” voltage. Almost all higher powered devices will have the case or something similar tied to ground. This way a malfunction will likely (not always!) cause a short circuit to ground and trip the circuit breaker rather than allow the device itself to present a dangerous voltage to the unsuspecting user.
4) For higher voltage consumer products (ovens, boilers etc), neutral may not be equivalent to ground. As the neutral could be tied to a different phase on a 3-phase system.
In any case, wiring simple home devices is rather straightforward and generally safe with a little precaution. So a PSA is to never try to meddle with high voltage and high power devices unless you understand at least the basics of power systems.
in north america we use split phase alternating current to get 110v. this is used for low draw appliances and lighting, etc. this is supplied by one hot and one neutral wire.
the full phase gives 220v and is used for high draw appliances like the stove, clothes dryer, heater/hot water, air conditioning, electric car charging, etc. this requires two hot wires (of opposing phase) to be run to the appliance.
inside the panel are two hot supply lines, red and black. the neutral is white, and ground is bare or green. red+white makes up 110v. black+white also has 110v but is opposite phase. red+black gives 220v.
Voltage is a difference, like height. When you measure how tall you are, you start at the bottom of your feet, and go to the top of your head. Only having a hot is like only measuring from the top of your head: there’s nothing to measure it from, so there’s no height.
This is also why birds can land on a power line and not get shocked. When they touch the line, there’s no difference between them and the line for current to pass through, so they’re just at the same voltage as the line. However, if they were to also be touching something connected to ground (or another line), then there would be a difference in voltage which would allow current to pass through them and they’d be shocked.
Latest Answers