Electronics > Beginners

How is Neutral Wire Neutral in Mains Electricity?

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KeepItSimpleStupid:
My turn.

There is a REFERENCE (Earth/ground), a RETURN (neutral) and at least one live.
there is one point for a dwelling where the reference and the return are connected together.
The reference/ground is not supposed to carry current except during a fault.

Now we can also talk about bonding.  Your copper plumbing gets connected to earth/ground.  There is no current flow normally.

If you touch the washer and the water pipes, you don't get shocked.

Now you have to analyze faults.

A hot wire touches the grounded case of the washer.  Large current through the ground wire which is an alternate path to the neutral and the breaker trips.

insulation breakdown in the washer and a human touches the washer.  A little bit of current >7mA goes through the human.  The GFCI (RCD) breaker detects this by measuring the difference of currents between hot an neutral.  Human doesn't get harmed.

A poor connection an a lamp starts to arc, so the AFCI (Arc fault catches that).

Let's go back to this EARTH thing again.  During a lightning storm, the potential across the earth changes.
So, your garage might be at a different potential than the house and it might be from static electricity, so we put a ground  rod at the detached garage.

Now lets add the complication of a sidewalk surrounding a swimming pool.  Guess what, the  sidewalk is done differently than the sidewalk in front of your house.  You have to prevent a shock between the pool water and the sidewalk during a lightning storm.

Maybe this helps.  Maybe it doesn't.



PS: I'll add a comment about hopitals, transmitters and daisy chaining power outlets and isolated ground outlets if this is understood.





IDEngineer:

--- Quote from: help_me_pick_username on January 14, 2020, 08:22:21 am ---I am kind of confused about the Live and Neutral wires in Mains wiring. I've seen YouTube videos where someone touches the Neutral wire, but it doesn't shock them, but touching the Live wire does.
--- End quote ---
Aw, what the heck, I'll wade in here too....

1) From Kirchhoff's current law, we know that the current into a mode must equal the current out of that node. If you have a load (light bulb, motor, heating element, whatever) between the "live" (black) and "neutral" (white) AC wires, both wires are carrying the same current. Yes, it's reversing 60 times per second (in the USA), but the current has to be the same. And if the current is reversing... which side is the "source"? The electron flow isn't "stronger" when the black wire happens to be positive, or vice versa. That white "neutral" wire is carrying current and can shock you. Does this change if it's connected to a copper grounding rod? Does the distance from said copper grounding rod matter? What is the inductance between the node in question and the copper rod connection? Are you CERTAIN there are no wiring errors in your house? We can argue about the finer details but are you willing to bet your life on it?

2) To understand where the concept of "neutral" originates, think of a center tapped transformer with its 240VAC output connected to your breaker panel's input. If you connect a 240VAC load (like an electric furnace, electric oven, electric dryer, etc.) then the reversing electron flow occurs between the two "live" wires (black and red) and the center tap (white) would see no current flow. Indeed, many such appliances don't even require a connection to the center tap (white) for precisely this reason. But what happens when you connect a 120VAC appliance? Now the circuit is closed by the center tap (white)... and that center tap is conducting current, as explained in #1 above.

3) All of the above is based on relative voltage potentials. For current to flow, there must be a voltage differential - and a voltage is always measured relative to some reference point. From one point of reference, the center tap (white wire) sits at zero VAC because it's the center tap of the power company's transformer and the 240VAC legs (black and red) oscillate around it. However, if you connect an AC voltmeter from black to white, or red to white, you'll still measure AC voltage. Go ahead, reverse the leads of the voltmeter - the AC voltage is still there! {grin} And if YOU are a current path between any two nodes of differing voltage, you can conduct current. The electrons don't care that you're a human. They don't care that there's another, perhaps lower reactance path in parallel with you. Some of those electrons will be happy to use you as their path to complete the circuit, and your body won't be happy that they do. We must respect electricity because electricity has zero respect for us.


--- Quote ---I've seen YouTube videos where someone touches the Neutral wire, but it doesn't shock them, but touching the Live wire does.
--- End quote ---
YouTube is the modern-day equivalent of Darwin, always trying to clean up the gene pool!

Under certain circumstances, if you were fully and completely electrically isolated from everything else, you could touch even the black or red wire - by itself - and not get shocked. Technically your body would be raised to the same AC voltage potential but since you'd only be in contact with ONE point, there'd be no opportunity for current to flow. This is why birds can sit on power lines, Tesla has those photos of himself being raised to hundreds of kilovolts, etc. It's not solely about being at any given voltage potential... it's about current flow. Current is what kills you, voltage just makes the current possible.

Bottom line: It is NOT inherently safe to cavalierly touch the center tap. When I'm working on AC power, I presume ALL wires are potentially lethal. I kill the breaker in question and then I ALSO check using a handheld "power checker" (electric field detector) that I've tested on a known-live circuit moments before. And even then, I don't touch more than one wire at a time if I can help it. This is your life you're gambling with.

IDEngineer:
So many triggers in a single message!


--- Quote from: KeepItSimpleStupid on January 17, 2020, 05:53:02 pm ---A poor connection an a lamp starts to arc, so the AFCI (Arc fault catches that).
--- End quote ---
AFCI's also "detect" welders and happily turn off to "protect" you from them. Grrrr. A recent remodel in my house required me (per NEC) to use all AFCI breakers. First time I connected my MIG welder, the freakin' 10X-as-expensive AFCI simply refused to let the welder operate. (It's an "arc" welder! It's literally how they work!) I ended up running a 12ga extension cord to another, distant outlet that wasn't part of the remodel. Of course, NEC also wants to minimize the use of extension cords (they regularly decrease the allowed distance between outlets for precisely that reason). So which is it? They literally want it both ways.


