Why does the power company provide a neutral line?

Question

I understand that power is often transmitted in 3 phases (with no neutral). Then, when we get to a certain substation, the power company basically gives house 1, L1, house 2, L2, house 3 L3, and connects them all to a common neutral line. That is, each house gets one phase of power and a neutral shared between all 3 lines. Then, that neutral is grounded to earth at the substation.

It is also my understanding that in the main panel in my home the neutral is tied to ground. It seems to me, if we are going to do this, why do we need the neutral wire at all. In fact, in any system, if the power company actually grounds the neutral wire at the substation, why can't each individual house simply provide its own neutral (i.e. each house has a single phase and a metal pole or two out in the back in the ground that serves as a neutral (current carrying earth ground) and a ground (for safety)). It seems to me this would save the electric company from having to provide a neutral wire. My point is is that the neutral wire is earth grounded at the substation and in each home, so why is it even necessary to provide it?

In my setup, current would flow from the 1 wire coming into the home from the pole (single phase) and current would flow to an earth ground provided at each home. There is no reason for the power company to provide the neutral.

Can you fix my misconceptions please? I have read many posts and I get conflicting or contradictory information about this.

Answer 1

A voltage is a potential difference between conductors. For instance the difference between the live and the neutral wire.

The power company delivers these two lines to ensure the potential difference to the customers. If it would only supply the live wire and the ground of a local customer is used, it's unknown which voltage the customer receives.

It is because there is no knowledge about the potential difference of neutral between the power facility and your house. It can be higher or lower. And then the resulting voltage on your devices can also be higher or lower.

Answer 2

I think part of the reason you're seeing contradictory information is that practice differs in different countries. So if you pick up part of the picture in one place and part in another it's not surprising you'll see contradictory information.

Stick to one location and understand how its practice works, and you'll be in better shape. In the UK, practice should follow BS7671, formerly known as the "IEE Wiring Regulations", currently at the 17th edition. The first edition dates from 1882.

Your first paragraph summarises it well : each house gets a phase and neutral, and the neutral is grounded at the substation.

In this system, the ground at your house is isolated from neutral, not connected to it, and thus carries relatively little current. However it should have low enough impedance to carry enough current to keep you safe in the event of a fault, until the main house fuse blows, or breakers trip. This ground impedance is assured - traditionally, by connecting to the cold water pipe in the days when they were metallic - and it is tested - for new houses, before you move in.

In fact, in a house protected by modern breakers - RCDs in the UK, or GFCIs (US) - if you divert as little as 20mA from neutral to ground anywhere in the house, the breakers will detect the imbalance and disconnect power, in case that 20mA was flowing through you.

Consider what would happen if you economised and only used one conductor for both neutral current, and safety earth. Now imagine excessive current flows because you overload the circuit, or a bad connection develops high resistance and overheats, and the neutral/earth conductor melts before the live...

Everything in your house (including the neutral/ground wire, and thus all exposed metalwork), is live, and you are wandering around in the dark trying to fix it. How happy are you about that?

Answer 3

You could disconnect the neutral line and use the earth to carry the current back to the power company.... as long as you only use a very little amount of current. If you are brave, take a small LED night night and connect one of it's prongs to the hot side of a power outlet and the other to a rod driven into the ground. It should light up. (Be really careful if you are going to try this - it can be dangerous if you don't know what you are doing!)

Why only a little night light? Why not disconnect the neutral from the power company from my house and run my blender using only a neutral connections from my house to the rod in the earth?

Because dirt has a much, much, much higher resistance than wire and the power station is far away. The more current you try to send though the dirt, the higher the voltage drop there will be across the dirt. There will not be enough power getting though the earth to run the blender.

Note that there actually are some "earth return" distributions systems that use the ground as one of the conductors- they just typically run at much, much higher voltages than your house. Because the voltage is so high, the current is low (ohms law) so the resistance of the soil is less of an issue.

https://en.wikipedia.org/wiki/Single-wire_earth_return

Answer 4

Anybody whom has ever seen what happens when a 12V car battery is cabled to two stakes driven in moist ground, will understand that wide-scale, high-power Earth conduction paths are not feasible. (In fertile soil, the earthworms will wriggle right out of the ground - which is great for catching fishing bait, but bad for the soil.)

Using soil as a conductor on a large scale would disrupt many burrowing creatures.

Answer 5

Why do we need both ground and neutral in the US?

Well, in the US, the neutral is a current carrying conductor. It is no different from the "hot" line except that it happens to be bonded to earth (in exactly one place). The ground wires carry current ONLY in the event of a fault. This scheme works well. The ground wire is bonded to all externally accessibly metal of all line powered equipment. Normally no current flows in the ground wire. But if the hot wire comes loose inside the equipment and shorts to the metal case, it will cause a large fault current and trip the circuit breaker.

What if neutral is shorted to the case? In principal, since current flows in the neutral wire, you could still get a shock in this scenario.

But most power outlets now have ground fault interrupt (GFI) circuit breakers. What these do is compare the current flowing in the hot and neutral conductors. If the currents are not exactly equal in magnitude (within a few mA) then the GFI trips. This is possible ONLY because both ground and neutral are used. And it is a great safety benefit.

So that is why we need both.

Answer 6

In a grounded system, electricity is always looking for the shortest(least resistive) path back to ground. It makes no difference what the resistance of the actual soil is if you are the only path to ground. One of the primary reasons for good grounding (low resistance) is so that electricity chooses a safe system ground path, rather then going through your body and killing you.

Answer 7

In U.S. high voltage 3 phase is sent across country. Voltage is stepped down through a series of transformers the last of which is local often pole mounted on street for residential service and is single phase. That last transformer's secondary coil is isolated from all system before it and is tapped at each end of the coil and at the center of the coil.

These three wires enter the residence l1, l2, and neutral which is the center-tap lead. The voltages between l1 or l2 and the neutral is in each case 120 volts, and that is the 120 volt service. The voltage across l1 and l2 is 240 volts and is used for higher power things like ovens and stove-tops.

The neutral is grounded [earthed] at the service panel directly. Also most homes have a separate household grounding system for safety.