If AC is alternating (+120v) - (-120v) then why we don't get shock by touching neutral wire

Question

If AC is alternating the current between phase and neutral then why we don't get shock by touching neutral wire?

Answer 1

Because neutral is the same potential as earth (but it is not the same as earth) and voltage on line relative to neutral swings +/- , while neutral stays at the same potential.

At the same time, the same current flows through N an L, but the potential of N does not change.

Answer 2

Assuming there aren't any issues with the wiring, the electric potential between ground and neutral is virtually nothing, thus there wouldn't be any current that would flow through you to ground. You'll only get shocked if there was an electric potential, such as touching live/hot wire and grounding yourself.

Answer 3

We should start by understanding that voltage is not an absolute quantity, but rather a measurement relative to some reference - i.e. it is a measurement between to points.

In mains wiring, we define this reference point as "earth". While you may not be touching earth, e.g. if you have insulated shoes, but it is highly likely that you may touch some exposed earth (pipework, metal enclosures, etc), so that is why it is picked as the reference.

Now if we were to measure the voltage (potential difference) between earth beneath your feet, and the earth between my feet, it will be some non-zero value unless you are stood next to me. But that doesn't matter, because it's the earth under your feet that you are touching, so we will define that as the reference point.

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In many places in the world (not sure if all), the neutral wire in standard home and commercial wiring is bonded (physically connected) to earth at some point in the property (usually the main service panel). There should be a low resistance connection between the earth wiring in your house and neutral wire.

That means if you touch the neutral wire, and touch earth, the voltage should in a properly wired setup be very low, usually no more than ~10s of volts depending on the length of the wiring run and current draw.

It further means that the live wire will then be oscillating positive and negative relative to both neutral and earth by the full line voltage.

However this assumes proper wiring, no broken or reversed wires, and no long wire runs that can develop high voltage drops. There is no guarantee that neutral is safe to touch. If the earth bonding fails, or the neutral connection in a split phase system is lost, the neutral wire can rise to a dangerous voltage. It is therefore not necessarily safe to touch.

In fact for this very reason many wiring codes require a separate earth wiring, such that appliances are bonded to this earth wire and not the neutral wire for extra safety.

Answer 4

In order to get shocked you would have to have a current pass through some part of your body. Typically this happens when you are standing on the ground and then touch a wire that has a voltage respective to ground. In this case you would have a potential difference between your feet and your hand, and current would flow through you.

If you are insulated from the ground, you can touch a live wire and not get shocked, because there is no path for current to flow. Likewise if your feet are at ground potential and the wire you touch is also at ground potential no current would flow because there's no difference in potential. This is generally, but not always the case with the neutral, it is typically at ground potential so touching it usually won't cause a shock. Usually.

Imagine taking a 120 V battery, connecting two wires to the negative terminal, and holding one wire in each hand. Since both wires are connected to the same potential you won't get any shock. Now connect the wires to the positive terminal, the same thing would happen, same potential, no shock. It doesn't matter if the potential is positive or negative with respect to the other terminal, as long as you don't connect yourself to both terminals at the same time there will be no path for current. This is the same for AC, it doesn't matter if the polarity is reversing, as long as there is no difference in potential across your body there is no current.

(Disclaimer: I'm not suggesting you actually try any of this, doing it incorrectly is likely to be fatal. Line voltage is nothing to play around with.)

Answer 5

Neutral does not shock because it is a 0V wire, it does not alternate between +120V and -120V.

It's the Live/Phase wire that alternates around 0V.

And for AC it is RMS voltage, so for 120VAC the amplitude is 170V so it alternates between +170V and -170V.

Answer 6

Because the NEUTRAL wire is at the same POTENTIAL as earth and you
(as you are usually in touch with earth or ground, either with your feet/skin directly, through moist socks or shoes or through the capacitive effect between your feet and conductive surface you are standing on).

By the way, it's not ±120 V; that is the EFFECTIVE or RMS voltage.
The actual value is about ±168 V peak, meaning it goes up to +168 V RELATIVE to ground or neutral wire, and then down to -168 V RELATIVE to neutral.
Like I said, since you usually in touch with ground in some way, there is no VOLTAGE affecting you.

VOLTAGE is the POTENTIAL DIFFERENCE or DIFFERENCE BETWEEN TWO POTENTIALS.
Since you and neutral are at about the SAME POTENTIAL at all times while the LIVE/PHASE potential changes RELATIVE TO NEUTRAL there is no POTENTIAL DIFFERENCE and thus NO VOLTAGE between you and neutral, thus no current will flow.