--- Quote ---So, your garage might be at a different potential than the house and it might be from static electricity, so we put a ground  rod at the detached garage.
--- End quote ---
Yep, that's what I do! Except that NEC doesn't allow that anymore. A few years ago, I helped my brother wire up his brand new detached garage. Thinking like an Engineer (and not like an Electrical Inspector!), we drove in a 10 foot copper grounding rod to provide a separate, dedicated earth reference for the garage. The three wires of the 240VAC service (black, red, and white) came through conduit that we buried 18+ inches below grade, per NEC. We bonded the neutral to the earth reference at the garage breaker panel. Everything worked. But when the Inspector came for the final, he refused to sign off because NEC required that we run a ground wire between the buildings and not use the separate ground rod. The detached garage was only to have its earth reference from the other building. I tried to reason with the Inspector, pointing out that a separate ground rod - closer to the breaker panel in question - was actually BETTER and SAFER than running a separate copper wire nearly 100 extra feet to the breaker panel in the main house. How was the connection from the power company to the house - a three wire connection with its own copper ground rod - different from the garage, which treated the house like the power company and received a three wire connection with its own copper ground rod? The logic finally got him, and he devolved to (actual quote) "Unless you do it my way, I'm not signing it".

So we did it his way. We pulled a fourth, bare copper wire through the underground conduit. And after he left we reconnected the second ground rod. We left the connection between the two buildings too. More is better, right? Grrrr.

Monkeh:

--- Quote from: IDEngineer on January 20, 2020, 06:36:46 pm ---Yep, that's what I do! Except that NEC doesn't allow that anymore. A few years ago, I helped my brother wire up his brand new detached garage. Thinking like an Engineer (and not like an Electrical Inspector!), we drove in a 10 foot copper grounding rod to provide a separate, dedicated earth reference for the garage. The three wires of the 240VAC service (black, red, and white) came through conduit that we buried 18+ inches below grade, per NEC. Everything worked. But when the Inspector came for the final, he refused to sign off because NEC required that we run a ground wire between the buildings and not use the separate ground rod. The detached garage was only to have its earth reference from the other building. I tried to reason with the Inspector, pointing out that a separate ground rod - closer to the breaker panel in question - was actually BETTER and SAFER than running a separate copper wire nearly 100 extra feet to the breaker panel in the main house. The logic finally got him, and he devolved to (actual quote) "Unless you do it my way, I'm not signing it".
--- End quote ---

Better and safer? Did you verify the loop impedance of your earth rod? No, then how do you know the breakers will operate correctly? What you did was completely change the earthing arrangement!

IDEngineer:

--- Quote from: Monkeh on January 20, 2020, 06:44:02 pm ---Did you verify the loop impedance of your earth rod?
--- End quote ---
Do you think the Inspectors measure the loop impedance? Do you think the Inspectors even understand the concept of loop impedance? I can guarantee you this one didn't, and none of the others I've worked with over the years did either. Some of them have been very good and very well informed, but the heart of their knowledge is the code - not pure electrical theory.

Think about how power wiring works. In the original house, a three wire connection came into the panel (two phases and neutral). The earth reference came from a copper rod in the ground next to the house, which is bonded to neutral at the breaker panel.

What I did for his standalone garage was EXACTLY the same thing. We brought a three wire connection into the panel (two phases and neutral) and the earth reference came from a copper rod in the ground next to the garage, bonded to neutral at the breaker panel.

What the Inspector demanded was to abandon the (local) earth reference, and instead rely on a nearly 100 foot long run of copper wire back to the main house panel. When discussing circuitry on this forum, we often worry about a few centimeters of copper trace introducing excessive and unnecessary impedance into a ground reference carrying milli- or microamps. Here we're talking dozens of feet, potentially carrying tens of amps if a fault occurs. I'll take the closer, shorter, lower impedance earth reference.


--- Quote from: Monkeh on January 20, 2020, 06:44:02 pm ---how do you know the breakers will operate correctly?
--- End quote ---
Breaker operation is not affected by the earth reference. Even the most recent AFCI/GFCI breakers do not have a connection to earth - only to neutral. And while neutral is bonded to the earth reference at the panel, that is outside the circuit being monitored by the breaker.

EDIT: It might pay to review how breakers work.

GFCI's monitor the current flow on both sides of the circuit (live and return). Those should be equal under normal conditions. If the GFCI detects a differential current above some threshold, it opens the breaker because some of the current is following a path other than back through the breaker - and that path might be a human body.

AFCI's monitor (very roughly speaking) the rate of change of current flow. This allows them to detect arcs regardless of the absolute amount of current that is flowing.

The two types of detection are not interchangeable, which leads to three individual types of breakers: 1) Current-only (traditional), 2) GFCI's, and 3) AFCI's. There is a fourth style that is a combo AFCI/GFCI. Note that the three latter types incorporate the first, in that all of them will trip on excessive current. The point of the latter three types is that there are fault conditions that do not necessarily involve excessive current; you can kill someone without 15A of AC, and you can start a fire from an arc sustaining far less that 15A too.

None of the above involves nor requires an earth reference.

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