Basic Electricity: What is Neutral Wire & Why We Need It

[Russell Ensslin]

What is the neutral wire? Why do we need that?

 

[Jony130]

Neutral wire provide a path for a returning current to the "source" (neutral is a wire that carrying the full load current).

 

[Russell Ensslin]

Thank you for your response.
Some you tube videos say alternating current changes directions frequently. Does it change back and fourth through the two hot wires? why doesn't the neutral cause a short? Other sources say high-tension lines can overbuild up too much charge causing risk of "static" discharge; would this be a short? or Ground?

 

[Windadct]

Hello Russel and Welcome - I disagree a little with Jony, the Neutral wire does provide a path back to the source, but does not necessarily carry the full load current. Think of the the Neutral as a Neutral point in the system. In most (not all) systems the neutral is grounded near the source. This allows for a path of current back to the source, but keeps the ground out of the normal path - there are many reasons for this being beneficial.

And yes - Alternating Current "alternates" direction but this is both current and voltage - So power can be delivered TO the load with the current flowing in both directions - this is represented in P = V * I .

So a load can be load can be connected between a "hot" leg and the neutral - the current then flows through the load and the neutral. But you would not connect the Neutral to the Hot leg - this would cause a short - the idea is to deliver energy to a load.

 

[Jony130]

As for the AC current we simply measure all the voltage with respect to the neutral wire (neutral wire is our reference point).
View attachment 85300
And this means that

The current flows out of HOT and into NEUTRAL (through the load) while the voltage on HOT is POSITIVE. When the voltage on HOT momentarily goes to zero there is no current flow. Then, when the voltage on HOT goes negative the current flows out of NEUTRAL and into HOT (through the load). The current can flow into or out of NEUTRAL (through the load) depending on whether HOT is positive or negative voltage. The current can flow in either direction through the load. In almost all cases it doesn't matter to the load which way the current flows, which is why it can accept AC voltage directly, like a toaster or a filament light bulb. (we will skip the exceptions for now).

And we in Europe use this grid system.

 

[Russell Ensslin]

So if the Neutral is grounded near the source, won't that break the circuit? In other words; if the load comes from the hot leg to the resistance (application) then to the Neutral (ground) why do we call it the Neutral not Ground? And why can't we ground it at the resistance like DC?

 

[Russell Ensslin]

So let's see if I have this straight. The application doesn't care if the load is positive or negative, as long as there is current carrying voltage back and fourth; or in one direction for that matter, as long as there is current.

 

[davenn]

Greetings Russell
welcome to PF

So if the Neutral is grounded near the source, won't that break the circuit? In other words; if the load comes from the hot leg to the resistance (application) then to the Neutral (ground) why do we call it the Neutral not Ground?

The neutral is connected to ground in several common places ...
1) at the power station,
2) at the transformer pole in your street/suburb,
3) at the power distribution board in your house or business

we call it neutral, because we ALSO have an Earth (ground) wire in a standard 3 pin plug/socket ... Some appliances only have a hot (phase/live) and neutral.
Others have an Earth ( ground) wire as well that connects to the appliances metal chassis/casing

And why can't we ground it at the resistance like DC?

have no idea what you mean by that ... a DC circuit also uses 2 wires the positive wire and negative aka 0V wire

The application doesn't care if the load is positive or negative, as long as there is current carrying voltage back and fourth; or in one direction for that matter, as long as there is current.

a lot of confusion in there
voltage isn't carried anywhere. Moving electrons are the current. The electrons are the charge carriers
many AC appliances don't care about polarity ... a light globe, a heater ( in all its forms ... oven (not microwave), room heater, jug/kettle)
Other things require the correct phase and neutral connections ... eg motors

In an AC system, the electrons don't really go anywhere, they just move rapidly back and forward about a point/position ( oscillate)
Your light globe in your house will never see the electrons that are moving in the wire in the generator at the power station

It is the EM (electromagnetic) wave that carries the energy through the circuitDave

 

[Russell Ensslin]

Greetings Russell
welcome to PF
"The neutral is connected to ground in several common places ..."

If the neutral connects to the ground then why not ground it at the device? Why carry the load back to supply after the application if it's going to be grounded before it gets back to the transformer?

 

[davenn]

If the neutral connects to the ground then why not ground it at the device? Why carry the load back to supply after the application if it's going to be grounded before it gets back to the transformer?

because you DONT want the metal chassis to be part of the AC circuit

the separate Earth connection gives a safety factor, if the phase/live becomes dislodged and touches the chassis, there will be an instant fuse blow back at the power panel
D

 

[Russell Ensslin]

My question is; what is the difference between in neutral in the ground? Especially if you're going to connect the neutral to the ground after it's done whatever job it has to do? And how can it do that job if it gets connected to ground before it reaches the transformer?