If you happened to be standing on a surface connected to LIVE wire (and not in contact with any other potential, neutral or ground), and then you touch another LIVE wire that is at the same POTENTIAL (same phase), there will be no potential difference, i.e. no voltage involved between those two wires, and thus no current through you and no shock.

^{simulate this circuit – Schematic created using CircuitLab}

The resistors R1, R2 and R3 represent you and your resistance.
VM is voltage meter, each one measures voltage across its corresponding resistor, in other words the voltage between your hand that's touching + or - side of the batteries, or touching the neutral.
AM is ampere-meter, measuring the current through its corresponding resistor, in other words through you when you touch the corresponding wires.

I hope that my explanation is clear enough.

Answer 7

You get shocked by potential difference. The neutral wire is connected to the Earth somewhere in the system, as is your body, so when you touch it, the difference in potential between your body and that wire is very close to zero, and you feel no shock.

The live/line/hot/phase wire is not connected to Earth anywhere. The live wire rises and falls in potential with respect to neutral. One moment it is 170V more positive than neutral, the next moment it is 170V negative with respect to neutral. Consequently it oscillates between ±170V with respect to your body too, which is at the same potential, or near to the same potential, as neutral and earth. When you touch live, you will have 170V between your finger and your feet, and you will definitely feel that.

Answer 8

tl; dr: if everything is working properly, neutral is the same voltage, or close to the same voltage, as safety GND. You won't receive a shock because they share a common Earth GND reference at the panel.

But don't bet your life on it: neutral presented at outlets is energized and carries current, too.

On a North American style 120/240V panel neutral and GND are tied together at the entry panel to a common bus bar. The bus bar is, in turn, is connected to Earth with one or more grounding rods. Connected this way, if everything is working properly you won't receive a harmful shock by touching neutral. If there is a break in neutral, or some other fault, there could be hazardous voltage on neutral, too.

Maybe the thing that's confusing you is the idea that a wire that's carrying current can have no voltage. Well, it can, if:

• the neutral wire connects to the transformer neutral (in 120V regions, the center tap)
• the neutral wire connects to Earth GND
• the neutral wire has low resistance
• the neutral wire's current rating isn't exceeded

Any one of these conditions not being met could cause a dangerous voltage to be present on neutral.

Let's step back a bit. Here's what the service entry looks like:

From here: https://heatzone.ca/project-view/basics-of-homes-electrical-system/

What do the house wires do?

• Hot, "line 1", L1 (black): swings +/- 170V (120rms) compared to Neutral. Carries current.
• Hot, "line 2", L2 (red): swings +/- 170V (120rms) compared to Neutral. opposite phase to L1. Carries current.
• Neutral (white): tied to a bus bar with GND at the panel, to a grounding rod, then to neutral center tap back to the pole. Has almost no voltage. Carries current.
• GND (green): has no voltage. Tied to same bus bar (and only at the bus bar) as neutral. Does not carry current unless there is a fault.

A one-leg load connection will take current from L1 or L2, and return it back via neutral, through the grounded bus bar, then back to the transformer center tap. That transformer is floating, except that center tap (neutral), too, is grounded at the pole. While there is current on neutral, it all goes back to the transformer.

We see that because neutral and Earth GND are bonded together, in the home neutral is same voltage as GND, sort of. It will be if the neutral connection all the way back to the panel is good and it doesn't have excessive voltage drop, often called 'IR drop'.

Electrical code allows IR drop to be up to 5% of the voltage. Each conductor will be half the drop, or 2.5%, which means you could be seeing as much as 3Vrms (8.4V p-p) neutral swing on a fully-loaded branch. Will you get shocked if you touch that? You might feel a 'tingle' on it if the feed leg has a big load that's causing IR drop in the wiring.

You can see the IR drop effect for yourself if you have a high-watt appliance like a portable heater or hair dryer as a load and a voltmeter to measure GND to neutral. Switching on the load will increase the small GND to neutral voltage.

What if there is a break in neutral somewhere, such as a bad connection in a junction box? A load test will show neutral swinging at the full 120V line voltage when the load is switched on. If you've worked on house wiring, you've probably seen how it can be done badly, by DIY'ers and lazy electricians alike. It's very easy for a mis-sized wire nut to come loose, or for a socket wire-through to fail. Workmanship, experience and attention to detail matter.

Tip: if you ever see more than 3Vrms between neutral and GND, your wiring should be checked by a qualified electrician.

So, can you really safely touch neutral? Again, if its working properly, yes. If it isn't working (example, has a break in it as I discuss above) it could become live and shock you. Even if the neutral itself is good, it's also possible that the outlet or something connected to it (like, a lamp with an Edison base) could be miswired.

For saftey's sake then, it's best practice to treat neutral as if it were also live, until you can verify that it isn't.