 

[Russell Ensslin]

So the grounded conductor is a phase of the circuit ? And the ground is a safety from fault?

 

[jaus tail]

The ground provides path for fault current. This happened at a site where i worked. My laptop would give me electric shocks. There was no ground at the socket where I would plug the charger. If i'd touch the metallic edge of the laptop I'd get small currents. I made a practice of using a page to flip open the laptop n was careful to restrict touching only the keyboard.

if there is any insulation failure in the appliance n if you touch the metallic edge, then leakage current may flow from appliance edge to ground through your body. but if the appliance is grounded, then the leakage current will flow through the ground wire.
a ground wire isn't supposed to carry current under normal conditions. though the secondary of transformer is grounded, current won't flow through it under normal conditions.

 

[dlgoff]

if there is any insulation failure in the appliance n if you touch the metallic edge, then leakage current may flow from appliance edge to ground through your body. but if the appliance is grounded, then the leakage current will flow through the ground wire.
a ground wire isn't supposed to carry current under normal conditions. though the secondary of transformer is grounded, current won't flow through it under normal conditions.

Yes. This is correct. I just wanted to post a couple of my favorite images from http://ecmweb.com/bonding-amp-grounding/grounding-vs-bonding-part-1-12

This one shows the currents path of a fault through the safety ground.

This one shows the "touch potential" due to an inadequate ground.

 

[jaus tail]

Thanks for the image dlgoff. I have one question. Current always flows through least resistive path. So in the first image, if we look to the left, then at the secondary of the transformer why does current flow X1---meter---main---panel---outlet---load---X0 instead of X1---X0---ground, since X0 is grounded?

 

[Windadct]

J-T -- the current needs to flow in a loop. If it goes X1 - X0 - Ground- then to where? -- Be careful with the thought "Current always flows through least restive path." - this can be misleading thinking that ALL of the current flows in the least restive path; when we have 2 or more paths the current will divide and the least restive will carry more current than the higher restive path.
Russel - the Neutral is part of the circuit normally, it is A (as opposed to the only) return path to the source. The process of grounding ( earthing) is primarily for safety, but has other benefits when well applied, such as affecting power quality, noise, etc.

 

[dlgoff]

Current always flows through least resistive path.

From the same website that I posted the previous images from ...

One of the most dangerous ideas circulating in the electrical industry is that electricity follows the path of least resistance. But if it did, not a single one of our electronic gadgets would work.

Understanding how electricity flows through a circuit

 

[Russell Ensslin]

So I've been researching and have found that the load tries to get back to it's place of origin; not the ground. That's why its called the neutral.

 

[jaus tail]

Thanks for the X1 to X0 to Ground then where? That what i was thinking while posting what i said. But then what happens in case of fault? Like when lightning strikes or in case of insulation failure of appliance where current leaks to the appliance body and could give the user a shock if the appliance isn't grounded.

Like the current indicated by green arrow in the diagram. That current goes to ground but doesn't return to the appliance from where it is leaking.

Maybe there must be two separate grounds: one at load end and one at secondary transformer:

Any insight would be helpful...

 

[dlgoff]

Thanks for the X1 to X0 to Ground then where? That what i was thinking while posting what i said. But then what happens in case of fault? Like when lightning strikes or in case of insulation failure of appliance where current leaks to the appliance body and could give the user a shock if the appliance isn't grounded.

Like the current indicated by green arrow in the diagram. That current goes to ground but doesn't return to the appliance from where it is leaking.

Maybe there must be two separate grounds: one at load end and one at secondary transformer:

Any insight would be helpful...

You are asking a very important question here. You should go through all of the Grounding vs Bonding parts. I'll provide some quotes, followed by images, that I consider high points from those parts.

The Earth's resistance is too great for it to be an effective bonding jumper. Very little fault current returns to the power source winding if the Earth is the only fault-current return path. ...

.
.
.

Properly grounding and bonding service equipment improves safety while eliminating a common cause of power quality problems. You just have to make the right connections in the right places. If you think in terms of providing a low-impedance ground-fault path back to the source, you'll have no problem.

The benefits of proper bonding include clearing faults, preventing shock, and reducing fires. Grounding provides a low-impedance path for lighting, and properly grounded systems operate as intended. Improperly grounded systems create fertile ground for power quality problems.

The preferred practice is to not use the grounded conductor (neutral) as the effective ground-fault current path. Instead, you should install an equipment grounding (bonding) conductor with the feeder conductors to the building or structure ...

... electrons that leave a power supply are trying to get back to their source, not back to the earth. However, lightning is trying to get to earth.

this quote and image is from http://ecmweb.com/content/earth-not-bonding-jumper

... bonding jumpers tie the grounding system together through a reliable low-impedance path.

I hope this will give you an idea of the considerations. That's why there are electrical codes ... to minimize risks.

 

[Russell Ensslin]

J-T -- the current needs to flow in a loop. If it goes X1 - X0 - Ground- then to where? -- Be careful with the thought "Current always flows through least restive path." - this can be misleading thinking that ALL of the current flows in the least restive path; when we have 2 or more paths the current will divide and the least restive will carry more current than the higher restive path.
Russel - the Neutral is part of the circuit normally, it is A (as opposed to the only) return path to the source. The process of grounding ( earthing) is primarily for safety, but has other benefits when well applied, such as affecting power quality, noise, etc.

What is X1? X0? Is there an image somewhere?

 

[Windadct]

I was responding to J-T (p15) asking about the top image in P14

 

[jaus tail]

I was asking regarding this image: like in case of fault at the load end(extreme right side) the fault current(green line) flows from appliance to outlet to panel to main to meter to X0 and then to ground
Why does this not happen in normal working case like: X1 to X0 to ground or X2 to X0 to ground.
thanks for pointing out that lightning wants to reach the ground.

I guess both the source and load ends must be grounded and the ground will act as return from load to source in case any insulation breaks at load end.

 

[Windadct]

I really like your second image ( but apparently my opinion here sucks) - if I read this right you answered your own question?

 

[dlgoff]

( but apparently my opinion here sucks)

Why would you think this? This is a discussion forum that values reaching clear understandings of scientific concepts.

 

[Russell Ensslin]

I hope you guys know that you help a lot of people. Just because someone gets on here an asks a few questions and then you never hear from them again doesn't mean you haven't helped them. #155 posts! 866 posts! I bet more than a few of those posts have improved the lives of more than a few people.

 

[Russell Ensslin]

So the un-spent electrons need to get back to the source? Is that it? If the electrons come out of the hot leg and go into the appliance and get spent as work why then does anything need to get back to the source? Why not just the hot leg to the appliance and the appliance burn the amps in the form of current? Why neutral? Does the returning neutral current have less current? Does the ampacity start out with say 20 amps and then the vacuum cleaner use say 3 amps and then the remaining 17 amps return as spent current via the neutral to re-unite with the other electrons back at the transformer? Why not just stay in the hot leg wire until their turn to power the vacuum cleaner? Back at the transformer how does the spent amps get back over to the hot leg? Why don't they drain out into the ground electrode?

 

[Russell Ensslin]

These must sound like stupid questions but I'd rather ask and look silly than not ask and actually be.

 

[davenn]

So the un-spent electrons need to get back to the source? Is that it? If the electrons come out of the hot leg and go into the appliance and get spent as work why then does anything need to get back to the source?

No, this is incorrect, particularly as far as an AC system is concerned
A way back in post #8 I commented that the electrons don't really go anywhere ...

In an AC system, the electrons don't really go anywhere, they just move rapidly back and forward about a point/position ( oscillate)
Your light globe in your house will never see the electrons that are moving in the wire in the generator at the power station

Does the ampacity start out with say 20 amps and then the vacuum cleaner use say 3 amps and then the remaining 17 amps return as spent current

no, The resistance of the circuit ( wire resistance + the load) determines the current flowing in the circuit
The wall outlet may be rated at 20A ... that's the max rating, it isn't supplying that amount all the time
lets take the 240VAC mains where I live, and you plug in a 100W light globe,
I ( amps) = Power (Watts) / V (Volts)
I = 100 / 240
I = 0.416 A being drawn even tho the outlet can supply 20 A

another example
a 1000W room heater
I = W/V
I = 1000 / 240
I = 4.16 A

now knowing the current being drawn by the heater element you can now work out its resistance

R (resistance) = Volts / Amps

work it out

so you see there are no spent or unspent amps cheers
Dave

 

[davenn]

Now I should also mention that, because of the heating of the lamp filament and the heater element wire
their resistances will change a bit ... but we won't go down that path at the moment

You need to grasp hold of the basics first D